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Old 02-21-2021, 07:55 AM is offline
Join Date: Oct 2020
Location: Malden, MO
Posts: 34
Default Beep....beep...

Thread drift...a trim "beep" tone when the trim motor runs has been pretty much standard on (almost) all the large aircraft I’ve flown. Where would an experimental aircraft equivalent fall on the technical/practical/Human factors/“worth the effort” scale?
Initial thoughts (in answering my own question I doubt the "beeps" would help much in a runaway situation where the trim was commanded to run at full speed while on autopilot...but if you're at cruise and you hear more than a very few and very occasional beeps, you’d likely know something is wrong before it got ugly. If the VP-X (or similar device)is set up to run trim speeds at substantially slower rates when above 100 knots, a lot more beeps would have to be ignored before things got ugly. ?

(Thanks to the OP for starting the thread)
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Old 02-21-2021, 09:57 AM
pvalovich pvalovich is offline
Join Date: Dec 2005
Location: Ridgecrest, CA
Posts: 439
Default Thoughts on Runaway Trim

Much of my flight time before starting my -8A build was in the Navy A-4 Skyhawk, where runaway elevator trim was an immediate action emergency. Gave a lot of thought to that in my panel design.

Forward three switches on my switch panel: Elev Trim Power; Aileron Trim Power; Autopilot Power. Have never had reason in 1300+ hours to activate the first two in flight, but have used the A/P power cutoff. If building now, would eliminate the first two. Didn't think the power cutoff switches could be activated in time to stop full runaway, but could prevent further random trim inputs caused by uncontrolled electrons.

Didn't have the Flight Test docs in Phase 1 in 2012 but one of the early tests I conducted was full nose up trim and full nose down trim over slow to fast cruise and landing speeds. For nose down scenarios I let the nose drop to various attitudes before initiating recovery. For slow nose up scenarios, immediate unload to zero g, stop any roll, get the nose down then sort things out.

The -8A was controllable - admittedly sometimes uncomfortably so - at all test points.

The key - just as was shown in the 737 Max debacle - was to control the nose-low airspeed by reducing power to idle as soon as possible. With a constant speed prop, airspeed reduction is pretty quick. I never got near Vne.

This was for an -8A with fairly forward CG. Your results may vary.
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Old 02-21-2021, 10:18 AM
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mburch mburch is offline
Join Date: Feb 2005
Location: Oregon
Posts: 1,316

Don't forget that if you are like the OP, and you have a Garmin G3X or G5-based autopilot with GSA 28 autopilot servos that drive the aircraft's DC trim motors, then simply pressing the AP DISC button on the control stick will interrupt power to both the autopilot servos and the trim motors.
Matt Burch
RV-7 (last 90%)

Any opinions expressed in this message are my own and not those of my employer.
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Old 02-21-2021, 03:12 PM
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MCA MCA is offline
Join Date: Jun 2005
Posts: 710


The VP-X and other modules that run trim don't normally directly connect to the audio system. In the case of the VP-X, it sends a signal to the EFIS whenever the trim is moving, and then it is up to the EFIS (which is normally connected to the audio system) to generate a tone if that vendor so chooses.
Marc Ausman
RV-7 980 hours, IO-390, VP-X (sold)
RV-8 (flying a friend's)
Thinking about low and slow backcountry build.
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Old 02-22-2021, 08:13 AM
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Tankerpilot75 Tankerpilot75 is offline
Join Date: Jan 2015
Location: Oklahoma City, OK
Posts: 681
Default Runaway Trim vs Disconnected Manual Trim

Manual trim malfunctions can be really scary too (and yes, they do occur)!

During my first two years of ownership my RV7A had manual trim. Twice during that period the trim cable became disconnected from the trim knob where the two join. Both times this occurred immediately after takeoff when I was adjusting for climb out speed.

The aircraft immediately acted as if it has full nose up trim and became a handful to fly and created a real interesting landing problem. Of course the faster one flies the greater the out of trim pressures. There’s nothing like landing an airplane where you release forward pressure on the stick to touchdown. Not a pretty landing!!!

