Piper’s long-running preference for stabilators began with the PA-24 Comanche. A&P/IA KRISTIN WINTER discusses the stabilator system and its most common problems, as well as what you can do to keep your tailfeathers airworthy.
The Piper PA-24 Comanche was the first Piper production aircraft to use an all-moving stabilator instead of the more common horizontal stabilizer and elevator combination. Piper obviously liked the result, as they used stabilators in the Aztec and in all the Cherokees and their derivatives.
Piper stabilators have proven to be fairly robust, with only a few issues that can easily be addressed with heads-up maintenance. In this piece, we will look at the known issues with Comanche and Twin Comanche stabilators.
Torque tube horn cracking
The stabilator system had been reasonably AD-free until 2012, when the FAA issued AD 2012-17-06, which addresses cracking in the torque tube horn that attaches the counterbalance arm to the torque tube, to which in turn the stabilator halves are attached.
The cracks propagate from the inside, as all three of the cracked horns in the photo show (See photo below).
The cracks progress from there to the outside, and eventually, a secondary crack will propagate on the aft side, as cracks in the front have now progressed to the point there is relative movement inside the horn. Once this happens, complete failure of the horn is likely but a few flight hours away.
Were the horn to completely fail, leaving two halves and counterbalance arm effectively disconnected, it is expected that a violent flutter would ensue, leading rapidly to an inflight breakup. The consequences of the horn failing made an Airworthiness Directive inevitable.
AD 2012-17-06 requires an initial and repetitive inspection every 500 hours or five years for the PA-24-180, -250 and -260 models. Alternately, the AD allows for installation of a new horn which then does not have to be inspected until 10 years or 1,000 hours time-in-service, whichever occurs first. A recent Piper Flyer article written by Steve Ells discusses the stabilator horn inspection procedure in detail. (See link in Resources.—Ed.)
The PA-30 Twin Comanches and the PA-24-400 Comanche 400 were spared from the AD. No horns on these models have been found to be cracked. Also, they have torque tubes with thicker walls.
One theory on why some horns have cracked is that the two bolts that hold the horn in position on the torque tube were overtightened during manufacture, causing the horn and torque tube to be pulled as much as 0.010 inch out of round and creating internal stress in the horn. FAA metallurgical tests confirmed that the cracks were due to stress corrosion in horns.
The thicker tube of the twins and 400s dramatically reduces the potential for an out-of-round condition to be induced and to date, no reports of cracks have been made in the 400 and the twins.
A group of Australian Comanche owners engineered a replacement horn. They have gained an STC. It is made of a different aluminum alloy and is machined rather than forged. The FAA has stated that the installation of the “Aussie Horn,” as it is commonly called, terminates the AD.
That declaration was unnecessary as the AD listed the applicability to the part number of the Piper horn. Once that particular part number is no longer installed in the airplane, the AD is automatically terminated.
Johnston Aircraft in Tulare, California, is the North American distributor of the horn. Other well-known Comanche-savvy maintenance shops advertise that they stock the kits for customers wanting to eliminate the AD.
One note of caution is that the STC requires a 100-hour visual inspection of the Aussie Horn. It is not a concern for owners who fly less than 100 hours a year, as any modestly competent annual inspection would make a visual inspection of such a critical piece of the control system.
However, for those flying more than 100 hours a year, in order to maintain complete and verifiable airworthiness, owners need to have that inspection made and accomplishment of such entered into the aircraft logbooks. It would also be wise to have that inspection entered as a separate item for the annual inspection as well.
Torque tube corrosion (Piper SB 1160)
A related issue that comes up when complying with the horn AD is corrosion on the torque tube. Piper issued Service Bulletin No. 1160 to address this.
When it first came out, Piper was offering the torque tube for less than $300, which was very reasonable. Currently, it isn’t listed as being available, though that might be a computer glitch. The most recent price I’ve heard is over $1,000 for a new torque tube.
