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Researching the Regs: Owner Produced Parts

Researching the Regs: Owner Produced Parts

An in-depth look at FAR 21.9 and Advisory Circular No. 23-27 by aviation legal consultant and A&P/IA Kristin Winter

The FAA keeps an iron grip on the supply of approved replacement parts for Type Certificated aircraft. Replacement parts generally must come from the airframe, engine or propeller manufacturer, or from an approved source that has been issued Parts Manufacturing Approval, commonly referred to as a PMA. 

There are some other limited exceptions for what the FAA refers to as “standard” parts, such as nuts, bolts and other hardware manufactured under an industry standard such as AN (Army-Navy) or MS (Military Standard), or parts manufactured by a repair station. 

There is one major exception to the FAA’s tight grip, and that is the owner produced part. Owner produced parts are commonly used by the airlines, which often have a large fleet of the same or similar types of aircraft. 

Like the owner of a General Aviation aircraft, an airline often wants to avoid the high cost of commonly used parts from the original equipment manufacturer (OEM), so it will reverse engineer and produce batches of parts that are
then used in its fleet. An example might be landing gear bearings that wear
out frequently. 

The availability of owner produced parts appears to go back for decades, though the origin is unclear. As it is of biggest benefit to airline operators, there is a major constituency to make sure that owner produced parts remain an available solution for all aircraft owners and operators.

FAA standards and definitions

The FAA sets out the limitations on replacement parts in FAR 21.9. 

Paragraph (a)(5) provides that one type of approved part is one that is “[p]roduced by an owner or operator for maintaining or altering that owner or operator’s product.” 

This simple statement leaves lots of questions unanswered. One common question is: must the owner or operator physically produce the part themselves? Most GA aircraft owners may not be equipped or sufficiently skilled to make a part in their basement. Fortunately, the friendly FAA has provided an interpretation. 

When the FAA renumbered and revised Part 21 in 2009, it specifically made mention in the Federal Register that the interpretation memorandum issued on Aug. 5, 1993 was still operative. 

The answer to the first common question as to whether the owner/operator must physically produce the part is clearly no. The FAA memorandum states that “An owner would be considered a producer of a part if the owner participated in controlling the design, manufacture or quality of the part.” 

The memorandum goes on to provide five nonexclusive indicia that an owner “participated” in the production of the part (italics added):

1. The person provided the manufacturer with design or performance data from which to manufacture the part. (This may occur, for instance, where a person provided a part to a manufacturer and asked that the part be duplicated.)

2. The person provided the manufacturer with materials from which to manufacture the part.

3. The person provided the manufacturer with fabrication processes or assembly methods to be used in the manufacture of the part.

4. The person provided the manufacturer with quality control procedures to be used in the manufacture of the part.

5. The person supervised the manufacturer of the part.

Responsibility of owners, responsibility of mechanics

So what does this mean to our aircraft owner faced with the unavailability of a critical part, or who is suffering from cardiac arrest at the cost and time delay of obtaining the part from the original equipment manufacturer? 

It is important to keep in mind that it is the owner or operator’s obligation to produce a part that is airworthy, meaning that the part conforms to type design and is safe to install in the aircraft. 

The installing mechanic’s responsibility is only to make a reasonable assessment that it is an airworthy part and to install it properly. (It will likely help the mechanic feel comfortable if they are provided with a copy of the drawing, the specifications, and/or have been part of the process from the beginning.)


Ways an owner can participate

Two options for owner participation jump out of the memorandum on first blush. 

First, the owner can provide the part to an appropriate manufacturer (such as a machine shop) and ask them to duplicate it. 

The other option is if an owner supervises the production, which might involve working with the mechanic to fabricate the part. Supervision would not likely require an owner to stand there every moment, but to be reasonably available to provide supervision or answer questions. 

In practice, owner supervision might be a little difficult given the likely disparity of knowledge between the owner and the mechanic. However, if the owner is willing to certify in the logs that he or she supervised the production of the part, it is unlikely to be challenged.

FAR 21.9 put into practice 

So let’s look at some practical applications. Not long ago, I determined that it was time to replace the flap tracks on my Twin Comanche. I had spoken with another owner who had the same problem, and we agreed to pool our resources to obtain some owner produced parts. 

I obtained an exemplar track and sent it to a metallurgical lab for analysis to determine the proper material. The lab charged a bit over $200 for the testing and provided a formal report. The other owner produced a drawing of the part. 

Armed with material and drawing, we had a couple of ship sets made. The cost was about $75 each, and the sets were created in five working days (turnaround can vary depending on how busy the shop is), compared to a cost of over $300 each from Piper and a wait of unknown duration. 

We clearly met the first example of conditions that qualified me as participating in controlling the design of the part. (See photo, top of page 26.)

Some considerations 

One of the most useful options for the aircraft owner is the specific acknowledgement by the FAA legal memorandum that an owner may provide the part and ask that it be duplicated. 

There is one complexity here in that many machine shops can duplicate a part, but are not equipped to determine what material it was made from. That might mean that the owner will need to resort to the metallurgical testing lab as we did with the flap tracks. 

Consideration must also be given to whether the part had any protective coating which should be duplicated. This could mean having the completed part anodized or cadmium plated. 

All of this may make it uneconomical to use the owner-produced exception if one is simply trying to avoid purchasing an overpriced bushing from the manufacturer. 

This is an area where an active type club that maintains a database of parts that have been owner produced—and possibly test results, and even CNC programs—can be most helpful. There appears to be no requirement that participation in the design requires an owner to reinvent the drawing, material specs, etc.

Gray areas remain

There are some gray areas still, even with the FAA’s memorandum. Take, for instance, a retractable gear single with a loose bushing where the nosegear pivots. The boss in the mount is worn slightly oversized so that bushing is no longer a press fit. 

To use a new OEM bushing would require removing the engine, removing the mount and sending the mount out for repair. The cost could easily exceed $5,000. 

A repair involving an oversized bushing might be a cost effective solution, provided your mechanic is comfortable making that repair. 

As the owner, you send the bushing to the lab and sketch out a drawing with the dimensions that have the bushing .001 inch wider than the factory. 

With the material specification in hand and the dimensions, you may then have a machine shop fabricate one. I have done this with great success and the only way I could tell a new OEM bushing from the oversized one was to get out a micrometer. (See photo, bottom left.) 

Success here will require a mechanic comfortable with the oversized bushing being an acceptable minor repair, so it is important to discuss this with your mechanic before embarking on such a repair. 

It is not unreasonable for the mechanic to ask the owner to make a logbook entry that they provided a part produced under FAR 21.9(a)(5) and provide the information used for its manufacture.

Why not use a commercially available part?

Any discussion of owner produced parts seems to raise the question about whether an owner can just go and buy the part which is commercially available. 

A good example of this might be a wheel bearing which is frequently a standard Timken bearing. From Piper, that bearing is $110.24; the parts catalog even identifies it as a Timken 13889 bearing. One can likely get the same item from the local auto parts store or a bearing supply company for $25 to $30—but is that legal?

As an owner produced part, the interpretation seems to suggest that it is not, though to my knowledge that hasn’t been clearly addressed—especially in the context of Piper identifying the actual vendor part number. 

If the aircraft owner got out the calipers and confirmed that the auto parts bearing was the same size, number of rollers, etc., arguably that would qualify as participating in the quality control of the part. This is another gray area. 

But there is one more option. Fortunately with this example, McFarlane Aviation has already obtained a PMA for the bearing, so for $40 instead of $100, one can obtain a bearing with a PMA stamp. 


Substitute parts under Advisory Circular No. 23-27

Advisory Circular (AC) 23-27 provides information on using substitute parts for small, unpressurized aircraft Type Certificated before 1980. That includes most standard Piper airplanes. 

Note that the operative deadline is not when the aircraft was produced, but when it was certificated—so even a late model Piper Archer is going to qualify, as it was Type Certificated before 1980.

While a bit confusing in its applicability, this AC appears to provide an approval for parts substitution if such would be considered a minor repair or minor alteration and to provide the basis for a field approval if a major repair or major alteration is required. 

Directly applicable to our example is the provision that states that “You may substitute parts where a direct substitute for a part/material can be found under manufacturer part number, military specification, or other recognized standard, such as the SAE.” 

For most aircraft owners, AC 23-27 can provide a route for substituting an industry standard part for an OEM part which may no longer be available in a timely manner or for a reasonable price. Mil-Spec switches, SAE alternator belts, batteries, etc. can often be used under the guidance of this Advisory Circular without resorting to an owner produced part.

For those of us who cannot afford a new or nearly new aircraft and who don’t have the luxury of just dropping off our plane at the local OEM service center with the keys and the Visa card, FAR 21.9 and AC 23-27 can be helpful in keeping aircraft maintenance cost effective, while still meeting the regulatory requirements.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She uses Maggie in furtherance of her aviation legal and consulting practice; she also assists would-be Comanche, Twin Comanche, and other Piper owners with training and pre-purchase consulting. Send questions or comments to .


Further reading
FAA Memorandum, Aug. 5, 1993
“Definition of ‘Owner Produced Part,’ FAR 21.303(b)(2)”
Advisory Circular No. 23-27,
May 18, 2009
“Parts and Materials Substitution for Vintage Aircraft”

Both documents are available at PiperFlyer.org/Forum under “Magazine Extras”

PMA wheel bearings
McFarlane Aviation Products
– PFA supporter
Lock Haven Low Wings Comanche Aileron ADs 36

Lock Haven Low Wings Comanche Aileron ADs 36

This detailed report from a Comanche expert spells out exactly how to check for compliance with two Airworthiness Directives. 

The Comanche’s ailerons have been a source of trouble from the beginning. It was not a good bit of detail design. Poor loads analysis led to component cracking in two locations.

One issue was the outer hinge bracket concentrated stress in the aileron spar and caused cracks. The other was the nose ribs used to attach the aileron counterweight to the aileron spar were cracking. These issues apply to both single Comanches and Twin Comanches.

The aileron issues date from the early days of the Comanches. The first Piper service information regarding the ailerons was issued in 1959. Additional bulletins came out in the late 1960s. A pair of Airworthiness Directives provided a final resolution in the late 1970s. 

Piper did step up nearly 40 years ago and offered a kit which resolved the issues. Unfortunately, not all aircraft have had the kits installed and thus require additional inspections. 

Both of those ADs are current and recurring at 100-hour intervals until a kit is installed. The kits resolved the issues with the Comanche ailerons. Any further problems are rare or nonexistent for modified aircraft. 

It is very common to find Comanches that have not had one or both of the kits installed, hence removing the ailerons every 100 hours is required. I have often found that the logbooks claim the ADs have been complied with, but do not specify the method of compliance. 

Vague logbook entries can lead less-careful IAs to terminate the 100-hour inspections thinking that the correct kit was installed, when in fact it had not been. I will discuss each AD and explain how to make a visual inspection to determine whether the ADs were in fact properly terminated by installation of the correct kits.

