Professional Curiousity - Runaway Elevator Trim

[AirSic]

I've started this thread because I asked the question in the thread relating to the Chieftain that bingled in NT after an aborted take-off. The remark was made that a "runaway trim" may have been the cause of the abnormality requiring the abort. The additional comment was made that the trim caused a Nose-Down attitude.

The Question(s) is/are:

Never flown one, but would a "runaway" elevator trim catch you out that quickly, and can you over-ride one manually? How does a trim get to "runaway"? I would have thought that the slip stream would keep it in check and if it failed gravity would pull it down creating a nose high attitude.
Professionally curious.... how does this happen?
I am a low timer and am curious for the sake of safety and experience in the future.

[bushy]

Airsic

I have never heard of an electric trim runaway in a chieftain, although it is theoretically possible. The electric trim switch in a chieftain is a double switch. One is activated by pushing down on it, and the other by moving it forward or rearwards. The "pushing down on it" is an instinctive action, and many pilots do not realise they are doing it. A trim runaway caused by a switch malfunction is very unlikely, as both switches would have to malfunction at the same time. There could, of course be other causes of a trim runaway. No, gravity and /or slipstream would not fix it. The pilot wpuld have to.

Should it happen, I have no doubt it would not be nice. When we first got Partenavia's one of the things I checked out was what would happen if the electric trim malfunctioned, and wound full "nose down" We diid this in flight with two pilots, and I can tell you it is very difficult to overide by brute strength. You need two hands on the yoke.

We never did this in the chieftain, but I never had a trim problem in a chieftain, in about ten years of flying them.

[flywatcher]

Pull the two circuit breakers on the panel under your left elbow- quickly.

[maxgrad]

Have had instances in the past where whilst on auto pilot in a chieftain (and other types) the trim has runaway but in each of those have had time and been able to disconnect the AP which in turn stopped the runaway condition

[scrambler]

I believe that this problem cause the demise of a few Partenavia's in the past. I once heard of a case where full forward trim was overcome by pilot & instructor some years ago. The Partenavia now has lots of ways to turn the auto pilot off in a hurry.

[gaunty]

I am not all that familiar with the Chieftain but it is a certification requirement for an Autopilot and electric trim disconnect switch independent of the regular on/off control.

I think in the case of the Pipers this is why the double switch as described is used and is the same?? in the Partenavia. In the Cessna piston and turboprop series it is/was a red button on the control wheel near the trim switch.

In any event it is or at least should be a regular part of the after start/pre take off checklist for proper operation, the failure of which is a no go item for the use of either A/P or Electric Trim and IMHO further flight.

Thats not to say the trim switch can't subsequently fail sometime after but the pulling of the relevant breaker should be the last not first resort.

With most of the aircraft now waay past their regular use by date the possibility of dodgy or corroded shut breakers can not be discounted as a result even that remedy may be unavailable.

Either way if you do get a runaway trim you are going to need both hands and some fumbliing around the CB panel looking for the switch is not a desirable option.

Flying any aircraft without an electric trim is a chore, but your willing helper can also be your executioner, make sure you understand the system thoroughly especially the emergency disconnect or protection procedure, exactly where the important CBs are and ensure they themselves are serviceable..

With the large speed ranges of modern aircraft especially the turboprops and jets trim, trim, trim, trimming all the time is the word, what may have in the past been a looxury is now a requirement.

At the risk of repeating myself the Autopilot/electric trim mechanism/function is an equally important a part of your pretake off checklist as is "controls full and free" , remember whatever is there to "help" you in flight is attached to the very same control cables:ok:

[splatman]

In September last year I had the misfortune to have a runaway trim in an EMB110 on takeoff at Bankstown. The rotation was normal and as the aircraft accellerated it started nosing over uncontrollably. Application of nose up electric trim did nothing, manually trying to move the trim was unsuccessful. At the time we diagnosed a failure of the trim system through the screw jack or some other component ( the trim appeared stuck).

The only way to maintain some control was to reduce power and airspeed to a point that subsequently allowed both crew to overcome the aerodynamic forces. (FO with feet on the instrument panel pulling on the column for all he was worth - Nice work on his part if I do say so!)

The initial findings were ones of the Captains trim switch sticking irrespective of selected position and the elevator trim torque motor failing a breakout test thus preventing the crew from overpowering the electric trim.

The aircraft had no switching to enable the turning off of the electric trim system other than pulling CB's.

So to answer the question, yes trim runaways can occur, they are a serious event and not always that easy to detect. If your in straight and level flight and the nose starts to pitch down or up then its generally easy to diagnose, but in a state of aircraft acceleration it can be difficult to determine what is a changing aerodynamic force associated with speed to one of a trim change.

