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Run away trim

Jkkinz

Well Known Member
I was talking to a local Airbus pilot today about my 7A. I told him that I was installing two axis electric trim. He asked if there was also manual trim in case the trim motor goes haywire and forces the trim into full deflection. I told him that the Rv's only have either electric or manual, not both. He was flabbergasted that there was not both manual and electric like the commercial built have. (eg Cessna)

That got me to thinking. I have been building for 3 years and reading the forum for longer than that. I cannot remember reading about anyone having a run away problem with the Ray Allen electric trim on a RV. Have any of you high or low timers had any problems with the electric trim motors supplied by Vans? If so, what was the cause, results of the malfunction and forces necessary to maintain control of your out of trim RV?

Just pondering
Jim Kinsey
7A Fuselage
 
Thanks to light control forces on an RV, you can easily overpower full trim deflection. No biggie if it ever happened. It is a bit mean at higher speeds but no problem in the pattern.
 
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Runaway trim

It's not a serious issue with an RV. But, just in case,I ran my trim and autopilot circuits thru switches by the throttle. If either acts up, I can remove power instantly. In other aircraft this is done by pulling a circuit breaker but since mine are behind the panel I can't do that.
 
My approach was to put my trim on my main buss (i.e. not on my essential bus). If the trim runs away I'll just engage the e-buss. Now, that won't get me back to level trim but it will stop the runaway.
 
I think I can remember only one post about a runaway condition, and yes - it is absolutely controllable. The more likely case is that the relay slowly dies, and won't let the trim run in one direction. What happens then is that in the process of playing with it, jockeying it back and forth, you drive it farther out of trim in the opposite direction. I've had that happen, and gotten stuck at one end of the range, and yup - the airplane is quite flyable...not fun, but flyable!

Oh, and these aren't Airbuses! ;)

Paul
 
What was his reaction when you mentioned your RV only had 1 engine, 1 pilot and no flight attendants?

Jekyll
 
Trim cutoff

I have a trim cutoff switch for this reason. Practice with full trim deflection so you aren't surprised if it happens.
 
RV7Guy said:
I have a trim cutoff switch for this reason. Practice with full trim deflection so you aren't surprised if it happens.

I installed a "trim disable" switch with a red barrier around the switch.

There are also two pullable circuit breakers for elevator & aileron, which are down line from the switch; while most circuits are using fuses behind the panel.

If there is a problem, all trim is easily & quickly disconnected, and yet can be isolated, to at least use one or the other.
 
Trim Backup

As others have said, the RV is controllable with fully deflected trim. Still, since I have heard of a couple runaway trim situations (both caused by poor wiring) I put a collat on my trim CB so I can pull it in a hurry if need be.

As a side note, my "Day job" is flying Challengers with electric trim and no manual backup. I have also flown a couple AF jets without a manual backup.
 
Ask him how the electric trim works in the Airbus...'cuz it ain't got one (unless he's flying an older 300). You set it manually after engine start and the computers do all the rest.
 
I have found this discussion interesting, along with the article that was attached. My question is more pilot related. It seems that those with a concern are planning on pulling a breaker or throwing a switch to stop a runaway. However, while on autopilot, I can't help but wonder if by the time you detect the problem, your brain goes "what the heck?" and you recognize that you have a runaway trim that you won't be approaching full trim anyway. I only see pulling a breaker or throwing a switch as an action to keep from smoking the trim motor. In my mind, ability to safely fly in any CG at full trim would be an absolute requirement if lacking a manual backup control. Am I way off base here?

DJ
 
Phyrcooler said:
In my mind, ability to safely fly in any CG at full trim would be an absolute requirement if lacking a manual backup control. Am I way off base here?

DJ

Not off base at all DJ - I tested the Val at all extremes of trim and found it controllable without too much difficulty. Sure isn't enjoyable, but not a real issue. My "trim runaway" prevention is pulling the breaker BTW. My AlTrak will start flashing a light if it can't handle the out of trim condition...and it isn't very strong, so you get a VERY early warning.

People that are worried about this should try to get a ride in an RV and try it out of trim - I can think of a lot worse failures than this.

Paul
 
Thats what I like about this forum, you bring up a subject that hasn't been discussed and it gets plenty of attention, suggestions and always someone that has tested it to the extreme. Thanks guys, I appreciate all of your input.

Jim Kinsey
7A Fuselage
 
Jkkinz said:
Thats what I like about this forum, you bring up a subject that hasn't been discussed and it gets plenty of attention, suggestions and always someone that has tested it to the extreme. Thanks guys, I appreciate all of your input.

Jim Kinsey
7A Fuselage
Yup! Thanks Paul and others. DJ
 
Run away trim on today's flight

Had elevator trim run all the way up today and it was certainly controllable but not comfortable. Older, well built RV6 with Ray Allen servo and relay deck.
Bounced the landing pretty good but probably nerves more than anything else. This will be my third relay deck replaced. Last one was aileron trim that just stopped in one direction in flight. First one quit before takeoff. All at least 8 yrs old and more than 400 hours. Replaced last one with same manufacturer's newer model and is working fine. I love flying RVs
 
had mine runaway years ago the day before heading south. no problem. adjusted it manually with a few flights, then with a small bungy cord fine tuned it for the flight home the next day. love these simple efficient planes. ;)
P1020131.jpg
 
The TCWTech safety trim is a nice little gadget.
You don't really need it until you need it, kind of like insurance.
I do also like the 2 speed feature.
 