Those two experiences convinced me that “runaway trim” couldn’t be any worse than a manual trim cable disconnect. At least with runaway trim I should probably have some indications that trim is moving in a direction that I don’t want. With a manual trim cable disconnect it happens suddenly!

Those two incidents convinced me to convert to electric trim and I haven’t regretted the decision. I did use the Pat Hatch UTRB module with Tosten military style grips. I’m also getting ready to install the TruTrac Autotrim Module that a friend (who has manual trim) didn’t use when he installed his autopilot.

Let’s face it. There isn’t a system designed by men that isn’t subject to unexpected failures. Just be prepared to handle them when (not if) they occur. As the saying goes: sh** happens!
Jim Harris, ATP, T38, EC/KC-135A/E/R, 2008 RV7A, 2nd owner, N523RM (2015)
Superior XPIO-360, Hartzel CS prop, Dual GRT Horizon EX with ARINC, EIS, Garmin 340, 335 w/WAAS gps, Dual 430s (non-WAAS), TruTrak 385 A/P with auto-level & auto-trim, Tosten 6 button Military Grips, FlightBox wired to EX, Dynon D10A w/battery backup, 406 MHz ELT. Custom Interior, New TS Flightline hoses, Great POH!
Retired - Living the dream - going broke!
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Old 02-22-2021, 09:04 AM
dweyant dweyant is offline
Join Date: Sep 2012
Location: 07TS
Posts: 505

Originally Posted by mburch View Post
Don't forget that if you are like the OP, and you have a Garmin G3X or G5-based autopilot with GSA 28 autopilot servos that drive the aircraft's DC trim motors, then simply pressing the AP DISC button on the control stick will interrupt power to both the autopilot servos and the trim motors.
That is not correct.

In my case the AP had nothing to do with the trim issue. They are two different motors, and while the G3X will drive the trim.

Disconnecting the AP would have done nothing. In fact the GARMIN setup immediately disconnects the AP if it detects a trim input from one of the switches. That was in fact my first indication that I had a problem.

Dan Weyant
Done and Flying 4/30/2015
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Old 05-09-2021, 07:16 AM
RViator60 RViator60 is offline
Join Date: Nov 2019
Location: Southport, NC
Posts: 87

Thanks Dan for starting this instructive post on runaway trim, something that until now I didn't think much about. I haven't purchased/wired my grips yet but my panel is fully installed and wired. Here's what I took away:

- Get to know how your plane handles with fully deflected trim at various airspeeds so your prepared when this happens.

- If this happens, slow down immediately and focus on flying the airplane

- Consider this an emergency and land asap. Dan's interaction with ATC was a great example of cooperation and keeping your cool under pressure, very impressive.

- I won't rely on back-up systems and complicated electric solutions. Assume some fault, probably a short, caused the trim to fully deflect and stays put, certainly if power is eventually cut to the trim motor

- I imagine any short is most likely at the molex connecting the stick to the main harness. This connection moves continually so I will double check that all terminations are securely made and that there is sufficient strain relief. Probably also add this to my maintenance inspection plan.

- I still plan on the Tosten grip with the 4-way hat switch but only for the pilot side.
Marty Guinn
Based at KSUT, Southport, NC
- flying 1973 PA28-180 (N55087)
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Old 09-13-2021, 12:35 PM
georgemohr georgemohr is offline
Join Date: Aug 2021
Location: Ewing Township
Posts: 23

Is anyone aware of a good grip that includes split switches for pitch trim? Also, can either the VPX accommodate needing parallel switch input to run the trim? As this switch or the wires serving it are the most likely culprit on a runaway, I'd love to plan for this on our 14.
Edit: I think the pinout of the VPX suggests it will support split switches. There's poles for the trim position and trim power circuits...

Last edited by georgemohr : 09-13-2021 at 01:46 PM.
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