Piper gives a minimum dimension for outside diameter and for wall thickness. The corrosion happens between the two saddles that are inside the stabilator. My view is that the outside dimension is most critical at those four points, and the two points where the bearings sit. For the strength of the tube, the minimum wall thickness must be maintained throughout.
I have personally witnessed at least half a dozen torque tubes on PA-24 series aircraft that have wall thickness of approximately 0.190 inch, which substantially exceeds the minimum for any of the tubes.
Any doubt about the airworthiness of the torque tube should be resolved by replacement. Loss of part of the tail never ends well.
Trim system play
There are a few additional issues to be aware of related to the stabilators. One is excessive play in the trim tab hinges and mechanism. Piper has set the maximum play of 0.076 inch. There are several bolts and bushings in the trim mechanism that, when worn, contribute to the play.
However, the biggest culprit is usually the trim drum mechanism. The Oilite bushing in the top and bottom of the housing wear, and the drum itself wears a groove where it contacts those bushings. The bushings are obtainable, as Piper Part No. 452-376. These are pretty standard, so they may be available from sources other than Piper.
There are two common approaches to repairing the wear in the top and bottom of the trim drum. One method is to use welding to replace the missing material and then machine it smooth.
Another common repair is to machine out a space to press in a stainless-steel washer that the bushing can ride on. This is a bit more permanent, but requires precision work, as ideally, one wants an interference fit that requires heating the drum and chilling the washers so that they are held in position when everything normalizes.
Less often, it is the hinges that contribute significantly to the play in the trim tab. They are reasonably available and not difficult to replace. For single Comanches (except for the 400), the hinges are Piper Part No. 20194-18. For the twins and the 400, use Piper Part No. 16914-00.
Repairs to stabilators
Sometime after 1965, Piper added a prohibition in the Service Manual to any patches or splices in the stabilator. This prohibition does not apply retroactively, as a matter of law. (See the sidebar “Is it true that aircraft must be maintained in accordance with the most recent service manual?” for a discussion on the legalities. —Ed.)
I keep hoping to find out exactly what year Piper came out with that prohibition, so we could know which models are affected.
One common repair is a splice of the outboard panels, from spar to trailing edge and outboard of the trim tab. For sure this is a legal repair on 1965 and earlier aircraft. It also meets all the requirements of Advisory Circular 43.13 which sets forth acceptable repair techniques.
Not all patches meet the standard of 43.13, which means that in absence of a field approval from the FAA or an FAA Designated Engineering Representative, the repair would be illegal and the stabilator will need to have a proper repair done.
The splice shown in the below is the only one that I would be comfortable doing, and then only on model years that don’t bear the Piper prohibition. I know of one aircraft that has had its stabilator spliced in that manner for 58 years and it is just fine.
Comanche 400 and Twin Comanche Hi-Shear rivets (AD 94-13-10)
The stabilator attaches to the fuselage with four large bolts that go through the blocks that hold the torque tube bearings, and then pass through the aft bulkhead and into four L-shaped blocks that are attached to the longerons in the empennage.
In the PA-24 series, with the exception of the Comanche 400, these blocks are attached to the longerons with standard rivets. Problems are rare with the PA-24s with the standard rivets.
The 400 and the twins used a different fastener. In that application, Piper used what is known as a Hi-Shear rivet. The Hi-Shear is a steel pin with a soft aluminum collar to hold it in. The aluminum collar makes the Hi-Shear rivet unsuitable for any application which induces any tension loads. The Hi-Shear rivet is purely for a shear load.
The problem with using the Hi-Shear in this application is that with all the different loads present in the tail of an aircraft in flight, there are likely some tension loads present in the joint between the “L”-blocks and the longerons.
The above is not theoretical. L-blocks have worked loose on some twins and 400s. Piper issued Service Bulletin 411A and the FAA followed up with AD 94-13-10. The SB and the AD mandate a 100-hour check of the blocks for looseness. This is an easy check to make as it takes less than five minutes.
