Runaway trim! Scary stuff!

[PeytonM]

1989 M20J. Century 2000 with electric trim. Beautiful flying day Monday. On approach to landing, I used my electric trim, adding a little more nose-up to fine tune my approach speed over the threshold. The airplane began to feel very, very nose heavy, diving for the runway!!  What the...?  I didn't know what had happened to cause the pitch force to become so heavy. A jammed control? I reached down and tried to move the trim wheel. No dice. I just focused on trying to land the airplane. I hit the CB. (Later I saw the trim interrupt on the yolk, which is on the inside of the left horn, kinda awkward place. I've been flying this aircraft for 20+ years!)

After I landed, I looked down to the trim indicator and saw it was full down! I did a couple of tests on the ground and found that the trim ran to full nose-down trim regardless of which way you moved the trim thumb switch. Both the yolk trim-interrupt and the panel switch/CB stopped the trim motor. 

Can you ever train for this?  Be familiar with all those buttons and switches. Keep your hand on the trim wheel when you're using electric trim. 3900 foot runway helped. What might happen in IMC on departure with runaway "up"electric trim?  From here on, I'll be using manual trim when I'm close to the ground!  

[thinwing]

I had a roll servo try to take me out while in IMC...it's amazing how quickly the barber pole appears!

[neilpilot]

Recently had my pitch servo rebuilt by sTec aka Genesis.  They wired it backwards during the rebuild.  My shop installed the rebuilt servo and confirmed it worked but did not confirm it worked in the correct direction.  You know how exciting it can get when your pitch servo runs in the wrong direction?

Genesis had this excuse:  the same servo is used in many aircraft, but needs to be wired (by them) in reverse if it goes in a Mooney.  The knew it was for a Mooney but failed to wire it accordingly.

My shop had no excuse.

[alex]

Glad you're ok Peyton...

[JKSmith]

Good thing you was on your toes and took care of the situation! Good job, and thanks for the lesson, Experience gives the test first and the lesson after and most lessons aren't survivable

 

[carusoam]

Peyton,

Thanks for sharing your experience. I put my hand on the trim wheel, while close to the ground, just to feel it moving.  

I have run out of trim before.  You won't know that you ran out, while you count seconds pass while operating the switch.

When Eyes are outside, it is impossible to know if the trim wheel is moving.

Knowing the disconnects is key!  A button on the yoke.  The CB(s) on the panel. And recognizing the tone of the AP when it shuts off.... all are helpful.

Thanks again,

-a-

[triple8s]

I had this happen once in an off brand, as a result, I leave the electric trim off for climb out, and I also have the A/P CB marked for easy ID along with the electric trim CB. This way you can pop them out quickly if needed. If the electric trim is left turned off on climb out your A/P will have to be tested inflight before it will engage and this means hand flying till that's completed. Yes this increases the workload while climbing out, communicating with tower departure, and cleaning up the airplane sure, but if that's too much for a pilot to handle..........well that says something about the pilot. 

[flyboy0681]

Same experience here but on takeoff, which I've written about many times. I was trimming for some nose down a couple of minutes after takeoff and the controls got heavy - real heavy, to the point where I had to place my elbows behind the yoke to keep her from nosediving into the ground.

As Carusoam shared, I now place my hand on the wheel to make sure it stops spinning.

The cause was a sticky trim switch of the King kind.

 

Edited October 7, 2016 by flyboy0681

[Oldguy]

3 hours ago, flyboy0681 said:

Same experience here but on takeoff, which I've written about many times. I was trimming for some nose down a couple of minutes after takeoff and the controls got heavy - real heavy, to the point where I had to place my elbows behind the yoke to keep her from nosediving into the ground.

As Carusoam shared, I now place my hand on the wheel to make sure it stops spinning.

The cause was a sticky trim switch of the King kind.

 

Sounds like a duplicate of the first IFR flight I had after my check ride. Glad I was still paranoid about doing something wrong so I caught it fairly quickly. Yup, sticky King trim switch.

[OR75]

there is a reason that an A/P requires a disconnect button on the yoke

there is a reason that an A/P can be overridden by the pilot at all time

 

[flyboy0681]

Just now, OR75 said:

there is a reason that an A/P requires a disconnect button on the yoke

there is a reason that an A/P can be overridden by the pilot at all time

 

Those are great features to have when you know what's happening when it's happening.

[Yetti]

I think it was my second solo cross country during training.  The King A/P would not disconnect. tried everything. finally pulled the CB.

The Mooney is such a joy to hand fly. 

