Electric trim failure

For the past six months, I've been dealing with an intermittent electric elevator trim problem cleverly masquerading as an autopilot problem -- when the electric trim would stick after disconnecting the autopilot (a misleadingly truncated version of the scenario), the cure was to cycle power to the autopilot servos. But today, I finally had a hard failure (fortunately, on the ground) and the problem was easy to find, especially when you could hear the elevator trim servo motor running and see that the trim tab wasn't moving. A stripped gear.

So this got me to thinking... probably most all of us learned to fly in factory built airplanes where electric trim, if installed, was an adjunct to the manual trim, so electric elevator trim failure was of little consequence and not taught. And in transport category aircraft, the concern is runaway trim, so I've always made sure in my airplanes that there was a way to power down the electric trim if it decided to get frisky spontaneously.

And when I think of total electrical failure symptoms in homebuilts, I've always thought about attitude information, a standby GPS, electronic ignition, and flaps -- but I've never thought seriously about the effects of stuck elevator trim, like you would get in an electrical failure.

So... Has anybody ever practiced landing an RV with a simulated failed elevator trim, like, stuck at cruise setting? Or done a full circuit with lots of nose up trim, like you'd have on landing?

For all you CFIs and checkout pilots, do you teach elevator trim failure? Any thoughts or comments?

Thanks,

Ed

I have been wondering about this quite a lot lately as well. I am curious if the plane is still manageable with full trim tab deflection. Would love to hear if anyone has experience with this.

jcarne said:

I am curious if the plane is still manageable with full trim tab deflection.

Click to expand...

Yes. Go fly and try it. And remember that you don't have to do everything at WOT.

It takes a LOT of force on the stick at cruise speed with a trim at full deflection, but you can still fly it. Once you slow down, it's no big deal.

Should be part of Phase I testing or transition training/aircraft type familiarization.

I had a stuck trim on my RV8 years ago, almost 1000 miles from Home.

It was mildly annoying, but certainly not a big deal.

It wasn?t stuck at either extreme, but at an approximate climb setting, which meant some continuous forward pressure on the stick at cruise, and some back pressure around the pattern. Workload was pretty low.

I?ve flown the airplane with the trims at either stop, and while annoying, the stick forces are light enough in the RV that it?s manageable.

This is emphatically not true of some other airplanes I fly, where a stuck or runaway trim is a real emergency, but at least in my RV8 it?s not a big deal.

I normally fly my rocket trimmed for cruise and I leave it that way. Landings and take off are no problem at all.

The only time I will adjust it is for a passenger, but again trimmed neutral in cruise.

I had a stuck pitch trim one time in a high trim up position. It took noticeable forward pressure to fly home. Slowing down helped. Upon landing, the position of the trim became less and less of an issue, and at touchdown, it wasn't really an issue at all.

Short answer is yes - I have flown all of my airplanes with "stuck" trim during Phase 1. That's one of the things that Phase 1 is for - fly it in all corners of the envelope. All of the RV's were flyable that way (not fun, but flyable). If you are trying a new design, sneak up on the condition, of course.

Paul

If I could piggyback on this conversation, I was forced to land with full aileron trim stuck in one direction recently. It was a challenge, but do-able. Basically, the servo extended when the hat switch was pushed to the right, but wouldn't retract when the switch was deflected the other way. My first hunch was that the switch was bad, but by playing around with it on the ground, I realized that if I very quickly deflected the hat switch in the good direction and then reversed direction immediately, the server would start working the other way. Anyone have any ideas as to whether this would be caused by the hat switch or a bad servo? It's standard Ray Allen stuff.
Thx.

j-red said:

If I could piggyback on this conversation, I was forced to land with full aileron trim stuck in one direction recently. It was a challenge, but do-able. Basically, the servo extended when the hat switch was pushed to the right, but wouldn't retract when the switch was deflected the other way. My first hunch was that the switch was bad, but by playing around with it on the ground, I realized that if I very quickly deflected the hat switch in the good direction and then reversed direction immediately, the server would start working the other way. Anyone have any ideas as to whether this would be caused by the hat switch or a bad servo? It's standard Ray Allen stuff.
Thx.

Click to expand...

I've had relay failures (the old RC Allen relays were known to go bad) that allowed motion only in one direction. Every failure I've had in these systems has been a result of those old relays.

I just recently did testing failed trim conditions.
I simulated runaway trim to full down with a forward C/G and at 120 KIAS the pull force on the stick was 22 lbs. So no fun for a longer flight.

To cover runaway trim and other hardware failures like a faulty hat switch I installed an alternate trim function which consists of a guarded selector switch (Dynon A/P panel versus power from main bus) and an extra springloaded switch for the trim function. Of course this doesn?t cover electrical failure or a failed trim motor.

Just curious...for the stock RAC pitch trim, has anybody ever had one run away?

IIRC, that would require a short to ground somewhere "after" the trim motor (i.e., wiring back from the motor to the switch or the switch itself) if it's wired up as RAC diagrams out.

Just curious if we're solving a problem which really doesn't exist. As always, adding "safety" mechanisms can often result in an actual *decrease* in the overall system safety...

For a short ckt to cause runaway trim it would have to occur in the control wiring related to the switch(es). A short in the servo drive wiring could not cause it.

My experience based on the few instances I am aware of over the years is that the typical runaway trim is from a stuck switch or relay and it usually occurs when the trim was being purposefully changed (so the pilot is not likely to be taken totally by surprise by it).

I would consider it not very common in properly built and maintained RV's, but it is a good idea to do a test flight and know what you will be up against if it ever does occur.

In an extremely out of trim condition, speed is your enemy.
The higher the airspeed the more difficult countering the out of trim condition will be.
If you ever have it occur, immediately pull the power back and reduce speed to help you get things sorted out. Then find a speed that is comfortable to fly for an extended period.

If the out of trim condition is nose up and you slow to at or below Vfe, you can use flaps to help relieve the trim force.
If the out of trim condition is nose down, it will probably be of value to land no flaps.

Manual Trim Failure

I converted to electric in my RV7A a year ago but only after experiencing a manual trim failure right after takeoff (twice). It seemed the elevator trim cable had come unscrewed from the elevator trim knob resulting in a "full nose up" trim condition. Needless to say, this situation got serious attention because in solo flight you are not going to be able to screw the trim knob back to the cable and it took a LOT of forward stick pressure to maintain level flight.

The first time it happened I didn't fully understand my situation until after landing and could investigate the cause. The second time it happened I was just angry with myself for not catching the problem during preflight.

However, I will say that if you keep your wits about you and "fly the airplane" sudden full nose up trim in the RV is very manageable. It just takes serious forward stick pressure until landing and then you "release" pressure "a little " to land. Expect a few bounces and try to avoid high crosswinds with gust!!!

I cannot imagine a situation in an RV with electric trim that would cause a "runaway trim" condition of full nose up or nose down trim. Electric trim does not move that quick. However, if you experienced a full nose up or down condition due to a parts failure then I guess anything is possible. However, I repeat, the aircraft is recoverable if you recognize your situation and keep your wits about you. The elevator has a lot of authority and you can safely land it. BUT, I don't recommend practicing this! It's not something to play around with.

I honestly don't know to this day why the cable became unscrewed (twice) but it did convince me to do a trim conversion I had been thinking about doing just didn't know if it would be worth the effort and expense. I now LOVE my electric trim and Tosten military style grips!

I strongly concur with Scott McDanials recommendations on how to handle an extremely out of trim condition. The first thing is to slow down to around Vfe - Speed Only Magnifies your out of trim condition. If possible pick a runway with light winds and take time to analyze your situation and how your going to land your aircraft.