Not an experience I care to repeat!

[Mr.Buzzy]

I remember it being common practice to wind full "nose down" then winding on 4 and half turns of "nose up" before takeoff in the PA31. People have been distracted in the past after performing the first half of this procedure and attempting takeoff.

ps. I am most certainly not suggesting this was the case in the NT recently.

[AirSic]

Got a PM from an "experienced" soul. So the trim doesn't flay around in the slip stream and doesn't go "dead" i.e. fall due to gravity.....a "runaway trim" continues to cycle up and down, causing an oscillation of the aircraft and some serious weight training of the biceps for the PIC and anyone else in the front. I wasn't aware that the problem extends from an electrical failure, not a manual linkage failure.
Worth knowing.

Cheers for the heads up, and the beer!:ok:

[rmcdonal]

I remember it being common practice to wind full "nose down" then winding on 4 and half turns of "nose up" before takeoff in the PA31. People have been distracted in the past after performing the first half of this procedure and attempting takeoff. Can very easily happen if you get distracted at that point of your checks. I would imagine it would be near on un-recoverable if it where to happen to. Especially if you have chewed up ¾ of the strip already in the T/O roll. :eek:

[Contract Con]

I agree with Flywatcher.

I have a vague recollection of it being a case of 3rd one in from the end???

It was made quite clear to me during my PA31 endorsement years ago, that this was the only way of stopping the electric trim, if it ran away.

I had one go for a bit of a wander with the autopilot engaged, but it stopped once I disengaged the A/P.:eek:

Someone current on type may be able to add more.

Cheers,

Con:ok:

[Peter Fanelli]

[quote=Mr.Buzzy]I remember it being common practice to wind full "nose down" then winding on 4 and half turns of "nose up" before takeoff in the PA31. People have been distracted in the past after performing the first half of this procedure and attempting takeoff.

I used to use 3 1/2 turns nose up in VH-NPD. The reason I started doing this was because the trim indicator would read inaccurately due to static electricity on the plastic cover over the trim indicator, wiping or cleaning this cover would couse large changes in the indicated trim setting.

[Captain Starlight]

I remember it being common practice to wind full "nose down" then winding on 4 and half turns of "nose up" before takeoff in the PA31.

This procedure was introduced as a confidence check when doubt existed as to the indication of the trim indicator.

The PA31 trim indicators are electric guages, not mechanical linkages.

There are three trim indicators, elevator, with which we are concerned, rudder and aileron.

The guages tend to be kicked and knocked while getting in or out of the crew seats,
and generally tend to be about as reliable as a GA single electric fuel guage.

"Trims set for takeoff" is a pre takeoff check item in any aircraft, if you don't trust the guage,
in the PA31, one can wind in fwd trim to the stop, THEN wind back trim to either 7 handfuls, or 4.5 complete turns,
thus ensuring it is in the takeoff range.

Given that the Chieftain in question may have arrived fully loaded and possibly with the CofG tending aft, it probably was in a trim fwd position.

Taking off empty (fwd CofG) and accidentally missing the trim check prior to takeoff could line up a few holes in the cheese.

Yanda Airlines demise may have been triggered by a similar event, check the ATSB website,
but it seems a slightly built lady pilot had to enlist the help of the front seat passenger to apply sufficient elevator force to control the aircraft.

It was eventually trimmed out ok.

It is understood that the passenger later made a complaint and the rest is history.

I guess the main point here is that there are no new accidents or incidents waiting out there to entrap us,
some one else has already "been there and done that".

It's up to chief pilots and check / trainers (and to some extent, CASA) to pass on the hard learnt lessons of the past and try to ensure
that pilots are briefed or reminded of these events or characteristics.

The comments on P68 are particularly relevant, the pitch excursions with flap, or excess trim, can be quite entertaining.
Most P68 pilots have been taught the correllation between pitch trim and flap useage, and a technique that negates it.

(co-ordinated and synchronised electric trim use opposite to flap switch use.)

[18-Wheeler]

I've never had a runaway elevator trim, but I was right-hand-seat in a Metro when coming out of maintenance the elevator trim switches were installed upside-down.
The first little bit of the flight was most unpleasant for a short while (http://www.billzilla.org/flying3.html).

[bushy]

Splatman, did you have a contortionist for an F/O?

[Capt Claret]

AirSic

I've never flown a chieftan but had a trim runaway in a Gonad (Nomad for the uninitiated). In the cruise to Katherine after a hospital transfer to Darwin, the nurse in the RHS was asking about instrument approaches.

