Piper PA-24 Comanches are subject to AD 2012-17-06, which requires inspection of the stabilator horn for cracks and corrosion. A&P/IA Steve Ells describes proper procedures for removal, inspection and reinstallation.
June 22, 2011, the FAA issued a Notice of Proposed Rulemaking (NPRM) that asked all concerned parties to comment on a proposed AD. The proposed AD would mandate an initial inspection of the stabilator horn assembly for cracks and corrosion on Piper PA-24, PA-24-250 and PA-24-260 Comanche single-engine airplanes. After the initial inspection, replacement or continued inspection would be required.
Since I am the happy owner of a 1960 PA-24, I followed this closely.
Comanche owners contributed many comments seeking to relieve and/or clarify the requirements of the original NPRM. Some suggested that the FAA make changes due to mitigating factors such as the wall thickness of the stabilator torque tube, nut-tightening torque values on the bolts that secure the horn to the torque tube and other concerns.
The NPRM was written because cracks had been found in several stabilator horns. The submitted comments were well-regarded by the FAA and raised valid points. After changes and updates, AD 2012-17-06 was issued Aug. 22, 2012.
The review of the initial proposed rule, the comments, names of the commenters and the FAA’s responses to the commenters can be read in full on the FAA’s website. (A link to the page is in Resources. —Ed.)
AD 2012-17-06 and Piper Service Bulletin No. 1189 (April 29, 2010)
A little over five years ago, in early January 2013, I pulled the stabilator horn assembly on my Comanche and drove it over to Johnston Aircraft in Tulare, California, for the initial inspection. In my opinion, Johnston ranks right up there with three other Comanche shops in the country.
I watched as Charles Gazarek at Johnston showed me how to remove the aluminum horn and then perform the inspection steps mandated by the AD. No cracks were found in the horn.
This horn is an aluminum part that connects the stabilator torque tube to the cables that control aircraft pitch through the horn assembly. The horn assembly consists of the horn, a tube (Piper Part No. 22880-00) and a balance weight (Part No. 23175-00). The torque tube is secured to the aft bulkhead of the fuselage by four bolts that hold bearing block assemblies in place. The torque tube rotates in two large sealed bearings. The stabilators slide over the torque tube and are secured by two bolts on each side.
The AD calls for an initial inspection, and then repetitive inspections every five years or 500 hours TIS, whichever comes sooner. The horn must be removed from the aircraft and inspected for cracks by dye penetrant or other means.
The timeline for complying with the initial inspection requirement of the AD varies based on the age and origin of the horn. Refer to the AD for details, but the short version is that if a horn has not been inspected since the AD was published in 2012, it is likely due for an initial inspection.
The repetitive inspection can be terminated by installation of what’s known as the Aussie horn, which is a stronger replacement horn that was developed in Australia and is approved for installation by a STC. (For more on this alternative, see the sidebar on Page 28. —Ed.)
Pull those tailfeathers
Removing my left and right stabilators was easy. (The author, Steve Ells, is an A&P/IA. Repair work such as this must be performed or supervised by an authorized mechanic. —Ed.) I removed the two close-tolerance (AN175) corrosion-resistant bolts on each side that I installed to comply with a previous AD (AD 74-13-03 R1) and slid both stabilators off the torque tube. They slid right off since I had cleaned and polished the tubes and applied a light coat of LPS-2 prior to reinstallation five years ago.
The stabilator control cables attach to the torque tube assembly at the forward end of the tube at the balance weight. I cut the safety wire and slacked both cables, then removed the bolt that connected the cable terminations.
Then I removed the two bolts that secured the balance weight on the tube, and slid the weight off.
Next, I removed the upper and lower tailcone fairings and the tail navigation light bulb socket after disconnecting the trim tab actuating arm assembly (Part No. 20828-00) and electrical wiring to get access to remove the torque tube.
After removing the two bolts of the yoke assembly that supported the trim tab drum assembly to the blocks that hold the torque tube bearings and lowering the yoke and drum assemblies, I removed the four bolts securing the bearing blocks and pulled the torque tube assembly out from the aft bulkhead.
Preparing for inspection
After removing the assembly from the airframe, I slid the two bearings and blocks off each end of the torque tube, then slid the aluminum horn off the tube for inspection.
None of this was difficult. Because the tube had been previously cleaned, the bearings slid off easily. I checked the bearings for ease of rotation. They were smooth and free. The left and right bearings had been replaced in the past. Newer bearings have a white Teflon seal; original bearings have a red seal.
After I heated the horn for a few minutes with my electric heat gun (an electric hair dryer will work), it slid off the tube.
I stripped off the “rattle-can” primer paint I had applied after the last inspection with off-the-shelf paint stripper.
Piper Service Bulletin No. 1189 shows two horns that had cracked. In each case, the cracks were through the bolt holes either at the front or aft side of the horn.
I looked closely at the horn with a very bright light but couldn’t see any evidence of cracking. The next step was the dye penetrant inspection.
It took me just over three man-hours to get to this point.
The AD requires an inspection in accordance with the instructions given in Piper Service Bulletin No. 1189. According to the bulletin, cracks start at the inner surface, so there’s no need to remove exterior paint from the horn to complete the inspection. Simply clean the inside of the horn with isopropyl alcohol prior to performing the dye penetrant inspection.