Outer aileron hinge bracket AD 77-08-01 and Piper Service Letter 787

At the end of 1976, Piper issued Service Letter 787, alerting the fleet to the possibility of cracks developing in the aileron spar at the outer aileron hinge bracket. SL 787 recommended a recurring 100-hour visual inspection which required the ailerons to be removed. 

SL 787 also provided for a terminating action by installation of Piper Part No. 760-914, Aileron Outboard Hinge Bracket Replacement. 

The FAA mandated these aileron inspections with the issuance of AD 77-08-01. This AD also approved the installation of kit 760-914 as a terminating action.


Complying with AD 77-08-01

You can tell if your aircraft has kit 760-914 installed by visual inspection of the outboard hinge bracket. The replacement bracket has a much larger base/reinforcement to spread the load over more of the aileron spar. In addition, the new bracket is made from steel instead of aluminum. CherryMAX rivets will have been used to attach the hinge bracket to the aileron. 

If you know what to look for, the difference between new and old brackets is immediately obvious. Until then, a magnet is a quick way to check if your aircraft has the new steel bracket. 

Most aircraft I see have had this kit installed. It was easy to accomplish, and doubtless rather inexpensive back in the 1970s. 

The 760-914 kits are very difficult to find these days. Piper doesn’t stock them. Piper’s price is about $600 per kit and two kits are necessary to do both ailerons. Piper’s delivery time is about four months. Installation should be about an hour per aileron.

(These brackets are difficult to find! If you read last month’s “The View from Here,” you know that as a Piper Flyer member, you have access to PFA’s parts locating service. Log in to PiperFlyer.org; from the “Members” menu, click on “Parts Locating” and fill out the form. We’ll get on it as quickly as we can. You can also email . —Ed.) 

Aileron nose ribs 

AD 79-20-10 and Piper Service Letter 850 

The aileron nose ribs attach to the counterweight that extends inside the wing. The shaft bolts to the nose ribs, with a lead mass at the other end. This puts a lot of stress on the nose ribs. 

Problems with the aileron nose ribs go back to 1959. Cracks started appearing very quickly. It took four versions of nose ribs before Piper got it right, and each was further reinforced from the previous version. 

Piper issued Service Bulletin 173 which mandated installing a second version of the nose rib with some additional reinforcement. In about 1968, Piper issued a Service Spares Letter and associated kit, Part No. 757-162. This kit further upgraded the nose ribs to the third version of the rib: Part No. 20234-31.

Several years later, the FAA issued AD 74-10-03 which mandated the installation of kit 757-162 or a recurring inspection. Subsequent history showed that cracks could develop even in the third version of the nose rib.

In 1979, Piper issued a Service Letter to Comanche owners, warning of further cracks in the aileron nose ribs and possibly the spar itself. Service Letter 850 sets forth recurring 100-hour inspections, which can be discontinued when kit 763-893 has been installed.

The 763-893 kit contained the fourth version of the nose rib, Part No. 20234-42. The FAA also issued another Airworthiness Directive, AD 79-20-10. 

AD 79-20-10 superseded AD 74-10-03 and included all aircraft—even the ones that had the previous kit (757-162) installed. AD 79-20-10 merely mandates compliance with Service Letter 850 and provides the same terminating action.


Complying with AD 79-20-10

Compliance with the nose rib AD is somewhat challenging to confirm without removing the aileron. However, the aileron nose rib can be inspected for compliance with a flexible videoscope. 

By going in through the inboard hinge and snaking the scope outboard in the aileron, one should be able to see enough of the nose rib to make a positive identification. An example of what can been seen is shown on Page 40. 

This is the only sure way of verifying that the Part No. 20234-42 nose ribs have been installed and that removal of the ailerons every 100 hours is no longer required. 

In my experience, most aircraft have not had this kit installed. I am skeptical as to whether the rest are receiving the mandated 100-hour inspection for cracks, as there is some confusion in the field. 

Often, IAs see that a kit was installed in the late 1960s or early 1970s and assume that terminates the AD. This has even confused some longtime Comanche-savvy mechanics; one in particular insisted that version three of the nose ribs (i.e., installation of Part No. 20234-31) terminated the AD. Piper confirmed to me by email that only version four nose ribs comply with SB 850; and hence terminate the inspection requirement of AD 79-20-10.

An IA must also be careful if an aileron has been replaced. I have seen cases where the logs stated that the 20234-42 kit had been installed in both ailerons, but I found them installed only in one. As it turned out, the aileron had been replaced with one from a salvage aircraft—and no one thought to retrieve the -42 nose rib from the removed aileron and install it on the replacement aileron.

Kit 763-893 installation

The Piper instructions for installing the kit require removal of the aileron and removing numerous rivets. This allows the skin to be peeled back and permits a mechanic to get to the back side of the spar to buck the rivets for the new nose ribs. 

There is an approved Alternative Method of Compliance that uses rivnuts and screws instead of rivets, which eliminates the need to unstitch most of the upper skin of the aileron. This is a big time-saver. (A copy of the AMOC can be found at PiperFlyer.org. —Ed.) 

As with the aileron kit, the nose rib kits have become very hard to find. Piper still lists them but doesn’t stock them. The last time I checked, the delivery time was listed at over four months and the cost was over $1,200. One kit covers both ailerons, unlike the kit for the outer hinge bracket. 

Occasionally, a nose rib kit comes up for sale as new old stock, but those have become rarer and rarer. The installation takes a fair amount of time; and the high labor cost is likely the reason that a large percentage of Comanches have not had the nose rib kit installed.

I installed kit 763-893 on my aircraft several years ago. I have done a few others since. The first time I installed the kit, it took me about 12 hours to do the first aileron, and six hours to do the second one, using the AMOC. Obviously, there is a learning curve here. If I had to pay shop labor rates, the return on investment would have been questionable. 

It takes only about an hour to remove each aileron, do the visual inspection and reinstall. However, only doing the inspection and not installing the kit raises the potential of a much more expensive repair later. The inspection finds cracks. Cracks mean that the repair might be more expensive than installing the kit in the first place.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She is a recognized authority on Piper Comanche aircraft. Currently she is serving as Director of Operations for a commuter airline in Southeast Alaska. Send questions or comments to .


AD 77-08-01 Aileron Spar Cracks

AD 79-20-10 Mandating Compliance with Piper Service Letter No. 850 rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/B81EE4072873C7688625699E004AF817?OpenDocument

PA-28 & PA-32 Wing Spar Cracks: What You Should Know

PA-28 & PA-32 Wing Spar Cracks: What You Should Know

STEVE ELLS delves into the history of Cherokee wing spar cracks and investigates inspection methods. 

By now, every Cherokee owner, from the earliest PA-28-180 to the most recent Arrow, has heard about the inflight wing separation that occurred April 4, 2018, to a Piper Arrow owned by Embry-Riddle Aeronautical University in Daytona Beach, Florida. 

A very experienced flight instructor/check airman and a student perished when the left wing of a 1997 Arrow with slightly more than 7,000 airframe hours broke off. 

The NTSB issued Preliminary Report No. ERA18FA120 following the April 4, 2018, wing separation. The report said, “The airplane entered the traffic pattern and performed a touch-and-go landing.” ATC issued a transponder code and the pilot asked for approval to turn crosswind. ATC told the pilot to continue his upwind heading. 

The next sentence in the NTSB report stated, “Radar data indicate the aircraft climbed to 900 feet MSL at a groundspeed of 80 knots and a heading of 240 degrees before radar contact was lost. According to multiple witness, all within 2,500 feet of the accident site, they saw the airplane flying normally, then watched as the left wing separated from the aircraft.”

Wing separation history

Unfortunately for Piper Aircraft, the FAA, and PA-28 and PA-32 owners, this is not the first wing separation in PA-28/-32 series aircraft.

The first recorded separation took place 31 years ago when the left wing of a PA-28-181 Archer II broke off. 

Following that accident, Jim Burnett, Chairman of the National Transportation Safety Board, sent this letter to Donald Engen, Director of the Federal Aviation Administration:

On March 30, 1987, a Piper PA-28-181, N8191V, crashed following an in-flight separation at the wing root attachment while in cruise flight at low altitude near Marlin, Texas. The airplane, which was owned and operated by Griffin Pipeline Patrol Company, was patrolling a pipeline right of way at the time of the accident.

The pilot, the sole occupant, received fatal injuries.

Although the investigation is continuing, preliminary examination by the Safety Board’s Materials Laboratory disclosed features indicative of fatigue cracking in the separated left wing main spar. Fatigue cracking initiated at two locations just outboard of the outermost forward attachment bolt hole in the lower T-shaped spar cap of the main spar. Fatigue propagation was upward through the thickness and chordwise completely through the forward leg of the lower spar cap (about 1.3 inches). A small area of fatigue cracking also was noted in the forward web fracture piece adjacent to the forward outboard attachment bolt hole. 

Examination of the left wing at the accident site disclosed evidence of an approximate 10-inch-long crack that had been stop-drilled in the upper wing skin. The crack was located forward of the main spar at the wing root and was oriented chordwise parallel to the fuselage.

The accident airplane had been flown 7,488 hours since new. Following the separation, the operator then inspected a second PA-28-181 with 7,878 hours and found upper wing skin cracks that the maintenance department had stop-drilled. When the wings were removed, a visual inspection of the spar caps at the outboard attachment hole showed “crack indications” in the same area. 

The Burnett letter also included this: “Representatives of Piper Aircraft Corporation (Piper) indicate that other Piper models have wing spar structures similar to that of the PA-28.”

The NTSB issued three recommendations to the FAA: 

1. Issue an airworthiness directive to require an immediate inspection of the main wing spars and upper wing skin at the wing root of Piper PA-28 airplane with over a specified number of service hours for evidence of cracking. Particular attention should be placed on inspecting the bottom surface of the lower spar cap adjacent to the outboard forward attachment bolt hole at the wing root attachment, as well as along the upper wing skin adjacent to the fuselage just forward of the main spar. (Class I, Urgent Action) (A-87-40)

2. Based on the inspection described in Safety Recommendation A-87-40, establish a recurrent periodic inspection of the wing root area for cracks by an approved method to identify those cracks before they become critical. (Class I, Urgent Action) (A-87-41)

3. Conduct a Directed Safety Investigation to inspect the lower spar cap and upper skin on other Piper model airplanes that have a similarly configured wing spar structure to that of the model PA-28 airplane. (Class I, Urgent Action) (A-87-42)AD 87-08-08 and Service Letter 997

The FAA published Airworthiness Directive 87-08-08, issued May 5, 1987, only 36 days after the wing separation.

Piper followed with Service Letter 997, issued May 14, 1987, which detailed the importance of proper wing removal procedures.

AD 87-08-08 applied to all PA-28 models, including the PA-28-201T Turbo Dakota. It also included PA-32-260 and PA-32-300 aircraft. Additionally, the AD applied to PA-28R retractable gear versions of the PA-28. The only model excluded was the PA-28-236 Dakota.

Aircraft with less than 5,000 hours total time in service (TIS) had to comply before reaching 5,050 hours; aircraft with more than 5,000 hours TIS had to comply within the next 50 hours of flight time.

