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Loose Elevator Trim Servo

Kevin Horton

Well Known Member
Motivated by elevator spar cracks found by one RV-7A owner , and Tony Kelly's tragic accident on Friday, I conducted a detailed inspection of the empennage on my RV-8 today. I'm very, very glad I did this inspection, as I found a significant, but easily fixed issue. I'm lucky to have found the issue before it progressed to its logical conclusion, because the results could have been very, very bad.

As I was laying under the left side of the tail, I flexed the elevator trim tab, and there was a bit more play than I expected. Looking closer, it seemed that the majority of the play was coming from inside the elevator. It looked like the trim servo was moving in its mount. I removed the panel that the trim servo is mounted to, and was horrified to see that three of the four nuts that secure the bolts holding the servo to its mount were missing. Two of the bolts were also gone. The only thing holding it in place was one bolt and nut, and a bolt without a nut on the diagonal corner. I shudder to think what would have happened if that last nut had come loose.

It looks like I used temporary hardware to attach the servo to its mount, as I obviously didn't use lock nuts or Lock Tite. I must have done a trial fit with temporary hardware, and forgot to later remove it to install the proper flight hardware. This would have been early in the project, when I still had the illusion that my memory was perfect. I later learned to never, ever assemble something with temporary hardware, or without the final torque, unless I added an unmistakable visual clue that more work was needed. I started hanging pieces of fluorescent tape from parts that had temporary hardware, or were not final torqued.
 
Good find Kevin, I removed the carb. on my SuperCub a while back. Removed the drain plug to get rid of the gas and reinstalled the plug finger tight. Big mistake that hopefully I will not repeat. Like you , luckily I found it on the second preflight by noticing no lock wire.
I now make sure I mark the detail needing work very clearly. You know I have always had a problem closing up a panel cover or even the cowl door, thinking what am I missing. After you look long enough , you just have to close the door and go flying.
Talked to your Dad at the air show last week. Always interesting to talk to, very friendly guy, must be where you get it from.
Ron
 
It looks like I used temporary hardware to attach the servo to its mount, as I obviously didn't use lock nuts or Lock Tite. I must have done a trial fit with temporary hardware, and forgot to later remove it to install the proper flight hardware. This would have been early in the project, when I still had the illusion that my memory was perfect. I later learned to never, ever assemble something with temporary hardware, or without the final torque, unless I added an unmistakable visual clue that more work was needed. I started hanging pieces of fluorescent tape from parts that had temporary hardware, or were not final torqued.

Trick I used was all temp hardware was painted fluorescent orange.
 
Trick I used was all temp hardware was painted fluorescent orange.

Great idea ... I'm getting a rattle can of the brightest color I can find.

In the satellite biz, the NFI (non flight item) are red; aluminum parts are anodized red ... good policy!
 
Been there, done that.

It happened to me.

My -10 has a bellcrank hidden under some structure in the tail, that reverses the push/pull rods direction. After takeoff, I needed some 'down' trim so I touched the button. Instead, the airplane pitched up...startling me. We had just made an uneventful trip to Key West and back, with four of us and baggage.

I immediately pulled power and U-turned back for a landing. The bolt had fallen out of the bellcrank so there was no reversal and trim worked backwards. The bolt, washer and nut were lying in the tailcone, directly under the bellcrank and no cotter key in sight anywhere!

My bet is that it was never installed and flew for 5 years and 600 hours like that!

Best,

ps Philip, I really like the bright orange paint on temporary nuts and bolts.
 
Thanks for the OP, Kevin ...

... I thought I was the only one that had "oversights" like that. At my first annual a few weeks ago, I found the nut on the bottom rudder pivot was not tightened down, though a bit more than finger-tight. It was a Nyloc, and not in much danger of coming off, but I could still see daylight between the washer and the hinge bracket. I was astounded, given how many times I had gone over all the tail hdwe and connections before first flight a year ago. For me, all of these instances underscore the wisdom of having "fresh eyes" do some of our annuals.
 
You can never have too many fresh eyes!

Team AeroDynamix members inspect each others' airplanes for just that reason. Sounds like a good reason to spend a day at the airport with an RV buddy-- going over each others' airplanes. We fly sometimes with safety pilots-- why not a "safety mechanic!"
 
I invite all my neighbors (A&P mechanics) over to my hangar, open up the airplane, and tell them that the person who finds the most or the most significant defect gets a gift certificate for two to a nice restaurant.

Best $100 you can spend.
 
Servo

Glad to see that you caught that in time Kevin.

Going down Thursday to check on my tail just to be sure.

This sure is an eye opener.
 
