Power trim emergency advice

I have an RV6A with a power trim, no manual. I have been unable to find published emergency procedures in the event of failure, either nose down or nose up

Go fly and give it a test. I think you'll find that with full up or down trim the plane is still very controllable.

The stick pressure gets pretty heavy. But it is still controllable. Slowing down the aircraft helps.

I know of a couple builders that installed a trim reverse and trim off switch due to having a run-away trim occurrence on more than one occasion. The reverse switch moved the trim in the opposite direction and then can be disabled.

Nice device that is designed to address your concerns.

I had it in the 10, will be putting one in the new plane also.

http://www.tcwtech.com/Safety-Trim Page.htm

I have kill switches for both my trim and autopilot. They are both simple on/off switches which can kill the power to the trim or A/P. You could do the same with pullable breakers.

Ted Westerman said:

I have an RV6A with a power trim, no manual. I have been unable to find published emergency procedures in the event of failure, either nose down or nose up

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Try trimming for neutral cruise flight. Then try an approach and landing with no changes. As you slow down and add flaps for landing, the nose gets slightly heavy. It is quite controllable and easy to land while holding back the nose down stick pressure.

As previously mentioned, a lot of builders have either a toggle switch or trim diss- connect button.

I normally leave my trim set for cruise and the power off. It makes for an easy preflight item to check the trim tab position and connection integrity while checking the other elevator items.

About the only time I have the power on for the trim is for tiny adjustments in cruise flight.

If you have a Garmin G3X system with autopilot, and the trim servos have been wired through the GSA 28 autopilot servos, then the Garmin system will perform a similar function to the TCW system suggested by Mike S.
Per the G3X manual:
"The Trim Motor Max Run Time item allows optional configuration of a time limit for use
with manual electric trim. When the manual trim input switch is pressed, the electric trim
motor will stop running after the time limit expires, and will not run again until the trim
input switch is released and pressed again. This can help prevent "trim runway" caused
by a stuck trim input switch."

Cheers,

Don?t want to be argumentative

But Jim, set up in fast cruise flight, trim the nose down continuously. assuming a stock Vans trim system, count the seconds before the trim pressure becomes so great that you could not comfortably overcome it. For me that was about 3 seconds. Enough time to reach a switch and secure power?

I suggest in that scenario, A run away stock trim is marginally controllable. In slow flight probably not a problem, but in fast cruise, I contend that a slower trim speed or some other risk mitigation method is very prudent

I don't remember if there's a good way to "limit" the ray allen servo to the max trim needed for slow flight.... or if that amount of trim at cruise is excessively difficult to hold the stick.
I guess I just need to go test by trimming for slow flight, and speeding back up to cruise to see how it feels.

It also seems that it should/could be possible to increase the arm distance at the trim tab pushrod connector to limit overall trim tab throw... but that could make trim adjustments slow (although precise). Has anyone tried that?

Since experiencing "runaway" down trim one time at cruise (my finger hit my trim switch - located on the panel next to my throttle) I'd really like to find ways to make sure a real runaway trim or another similar event doesn't end up causing a bad day. Once it happens to you, it really wakes you up.

I used a two part approach to mitigate risk:

1) I “biased” the servo/tab linkage by lengthening the threaded rod so that the servo provides just enough nose down trim to balance flight at Vne, no more is possible as the servo is at its physical limit. My reasoning was that a nose up runaway was a less dire situation, as a simple reduction in power would result in rapidly decreasing airspeed in a runaway induced climb, and as AS slows, stick force could easily overcome trim force.

2) install the TCW controller set on slow speed - trim operation is slow in some regimes, but certainly adequate.

I have a trim disable switch, but was not confident in my response diagnosis time in an extremis situation

Give it a real world test. See what you think

I think this question might be prompted by the recent trim issues with 737 MAX. Jets, entire horizontal stab moves, not a tab on the elevator.

The trim tab on an RV elevator can't make plane uncontrollable but could create high control forces at high speeds. Slowing down and man handling it will work.

gmcjetpilot said:

I think this question might be prompted by the recent trim issues with 737 MAX. Jets, entire horizontal stab moves, not a tab on the elevator.

The trim tab on an RV elevator can't make plane uncontrollable but could create high control forces at high speeds. Slowing down and man handling it will work.

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And if my pitot ices over, the minimum speed function on the G3 autopilot try?s to do an outside loop like the Max 8.