The good news is that when looseness develops, it develops slowly. The ones that I have personally observed as being loose were likely just a relative movement of a few thousandths of an inch. This inspection catches the problem long before it can become critical.
The Piper fix is to replace the Hi-Shear rivets with small AN21-15A clevis bolts and nuts. This involves removing the Hi-Shear rivets, reaming out the holes from 0.125 inch to 0.136 inch, and installing the clevis bolts and nuts. The Piper kit number is Part No. 760-835.
Currently, Piper wants $709 for a kit that contains five clevis bolts, five nuts, and 10 washers. Since four kits are needed, the cost of parts to do all four L-blocks is almost $3,000 just for parts.
It is interesting to note that Piper’s kit instructions state that only blocks with loose rivets should have the kit installed. This is a bit hard to fathom considering the work that goes into just getting access. I know of several owners and their mechanics who decided to replace all of the rivets, even though only one or two were loose.
Given the high cost of parts, I would be looking for alternative sources for the kit parts which are just basic AN/MS hardware, though not of a common size. Another option is an alternative method of compliance which allows the use of standard AN3 bolts and locking nuts. (The letter granting this AMOC is available on the Piper Flyer website. —Ed.)
My concern with that AMOC is that you are reaming the holes out to 0.1875 inch, which is removing a lot of metal on the blocks, particularly the forward one. If I were faced with the inability to find affordable AN-21-15A bolts or the entire kits, I would consider asking for an AMOC to use Hi-Lok fasteners which can be 0.1563 inch, which leaves more material on the blocks.
It is easy to tell whether any of the kits have been installed. Access the tail through the access panel which is on the right side of the empennage just forward of the stabilator. From there, you can see whether the Hi-Shear rivets have already been replaced.
The photo below shows the nuts clearly which means that the kit was installed on those visible L-blocks. I have never heard of any of these blocks coming loose after the kits were installed.
A further tip for owners and IAs—don’t assume that just because there is a signoff in the logbook stating AD 94-13-10 has been complied with, that all four kits were installed. Until all four have been installed, the AD has not been terminated and the check required by Piper Service Bulletin 411A must still be done on the blocks which have not had the kit installed.
Additional Airworthiness Directives (75-27-08 and 74-13-03)
Another stabilator-related AD is AD 75-27-08 which is an inspection for a manufacturing error. Apparently, some Comanches left Lock Haven with tail skin rivet holes drilled into the L-blocks and aligned with the rivets that hold them on.
This is a one-time AD and one must look at the Piper Service Bulletin No. 464A to really see what this is about. If all four of the Hi-Shear rivet replacement kits have been installed, then the AD doesn’t apply. If it is determined that Piper did drill into the blocks at their attach rivets, the same kit is the cure for that.
The last of the stabilator-related issues is a minor one. There are eight bolts that go through the torque tube. They are all subject to corrosion, particularly the parts that are exposed inside the tube. Four hold the stabilator halves to the ends of the torque tubes. These were called out for inspection in AD 74-13-03.
The requirement was to inspect within 100 hours and then every three years or 500 hours. If stainless steel bolts were installed, the repetitive inspection was terminated.
These are close-tolerance bolts, so when looking at a Comanche for the first time I look for the triangle marking on the head to make sure that a close tolerance bolt was installed. A small raised dash on the head confirms that the bolt is stainless.
The PA-24 series, other than the 400, use an AN174-C32A. The twins and the 400s use a larger bolt, the AN175-C32A. I haven’t seen a Comanche in the last few decades that didn’t have the stainless bolts.
There are two AN175-33A bolts that hold the horn onto the torque tube. These can be a pain to get off if they are not the stainless variety and they haven’t been off in a bunch of years. The middle part of these bolts rusts badly. This is also true of the two AN24-46A clevis bolts that hold on the stop collar.
I have found success with a good bead blast cabinet and a wire wheel on a long extension so that I could clean enough of the rust off the bolts to pound them out. Using Kroil, Mouse Milk, etc., is helpful, but I didn’t find penetrating oils enough on their own, especially as I didn’t want to risk damaging the aluminum horn.