Whilst explaining the rudiments of an NDB approach to him, I became aware that it seemed awfully noisy. Looking up, we were pitching nose down, the speed was increasing and the trim wheel was happily winding in more nose down trim. I don't recall sensing the pitching.

Recovery saw VNE exceeded but the tail stayed put, thankfully.

During stabiliser runaway training in the 717 sim last year, the control forces were so high that if it wasn't caught quickly, the only way to maintain a level of control if the pitch up force became too large was, to roll to about 90ish degrees of bank, allow the nose to yaw down, then roll wings level, then repeat.

I hope I never have a runaway stab like that for real. :eek:

[OzExpat]

Don't think the pax would enjoy it too much either, Clarrie! :}

[SmallGlassofPort]

Peter Fanelli, I dont quite get where you are heading.
So you only wound up 3 1/2 turns instead of 4 1/2 so that the guage would look correct due to guage error?
Or you only wound up 3 1/2 due to the C of G of the actual aircraft?
Thanks!!

[Peter Fanelli]

Peter Fanelli, I dont quite get where you are heading.
So you only wound up 3 1/2 turns instead of 4 1/2 so that the guage would look correct due to guage error?
Or you only wound up 3 1/2 due to the C of G of the actual aircraft?
Thanks!!

The issue is that the trim indicators can be quite innacurate.
By going all the way to the limit and then setting a known number of turns of the wheel you know you are setting the trim to the same place for each takeoff regardless of the state of the indicators.

3 1/2 turns was what I found to be about right based on watching the indicator over a period of time when it possibly was accurate and what felt comfortable.

Running a finger over the indicators plastic window was enough to cause considerable deflection of the indicator needle due to static electricity. It was possible to change the indicated setting just by rubbing the plastic such as might occur when cleaning the panel.

[tinpis]

Its been 30 odd years since I flew one but didnt the lekky trim run very slowly on a Chieftan?

[Peter Fanelli]

Its been 30 odd years since I flew one but didnt the lekky trim run very slowly on a Chieftan?

Yes, very slowly.

[gaunty]

Forgot to mention that, running the trim to both ends of travel as part of the preflight is a smart move for two reasons, check indication and that there are no restrictions.
Trim is a "control", full and free it needs to be.:p

While I'm at it the Navajo elevator system has a "bob weight" attached at the working end.
It is there to correct a pitch stability problem they didn't get right at the original design stage and it got worse when it was stretched to a Chieftain.

It is why the the controls generally flop to the full down position, or they feel "loaded" when you pull back when the aircraft is stationary. That is normal and part of the damping effect.

Its fairly well attached but given the age and serviceability of the average type nowadays it is something you need to watch out for. If it has or does become detached expect some lively flying characteristics, it will not necessarily be uncontrollable, but if you are overweight or at the extreme ends of the C of G envelope who knows, nobody else does either.

[Jarule]

Hope this helps answer the original question to this thread.

When the Chieftains' auto pilot is engaged, in alt hold or pitch mode, it must adjust the elevator trim automaticaly (auto trim) to keep the loads on the elevator servo to a minimum.

To auto trim the system senses the amount of electrical power required for the elevator servo to hold the elevator in a position corresponding to the required nose attitude of the aircraft.... with me so far? :confused: A bit like hand flying the aircraft when the elevator is out of trim.... your arm gets tired.:hmm:

The auto trim system senses the load on the elevator servo, then as if by magic, adjusts the elevator electric trim to reduce the load. Just like manually trimming in the climb:rolleyes:. You may be excused for thinking that if the elevator servo is electrical, how can it get tired!
Well, if you remember the auto pilot test you did before you took off, you may remember engaging the auto pilot and testing that you can "manually override" the auto pilot motor. This happens because there is a clutch in the system that disengages at a pre-set control force.

Now back to the auto trim......If the system doesn't auto trim, the clutch on the elevator servo would suddenly disengage and a very sharp nose pitch up or down would occur. In the case of a Runaway Trim with the auto pilot engaged. Simply the device that senses the load on the elevator servo has failed and commands the electric trim to run full up or down. Eventually the elevator servo cannot resist the loads on the elevator any longer, due to the out of trim situation, and lets the elevator go. resulting in a violent nose pitch up or down.

This happened to me once on a night freight run....scared the living SH1T out of me.

As for an electric trim runaway without the auto pilot engaged. This is simply a stuck relay or control circuit causing the trim motor to run in one direction continuously. Operating the trim switch in the opposite direction will trip the circuit breaker for the system and you can continue using manual trim (unless in a PA60):D :D :D

[AirSic]

JARULE, a most insightful explanation.
Thank you for you time and comments.
AirSic:ok: and thank you to all the others..it appears that this little issue crops up all too often...glad you are all still with us!:)