The AD required that both wings be removed. One shop owner I spoke with told me than his two-man experienced crew could support the fuselage and remove both wings in 16 man-hours. His shop already had the fixtures to support the fuselage and wings. A shop doing the inspection for the first time would need to build these tools.

Compliance required a visual inspection—using a magnifying glass of at least 10 power—“for cracks in the lower spar cap from the wing skin line outboard of the outboard row of wing attach bolt holes to an area midway between the second and third row or bolt holes from the outboard row.” 

The AD also mandated the use of non-destructive crack detection tools such as the dye penetrant method and eddy current testing to aid in the search for cracks. 

If no cracks were found, the wings could be reinstalled. 

If even the tiniest crack was found, the airplane was deemed to be unairworthy until a new spar or a wing with no spar cracks was installed. 

The upper wing skins were also inspected for cracks. If found, the skin cracks had to be repaired using repair methods acceptable to the Administrator.

AD 87-08-08 did not require further inspections. Apparently, the author(s) of the AD didn’t plan for any future cracking. That assumption was incorrect. 

In an effort to determine the extent of the cracking in the fleet, AD 87-08-08 also mandated that within five days of the completion of each inspection, that all inspection results be sent to the National Safety Data Branch of the FAA in Oklahoma City, Oklahoma.

AD 87-08-08 was rescinded Sept. 28, 1987, less than six months after it was issued. PA-28 and PA-32 owners and operators no longer had to pull the wings to inspect for cracks.

Piper Service Bulletin 886 

On June 8, 1988, Piper Aircraft issued Service Bulletin (SB) No. 886 entitled “Wing Spar Inspection.” (This and a related Service Bulletin are available under “Magazine Extras” on the forums at PiperFlyer.org. —Ed.)

SB 886 divided the PA-28 and PA-32 airplanes into two groups.1

Group I applies to all PA-28-140 through PA-28-181 Archer II; and PA-28R-180 and PA-28R-200 Arrow II aircraft.

Group II applies to all PA-28-235 airplanes; PA-32-260 and PA-32-300; as well as all PA-28R-201 Arrow III, PA-28R-201T Turbo Arrow III and PA-28RT-201 Arrow IV and PA-28RT-201T Turbo Arrow IV aircraft. 


It also includes this sentence: “To date, over five hundred (500) inspections have been accomplished. Only two (2) negative findings were reported on a pair of PA-32s operating in a severe environment and with considerable damage histories.”

The SB directs owners and operators to determine which “usage class” applies to their airplane.

This Service Bulletin provides instructions for:

1. determining the aircraft’s “usage class;”

2. determining the initial and recurring inspection times; and

3. accomplishing the wing spar inspection(s).

Determining aircraft usage class

The usage classes that Piper provides in SB 886 are Normal Usage (Class A), Severe Usage (Class B), Extreme Usage (Class C) and Unknown Usage (Class D).

The SB defines “normal flight training operations” as Normal Usage (Class A). 

Severe Usage is defined as “aircraft which have engaged in severe usage, involving contour or terrain following operations, (such as power/pipeline patrol, fish/game spotting, aerial application, aerial advertising, police patrol, livestock management or other activities) where a significant part of the total flight time has been spent at below one thousand (1,000) feet AGL.”

Extreme Usage is defined as aircraft that have been significantly damaged, such as damage which “required major repair or replacement of wing(s), landing gear or engine mount.”

Unknown Usage is defined as “aircraft and/or wings of unknown or undetermined operational or maintenance history.”

The SB warns owners: “However, if there is any doubt as to the aircraft’s operating history, it is recommended that the initial inspection be conducted in accordance with the UNKNOWN USAGE CLASS ‘D’ Compliance Time.”

Determining inspection compliance times

Once the Usage Class has been defined, it is used to “determine the applicable initial or repetitive wing spar inspection compliance time from TABLE 1.”

The compliance times differ somewhat between Group I and Group II aircraft. Based on engineering studies completed by Piper, all Group I aircraft in the Normal Usage category must have an initial crack inspection at 62,900 hours total TIS. The aircraft must also have repetitive inspections thereafter every 6,000 hours TIS.

All Group II aircraft in the Normal Usage category must have the initial inspection at 30,600 hours TIS, and the repetitive inspections every 3,000 hours TIS. 

Group I aircraft in the Severe Usage category must comply with the initial inspection requirement at 3,700 hours TIS with repetitive inspections every 1,600 hours TIS thereafter.

Group II requirements are initial at 1,800 hours TIS and repetitive inspections every 800 hours thereafter. 

Interpreting this complex bulletin

The catch here is that the Piper SB says that aircraft in the Unknown Usage category should have the wings pulled to complete the initial inspection within 50 hours TIS unless the crack inspection required in AD 87-08-08 has been completed. The repetitive inspection intervals are then based on which Usage Category applies.

What this means for owners is that if the usage history prior to their purchase of the airplane is unknown, this SB, which Piper considers mandatory, requires that the wings be pulled, and the crack inspection completed right now. 

I have purchased the ownership and major repair records from the FAA Aircraft Registry office—available to all owners on CD for nothing more than a phone call and less than $20—for all the airplanes I’ve owned. (For a link to the online request form, see Resources. —Ed.)

But these records only hint at how each airplane was flown prior to my ownership. Unless past ownership and flight conditions are known, an airplane is automatically in the “Unknown Usage” category. 

The smoking guns

In 1987, William Johnson was an A&P mechanic and station manager for Yute Air in Dillingham, Alaska. Johnson, in addition to holding an A&P, also holds an Inspection Authorization (IA), and is an Airline Transport Pilot (ATP) with over 20,000 Alaska flying hours. 

During my annual trip to the Alaska Airman’s meeting at Anchorage International Airport (PANC) in early May 2018, Johnson told me that every PA-32 that he and his crew inspected following the 1987 AD had cracks in the suspect area. 

“I sent in over 40 Malfunction and Defect (M&D) reports to the FAA about cracks,” he explained during a recent phone conversation.

At the time of the AD, Yute Air was flying 4 PA-32-300 Cherokee Six 300s. In addition to changing the spars on the company Cherokees, Williams also changed spars on other western Alaska Cherokees. 

Johnson told me that Ray Boyce of the FAA and a representative from Piper Aircraft visited him in Dillingham to view the results of his inspections. Comments from the FAA engineer seem to back up Johnson’s concerns that this will be a big problem.

Inspection methods

Since it takes about 16 man-hours to remove the wings prior to the inspection, an estimate of the costs for wing removal, inspection and reattachment, assuming a $75/hour shop rate, would probably start at $3,000. This unexpected cost could be daunting to many owners.

What if there’s a way to do the inspection without removing the wings? 

I wondered if an inspection panel could be cut that would expose the inspection area, so I asked this question of Paul New of Tennessee Aircraft. New is very experienced with structural repairs of Piper and Cessna aircraft. 

New said there’s no way to cut an inspection hole to perform the inspection.

However, in an Investigative Update to the original NTSB Preliminary report (ERA18FA120) on the April 4 wing separation, a second Embry-Riddle flight training PA-28R-201 Arrow (Serial No. 2844135) was inspected using an eddy current inspection (ECI) method. Wing cracks were found. The second airplane had 7,661 hours TIS. 

It was further reported that the cracked wings were reinstalled and subsequently inspected using a new ECI inspection procedure developed by Piper Aircraft. The new method, utilizing a bolt-hole probe inspection technique, was able to confirm the location and size of the previously identified cracks.

Based on the reported success of the bolt-hole ECI inspection, this method may be the fastest and least expensive method to inspect for wing cracks. Additionally, eddy current inspections are much more effective than dye penetrant inspections.

In every AD, there’s a paragraph saying that users are encouraged to submit alternate methods of compliance (AMOC) to solve the condition cited as causing the AD. The question now is whether there is a company that could create an eddy current inspection—or other definitive inspection technique that doesn’t require wing removal—for approval as an AMOC. Due to the equipment required and the possibility of stressing the wing structure during removal and installation, removal of the wing is the least desirable inspection mode.


Liability concerns

In another classic case of “be careful what you pray for,” and, “behind every survivor is a lawyer,” any change to an airplane structure runs into a liability glitch. 

In 1987, Cessna had stopped the manufacture of piston-powered airplanes as a statement to Congress; calling for the need for legal protection from enormous losses in civil courts. A common practice in aviation civil lawsuits to “name everyone who has deep pockets and has ever touched the airplane” meant Cessna was always listed as a defendant—even though it had not touched the subject airplane for decades, in some cases. 

Piper Aircraft did not stop production, but the effects of high product liability costs contributed to the company closing its Vero Beach plant in 1990 and declaring bankruptcy in 1991. 

The entire industry breathed a huge sigh of relief and celebrated Jan. 25, 1994, when Congress enacted Senate Bill 1458, also called the General Aviation Revitalization Act (GARA) of 1994. The bill was enacted to “amend the Federal Aviation Act of 1958 to establish time limitations on certain civil actions against aircraft manufacturers, and for other purposes.”

Cessna resumed production in 1996—a full nine years after it stopped production of piston-powered GA aircraft. 

The most important provision of GARA was the implementation of an 18-year window of responsibility by light airplane manufacturers. This “Statute of Repose” excused Cessna (and Piper) from involvement in civil suits in accidents involving airplanes that had left the factory more than 18 years before.

However, Paragraph 2 of the Act is worded that if Cessna (or Piper, or any other light airplane manufacturer) creates a significant change and mandates the installation of that change on an airplane outside the 18-year window, the company is again subject to civil lawsuits during a new 18-year window. 

In my opinion, installation of a new wing spar or installation of an airworthy used wing does not constitute a change that would reopen the statute window.

The takeaway

So that’s where Piper and the owners and operators of its PA-28 and PA-32 aircraft stand today. According to Piper SB 886 and 978A, if owners and operators can’t determine the usage history, a crack inspection should be performed immediately. 

If cracks are found, and the aircraft is returned to airworthy status by installing a used wing, that used wing must be inspected for cracks prior to installation. 

Given the gravity of a failure, and that 7,000 hours was the tipping point for the two spar failures, and that a well-maintained airplane was found to have cracks at around 7,000 hours, I strongly suggest that Piper Flyer owners start to budget for this inspection. I expect a new AD will be issued mandating inspection for cracks. Hopefully, a low-cost inspection will eliminate the need for wing removal and reinstallation to determine if cracks exist.  

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, California, with his wife Audrey. Send questions and comments to .

1 Service Bulletin 978A was issued Aug. 6, 1999 and includes serial numbers not manufactured in when SB 886 was issued. A note on page 2 of Piper Service Bulletin No. 978A reads, “This Service Bulletin is similar to Service Bulletin 886 issued June 8, 1988 with the identical purpose and has been released to add models and serial numbers not manufactured at the time of the original Service Bulletin.”