Like many others... it's a fantastic little box that I'm sure does a great job.

As long as it's working.

But why would I believe it's less likely to cause problems than a simpler solution? That's the question I have been asking myself while designing my own electrical system. Not just about that particular box, but every other option. I'd love to know what the long term reliability of something like that is as opposed to, say, a run of the mill automotive type relay or two. I've seen relays fail stick, I've seen electronic boxes of every conceivable type fail, I've even seen a toggle switch come apart at the seams. Of the three I'd say switch failures are the most rare, relay failures are not unheard of but somewhat unusual, and electronics failures have faithfully supported my family for decades now.
 
The electronic trim controllers (Like the TCW, one in the Dynon AP control panel, and VX-Aviation's) offer protection against some failures a relay deck can't.

A relay deck will run the trim anytime a button is pressed. The electronic controllers will time out and start ignoring that button. Now you can use the other button to move the trim back. A relay deck goes into "no motion" when two buttons are pressed.

The electronic decks also slow down trim as you go faster, so at cruise you will have more time to notice the trim moving.

So, while they are more complex than a relay in one sense, they do protect against failures a relay can't.

Given the question here was run-away trim, not total lack of motion in your trim, I think a good argument can be made that an electronic box protects against uncommanded motion more than a relay. If you want to talk about the trim system not working at all, then they may be pretty equivalent.

--Ian Jordan
Dynon Avionics
 
I have a pitch trim rocker switch on the panel beside throttle. I must depress a "pitch trim enable" button before power is provided to trim motor. I may use it 4 or five times during a flight. Simple, inexpensive and reliable.
 
Trim Travel!!!

... One of the first things I did, as soon as I began flying my RV was address the elevator trim. Firstly it was about two times too fast and extremely difficult to adjust at cruise speeds.Just tapping it as fast as I could was still to much. Secondly the amount of trim available was roughly three times too much up trim, and five times too much down trim. I went out and flew the pattern to establish a trim setting that was the most that would ever be required for up trim. full fuel, one person on board, full flaps, at 55 mph. (You will never need more than this) Without touching the trim again I landed, measured the angle on the trim tab and recorded this information. The next time the fuel was low, I went up with myself and a gravitationally challenged passenger to gather the down trim setting. We pointed the nose down, power off and trimmed for a 4000 ft per minute decent. I can't imagine ever needing more than this on down trim. We landed, measured and recorded this angle on the trim tab of down trim. I then looked at the travel on the servo motor, did the math for my desired tab travel and made a new even more robust trim tab lever arm on the tab that accomplished the desired result. Also by reducing this over travel, in the unlikely event of the trim running away will dramatically reduce control forces required to over power the system for control. Now full travel up and down stops at exactly the most travel ever to be needed. This also corrects the speed issue, slowing the travel as well. It takes unneeded pressure off the servo motor, and in landing configuration I automatically know to run the trim to its limit for slowing to final approach. I see people placing band-aids on this like servo speed controllers, voltage regulators, electronic travel position stops etc. This just adds considerable complexity, expense, and adds to the possibility of failures down the line. Simplicity is reliability! Thanks, Allan:D
 
So, while they are more complex than a relay in one sense, they do protect against failures a relay can't.
Quite true, as long as they are working properly.
Given the question here was run-away trim, not total lack of motion in your trim, I think a good argument can be made that an electronic box protects against uncommanded motion more than a relay.
I think a good argument could also be made that either one is subject to failure modes that can cause, rather than prevent, uncommanded motion. Both have potential failures in both the input (trim switches) and output (relay contacts or driver MOSFETs/transistors) stages. I've seen MOSFET drivers fail shorted, which would be just as much of an in-flight annoyance as a sticky relay. Of course the trim controller has an added potential point of failure in the firmware, which we can assume is very unlikely to fail -- but it's still not impossible.

Don't get me wrong, I love my electronics. It's what I do. But I have learned not to trust anything (including relays). That said, the failure rate of any of the options is low enough that I wouldn't be overly paranoid about any of them. I would just caution against placing blind faith in a box full of electronics over some other solution. If your risk assessment would lead you to use a switch like Wayne mentioned for a relay deck, I'd say you should probably do the same with a trim controller.
 
Just to clarify failure modes...

Has anyone had a runaway trim condition *with just the simple trim switch*, not including any relays or other trim controllers? (I include in that either a set of buttons on the stick, a coolie hat switch, or the rockers on the panel).

Because for that to happen, the switch would have to fail closed, providing power to the servo.

Seems as though most, if not all, of the failures were not the switch failing closed or a fault in the servo, but rather due to the relay, relay deck or trim controller.

True?
 
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