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Scared me....any ideas?

patterson

patterson

Well Known Member
Patron
On a cross country yesterday, cruising at 7500', my airplane began to dive without my decision or consent! :confused:

I throttled back since the stick pressures were buildiing and quickly tried to figure out what was happening. Was something jammed inside the control system?.....did something on the tail leave the airplane?

Once slowed enough to safely override the dive with back pressure I discovered the trim had gone to FULL nose down with no input from me at all.
I used the switch (on my infinity grip) to again trim the airplane and all was normal. When I landed and inspected, I could find nothing obvious to cause this.

I'd appreciate any ideas or similar experiences and how you delt with it.
Ron
N8ZD - RV-4.......Flying fine for 8 months until yesterday.
 
Do you have relays in your trim system? Could be a bad relay? Maybe a bad switch?

I would switch on the master and trace the entire trim system wiggling the wires at each connection point and see if you can get any movement of the trim servo.
 
Runaway trim....

Runaway trim can ruin your whole day. In larger aircraft, these are common scenarios dealt with in simulators. If you have autopilot, you should have an autopilot/trim disconnect switch on your stick. If no autopilot, a trim disconnect switch or a small trim on/off switch on the stick grip is a good idea. When flying jets, you always know where the pitch trim circuit breaker is located, and lots of pilots have installed one of those yellow or red collars on those c/b's for quick identification and pulling.
 
Trim in motion

I agree with everything Ron said. The good news is that I doubt that an RV at full trim travel can create enough pressure that you couldn't overcome it with muscle. Never thought to try it however. :) I used to fly a large turboprop that had electric only (no manual backup) trim like an RV with electric trim. The Feds required a "trim in motion" warning, a beep tone while the trim was moving. The point being that if the beeping started and you didn't have your thumb on the switch, it gave you a few seconds to turn the system off.

And yes, the electric trim breaker in my '8' has a collar on it.

John Clark
RV8 N18U "Sunshine"
KSBA
 
Scary for sure

I have a trim disconnect switch to prevent runaways. You didn't mention the type of switch you are using. It could possibly be that. Check the easiest stuff first.

Good luck. Let us know what you find.
 
boy, this scares me. When I started my project, I didn't know much about electric trim -- except everyone seems to be putting it in -- but I'd read enough stories about runaway trim that I decided to put a manual trim instead. Is this a common (not in an every day sense but in the sense there's a fair chance of it happens) thing?

I'm curious why people opt for electric trim as opposed to manual?
 
Bob Collins said:
I'm curious why people opt for electric trim as opposed to manual?
Click to expand...
Ok, devil's advocate time.

If our electronics gremlins are so EAGER to rear their ugly heads, let's take a look at the following systems that are EQUALLY PRONE to such gremlinization..

- PTT transmitting uncommanded
- flaps coming down above Vfe
- starter engaging in flight
- and the list goes on...

Anybody who thinks electric trim is a "scary" choice, think about all the other C R A P in your cockpit that's switch-based that could cause potentially worse problems. Like having manual trim reduces risk significantly...

Gimme a break...
 
Why do people opt for electric trim? Because 99.9999999% of time when it's NOT failing, it's lowering pilot workload during critical phases of flight. That's all... :rolleyes:
 
Why do people opt for electric trim...?

Because Van's version of Manual trim is HOAKY!!! I would MUCH prefer having a system where I can set it AND it has electric, but failing that option without a lot of custom work, I decided to go electric. My main comfort factor is a buddy who took his RV-4 and landed it with the trim at BOTH extremes. Lesson learned, light stick forces=managable trim runaway.
 
electric trim

Electric trim is very convenient, particularly when it's on the stick grip. Anytime I fly one of our club's aircraft without it, I realize how much I miss the electric trim.
 