Took lunch at the airport today and checked all the items being discussed on the forumns lately. All looks good! One thing I did want to mention is that I have electric pitch trim and there is absolutely NO slop or movement in the trim tab! Will use this for future referance.

All of the events of late have caused me to modify my pre-flight walk around inspection. Now I have a flashlight in my back pocket to see those elevator and rudder rod end barings. I am also testing all surfaces for unusual movement or any sign of rubbing etc. Finally, I am listening during those controll surface test for any sounds that are not supposed to be there.

Getting serious about my preflight and not taking any of it for granted! Time to get professional.
 
It looks like I used temporary hardware to attach the servo to its mount, as I obviously didn't use lock nuts or Lock Tite. I must have done a trial fit with temporary hardware, and forgot to later remove it to install the proper flight hardware. This would have been early in the project, when I still had the illusion that my memory was perfect. I later learned to never, ever assemble something with temporary hardware, or without the final torque, unless I added an unmistakable visual clue that more work was needed. I started hanging pieces of fluorescent tape from parts that had temporary hardware, or were not final torqued.

Great find Kevin, and a great lesson for all. When building, in addition to leaving things OBVIOUSLY LOOSE that were NOT COMPLETED TO FINAL TORQUE, I used a check list with many many items to be checked off prior to first flight. Like this: "ITEM 87 - Torque pitch trim servo fasteners".

Now that I am flying, I often use a paper tag attached to my throttle quadrant that has a list of critical things I undid during maintenance to ensure they go back and get checked prior to flight.
 
Team AeroDynamix members inspect each others' airplanes for just that reason. Sounds like a good reason to spend a day at the airport with an RV buddy-- going over each others' airplanes. We fly sometimes with safety pilots-- why not a "safety mechanic!"

I share a hangar with a friend. We are always looking at each others planes with a critical eye. It's turned into a bit of a game, but we have both found things that needed to be addressed. Neither of us are offended by the others inspections.

BTW. I inspected the rod end jamb nuts on the tail of my -8 today and found two that were very loose and another that needed to be tightened. I put some torque seal on them to make it easier to see if they move.

RIP Tony. Thanks for everything.
 
Personally, I made it a policy to never put non aircraft grade / temp hardware near my kit. In Australia you have to go to a little bit more effort to buy imperial bolts and the like. Most of us don't have heaps of spares lying around. About the only real exception I can think of was spar carry through bolts.

IMO the risks of reusing a nylock before it has the chance to "age" in situ is far less than using unsafe hardware to fit things up. I just bought some extra nylocks and threw them out when necessary.
 
elevator / trim tap

I previously had a Zodiak 601 with the same MAC servo as the RV's

Experienced severe vibration (10Hz?) in the control stick and subsequenly found the servo connnector rod failed. E.g trim tap flutter. This was at 90 mph so I can imagine the effect would be at double that speed.
I will replace the link on my RV with steel forks, ipo nylon, and a thicker threaded rod.
 
What happens when it lets loose?



It's a very exciting ride...note that there isn't much attached to anything else. Left elevator Mk2 was built much better.

Carry on!
Mark
 
Does anybody have a RV8 drawing or photo of what the servo installation is suppose to look like by the plans?
 
No big surprise:

...and the reason for it coming loose???

Not following the assembly instructions. Harumph. I wonder who wrote those??:eek:

The tab actuator arm broke 1st, as it was not attached correctly. The elevator started dis-assembling itself right after that.

The Mk2 version (elevator AND tab) involved additional pieces being installed here and there to beef up what myself and a few eyeball engineers thought might be weak points (if the part was to be run past the normal Vne on a regular basis). Of course, having a part in front of you that did not make the cut is a good thing, if the parts are to be rebuilt differently. So far, so good.

Carry on!
Mark
 
Don't overate "Fresh eyes"

... For me, all of these instances underscore the wisdom of having "fresh eyes" do some of our annuals.
.

I think you as the builder with a thorough inspection and action worklist for the annual condition inspection can do the work best. You must focus on every item, the requirements must be unambiguous and there must be personal accountability for every item completed (initials or signature and date not just a check mark). Separate from the worklist you need a discrepancy report form with a unique number for each discrepancy that is correlated with the worklist. Every discrepancy recorded must have the corrective action included in an adjacent space on the Discrepancy Report form and closeout accountability on the right side of the page - you need to be able to look at all of the pages and verify closure reliably.

I read of Annual condition inspections completed in a day but mine take about a month and I have the complete records of every one.

Calling in someone else to take a look gives you nothing of the required rigor for this critical work.

Bob Axsom
 
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