Elevator trim tab safety

[wypaul 448]

Returned from the Idaho Backcountry last Saturday interesting trip to say the least.  On the way into JC, about 5 miles from Indian Creek my Avid cut loose with an extreme case of elevator flutter.  That got my attention right away.  My buddy confirmed that the elevator trim tab was the culprit. Jim I think that you mentioned that you had this happen one time.

Looking back in my manual I found a couple of pages labeled "Trim Tab Safety".  I did not find any of the parts to build the safety and thought that it applied only to the Bowden cable setup.  So my question is did anyone receive these parts with their MK IV kit and if so I would like a picture of the parts as installed or not install and what are the materaials used to build HT-10 and HT-11.  

I have not seen a service bulletin referring to this Safety but suggest anyone flying a MK IV or an Avid upgraded with the trim system, electric or mechanical look into installing the Safety on their plane.  I do have the drawings, parts list and instructions if anyone needs them.

So the failure occurred by way of the rivets failing that were holding the trim horn to the trim tab.  When I covered the tab I covered the rivets with the fabric flap so that they were not visible. A better practice would have been to install the horn after covering and painting was complete or not allowing the flap of fabric to cover the horn.  I have built a new horn and installed it with three SS Rivets and in addition used epoxy between the horn and the F-89 that it attaches to.  The horn is not easy to look at while doing the preflight but having lived thru my wake up call rest assured that is going to be a big priority in the future.

all in all the trip was way to exciting, my flying buddy had his engine lockup just after we came out of the Backcountry and by a stroke of luck and great skill was able to glide to a grass strip and a safe landing.  So we left our gear with his plane and returned home in my Avid.  Now we have a 1600 mile road trip in a couple of days to retrieve his plane.

Some of you will know where the attach photo was taken.

[EDMO 1078]

Paul,  Can you post the drawings?  Glad it was not worse for either of you.  I have seen drawings of a spring attached to top of tab to bring it to neutral if controls unlatch.   Some have added an arm with counterbalance weight to top of tab too.   EDMO

Edited 12 Jul 2017 by EDMO

[C5Engineer 1170]

Thanks for sharing. Please be sure to post the rest of the pics of your trip when you get home and settled.

[1avidflyer 1983]

Yes, I had the trim tab get loose and start to flop the elevator up and down.  Go my attention for sure.  Glad your episode ended well also.  I would also like to see the "safety" pictures or drawings.  JImChuk

[zadwit 105]

another way to  prevent this  is put a mass balance weight on the tab to balance it 100%. IE with the tab control disconnected, the tab should just  ride along. Other wise is can be a dangerous situation.. IT caused  many a crash on the beech craft Bonaze Vee tail because the 1/16" trim tab cables would rust and fail allowing the elevator to flutter causing  the stablizer to  flex wildly up and down and few times and rip off. It is nothing to mess with. I f you look  at teh Dehavilland HC2 Beaver, they have the mass balance weight on the ailerons.

The Taylorcraft L-2 has it as well on the elevator tab. Most Cessna 120, 140, 170, 172 dont need it because a cable chain drived a gear box that has a push rod that goes to the elevator tab.

[flywise 169]

Thanks for your input Paul, I checked my trim tab horn after Cowlove had the same happen to him on his Catalina.

Cheeers, Laurent

[cowlove 33]

As Laurent said, I had the same happen on my Catalina.  Mine is still just riveted, no spring or mass balancing, but it gets a very thorough looking-at on preflights now