Stick wiring...

patterson said:
On a cross country yesterday, cruising at 7500', my airplane began to dive without my decision or consent! :confused:
......
I'd appreciate any ideas or similar experiences and how you delt with it.
Ron
N8ZD - RV-4.......Flying fine for 8 months until yesterday.
Click to expand...
Ron,
One of the hazards of a stick mounted trim switch system is the small wires that need to travel up the stick. This bundle of wires is constantly flexing and it only takes one edge somewhere to displace some insulation and cause a short.

I would remove the wiring in the stick area and check it carefully for damage... most electrical failures are mechanical in nature (spoken by an EE.... :D ...)

The second area to check for wiring damage would be the bundle of wires going to the trim servo, if it's mounted inside the elevator.. same reasons as above...

gil in Tucson
 
Ron,

I think I would replace the switch for the direction of trim that when bad on you. I would also look at and maybe replace any wires in the stick that connect to that switch.

Either the switch stuck on you and when you worked it to correct the problem it started working again. Or the stick position changed and allowed the switch to start working again.

The switches are cheap and so would be replacing the wire.

Good luck.

Kent
 
dan said:
Anybody who thinks electric trim is a "scary" choice,
Click to expand...
I don't think you read my comments properly. What scares me is the original post and the concept of runaway trim.

Personally, I think there's a certain beauty in simplicity. But that's just me and my choices. I would never criticize -- let alone get exasperated with (or, for the other poster, suggest there's insanity involved) -- someone else's choice.

I think if someone has opted for electric trim; that's great. If they think the Van's design is "hokey," I think that's great too (I tend to agree, having tried to install the cable with an AFP pump).

I guess what I was looking for was simply information on why people made the choices so *I* could be more informed.

Unfortunately, in today's online environment, this is perceived as suggesting that your choices s*ck and then we're off and running with the usual "us vs. them"... electric folks over here... manual folks over there.... stuff.

Online discussions boards are a lousy environment for the Socratic method. Therefore, you may rest completely assured, I never employ it there.
 
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Personally, this story makes me feel a little better, believe it or not.

I wanted the ease of use that would come with electric trim and a trim switch on the stick, but two things scared me - runaway trim and the speed of the trim motor making it difficult to "fine tune" the trim.

The trim speed can be adjusted at the motor, as has been discussed here in many places. Runaway trim is an electrical hazard that cannot be completely eliminated, but the final situation is apparently completely manageable, if not comfortable - you can still fly and land the airplane safely with trim at either extreme. Worst case scenario is where runaway trim in cruise gets stick forces so far out of whack that the AP finally says "screw it - I quit" and disconnects, giving you a sudden AOA change. Even in that case, it does not appear that trim alone would be able to induce enough G-loading to endanger the aircraft - not at our speeds with our aircraft anyway. Other thoughts? Anyone document the resulting G-forces of max trim during flight testing?

This is actually making me lean more and more to electric trim with reduced trim rates. So what if I have to hold the button another second or two? At least I can set it right where I want it.
 
Slightly off topic so I apologize for the thread hijack...

A very important issue has been raised about the ability of you to fly the airplane with the trim extended to its most extreme limits. This absolutely should be one of the items you perform during your flight test period. If you haven't performed this test yet, do so the next time you fly. I think you'll find that you can overcome the trim with sufficient stick force.

BTW, I've accidentally extended the trim while flying with the A/P on by placing a book or magazine on top of the stick, accidentally pressing one of the trim buttons. It gets your attention when the A/P automatically disengages the the aiplane abruptly climbs or dives.

Now, on the issue of why have an electric trim, well Dan kind of put it all into the right words, didn't he? :D
 
Hoakey?

Manual trim was not very difficult to install. It is simple and it works. Might not be as good as electric for some folks during critical phases of flight, but it has never overworked me. Very small inputs required to obtain desired results. This may go back to your mission and the conditions you fly in. My plane is VFR only day/night and the manual trim has served me well. And, it has never moved by itself.
 
For your manual trim folks...how many turns of the "knob" are required to make small adjustments. I only ask because all I've ever flown are cessnas and warriors and reaching down between my seat to give the wheel a small adjustment has never seemed like a bother to me. ONe hand on the throttle..one on the yoke....take a hand off the throttle for a sec to adjust trim.