NTSB Safety Recommendation
(A-87-40, -41, -42)


Piper Aircraft Service Letter No. 997, “Wing Removal and Reinstallation”
Piper Aircraft Service Bulletin No. 886, “Wing Spar Inspection”
Piper Aircraft Service Bulletin No. 978A, “Wing Spar Inspection”
PiperFlyer.org/forum under “Magazine Extras”


Request Copies of Aircraft Records


Piper Wing Spars Explained - Video
Lock Haven Low Wings Type Certificate Data Sheets (TCDS) for Dummies

Lock Haven Low Wings Type Certificate Data Sheets (TCDS) for Dummies

Are you familiar with the wealth of information contained in your airplane’s TCDS? You should be! 

What, you ask, is a Type Certificate Data Sheet (TCDS)? If you are an owner, you need to know. If you are thinking about becoming an owner, you also need to know. The TCDS is something that IAs must to be aware of, but do not always properly utilize. Every certificated airplane has one, save for some antiques. 

The TCDS sets forth some of the critical parameters that the aircraft must meet in order to be considered airworthy. Before about 1960, the data sheets were called “Aircraft Specifications,” but the format is basically the same. The only exception to compliance with the TCDS is if the aircraft has had one of the TCDS particulars modified by a Supplemental Type Certificate (STC) or a field approval. 

What does “airworthy” mean?

Let’s take a quick detour and discuss what is meant by the term “airworthy.” You will not find it in FAR Part 1, which contains definitions for terms used in the FARs. In 2005, the FAA added the definition in FAR 3.5(a), which is likely broadly applicable, even though it could be read as being limited to that section:

“14 CFR §3.5(a) Definitions. The following terms will have the stated meanings when used in this section: Airworthy means the aircraft conforms to its type design and is in a condition for safe operation.”

The interpretation that this 3.5(a) definition should be read as the operational definition of “airworthy” for all purposes is supported by ICAO’s Annex 8 and several NTSB decisions which essentially give the same two-part definition. (ICAO, the International Civil Aviation Organization, first adopted Airworthiness of Aircraft standards (Annex 8) in 1949. —Ed.)

So, to be airworthy, the plane must conform to its type design as set forth by the manufacturer or as properly modified with an STC or a field approval, and it must be safe for operation. This article addresses the first requirement. 

Repairs must be made to certain specifications for an aircraft to adhere to the type design and thus remain airworthy. Changes that constitute a major repair need to be done with approved data. “Approved data” can be in accordance with FAA literature such as Advisory Circular 43.13-1B, the manufacturer’s structural repair manual or another FAA-approved source.

Diving into a TCDS

The starting point for determining whether an aircraft conforms to its type design is the TCDS. This article is limited to examining this important document. A TCDS can be just a few pages or it can be a very lengthy document. 

As an example, we will look at the TCDS for the Piper PA-28 series. (See Resources for a link to the FAA Regulatory and Guidance Library, where you can access the TCDS database. —Ed.)

A TCDS is generally divided into four parts. The first part is the specifications for each different model. The second is titled “Data pertinent to all models.” The third section is: “Equipment.” The fourth section is the notes.

All the PA-28 series share the same TCDS. There are 22 individual models covered by TCDS 2A13. The format for the specifications of each model is pretty standard. (See Part 01, Page 23.) The heading tells you Piper’s designation for the model—in this case, PA-28R-200—and the serial number range covered. This happens to be the Arrow II. 

Because it differed sufficiently from the previous years, the Arrow II got a new model specification. The changes made from the previous model are also listed in the header. 

Of particular interest are the engine and propeller options available. Most owners and many mechanics don’t realize that you can change between TCDS-listed propellers or engines without an STC or field approval. If the option is listed in the TCDS, then it is already approved—and the most that might be necessary is to obtain the installation drawing, if different, from Piper.

The next section contains the data that applies to all models of the PA-28. (See Part 02, Page 24.) Of most significance is the certification basis, which in the case of the PA-28, is different for each model, notwithstanding that the list is in the section that supposedly applies to all models. 

In addition to listing the regulations forming the basis of certification, it will also give the amendment date. For the PA-28, most are a mixture of CAR 3 and FAR Part 23. 

While all PA-28 aircraft are basically CAR 3 aircraft—even ones built today—the more recent the model, the more additional FAR Part 23 requirements have been added. As the models were produced over the years and especially when changes were made, the FAA would often impose additional requirements. That is how a CAR 3 aircraft might have some FAR Part 23 sections required as well. 

Many owners and pilots are not aware that there are two bodies of certification regulations that apply to common GA aircraft. Even many mechanics are really only aware of FAR Part 23, and may incorrectly assume that it applies to all aircraft. I have seen more than one maintenance bill for fixing something to comply with the wrong regulation. 

A common example is the requirement for the level of fireproofing of seat covering material. The older certification regulation is Civil Air Regulation Part 3, abbreviated CAR 3. In general, CAR 3 does not require burn certifications. Here’s why you must read carefully: the section of FAR 23 that requires burn certification has been added to the basis of some aircraft designs that were originally all CAR 3. 

It is good to know what actually applies to your model and serial number so that you can understand whether the maintenance personnel are applying the correct standards during annual inspection or repairs.

The next section pertains to the equipment. (See Part 03, Page 24.) For older aircraft that use Aircraft Specifications instead of a TCDS (like the PA-24 Comanche series), there is a list of individual pieces of equipment, engine options, etc. This can be detailed and require some level of approval to deviate from what is listed. While usually a PMA part or STC will cover that requirement, occasionally it requires a field approval.

Also in this equipment section is a listing of the AFM/POH required to be in the aircraft. It is worth checking to make sure you have the correct AFM/POH, as it is not uncommon to find them missing and some generic pilot’s guide in its place.

Lastly comes the notes. (See Part 04, Page 24.) The notes can cover a wide range of topics, some germane to a particular model and some more generally applicable. Important ones for the IA and owner are those dealing with required placards. You would probably not be shocked at the number of placards that disappear over the years.

Why is this information important?

For the owner, the TCDS for their aircraft is worth reading through at least once. 

It is also a source of good information when researching a model to buy. For example, sales ads often misstate the model year, believing that model years start January 1. Like in the auto industry, the beginning of a model year can vary, but is usually sometime in the fall. 

Knowing what the basis of airworthiness is for your aircraft better prepares you to understand the requirements to be a more active participant in the care and feeding of the plane.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She is a recognized authority on Piper Comanche aircraft. Currently she is serving as Director of Operations for a commuter airline in Southeastern Alaska. Send questions or comments to .


Type Certificate Data Sheets for Piper aircraft
under “Databases” on right, select “Type Certificate Data Sheets” and search “By Make (TC Holder)”
Installing Shoulder Harnesses

Installing Shoulder Harnesses

Adding shoulder harnesses in at least the front seats should be a must for any aircraft that does not have them. A&P/IA Kristin Winter reports on the recent installation of a three-point system on an aircraft that previously had only lap belts.

Before the early 1970s, Piper did not provide shoulder harnesses for its aircraft. In fairness, that was only slightly after the auto manufacturers did the same thing. 

Lack of shoulder harnesses have resulted in life-altering brain injuries from accidents in which the front seat occupants could have walked away virtually unharmed. Fortunately, a number of companies have STCs for the retrofitting of shoulder harnesses in Piper aircraft. 

Several retrofit options

The largest manufacturer of restraint systems is AmSafe. It supplies much of the OEM market, be it a GA manufacturer or a commercial aircraft manufacturer like Boeing. 

The only product AmSafe currently offers as a GA retrofit is its seatbelt airbags. (Winter has a set of seatbelt airbags she plans to install in her Twin Comanche, and will report on that project in a future issue of Piper Flyer. —Ed.)

The largest holder of shoulder harness STCs for Piper aircraft is Alpha Aviation in Minnesota. Alpha Aviation has STCs covering early PA-23, PA-24, PA-28, PA-30, PA-32 and PA-39 aircraft. 

B.A.S. in Washington has an STC for the PA-28/32/34. The kit offered by B.A.S. is a four-point shoulder harness/lap belt system. 

Aero Fabricators, a company affiliated with Wag-Aero, has several STCs as well. Wisconsin-based Wag-Aero offers kits for J-3, PA-11, PA-18 and PA-20/22 aircraft, as well as PA-28s. 

Univair in Aurora, Colo. markets AmSafe shoulder harness restraint systems for J-3s, PA-11s, PA-12s and PA-18s.

A brief look at the kits

Most of the STC kits have two sets of components. One is the belts and fittings. 

Some shoulder harnesses have inertia reels, and some include a fixed belt. The latter type generally costs less, but can be a bit less convenient for the pilot when he or she has to loosen the belt in order to reach something in the cockpit. 

The other set of components in a kit is whatever is required to provide the necessary structure in order to mount the shoulder harness with enough strength to provide the necessary protection to the user. 

This typically involves some reinforcements that need to be attached to the fuselage, usually involving riveting. For that reason, this is not a project for an owner alone unless he or she possesses an A&P license and the tools necessary for the job. 

In addition, a shoulder harness installation constitutes a major alteration, and requires that an A&P/IA inspect and sign an FAA Form 337.

Required tools and supplies

The tooling necessary to complete this project will vary a bit depending on what structure is required. Some airplanes may already have the structure installed because shoulder harnesses were an optional item for that particular model year; others may need quite a bit of reinforcing pieces installed in order to provide the necessary support. 

Regardless of the kit, the tools necessary for installing solid rivets—and possibly blind rivets as well—will be a necessity. 

Any practicing A&P is likely to have the necessary tooling, but for an owner that is interested in participating and wants his or her own tools, the major item is a 2X rivet gun. U.S. Industrial Tool, Sioux Tools and Chicago Pneumatic are just a few of several rivet gun manufacturers. 

The most important part of the rivet gun is a good “teasing” trigger that lets the operator control the force and frequency of the blows. The gun also needs a rivet set, which is the part that actually touches the rivet, and a spring retainer to hold it to the rivet gun. 

In addition, a selection of Cleco temporary fasteners and pliers will also be necessary. 

It goes without saying that the ability to drill holes will be key. A #30 drill bit is used for a 1/8-inch rivet and #40 is for a 3/32-inch rivet. There are some excellent YouTube videos done by EAA on the basics of sheet metal work. (See Resources for additional information. —Ed.)

This is a project that any owner with mechanical aptitude can tackle with supervision by an A&P. Doing so will be a great learning experience for those interested in understanding more about what is involved in aircraft maintenance.

A Comanche 250 project

Recently I participated in, inspected and signed the Form 337 on the installation for a 1959 PA-24-250 Comanche which had never had shoulder harnesses installed. The new owner was keen to have the safety advantage of shoulder harnesses. 

The Alpha Aviation kit for the Comanche 250 was very complete and of excellent quality. (See photo 01, page 22.) The kit included all required parts and hardware including restraints for two front seats, an 8130-3 Airworthiness Certificate, an installation manual and a copy of the STC and signed STC authorization. 

First steps

The first step is to gently remove the headliner from the area to provide access to the structure above the rear window. (See photo 02 on page 24, top.) Removing the headliner can be a challenging project and needs to be undertaken carefully to avoid damaging the headliner.

The structural portion of the kit consisted of a stringer and two doubler plates. The two doubler plates are riveted together with a carefully-laid-out pattern, and to this doubler plate assembly is mounted the attachment point for the inertia reel for the shoulder strap. Then a longitudinal stringer and the assembled doubler plate must be fitted and riveted to the airframe above the rear window. 