A dye pen inspection consists of cleaning the surface, then applying a coating of the penetrant, which is a very viscous red liquid. Leave the dye in place for a few minutes so it can penetrate any surface cracks, then clean the part completely. The dye pen kit I used consisted of a can of spray-on cleaner, a can of spray-on dye and a can of spray-on developer.
As you can see by the photo, I sprayed a generous coat of dye onto my horn. After a few minutes, I sprayed the provided cleaner on a clean rag and wiped off all the dye.
After drying the horn, I sprayed on an even coating of the developer. The dye is ruby red, while the developer looks a lot like spray-on talcum powder. The developer coated the surfaces of the horn.
I was looking to see if I saw red lines in the white developer which would indicate cracks. Luckily, there was no evidence of cracking. So, I cleaned the horn and started reassembling the torque tube assembly.
I again smoothed and cleaned the outer surfaces of the torque tube with a Scotch-Brite™ pad before applying a light coating of LPS-2. Then I slid the horn into position.
The most critical reassembly task was applying the correct torque to the two bolts that hold the horn in position on the torque tube.
I then slid the left and right bearings and blocks onto the torque tube. There was no need to remove the part called the stabilator torque collar. This collar, which appears black in the photos on bottom of Page 30, is part of the stabilator travel adjustment system.
The service manual calls for a dimension of 8.620 inches between the left and right bearing block. If that dimension is not attained, shims are installed to achieve it. Mine checked OK.
The stabilator horn assembly is mounted to the rear bulkhead with four 5/16-inch diameter bolts, two in each bearing block. The nuts are torqued to the standard torque (there’s a standard torque table in the service manual) for 5/16-inch bolts loaded in tension. That torque value is 100 to 140 inch-pounds.
Once the bearing block bolts were torqued, I moved the trim tab drum yoke into position and installed and safetied those bolts. Then I slid the left and right stabilators onto the torque tube and secured and torqued those bolts. Checking stabilator balance and travel
After the tail was reassembled but before I reconnected the pitch system control cables, I followed the procedure in Chapter 4 of the Piper PA-24 Service Manual to check the stabilator balance. First, I leveled my Comanche. A stabilator is in balance when it can be moved to any position throughout full travel and not move once placed in any position. Mine was balanced.
I then connected the control cables and positioned and tensioned the up and down cables to provide the mandated control wheel travel and the stabilator travel.
I rechecked my procedures and steps, then when satisfied that all was as specified, I reinstalled the tailcone fairings after reconnecting the power and ground wires to the tail navigation lights.
In all, the removal, inspection, reinstallation and rigging and travel checks took very close to a full day of work, or 8 man-hours. (For reference, AD 2012-07-16 estimates the entire inspection process, including removal and replacement, takes 12 man-hours. —Ed.)
After completion of the steps in the AD, I entered a maintenance record (logbook) entry that is signed and reads similar to this: “October 29, 2018: Airframe total time 3,255. Complied with AD 2012-17-06, dated Oct. 22, 2012 (g) (2) (i) and (5) and Piper Service Bulletin 1189 Instructions 1 through 6. No cracks found. AD is next due October 29, 2023 and 3,755 aircraft total time.”
Now my stabilator horn is airworthy for another 500 hours or five years.
Not too tight; it’s a shear load
Paragraph 5 in the “Inspection/Replacement” section of AD 2012-17-06 mandates that the bolts that go through the horn and torque tube be torqued to 120 to 145 inch-pounds or 10 to 14.5 foot-pounds. It doesn’t sound like much, especially for bolts that may at first seem to hold the tail together!
There are two reasons for what seems to be a low torque value. The first reason is because overtorqueing the nuts has been determined to be the cause of the cracking. There’s no need to apply a hefty torque since the nuts don’t need to do any more than prevent the bolts from falling out.
The second reason is because of the bolt loading. This bolt-nut combination is loaded in shear, not tension. The bolts transfer the up and down motion of the horn/tube and balance weight into rotary motion of the torque tube.
To apply the correct amount of torque, the friction drag of the nut’s locking element first must be determined. Common aircraft-quality self-locking nuts “lock” in position due to either a fiber insert in one end of the nut or by a slight out-of-round section of a steel locknut.
How many inch-pounds of turning force does it take to overcome that locking component? The easiest way to determine the friction drag is with a deflecting-beam torque wrench. The following is the equation as published in the AD:
The stated torque value of 120–145 in.-lbs. includes friction drag from the nut’s locking element, which is assumed to be 60 in.-lbs. The installation torque can be adjusted according to the actual, measured friction drag. For example, if the friction-drag torque is measured to be 40 in.-lbs. (20 in.-lbs. less than the assumed value of 60 in.-lbs.), then the installation torque will be adjusted to be 100–125 in.-lbs. of torque.
The steps are to first determine the friction drag of the nut’s locking element in inch-pounds and then adjust as necessary to get the final torque.
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 and the proud owner of a 1960 Piper Comanche. He lives in Templeton, California, with his wife Audrey. Send questions and comments to editor [AT] piperflyer [DOT] com.
Johnston Aircraft Service Inc.