Do you find more work is necessary in a manually trimmed RV? Moreover, do you find trim in an RV to be more sensitive than in a spam can?
 
I really like

having the electric trim on the stick when flying formation. One hand on the stick and one hand on the throttle. You don't want to let go of one or the other to make a trim adjustment when you're in close!
 
It may be just me personally, but making the manual trim control look like the throttle is what makes it hoaky. If you are going to make manual trim, I'd want to make it the industry standard wheel type, but that would require a bunch of modifications I won't mess with, so electric it is.
 
hecilopter said:
having the electric trim on the stick when flying formation. One hand on the stick and one hand on the throttle. You don't want to let go of one or the other to make a trim adjustment when you're in close!
Click to expand...
that makes perfect sense. I hadn't thought of that. Man, I don't know how you guys do it that fly formation. You are fabulous pilots.
 
Bob Collins said:
For your manual trim folks...how many turns of the "knob" are required to make small adjustments.

Do you find more work is necessary in a manually trimmed RV? Moreover, do you find trim in an RV to be more sensitive than in a spam can?
Click to expand...

Nope, no more work at all, it's just a different method. I'd say the manual trim is probably a bit more sensitive than a spam can, but that's only because the airplane itself is more sensitve. As far as number of turns neccessary to make a small adjustment, it's more counted in fractions of a turn; it doesn't take much, but it easily usable. The only time it takes multiple turns is to reset it back to neutral after a nose heavy landing (RV-8); I use quite a bit of up trim in the pattern and on final, so after landing it's maybe two full turns back to neutral. No big deal.

A previous poster called the manual system "hokey"... now I know that everyone is entitled to their opinion, but I'm not sure I'd call it that until I'd actually flown it. It's simple, reliable, easy to install; works great, lasts a long time. Once you fly it for a few hours its operation becomes second nature, no big deal.

Just 2 cents from an airline guy who's been flying electric trim stuff at work for 17 years, but used manual trim on my RV-8. I'm quite happy with it.
 
Manual Trim

Dont knock it until you try it.
I have flown both. My manual trim works well and I have no regrets. I would caution those that have not flown them to try it first before you judge. I have never heard of anyone that did not like manual trim when flying with it.
The system is simple, solid, works well, and is very consistent and reliable. The electric trim is equally simple, works well, and is also very consistent and reliable, once you get used to it.
I dont buy the work load increase argument, sorry, it is just not that tough to do and is part of the normal "flow" of any configuration change. I would say there is greater risk of accidently hitting the electric trim button causing a lot more issues than not being able to reconfigure and trim immediatley with manual trim. Stick pressures just are not that great on these machines.
Both systems are very good and I would not be scared of either.
 
I don't have a few hours in each. I have flown a -6 with manual and both a -7 and -6 with electric. Not MUCH experience, but enough where I'm comfortable with my decision and opinion.

Humans are amazingly adaptable, and can get used to anything. The manual trim is easy to get used to, but like the old piper crank trim, not horribly intuitive. Personal choice.
 
The manual trim in my 7A is so simple and quick to adjust I can't believe anyone could think it would add cockpit load. Chasing an electric trim servo is cockpit load.

Roberta
 
Bob:


I am one of those "hoakey" manual trim users. I personally like how it "feels" to adjust the trim. Just a bit more feedback in my opinion. I really don't think it is hoakey either. Maybe the better word is simple.

I do not notice any difficulty dialing in the amount of trim I need at any point in flight. Usually a turn at a time is all I need while in the pattern. When I taxi out for takeoff, I return trim to takeoff postion and that usually takes about 3-4 turns....not sure on that as I never really counted.

When I was ordering my kit at OSH, Ken Scott was taking my order and when the subject of manual vs electric trim came up and I ordered manual, he said, sort of under his breath, "good". Not sure if that carries any weight with you but at the time, it did for me.