Measuring and positioning 

Careful measurement is key to making sure that the stringer and doubler are properly positioned. This is done by riveting the bottom of the assembled doubler to the existing stringer that runs above the window, as shown in photo 03 (page 24, bottom). Note that blind fasteners were used here. 

The installation of the assembled doubler sets the position for the new stringer, which runs from the door frame back to the frame at the back bulkhead. 

The photo below shows the assembled doubler and the stringer fitted to the aircraft and held in with spring sheet clamps, referred to colloquially as Clecos. Clecos have been used since before World War II and are indispensable for aircraft sheet metal work. The most common type require a special set of pliers to install and remove them. 


Clecos are color-coded based on the size of the hole they are designed to fill, and are used to pull tight the two sheets of metal. As mentioned earlier, there are several good videos available from EAA that cover Clecos and other basic sheet metal techniques and tools. 

The temporary fastening process

As is good practice, the initial holes were drilled to a smaller size, in this case 3/32 inch, which later were enlarged to 1/8 inch as called for in the instructions. This technique works to clean up any shifting that takes place so that each hole is reasonably precise. 

Once the structure is fitted and all the holes are drilled, the parts are removed in order to clean off the burrs and excess material from around the rivet holes. The parts are then reassembled and held in place with the Clecos.

Anchoring with rivets, and final steps

Solid rivets are generally preferred and more economical, but are not always practical, and the kit from Alpha Aviation provides both solid rivets and blind CherryMax rivets. 

CherryMax rivets are souped-up pop rivets made to an aerospace standard and are designed with a locking collar to fasten the stem into the rivet as the stem forms much of the strength of the fastener. 

CherryMax rivets are used when you can’t get a bucking bar to the back of a solid rivet. They can be seen in photo 04 (below) as they are used to attach the lower side of the assembled doubler to the aircraft’s existing stringer. CherryMax rivets have a number of special pullers that can be used to install them; some are hand-operated and some are pneumatic. 

Once all the rivets are installed, a strong support base has been created to anchor the inertia reel and shoulder harness, as shown in photo 05 (below). 

Reinstallation of the headliner, carefully cutting a hole for the bolt, and subsequently bolting on the inertia reel completes the installation—save for the log entry and completing the Form 337. 


Labor may vary; added safety will not

The labor necessary to install the shoulder harness kit varies with the amount of structure that must be added. It can take anywhere from a handful of hours to a couple of days’ worth of labor, but shoulder harnesses are most needed in the kind of accident that can happen to even the best pilots. 

No restraint will help if you hit a mountain at cruise speed—but landing mishaps, loss of runway control, or even a controlled glide into favorable terrain after an engine failure are more common. These scenarios are where a shoulder harness might make all the difference. 

Know your FAR/AIM and check with your mechanic before starting any work.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She uses Maggie in furtherance of her aviation legal and consulting practice; she also assists would-be Comanche, Twin Comanche, and other Piper owners with training and pre-purchase consulting. Send questions or comments to .


Shoulder harness STCs
–PFA supporters
Alpha Aviation Inc.


B.A.S. Inc.


Univair Aircraft Corp.


The Wag-Aero Group


GA seatbelt airbags
AmSafe, Inc.


Fastening tools and supplies
Chicago Pneumatic


Cherry Aerospace


Sioux Tools


U.S. Industrial Tool & Supply Co.


Educational videos
EAA’s Sheet Metal Channel
What Lies Beneath: Interior Renovation of Aging Piper Aircraft

What Lies Beneath: Interior Renovation of Aging Piper Aircraft

Renovating an aircraft interior is a prime opportunity to see what’s really underneath all that ugly old upholstery. Whether you do it yourself, hire it out, or choose a combination of the two, you want to make informed decisions. Industrial designer and airplane interior expert Dennis Wolter is here with a series of articles to help you.

What a difference 50 years can make. During my college days back in the late 1960s, I had a job where I would occasionally be sent to ferry a new airplane from the distributor back to Lunken Field (KLUK, now Cincinnati Municipal) for delivery to its new owner. To this day, I can remember the look, feel and smell of a new, ferry-time-only airplane. 

Very few of us have the opportunity to own a brand-new airplane, but there are obviously many older airplanes out there in need of upgrades that can in many ways elevate them to exceed the design, comfort, performance and safety of a new airplane. 

After graduating from the University of Cincinnati as a newly-minted industrial designer, I tried two “real” jobs before deciding to pursue my true passion of renovating airplanes. The key word here is passion; I believe it is the glue that holds the General Aviation community together. 

Folks who own, fly and maintain these wonderful machines do it primarily out of love of flying. I am committed to helping save these irreplaceable machines. In coming months, I will submit a series of articles that will cover in detail the work required to truly renovate an entire cabin, from the firewall to the aft bulkhead. 

I plan to walk readers through each step in the process, with photos showing all of the tricks we’ve learned over the past 44 years—including the tools and supplies required—to thoroughly execute the job.

Common issues in aging aircraft

I often mention in seminars that the time has come in this world of General Aviation to either save these airplanes for future generations of pilots, or let them slowly deteriorate and end up in a salvage yard, where their corroded airframes will be picked clean of usable parts to support the dwindling number of still-airworthy airframes. 

Some readers may feel that this is an overly pessimistic comment, but in my business, I see far too many airplanes that are deteriorating before our eyes. Various aging-airplane problems seem to apply to all makes and models of older airplanes, no matter the value of the airframe. High-dollar Senecas are as susceptible as Cherokee 140s.

Installing a new interior in a Piper presents a great opportunity to really see into every nook and cranny of the cabin area with all interior components and insulation removed. 

About 15 years ago, I realized we were dealing with an increasing number of non-upholstery issues, such as corrosion (Photo 01, Page 55); questionable wiring (Photo 02, Page 55), leaking windows (Photo 03, Page 55), degraded static and fuel lines, and so on. 

These problems were becoming almost commonplace in the majority of airplanes going through our shop, so I decided to make it standard practice to invest the time necessary to repair all of the technical issues we discovered. 

To address corrosion, we started with antiseptically cleaning all floors, bulkheads and cabin skins, followed by a thorough application of corrosion-controlling zinc chromate. (The corrosion mitigation process will be covered in-depth in a future article. —Ed.)

Corrosion isn’t the only aging airplane issue that must be addressed, however. We often encounter other problems such as neglected or poorly-executed maintenance and carelessly-installed upgrades. 


Addressing problems takes time

This total approach to renovation has made a very pronounced change in both the time required as well as the cost involved in fully renovating the interior of a 40-plus-year-old Piper. When Air Mod first opened its doors in 1973, the scope of work required to renovate an aircraft interior was much less than the task we face today. 

I remember one of the first interiors we did in a six-place 1964 Aztec. Working 12-hour days, a part-time assistant and I completed the job in two weeks. This turnaround time was possible for a number of reasons. 

First and foremost was the fact that the airplane was only 10 years old, and it certainly did not present many of the issues we would see today in the same, now 54-year-old airplane. 

A second factor was that, at the time, our customers typically did not add items beyond what we included in a full interior renovation. Today, an extensive list of upgrade items can bring airplanes up to 21st century standards. 

These optional upgrades include safety-enhancing four-point inertia reel shoulder harnesses; more efficient, quieter ventilation systems; LED lighting; super soundproofing; thicker glass; fully-articulating seats; composite side panels with recessed armrests (see Photo 04, Page 56); and custom instrument panels with exotic wood trim, to mention a few.

A third issue adding to today’s longer downtimes involves avionics installations and maintenance. Our company partners with two companies located at Clermont County Airport (I69), and coordinating these projects while the interior is being renovated makes it convenient for a customer. 

But the bottom line is this: the Aztec interior that took two weeks to complete in 1974 has become an eight- to 10-week project in 2018.

Proper intervention is key 

Here’s the good news. For most of the airplanes in the fleet, the level of degradation is at a point where it can be stopped with proper intervention, saving these airframes for the future. 

The other good news is that 95 percent of what is required to mitigate corrosion and upgrade an interior can legally be done by an owner under the FAA guidelines of preventive maintenance, FAR Part 43, Appendix A(c). 

AC 43.13-1B is an excellent “how-to” manual from the FAA, and a must for every handy aircraft owner to have. Armed with this great resource, you will have concise and clear guidance as to how to properly perform many of the tasks that are required to keep your airplane in tip-top shape.

Steps in a cabin and interior renovation project

The following is a list of the steps we take during a typical renovation of cabin and interior in a 40-year-old airplane. In upcoming months, I will work my way through the major items on this list, with descriptions of processes accompanied by photographs. 

1. Remove and secure all documents and personal items.

2. Perform an ergonomic study of the pilot seating station. 

3. Test radios, intercom, autopilot, electrical components and lights for function. Report findings and recommendations.

4. Remove and evaluate existing side panels, seats, headliner, carpet, insulation, floor inspection panels and window trim.

5. Inspect all structures and skins for corrosion. Remove corrosion and glue from inner cabin skins, spar carry-through and related components. Treat all inner surfaces and appropriate components with corrosion-control materials.

6. Clean exposed antenna connections and inspect all systems and controls. 

7. Strip seats to bare frames to perform a complete mechanical and structural inspection. Repair as required.

8. Install heavy sling reinforcement straps on seat frames and install a new seat sling. The reinforcement keeps the sling from stretching or coming loose from the frame and prevents future sagging.

9. If requested, build the height of the seat back structure to accommodate the stature of the pilot. Many of our customers choose this option as an alternative to a headrest. (Note: this requires appropriate FAA approval and a Form 337.)

10. Build new seat foam, shaping with several densities of flame retardant urethane foam. The new foam is contoured to fit the customer as determined by measurements taken earlier.

11. Clean, mask, prime and paint all seat frames to match the new interior color scheme.

12. Sew and fit the new seat upholstery. Shaping is done with hidden sewn-in rods and pulls to insure long-lasting structural integrity, eliminating sagging and shifting. All seams are double-lock stitched to prevent seam failure. All seat panels have backing foam and backing fabric to insure proper fill.

13. Strip side panels to bare metal. Factory cardboard panels are replaced with new aluminum panels; existing metal panels are repaired to be in like-new condition or made new. 

14. Temporarily install the non-upholstered panels. Check for fit and layout of the new design; modify as necessary to ensure ease of installation and removal when upholstered. Fill in ashtray holes if requested.

15. Sew and mount side panels using new high-density, flame retardant backing foam and upholstery material of choice.

16. Prep, paint and placard plastic trim, door and window trim, and cabin components. 

17. Clean, mask, sand, fill and paint all door frames and related interior airframe trim with custom-matched interior paint.

18. Install new insulation behind side panels, in doors and behind headliner. 

19. Clean below floors and behind rudder panels as required.

20. Clean all seat tracks. Buff or paint heater outlets and similar components.

21. Install new windlace cord on doors; install reupholstered side panels using new hardware.

22. Strip headliner panels and repair or replace as required. Plastic headliners are re-formed and reinforced with aluminum as necessary to prevent future sagging or warping.

