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Lock Haven Low Wings Comanche Aileron ADs 36

Lock Haven Low Wings Comanche Aileron ADs 36

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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 .

RESOURCES >>>>>

AD 77-08-01 Aileron Spar Cracks
rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/23E39A518733EFE28625699E004A9AD5?OpenDocument

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


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