Regards,
 
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I've flown a lot of RVs with both electric and manual trim. My -6 just turned 14 years of age last Sunday with manual trim and if I built another tomorrow, it too would have manual trim.
If you like the electric trim, then by all means, go for it. That's why we have choices.
 
painless said:
Bob:


I am one of those "hoakey" manual trim users. I personally like how it "feels" to adjust the trim. Just a bit more feedback in my opinion. I really don't think it is hoakey either. Maybe the better word is simple.

I do not notice any difficulty dialing in the amount of trim I need at any point in flight. Usually a turn at a time is all I need while in the pattern. When I taxi out for takeoff, I return trim to takeoff postion and that usually takes about 3-4 turns....not sure on that as I never really counted.

When I was ordering my kit at OSH, Ken Scott was taking my order and when the subject of manual vs electric trim came up and I ordered manual, he said, sort of under his breath, "good". Not sure if that carries any weight with you but at the time, it did for me.


Regards,
Click to expand...

Ken is my friend and his words carry a lot of weight with me. Cant remember whether I asked him about the trim or not, but his 6 is one of the first that I right seated with manual trim.
 
robertahegy said:
The manual trim in my 7A is so simple and quick to adjust I can't believe anyone could think it would add cockpit load.
Click to expand...
If you've done formation landings, you can understand how HOTAS is a high priority. Anything other than HOTAS in formation is increased workload. Plain and simple.

If your mission isn't formation flying, forget it. Install whatever you want. But if you want to become Sierra Hotel formation stick, consider on-stick/on-throttle controls.
 
!

lovin da hokey trim-lovin da wood prop- in arkansas- what do you expect
rv8a-nosedragger, O-360 --kept it simple --less is more to me!
 
Yep I did it

dan said:
Why do people opt for electric trim? Because 99.9999999% of time when it's NOT failing, it's lowering pilot workload during critical phases of flight. That's all... :rolleyes:
Click to expand...
Correction 99.9999999999999999%.

The trim went down with out my knowledge once, but I did it accidentally.

The MAC grip uses flat buttons on top for trim. I accidentally allowed my chart book to touch the stick grip, hitting the nose down trim. I was flying with the stick between my knees/thighs and noticed it, but the trim did put the plane in a steep descent. Let long enough it would have been bad news.

I realized it fairly quickly and just corrected, but my trim is now on the panel.
 
dan said:
If you've done formation landings, you can understand how HOTAS is a high priority. Anything other than HOTAS in formation is increased workload. Plain and simple.

If your mission isn't formation flying, forget it. Install whatever you want. But if you want to become Sierra Hotel formation stick, consider on-stick/on-throttle controls.
Click to expand...
I have flown several RVs with both manual and electric trim and I prefer the manual and that's what I installed in my 7. As far as formation flying I guess the several hundred hours of formation and formation aerobatics I have done in manual trim airplanes (T-6, Christen Eagle, Acroduster, Pitts and several others) must have been dangerous without HOTAS. :) I just find that the Macs electric trim is just not that precise and you always have to chase it back and forth a few times to get level flight. Don
 
Keep it simple

If the idea of runaway trim worries you (and it did me), why not eliminate failure points by locating the switch on the panel instead of on both sticks? That way you don't need relays and connectors and you don't have a whole bunch of tiny 26ga wires threaded through two control sticks that get a workout every time you turn.

Would runway trim be a problem? Yes, you can physically overcome a full-trim condition, but the stick force is way beyond anything you're used to. I could land with full-up trim if I had to, but it wouldn't be pretty and it's not not something I'd risk practicing.

But as others have mentioned, the chance of failure is actually very low. For me, keeping it simple with the standard panel-mounted switch was a reasonable compromise between safety and convenience.

And by the way, the concerns about chasing the trim are way overblown. You don't need any special circuit to reduce the sensitivity. By the end of your first flight you will love it. It takes about 30 seconds to get used to tapping a button instead of spinning a wheel.
 
Once you take the switch off the stick, it eliminates a major advantage of the Electric Trim. China Hat style would fix a lot of those problems.
 
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