23. Fit, sew and install new headliner and reupholstered sunvisors, if applicable.

24. Cut and fit new carpets for cabin floor, baggage compartment and related surfaces. Special attention is made to allow for future removal and reinstallation without removing seats.

25. Serge all carpet edges; sew on Velcro and heel pads. Insulate the underside of the floor carpets with heavy density, flame-retardant foam. Bond Velcro to floors and install the new carpet.

26. Reinstall existing lap belts and shoulder harnesses, or install new and/or re-webbed components. 

27. Install cabin seats. Lubricate all door and seat latches.

28. Perform a safety and function check on the interior, radios, flight controls and electrical components.

29. An A&P mechanic will make all necessary logbook entries and weight and balance changes, and check that all placards are in place.

30. Wash and ground-run aircraft to prepare for customer delivery.


Other items that may be addressed

Some additional items that may be addressed during a renovation include: reinforcement and repair of aluminum and plastic cabin components; replacement parts; painting and placarding of instrument panel, pedestal and circuit breaker panels. 

Other items might include installation of special composite insulation and soundproofing; glareshield modification, repair and upholstery; repairs to or replacement of side panel components and repair or replacement of damaged floor boards. 

In addition, the owner may choose to add extra map cases, storage boxes, cup holders and gooseneck maplights and install a new windshield and/or windows.

Things to consider

Some owners will read through this list and realize that some of the work is beyond their ability. (Sewing seats and headliners come to mind.) Fortunately, there are companies who can provide quality interior kits and components with good product support for those wanting to install a mail-order interior. 

I highly recommend that any owner undertaking interior renovation work seek the advice of his or her A&P mechanic and arrange for that mechanic to inspect the stripped-out cabin structure, systems and seat frames for signs of any airworthiness issues. These areas can be hidden from view during routine maintenance when interior components and insulation remain in place.

Many owners may choose to renovate their airplanes incrementally, removing one side panel at a time and cleaning, chromating and insulating the exposed structure of that one area. I have mentored a number of people through this process over the years. 

Whether you are planning to have a professional shop renovate your airplane, or you plan to do part or all of it yourself, stay tuned. Upcoming articles should help guide you in your decision-making. 

Until next time, fly safe!

Industrial designer and aviation enthusiast Dennis Wolter is well-known for giving countless seminars and contributing his expertise about all phases of aircraft renovation in various publications. Wolter founded Air Mod in 1973 in order to offer private aircraft owners the same professional, high-quality work then only offered to corporate jet operators. Send questions or comments to .

It’s Re-Bladder Time

It’s Re-Bladder Time

Contributing editor and A&P Steve Ells recently installed an FFC fuel bladder in the left tank of his 1960 Comanche.

I open the cabin door of my airplane to start my preflight inspection I’ve been catching a whiff of Avgas. 

I checked around the shaft of the fuel selector thinking that fuel was wicking up past the selector shaft seal, but didn’t see the telltale blue stain. I pretzel-ed myself into position to double-check, and risked (more?) brain damage by sucking in a big whiff. 

Nope, that wasn’t the source of the leak.

I looked in the engine compartment—maybe there was a leak there? But a thorough visual inspection didn’t reveal any blue staining. 

Uff da, I thought to myself. This lack of evidence narrows the source of the smell to one of the fuel bladders—one of the “rubber bags” snugly hidden away inside the fuel tank bay of the wing. 

I removed the wing root fairing and saw it: a small blue stain. 

It wasn’t very big, yet it was there.

Bladder backstory

According to one source, fuel bladders were installed in Piper airplanes for two reasons. First, the military used bladders because it thought that the bladder—being flexible—was much less likely than an aluminum or integral tank to burst during a crash. 

And second, installing a bladder—even the semi-stiff bladders manufactured 70 years ago—took less time than manufacturing and welding up a rigid tank or sealing the wing structure to create an integral tank. 


Causes of fuel bladder failure 

The most common cause of bladder failure is porosity of the upper surface. 

It’s pretty well known that fuel bladder life is maximized by keeping the tank as full as possible. This lessens plasticizer loss by keeping temperatures moderated. 

But the breakdown of the upper surface is inevitable and when it gets to a certain point, fuel starts to evaporate out of the tank. 

I had noticed that the fuel level in the left tank had dropped by an inch or so between flights; another sure sign the fuel was evaporating out the porous top surface of the tank. 

I needed a bladder and after checking with a couple of trusted techs for suggestions, ordered one from Floats & Fuel Cells (FFC) in Memphis, Tenn. 

FFC technology

A few days after ordering, a big box arrived. I was surprised at how light it was, but after opening the box and inspecting the new bladder, I understood why. 

New fuel bladders from Floats & Fuel Cells are very flexible and are visibly smoother than the old, semi-rigid bladder. Sort of like the difference between a 1960 Cadillac El Dorado and a 2015 Tesla. 

Each corner of my new bladder tank is rounded and each potential wear point and corner is smoothly reinforced. According to Brewer, this is because bladders from FFC are cured in an autoclave that melds all the parts together by utilizing temperature and pressure to create a one-piece unit.

And it’s light. The weight of the 30-gallon bladder is 4.9 pounds.

Modern bladder-style tanks from Floats & Fuel Cells are constructed of a proprietary P2393 nitrile rubber. Nitrile combines “excellent resistance to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water and alcohols with a good balance of such desirable working properties as low compression set, high tensile strength and high abrasion resistance,” according to one reference book. The only drawback is a low resistance to attacks by ozone. 

FFC’s bladders consist of a four-layer construction. The layers (from inner to outer) are a nitrile layer that retards permeability; a transparent fuel vapor barrier that’s bonded to a nylon fabric layer; and an outer layer of nitrile rubber formulated to resist ozone attacks. 

Remove and replace

I flew the fuel level in the left bladder down to six gallons before siphoning the remaining fuel into grounded fuel cans. (Important: Always establish a ground between the aircraft and the fuel cans to prevent static electricity from spontaneously igniting the fuel vapors.)

I next removed the large access (top) plate and the fuel quantity sender from the top of the wing and removed the screws and bolts connected to the reinforcing/mounting ring molded into the top of the bladder. 

I tried to wrestle the old semi-rigid bladder out and up through the access hole, but soon realized there was a vast difference between the new, flexible, lightweight FFC bladder that I had just unpacked and inspected and the semi-rigid heavy bladder that was leaking. 

I needed enough room to shove most of my right arm down inside the old bladder to free the feed nipple and pull loose the clips holding it in position. 

I took the easy way—I cut the old bladder into pieces. It was a good decision; a box knife and scissors was all it took. That transformed the process of working in the tank bay from an arm-bruising struggle into an easy job. 

The rate-determining step in the removal of older bladder tanks is getting the fuel feed nipples that are molded into each bladder to release from the rigid airframe fuel feed lines. The bladder in my Piper Comanche has a single feed nipple located in the aft inboard corner of the tank.

Due to my prior experience in struggling to pull an old bladder free of the feed line, I spent at least an hour using my box knife to cut away sections of the nipple from the feed line. It wasn’t difficult because the nipple was old and brittle. 

Not content with that, I then spent time spraying Kroil between the nipple and the feed line after I had shoved a small pointed punch between the two surfaces. (This penetrating oil spray is designed to free up frozen hardware, but I figured that since it penetrates so well it would work to break the seal between the nipple and the rigid feed line.)

I worked the punch—and sprayed the Kroil—between the feed line and the nipple from the wing root and from inside the tank. Then I went home for the day.

The next day I applied straight pulling pressure and to my surprise the bladder nipple slid easily off the fuel feed line and over the coarse screen at the end of the line. Removal complete!


They don’t last long? 

There’s a tale that has been passed along at the preflight planning table and in the pilot’s lounge that bladders don’t last long and that they’re very hard to change. Hogwash. 

Six years ago, I changed the right bladder in my airplane. This year, I changed the left bladder. 

The left bladder was manufactured in 1957 and installed in 1959 in my 1960 vintage airplane. There’s a maintenance logbook entry citing, “Replaced fuel tanks” in late 1988. 

This led me to believe that both bladders had been removed, sent out, repaired and reinstalled, since both bladders I removed had the original U.S. Rubber part numbers, serial numbers and acceptance dates. Thirty years before the first repair is not short-lived. (For more information, take a look at “Refurbish, Repair or Replace: What do to when your fuel bladder fails” in the December 2013 issue of Piper Flyer. —Ed.)

Some are repairable (and some aren’t)

There were at least four different bladder tank manufacturers in the past. These companies included industry powerhouses such as U.S. Rubber, BF Goodrich, Firestone and Goodyear Aerospace. 

There were good construction methods and some that weren’t so good—some bladders are very repairable and some are not. Companies in the refurbishment business know which bladders are good candidates for overhaul. (For a link to table on the FFC website with specifics on which brands/types can be repaired, see Resources at the end of this article. —Ed.)

My Comanche’s original tanks had a cotton-based construction. According to Kevin Brewer at FFC, those U.S. Rubber 584 bladders would not be repaired today. The right one lasted 22 years after being refurbished, the left one 28 years. 

There are at least two other companies that produce and sell bladder-type fuel cells. 


Clips and hangers

Bladder-type tanks are fabricated to fit as perfectly as possible within the fuel tank bay inside the wing. The Floats & Fuel Cells bladder arrived with a new gasket, a roll of industrial-strength, fabric-backed tape and nine button-type spring clips. 

The tape is used to prevent tank abrasion. It’s laid over all rivet and screw heads and all seams within the fuel tank bay prior to installing the bladder. The tank bay of my Piper was very clean, so all I had to do was remove the old tape and install new tape.

The clips slip under a reinforced ring attached to the outside surface of the bladder. There’s a ring/clip in each bottom corner of the tank that’s there to keep the bottom of the bladder wrinkle-free. 

The bladder in my airplane holds 30 gallons. It’s shaped like a rectangle except for the forward corner of the inboard section which extends forward; it’s like a triangular piece was grafted onto the rest of the rectangle. 

There are five clips that need to be pushed into receptacles to hold the upper surface of the bladders in position. All of the removal, installation, flow nipple and clip installation work has to be done through the access hole in the top of the wing. 

It’s important to use the new clips and to get them snapped into the receptacles without bending them.

AD 68-13-03 applies to the bladder tanks in my PA-24 Comanche. It requires a visual inspection every 100 hours to check the condition of the clips. So installing new clips—and installing them correctly—is important.

Since the bladder in my Piper is small, I didn’t have any problem with arm length/finger strength issues during clip installation. 

Finishing up

One part of the installation is difficult. Two half-inch (inside diameter) flexible tubes need to be slid over tube ends attached to the metal top plate and tube ends mounted in the wing. These tubes connect the top plate fuel spill drain-off tube and the fuel tank vent line tube. 

Once these tubes are slid into position, the reinforcing/mounting ring of the bladder has to be pulled up into position so the screws connecting the two can be started and torqued. 

My solution to pulling the bladder reinforcing/mounting ring up into position—since the top plate completely fills the access hole, thereby cutting off access to the bladder—is to fabricate two long aligning pins out of bolts. 