Any reader of this might get the general idea that a fairly well-equipped shop is a necessity for diving deep into the Comanche tail. If so, then I haven’t failed as a writer.
While a shop doesn’t need every gadget known in the industry, someone who knows and works on a lot of Comanches is likely to have acquired what is needed. There is no real substitute for experience in the particular aircraft.
After reading this, you should have an understanding of what the issues are and what maintenance you can perform if rated (or oversee as an owner) to ensure that your Comanche’s tail is in airworthy condition.
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 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 editor [AT] piperflyer [DOT] com.
From March 2019 Piper Flyer
Johnston Aircraft Service Inc.
Federal Aviation Administration
Federal Aviation Administration
Federal Aviation Administration
Federal Aviation Administration
Piper Service Bulletins
No. 411A, 464A, 1160
Piper Flyer Magazine Extras
PDFs available at piperflyer.com/forum
AMOC for AD 94-13-10
Piper Flyer Magazine Extras
PDF available at PiperFlyer.or
FAA maintenance legal opinions
(Dec. 5, 2008; May 21, 2015)
Piper Flyer Magazine Extras
PDFs available at piperflyer.com/forum
by Steve Ells, December 2018
Is it true that aircraft must be maintained in accordance with the most recent service manual?
For years, it was interpreted by FAA frontline inspectors that the most recent service manual is the one to be used, even though the aircraft being serviced was much older. The industry lived with this interpretation for years.
This interpretation hit older aircraft particularly hard. With the Comanche, it created endless controversy about repairs to the stabilator, which is prone to hangar-rash-type dings on the trailing edge.
Some countries require aircraft owners to comply with any new recommendations that come from the manufacturer. In effect, they allow manufacturers to issue regulations that owners must follow.
In 1946, the United States Congress passed the Administrative Procedures Act (APA). This act prevents federal agencies from delegating rulemaking authority and has other limitations, such as requiring notice and comment period.
In answering a request for interpretation from the FAA’s manager of the maintenance division, the FAA’s Assistant Chief Counsel issued a written opinion Dec. 5, 2008, which clarified that the term “current” only meant current at the time that the aircraft was produced or when the operator adopted the manufacturer’s progressive inspection regime.
The interpretation cited the APA’s limitations on federal power to delegate its rulemaking authority to third parties. (The FAA Office of Chief Counsel issued this interpretation in response to a request for legal interpretation from Tennessee Aircraft Services regarding Cessna 210 aircraft. —Ed.)
On May 21, 2015, the FAA Office of Chief Counsel applied that same rationale to changes in the service manual, even when they were in the limitations section. The limitations section is the only section of the service manual that is approved by the FAA and is regulatory for aircraft produced under that version of the type certificate.
In this case, Cessna had put structural inspection requirements in the most recent version of the Cessna 210 service manual, even though they hadn’t built a Cessna 210 since 1984. The FAA confirmed that for Part 91 aircraft, “current” meant current at the time the aircraft was produced.
That brings us to Comanche stabilator repairs, like those mentioned in the article. Sometime after 1965, Piper added the prohibition against making any splices in the skin, rather requiring the replacement of complete sheets.
Based on the above interpretations, it is clearly legal to make a splice repair to a stabilator skin if the aircraft was 1965 production or older and it was done with approved data, such as AC 43.13. The prohibition on stabilator repairs occurs later (sometime after 1965 —Ed.). I haven’t seen copies of the service manual from between 1965 and 1972. The latter (1972) contains the prohibition.
Arguably, it would be legal to ignore Piper even for later produced aircraft as it is not in the limitation section. However, the FAA has not specifically given their interpretation of whether a repair done in accordance with AC 43.13 can be used when the manufacturer prohibits such a repair. There is also the issue of civil liability when knowingly making a repair that the FAA might bless, but Piper does not.