Taming Runaway Trim

[dmattmul]

OK, I've never thought of this before or felt it could happen but seems it could. A recent AOPA article discussed an issue if a pilot disconnects the autopilot while there is still pressure on the yoke (say forward or back pressure) the servo might not mechanically disengage. I am about 2 weeks from Stein starting my panel and now would be a good time to make the change the article suggests. Adding a "Electric Trim in Motion" light to the instrument panel.

I know the G3X can be set for:

"The GSA 28 servos can limit the maximum continuous run time of the trim
motor to help prevent the potential for a trim runaway situation. The Trim
Motor Max Run Time setting is the maximum amount of time the trim servo
will run continuously when a manual electric trim input is detected. If the
maximum time is exceeded, the pilot must release the Manual Electric
Trim (MET) control and then reassert it to continue running trim."

Is the above enough or should I think about the "Electric Time in Motion " light.

[dlloyd3]

I have experienced a trim run problem on a Bonanza. Within a couple seconds I recognized what was happening. I took another second to reach across and pull the breaker, the only breaker I consciously memorized the location. Airspeed decayed from 100 to about 85 in those seconds. I think the nose up trim when the breaker was pulled would have been enough to stall without a healthy push on the yoke. Unlike most Van's, I could then manually crank in nose down trim before the force became a burden. Yes, I would put in the trim in motion light. I would set the trim to shut off after so many seconds. Put the odds in your favor.

[flyvans.com]

During our test phase, i performed landings on our RV-7A with max trim "runaway" in either direction (but not in motion).
Relatively easy to handle, although with a very "abnormal feel".
But not really a 737max-ish showstopper regarding forces and controls...

One very valuable thing regarding the trim we did was retrofit the trutrak autotrim module.
1. The autopilot dynamic performance is better with a trimmed airplane (phugoids and such)
2. trimming out manually while the AP is on is not that easy.
3. it's a much safer situation during an autopilot disconnect (be it intentional or not), taking over control of a trimmed airplane vs. one that wants to pitch away to either side immediately.
4. the autopilot without the autotrim could slowly "creep" upon you, e.g. appearing to follow a glide but slowly descending below due to limited force and e.g. a speed reduction without any kind of alert!

and we have already had an aural AP disconnect alert (a feature which i still can't believe was missing from the trutrak) from the very beginning, another safety critical feature in my opinion!

Anyone explored this runaway scenario in a -14 or a -10 and could comment?

Based on the -7A i would focus on the autopilot trim interaction but not so much on limiting / recovering extreme trim positions.

good luck!

[dmattmul]

I'm using Vertical Power and not sure how long "pulling the breaker" will take. I've never used VP but respect the difference. I do have breakers for my EFII that I can obviously add a breaker as needed for the servos.

[flyvans.com]

The VP200 had a trim runaway protection logic even on the software side of things.
may even have been carried over to the VPX?!

[mdevans9]

Bernie,

How did you add "an aural AP disconnect alert"? My Trutrak has disconnected three times on long trips. It does so very smoothly. Very hard to detect - it helps to have an alert son in the right seat who asks, "Where are you going?"

Thanks,

Merrill

[supik]

For pitch trim action monitoring I would consider a tone /aural warning during triming. -you would notice even when looking out..ideally the tone would come on when trim moves longer than 3seconds to avoid nuisance warnings. Anyway, I think the G3X time cutout is a good enough prevention.

With the G3X you can add a switch for fast trim and flaps motors disconnect.

Or put a collar on the associated CB for quick identification.

[Bill.Peyton]

From the vertical power manual



10. 5b Runaway trim and flaps
Runaway trim or flaps is indicated by both switch inputs being active at the same time. This can be caused by a stuck switch, a shorted wire, or various other causes.
If you discover the trim or flaps running un-commanded, push and hold the opposite button to immediately stop the motor. The input switch pairs are as follows:
Pitch trim: up down
Roll trim: left right Flaps: up down
After 3 seconds, the affected circuit faults and you can release the button. A faulted circuit does the following:
? The input switches for the faulted axis are disabled
? An alarm message is shown on the screen
? The EFIS may display soft keys that allow you to run the trim from the display.
For example, if the pitch trim begins to ?run away,? hold down the opposite pitch trim switch (a natural reaction, by the way) until the fault shows on the screen. When it does, the switches are disabled.
After a runaway condition, you can re-enable the trim or flap circuit by selecting it from the list of items on the EFIS electrical page and press the ?Re-Enable? soft key. You cannot re-enable the trim or flaps if a switch input is active.
10. 5c Trim and flap operation with a faulty position sensor
The position feedback is ignored when the trim and flaps are operated

[bpattonsoa]

I have a trim limiter in the RV-10 and more importantly a “Trim Power” switch in my “7 up” switch bank required for Take off and landing.

I did a trim runaway test in the -10 and can say it is flyable but not for long. The -10 trim is powerful.

[penguin]

I would not add a 'trim in motion light' - just more complexity. My advice would be to carefully consider what you want to achieve.

VPX and G3X both offer similar functionality so be careful what you wire up. I have to admit I'm not a VPX fan, so I would not use the VPX trim (or flap) features. The G3X does everything you need for the trim (and flap) so use that (it is also much better developed than the VPX). I would power the whole G3X system from c/bs that are independent of the VPX (but I understand that will not be a view held by all).

The G3X manual is comprehensive, if quite extensive http://static.garmin.com/pumac/190-01115-01_al.pdf. Do you have a GAD27? Perhaps a good idea to add one of these to handle all the trim mixing and limiting duties, no need to involve anything else. If the AP is off I understand the trim commands are routed direct to the trim servo by the Garmin AP servo (GSA 28). Stick with the system Garmin designed, they have put plenty of development time into sorting out the problems. Will the GDU display what the trim system is doing?

Autopilot audio is available from the GMC 507 (AFAIK TT doesn't provide disconnect audio).

Pete