These are screwed into nut plates in the bladder mounting ring prior to sliding the flexible tubes onto the wing mounted tubes. 

Once the top plate is in position I pull up on the aligning pins—which are nothing more than long bolts with the heads cut off—and start screwing in the screws that hold the top plate and the reinforcing/mounting ring together. 

Calibrating a dipstick

After the bladder is in and is deemed ready, I like to fill the tank in five-gallon steps—with the airplane on level ground and the landing gear struts and tires at normal inflation—for two reasons: this is the perfect time to make up a fuel tank quantity dipstick (I know I sound like a dipstick salesman, but they work and they are a simple safety tool) and it allows you to make sure you know exactly how much fuel your new bladder holds. 

A new bladder should provide good service for at least 20 years, and likely much longer if steps are taken keep the top surface of the bladder cool. 

Do this by shading the top surface of the wings whenever possible and by striving to top off the fuel tanks after each flight since the fuel will moderate bladder surface temperatures by acting as a heat sink. Hangaring an aircraft is the most effective method for preserving its fuel bladders. 

Know your FAR/AIM and check with your mechanic before starting any work.


Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, Calif. with his wife Audrey. Send questions and comments to


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Airplane Maintenance for the DIYer: Changing a Tire

Airplane Maintenance for the DIYer: Changing a Tire

In this article—the first in a series describing various preventive maintenance actions—A&P Jacqueline Shipe goes through the entire process of removing and reinstalling an aircraft tire. 

All aircraft owners will periodically have to replace a tire. Even planes that don’t get used much eventually require tire replacement due to dry rot and sidewall cracking of the rubber. 

Tire replacement is one of the items the FAA considers preventive maintenance that owners may legally perform on their aircraft. Changing a tire isn’t mechanically complex, but it does require the owner to use some caution. 

Proper aircraft jacking procedures have to be used, and if the plane is jacked outside, first check the weather conditions to ensure that the wind is not going to be too high. Major damage can be done to the airframe structure whenever a plane falls off a jack. 

Once the plane is properly jacked, the removal of the wheel assembly can begin. (For information about aircraft jacks and proper procedures, take a look at Shipe’s “Airplane Maintenance for the DIYer: First Steps” in the May 2016 issue. —Ed.)

If the plane is equipped with wheel pants, the wheel pant has to be removed first. This is usually a pretty straightforward process, just be sure to keep the removed screws and bolts identified as to which receptacle they came out of, because sometimes several different diameters and lengths are installed. 

Removing a main wheel

The next step is to remove the outboard brake backing plate (if the wheel is a main wheel) so that the brake disc will be free to slide off the axle with the wheel assembly. 

There are two or more bolts that connect the backing plates to the caliper that have to be removed. They may or may not have safety wire on them, depending on the design. 

Once the outboard brake is off, the next step is to deflate the tire by removing the valve core. It is important to do this before removing the big axle nut, because if any bolts holding the wheel halves together are loose or damaged, the wheel assembly could blow apart as the axle nut is loosened. 

Most axle nuts have either a clevis pin with a small cotter key or a single large cotter key to keep them from backing off. 

When the axle nut is removed, the wheel assembly will slide off. Some manufacturers employ a spacer that may or may not slide off with the wheel; care needs to be taken to ensure it doesn’t get misplaced.

Removing a nosewheel

Nosewheels usually have a removable axle that slides through the wheel and nose fork. This is generally held in place with a single long bolt and lock nut that secures two cup-shaped retainers. 

Nosewheels are fairly easy to remove unless the steel axle is corroded and frozen in the aluminum fork, in which case the axle has to be driven out. A wooden dowel should be used to drive out the axle because it won’t gouge or mar the aluminum fork.


Breaking the tire bead

Once the wheel is removed, the tire bead needs to be broken from the rim of the wheel halves. Get a piece of plywood to lay the wheel on (so it won’t get marred), then forcefully push the tire down all the way around. It will eventually pop down off the rim. This process has to be repeated on the reverse side as well. 

Some folks use flat blade screwdrivers or pry bars to pry the tire away from the wheel, but this can result in major scarring of the relatively soft cast aluminum that the wheels are made of. 

If the bead is really stubborn and just won’t break loose, you may have to enlist the help of a mechanic and a bead-breaking tool made specifically for the task.

Splitting the wheel and removing the tube 

Most wheel assemblies are two-piece, and the wheel halves are split by removing the through bolts and nuts holding them together. After separating the wheel halves, the tube can then be removed from the tire. 

Some mechanics replace the tube with every tire change, and some re-use the old tube as long as it looks good. Tube manufacturers recommend always replacing tubes when replacing a tire because they stretch while in use. 

Once an old tube is removed, it can be barely inflated—just enough to expand it a little—so the entire exterior can be inspected. Tubes with deep wrinkles or that have signs of damage or age, such as dry rot cracks around the valve stem, should always be replaced. 

When ordering a replacement tube, pilots may want to get the type that doesn’t lose air, such as a Leakguard or Airstop inner tube. They do reduce the frequency of having to air up the tires by quite a bit, especially if they are serviced with nitrogen instead of compressed air. 

Reinstalling the tube

The outside of the tube and inner part of the tire need to be coated in talcum powder before installing the tube. The powder keeps the tube from sticking to the sides of the tire and helps prevent chafing. 

The tube should have a balance mark on it. This needs to be aligned with the balance mark on the tire, which is generally a red dot. 

In the absence of a balance mark on the tube, align the valve stem with the red dot. This matches the heaviest part of the tube with the lightest part of the tire and makes it much easier to balance.


Reassembling the wheel

Once the tube is installed in the tire, the wheel halves (and the brake disc, if it is a main wheel) can be assembled together. Slightly inflating the tube a tiny amount helps to ensure it won’t be pinched between the wheel halves. 

Place the wheel halves together so the bolt holes align. The bolts can then be slid through and the washers and lock nuts installed. Lock nuts should have enough tension on them so that they cannot be tightened by hand, otherwise they should be replaced. 

The correct torque should be observed when assembling the wheel halves. These are made of cast aluminum and are strong, but over-tightening the nuts and bolts can lead to cracking. 


Balancing the tire

A wheel balancer is a fairly expensive tool to buy, and tires aren’t usually replaced on an individual airplane often enough to merit owning one for most folks. Large imbalances in a wheel assembly can be detected by mounting the wheel on the axle and installing the axle nut, but leaving it slightly loose so the wheel rotates freely. 

Once the tire is spun a few times, if the same spot always ends up coming to rest on the bottom, this indicates an imbalance and weights will need to be added to the light side. Using stick-on lead type weights purchased from an aviation parts warehouse or automotive store, add enough weight so that the wheel comes to a stop in random places as it is spun freely on the axle. The balance is more critical on the nosewheel assembly because it will cause a shimmy if there is even a slight imbalance. 


Reinstalling the wheel on the aircraft

Once the wheel is balanced, it is ready for installation. The axle should be wiped off and greased, and any corrosion should be removed with an abrasive cleaning pad. 

A general-purpose Scotch-Brite 7447 pad (maroon color) works well and can be purchased anywhere automotive paint products are sold. These pads are abrasive enough to clean off rust, but not so abrasive so as to scratch the metal. Use elbow grease to scrub the axle until it is shiny.  

The nosewheel axle is prone to rusting internally. Any rust should be removed here too, and the internal part of the axle should be either painted with a rusty-metal primer, or coated in LPS 3 or other corrosion-inhibiting compound. 

During the final installation, the axle nut needs to be tightened enough so there is no free play detected as the tire is grasped and pushed inboard on the top while pulling outboard on the bottom, or vice versa. (This checks for side-to-side free play, and there should be none.) 

The wheel should spin somewhat freely, but there needs to be a slight amount of tension on the axle nut. If the nut is too tight, the wheel won’t spin much at all by hand, and the wheel bearings will be more likely to fail from having too much of a pre-load placed on them. 

Once the correct tension is achieved, align the cotter key hole in the axle with the opening in the nut and install the cotter key. Be sure to bend the edges of the cotter key in such a way that they won’t get entangled in the valve stem or rub on the wheel bearing retainer. 

After the wheel is secured in place, the valve stem should be removed and the tube inflated and deflated two or three times to remove any wrinkles. Then the valve stem can be reinstalled and the tire inflated to its proper pressure. (Correct tire pressures are found in the POH.) 

After the outboard brake and/or wheel pant are reinstalled, the tire change will be complete. The aircraft owner will also have the satisfaction of having completed the work himself (or herself)—and will have hopefully have saved a few bucks in the process.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe attended Kentucky Tech to obtain her A&P license. She has worked as an airline mechanic and on a variety of General Aviation planes, and has logged over 5,000 hours of flight instruction time. Send question or comments to .


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Wheel Bearing Service: Why & How

Wheel Bearing Service: Why & How

A&P Jacqueline Shipe describes how to service wheel bearings in this article, the second in a DIY series for pilots who wish to take on preventive maintenance of their aircraft.

FAR 43 Appendix A lists the preventive maintenance items owners may legally perform on their planes. This list is fairly long—and some of the items are a little involved for a person to perform the first time by themselves, while other tasks on the list are pretty straightforward. 

There are several preventive maintenance tasks pertaining to the landing gear, including tire changes, strut servicing and servicing the wheel bearings. (Last month, Shipe discussed the steps involved in changing an aircraft tire. See the June 2016 issue for more information. —Ed.) 

Bearings: small but mighty

While cleaning and greasing wheel bearings doesn’t seem like too difficult a task, there are some guidelines that need to be followed. The failure of a wheel bearing can cause major damage to the wheel and can even allow the wheel assembly to slide off the axle.

Wheel bearings are relatively small, but are incredibly strong. They have to support the weight of the plane while allowing the wheel to spin freely in all types of temperatures and conditions. In addition, wheel bearings and races on airplane wheel assemblies also have to be capable of withstanding hard landings and both vertical and horizontal loads without failing. 

Types of bearings

The bearings on most airplane wheel assemblies are the tapered roller-type. The outer part of the bearing is larger than the inner part, and the rollers are installed at an angle. 

The bearing itself rides in a metal cup called a race. The race has a “pressed in” fit in the wheel half, and is tapered on the inside to match the bearing. The biggest advantage of tapered bearings is the high load capacity that they can withstand. 

Automotive wheel bearings, on the other hand, usually use spherical rollers (i.e., balls). Ball bearings can withstand prolonged high speeds without building up too much heat, but cannot take high impact loads. 

Tapered bearings will bear up under the not-so-good landings that occur from time to time with an aircraft. In addition, proper servicing of these bearings will keep the wheels spinning freely and will last for a long time. 

Removing the clips

Once a wheel assembly is removed from the axle, the wheel bearings are easily removed by taking out the metal retaining clips that secure the bearings and grease felts. 

There is an indention in the outer part of one end of the clip to allow a screwdriver to be used to pry it out. The clips don’t have a lot of tension on them and can be easily removed. 

Once the clip is off, the bearing, metal rings and grease felts can all be lifted out together. 

