Trim Servo Failure

[DonFromTX]

Trim Servo Installation Mistake

My friend with only about a couple dozen hours on his 12 had a harrowing day. He encountered violent vibration and control problems, slowed down to 80 and regained control, and landed without bending anything.
Postflight exam revealed the trim servo had busted its threaded rod from the servo, PROBABLY secondary to the nut coming off of the pivot bolt, and the pivot bolt moving out 3/4 of an inch or so. It is my guess that the cotter pin was never placed properly in the pivot bolt.

 

[RV10inOz]

Good post Don,

And a timely reminder to have a A&P (LAME in some countries) do a dual inspection of the entire aircraft. If you are a A&P, get another one to check you.

We have a REQUIREMENT here as I assume the US does for dual inspections on all flight control systems prior to first flight and whenever any maintenance is performed on a flight control item. This includes trim servo's Autopilots or adjusting a aileron or flap.

Even if it is not a legal requirement it is prudent to do so.

It is alarming the things a trained eye spots and the builder is blind to it. Worth spending a few hundred bucks on doing so.

 

[DonFromTX]

This is a case where the current owner purchased the completely built RV12, lacking only an engine and instruments.
I was double checking the instructions (11-07 and 08), it is interesting that all the drawings show the cotter pin just slipped thru the hole, no spreading of the ends! (Maybe someone was just following the drawings to the T)
I also noticed they call for only slightly spreading the ends at this stage, to be finalized after the tailcone is attached. Anyhow, just an area you might want to look at one more time to be sure.

 

[rvbuilder2002]

Maybe a more appropriate heading would be "construction error / post assembly inspection failure"?

It is no fault of the trim servo if it fails because fasteners fell out and parts began flailing around...

 

[DonFromTX]

You are quite correct, I altered it a bit to be more informative.

Maybe a more appropriate heading would be "construction error / post assembly inspection failure"?

It is no fault of the trim servo if it fails because fasteners fell out and parts began flailing around...

 

[BobTurner]

1. Here in the US the builder inspects prior to flight. A good DAR will, too, but is not required to. If the FAA comes out they may or may not inspect throughly.
2. NTSB found the probably cause of the P51 crash at Reno a couple of years ago to be a trim tab fastener coming loose. Your friend is lucky.
3. Every builder must or at least should adopt a system to prevent hoses put on finger tight but never torqued; cotter pins inserted but not spread; etc. Personally I used red tape as a flag on anything which looked done but was not.

 

[DonFromTX]

I think old age makes it worse (I am 76)! If I am about to put in a cotter pin, and the phone rings, it may never get put in. I try to use tape flags and notes, but it still gets by me sometimes.

 

[rvbuilder2002]

3. Every builder must or at least should adopt a system to prevent hoses put on finger tight but never torqued; cotter pins inserted but not spread; etc. Personally I used red tape as a flag on anything which looked done but was not.

Very good advice.

In my shop we use florescent green tape to mark anything not finished permanently, during maintenance or construction.

Because we often have multiple people working on an airplane build there is a high danger of "I thought you finished that" so we also use torque seal when final torquing any fastener, hose, etc. When it gets to final inspection before flight, any fastener or hose end without torque seal gets checked with the appropriate tool. A good practice even for builders working solo.

 

[RFSchaller]

During building I used to put a "T" on the heads of bolts after I torqued them. A Sharpie works well and a little alcohol removes the mark if you have to loosen it.