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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

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)”
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