Be sure to keep all the rings and clips organized so they can be reinstalled into the same wheel half and in the same place. The metal rings that retain the bearing are sometimes slightly smaller on the outer half than the double rings used on the inner half, and can be easily mixed up. 

Cleaning the parts

A small bucket with 100LL Avgas works well to clean the bearings. Swishing the bearing around and spinning it by hand while it is submerged will clean all of the old grease and gunk out. 

The metal rings and clips should also be cleaned, but the felt material needs to be set aside; it should not be submersed in anything. There is really no way to clean the felt, anyway—as long as it is still in one piece, it’s good to go. Any grease felt that is torn or missing a section needs to be replaced. 

Once all the parts are cleaned, they should be blown out with compressed air (if available) or laid out on paper towels to dry. The parts need to be thoroughly clean and dry before fresh grease is applied. Inspecting the parts

After the bearings, metal rings and clips are clean and dry, the bearing and race should be inspected for pitting or damage. If the race is smooth and has no corrosion, the bearing is generally corrosion-free as well. 

Races that have light surface corrosion can sometimes be smoothed out with a piece of light grit sandpaper (800 to start and 1200 to finish). Deep pits in a race mean replacement is needed. 

Discoloration on the bearing or race, such as a rainbow or gold color, can be a sign that these parts have generated excessive amounts of heat, in which case they should be replaced.

Preventing corrosion

Wheel bearings typically fail for two reasons: corrosion or overheating. 

The greatest threat to airplane wheel bearings is usually corrosion. Almost all bearings and races will eventually require replacement due to water getting past the grease seals and accumulating in the bearing cavity, causing rust and pitting. 

When cleaning a plane, strong degreasers should not be used on wheel assemblies and wheels should never be sprayed with a water hose. The pressurized water will get past the grease seals and ruin the bearings. 

Folks that want their wheels clean can wipe them out with a rag that is lightly moistened with a little Gojo original white cream hand cleaner (the non-pumice kind). Then the wheels can be wiped clean with a dry rag. 


Replacing the races

Wheel bearing replacement is easy, but replacement of the races is a little tough to do without the proper tools. 

Because the race has a pressed-in fit in the wheel half, it has to be driven out. This can be accomplished by using either a hammer and punch or a bearing driver tool. 

Occasionally a person encounters a wheel assembly with a race that has broken loose and is spinning in the wheel half itself. In this case, the wheel assembly has to be replaced; there is no permanent way to hold the race in place if the wheel assembly has lost enough metal that the race is no longer fitting tightly. 

The wheel is made of cast aluminum. When reinstalling the steel race, it is very important that it be driven in straight. If it gets cocked—even a little—the much softer aluminum will be gouged and damaged. 

The best tool for the job is a bearing driver, as it allows each blow of the hammer to be applied equally around the circumference of the race. 

Once the race is almost near the bottom of its recess, very light blows should be used to seat it in the wheel half. Many mechanics have driven the race in too far and cracked the fairly thin aluminum ring that retains the race. 

The wheel should always be thoroughly inspected for any sign of cracking on the front and back sides, whether or not a race is replaced.


Packing the bearings and reinstalling

Once all of the races are installed and the wheel halves are inspected, the bearings are ready to be packed and installed. A high-quality wheel bearing grease that has good water resistance should be used. 

The grease has to be pushed up through the bearing until it comes out the top between each roller. If it doesn’t squeeze through each opening, the inside of the bearing will have gaps and inadequate lubrication. 

It takes a little while to pack a bearing by hand. There are bearing packers sold in almost any automotive store that make the job a little faster and a little less messy. 

Once the bearing is packed, apply a layer of grease to the entire surface of the race to ensure it is covered as well. 

The bearing can then be reinstalled along with the correct order of retaining rings and grease felts. 

Lastly, reinstall the clip. It is a good idea to make sure the clip is pressed into place all the way around by pushing it outward with a screwdriver. 

After all the clips are in, the wheel bearing service is complete.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Strut Servicing: The Ins & Outs

Strut Servicing: The Ins & Outs

In the third article in a DIY series for pilots, A&P Jacqueline Shipe goes through the steps an owner can take in order to properly service the struts on their aircraft.

Among the preventive maintenance items listed in FAR 43 Appendix A that pilots may legally perform on an airplane that they own is strut servicing. 

The struts on any airplane serve a critical purpose. They provide the shock absorption necessary to prevent the airframe structure from enduring too much stress from the impact loads incurred on landings. 

Even taxi operations impose stress on an airframe every time the gear hits a bump or uneven surface. 

The strut absorbs the bulk of these loads and prevents them from being transmitted to the airframe. 

Types of struts

There are several different kinds of struts used for shock absorption. Over the years aircraft manufacturers have used different materials to limit the stress from the impact of landing. Some have used rubber biscuits, bungee cords and spring steel. 

The most common type found on most planes (and the only type used on fairly heavy planes from light twins all the way up to airliners) is the hydraulic air/oil cylinder, also referred to in some manuals as oleo struts. The oleo strut is very reliable, can withstand tremendous loads and is fairly simple in its design. 

The oleo strut uses air pressure and hydraulic fluid to create a spring effect. The strut consists of an outer housing called a cylinder and an inner piston that is connected to the nose fork or to the main wheel axle. The piston portion of the strut is the part that actuates up and down. 

There are different styles and configurations, but all struts house hydraulic fluid in the lower section of the strut and compressed air (or nitrogen) in the upper section. As the piston is driven into the cylinder upon landing, the fluid is forced through an opening called an orifice that slows the rate of the flow. 

Some manufacturers make use of a metering pin connected to the piston. The pin is mounted so that it is forced upward through the orifice along with the fluid. It protrudes up through the orifice, is slim in the middle and wider on both ends. 

Its shape is tapered so that as the piston reaches the top portion of its travel, less and less fluid can fit through the opening. This gradually slows the fluid flow and decelerates the piston. Meanwhile the pressure of the compressed air is being steadily increased as the piston travels upward and reduces the volume of space in the upper chamber. 

Eventually the increased pressure of the compressed air overcomes the decreasing fluid pressure and forces the piston to extend. As the fluid flows in the other direction, its flow is impeded at a steady rate by the opposite end of the metering pin, gradually slowing the fluid flow in the opposite direction. This results in dampening out any oscillations and returning the airplane to its normal static height above the ground.

Some models don’t use metering pins but have metering tubes with various sized holes in them that slow the rate of flow as the piston reaches either end of its travel. Some manufacturers don’t use either metering pins or tubes, but instead use restrictor plates with orifices in them to produce the same effect. 

Fluid and air: both are vital 

On any model, the strut has to have the correct amount of fluid and air to work properly. The fluid used for strut servicing is MIL-H-5606 (red) mineral-based hydraulic fluid. 

5606 is sold by the gallon and in quarts. It is nice to keep a supply on hand not only for struts, but also to refill brake and gear reservoirs. Typically it takes around a gallon of hydraulic fluid to service three struts.

Nitrogen is better than compressed air for strut servicing because it is drier and doesn’t vary in pressure as much as air; it is also less corrosive to the inside of the strut housing. 

However, nitrogen is not always readily available. A person needs a regulator and high-pressure hose in addition to a nitrogen bottle, and the cost for all the items can exceed $500. 

If nitrogen is not available, air pressure from a standard air compressor is usually sufficient to air up a nose strut. Nose struts don’t require as much pressure as main struts. 

Typically the main struts on Piper aircraft require at least 200 psi to inflate the strut to its proper level. Most standard air compressors don’t get that high. There are boosters that a person can purchase for around $200 that will increase compressor air to a high enough level to inflate the struts.

It would be best on a twin Piper (or any twin, period) to always use nitrogen, due to the increased weight of a twin engine plane and the more extreme temperature changes.

Servicing a strut

The tools a person needs to service a strut include about three feet of clear flexible tubing with a ¼ inch (inside diameter) opening to fit over the Schrader valve; a valve stem tool; and an empty gallon size container to catch the old fluid. 

To properly service a strut with fluid and air, the airplane needs to be jacked, or at least have the nose raised if only the nose is being serviced. 

With the airplane jacked, remove the valve stem slowly from the filler valve in the top of the strut. It is best to loosen it enough to release the air pressure, and then remove it the rest of the way after the pressure has bled off. A small spray of hydraulic fluid comes out with the air pressure, so it’s a good idea to have a rag handy. 

Once the valve stem has been removed, push the tubing over the open Schrader valve and insert the other end into the empty container. Next, push the strut up to its fully collapsed position. Any old fluid will be shoved out. 

Then remove the container with the old fluid and insert the hose into a can with at least a half-gallon of clean new hydraulic fluid. Next, pull the strut down to its maximum extended position. The suction will pull in the fluid; it will continue to siphon for a few seconds after the strut is fully extended. 

Next, slowly push the strut up to its fully collapsed position. As some of the fluid is pushed back out, air bubbles will come out too. 

Extend the strut again, and repeat the process until all of the fluid comes out as a solid stream on the compression stroke. 

Once all of the air bubbles are removed, the strut will be considerably more difficult to push up to its collapsed position. Once the strut is fully collapsed, the hose should then be removed from the valve and the valve core reinstalled. 

This process is called bleeding the strut, and it’s the only way to get the proper amount of hydraulic fluid into the inner chambers of the cylinder. 

There is no way to simply pump a little fluid in to the strut; the strut must be filled using this bleeding process. If the process isn’t followed, large air pockets in the lower chamber can cause the strut to collapse under a load. 

Once the strut is filled with fluid, it can then be aired up with either nitrogen or compressed air through a strut booster. 

After the airplane is lowered off the jacks, the final adjustments can be made by releasing a little of the pressure by depressing the valve core for a split second at a time. 

Generally main struts should be inflated so that around five inches of the piston is exposed, and nose struts to around four inches. 

The exact range for each model can be found in the service manual. The struts should be inflated so that they are within the proper range even when the airplane is fully loaded. 


Troubleshooting struts

Properly serviced struts should have a certain amount of buoyancy about them. 

Struts that are filled with air pressure but are low on hydraulic fluid tend to stick in place. Struts that stay extended for a period of time after a plane has landed and then suddenly collapse are also typically low on fluid. 

Any sort of a knocking noise from the nose strut during taxi operations or upon landing is an indication that it is bottoming out due to it being low on fluid, air, or both.

If a strut is low on fluid, it is usually because the rubber seals have gotten old and hardened. There is generally a rubber wiper ring and a large rubber O-ring with one or more backup rings in the strut housing. These rings harden and become brittle over time, especially in cold weather. 

Granville’s Aircraft Hydraulic and Strut Sealant is an FAA approved product that can be mixed with hydraulic fluid and added to the struts during the servicing process. It doesn’t cause the seals to swell, but it does cause them to soften and become more flexible, much as they were in their original state. 

This additive works well, once enough of it gets into contact with the seals. After it is first added, the strut may still go flat a time or two and need to be re-aired before it finally holds. 

Properly serviced struts help to soften landings and prevent damage to the airframe, and keeping the struts in good shape will pay off big in the long run.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Appendix A to Part 43, “Major Alterations, Major Repairs and Preventive Maintenance”
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