Dear Mooney Owners,
Nothing is more frustrating than being greeted with a nasty Service Bulletin, and have no history behind it. Therefore, here’s the rationale behind the required inspection:
In late January, I received a report from an Acclaim owner in Poland that they had experienced a separation of the Empennage hinge assembly from the Empennage bulkhead. It happened shortly after takeoff. The pilot reported that at about 200’ AGL, he encountered some magnitude of vertical shear, heard a loud bang in the tail, and the aircraft pitched nose up. With the assistance of another pilot in the right seat, the two of them managed to push the nose down enough, with throttle control, to land the airplane safely. My hat is off to these two pilots to say the least. Upon examination, it was found that the rear hinge, filler plate, and Jack screw lug had pulled apart from the Empennage bulkhead, Huck pins and all. The collars were found inside the stinger. I had the hardware shipped to me for investigation. I had pictures of the empennage sent too. The pictures revealed that the filler plate had been installed on the wrong side of the hinge. In the correct stack up, the hinge has a .032” surface milled above the hinge pivot. The hinge is then installed with a .040” filler plate that sits between the hinge and the bulkhead. The purpose here is twofold: One, it holds the hinge pivot off the bulkhead .008” to allow rotation of the hinge plate without galling the bulkhead. Second, it keeps the hinge parallel to the bulkhead, minimizing shear forces on the Huck pin. Our working theory was that without the filler plate between the hinge and bulkhead, the installed pins would not pull up perpendicular to the bulkhead, and the collars would not swedge properly. Also, with the .040” filler still in the stack, there was now an added .032” gap added to the lower row of Hucks, changing the grip length. Our theory was that this had allowed the hinge to “work” on the bulkhead, thereby allowing the Huck pins to loosen in their holes. Add a vertical gust load, and the lower Huck pins and collars would have failed first, then pulling the upper pins loose due to the lever action.
This was done in conjunction with FAA Engineers, whom I contacted immediately after receiving the hardware and pictures from Poland and determining there was more to this than wind shear. I am required (and would do it anyway, ask anyone who knows me) to report a failure such as this to the FAA within 24 hours of determining that the failure was one that is covered by FAR 21.3.
Mooney and FAA agreed that an AD should be issued, and due to the failure mode, it should be an immediate Emergency AD based on our Service Bulletin M20-313. With that accomplished, it was discovered that the next sequential S/N produced after the Polish airplane was incorrect also. Service Bulletin M20-314 was issued to put the airplane(s) back to Type Design. This new result was not like the previous, as there were no signs of movement in the assembly, it was just put together improperly. The only common factor we could find was that the same assembler has assembled both tails on the same day, and the same inspector had signed off on the installation. So we set the effectivity at the beginning of this installer’s tenure in the Empennage shop, and went back from there 10 airframes, just to be sure. We also made test coupons and tested our Huck guns for proper pull and swedging. All passed.
Weeks went by, and the Confirmation cards flooded in. All were reported inspected and proper, with good collar swedges on the Huck collars. I was starting to thing we had caught the only two, when another TN was reported to be found incorrect. This one was built by the same assembler, exactly one year to the day before the original two were produced. I have no idea what that tells us, but the confident feeling was leaving me. Two days later, an Eagle eyed mechanic at one of our Service Center’s called me to tell me that he was doing an Annual on a 1990 J Model, and thought he’d just take a look at the hinge. It was incorrect. This airplane had 1400 hours on it. I visited the airplane, and found no movement of the hinge, and good Huck bolt installations. My feeling is that it would have flown forever in that condition, no problems. However, it wasn’t right, it wasn’t to Type Design, and it had to be corrected. So, yet another call was made to FAA, their Engineers came out again, and we now had to try to isolate an effectivity once again. Inspection records from 1990 showed that another assembler had built the J Model’s tail. That means we had lost the common denominator used to set the original effectivity. Next stop was the Stinger assembly drawing to see if there was a change in construction in the past that would allow us to point at a specific time when we knew that all was well. Turns out that the hinge mounting hasn’t changed since 1961, with the introduction of the M20B.
This is where my job gets hard. We have nothing to correlate the improper assembly to. We still do not know the root cause of the failure in Poland, or why the others that have been identified have not loosened up like that one did. I have nothing that I can set an effectivity to after the initial design that is in use today. Therefore, I have decided that ALL airframes need to be looked at. The design is better than good, calculated to withstand 10 times ultimate load, and actually tested to 1.8 timed positive and negative gust Ultimate load. There has NEVER been a reported failure of this assembly, until the one in Poland. Even that one was flown to a safe landing. The chatter about the Empennage falling off is incorrect. The primary attach points are the Empennage pivot point and the elevator control rods. The hinge’s primary job is to allow for longitudinal trim, and to stabilize the Empennage laterally.
I have had to do this before. But I did it without hesitation, because it was the right thing to do.
Inspecting your airplanes is also the right thing to do. We absolutely have to know that this issue does not exist anywhere else. YOU absolutely have to know that this issue does not exist anywhere else. It’s going to cost you an hour of labor to have that peace of mind. You may curse me for making you do it. But at least you’ll be around to curse me, and that’s what matters most to all of us here at Mooney.
Director of Engineering
Mooney Aviation Company, Inc.
165 Al Mooney Drive North
Kerrville, TX. 78028
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