 

Edited October 7, 2016 by Yetti

[OR75]

52 minutes ago, Yetti said:

I think it was my second solo cross country during training.  The King A/P would not disconnect. tried everything. finally pulled the CB.

The Mooney is such a joy to hand fly. 

 

Something must be wrong with the install. Any autopilot servo should disconnect with a certain amount of force at the yoke. No need to be a pro wrestler to be able to disconnect . It is actually in most autopilot checklists 

[flyboy0681]

Just now, OR75 said:

Something must be wrong with the install. Any autopilot servo should disconnect with a certain amount of force at the yoke. No need to be a pro wrestler to be able to disconnect . It is actually in most autopilot checklists 

I didn't start to wrestle with it until the trim was to the stops. So by that time there was nothing for the A/P to disconnect.

[Pritch]

Mine sticks once in awhile also.  I just keep my right hand hanging by my side and feel the wheel as it is trimming (a leftover from my old days of manual trim).  I am thinking that it might be a buildup of crud in the switch after 32yrs.

Pritch

[Bennett]

About a year ago I had servo problems with my KAP 150. It would "hunt" in altitude hold mode, and it was driving me crazy. LAC Avionics in San Jose sent the pitch servo to Honeywell to rebuild. They sent the rebuilt servo back (ridiculously expensive) and it was installed and "tested" on the bench. Worked fine on the bench and it reinstalled in the airplane. Fortunately I hand flew the Mooney back to KSQL, with the idea I would test it for the hunting problem on my next flight. Fortunately I climbed to 3,500' before I turned on the autopilot and engaged altitude hold. What a wild ride that was! Up down, every excursion becoming larger and larger, and finally I was nose down and accelerating quickly. I had a hard time getting the autopilot to disengage - the yoke switches were ineffective, (trim and autopilot disengage) as was the autopilot on/off switch. I pulled the breaker at about 1000', popped the speed brakes, bled off speed, and landed ASAP. I admit to be badly shaken by the experience. I hand flew the airplane back to LAC Avionics where they found that Honeywell had wired it backwards (sound familiar?). They didn't believe the LAC technicians and so we shipped the servo back to them with a description of my wild flight. Honeywell to their credit, admitted their mistake, and sent us a brand new pitch servo. This one is steady as a rock, but I know where the AP circuit breaker is without looking for it.


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[flyboy0681]

Just now, Bennett said:

This one is steady as a rock, but I know where the AP circuit breaker is without looking for it.
 

So do I.

[Bryan1016]

I'm getting one of those yellow things.


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[LANCECASPER]

Red is also a good color for the A/P circuit breaker

http://www.aircraftspruce.com/catalog/elpages/edmobreakercaps.php?clickkey=516724

[NotarPilot]

Sound so like you followed lesson number 1. KEEP FLYING THE AIRPLANE.

Good show ol' chap.

[midlifeflyer]

On 10/7/2016 at 4:14 PM, flyboy0681 said:

Those are great features to have when you know what's happening when it's happening.

One of the "advantages" of the early training many have had in, well, not exactly new airplanes is, when they had an autopilot it was very finicky. That bred a healthy distrust in them. 

Even if we have a brand new ultramodern one, we need to maintain at least a "trust but verify" attitude. Know what the next thing the autopilot is supposed to do. If it doesn't do it, you know what's happening when it's happening,

it really easy for us to have a tendency to not only let Otto Pilot fly the airplane, but  to also abdicate some of our PIC responsibilities to it. The former is a great benefit, we can relax a bit, slow down our scan, save energy; the latter is potentially disastrous,

[OR75]

If you cannot overcome the autopilot or trim without pulling the circuit breaker , then something is wrong and should have the installation checked 

[Bennett]

If you cannot overcome the autopilot or trim without pulling the circuit breaker , then something is wrong and should have the installation checked 

Not necessarily true. When I took a lightning strike that fused much of my panel, and about all of the instruments that were powered, the pitch servo ran to full down trim and jammed in position. I finally was able to regain control by killing breakers , and forcing the manual trim wheel. In the case of the "rebuilt" pitch servo (different Mooney) the servo ran full down, and it too jammed. This time it was easier to manually force the trim wheel after punching out on the on/off switch, AP disconnect, and attempting to disengage the autopilot via the yoke electrical trim switch. When the servos jam in their fully extended position, you have a serious problem. A runaway servo is unusual, but it does happen. I spent a fair amount of time discussing this with a Bendix/King engineer as to their certification tests for the KAP 150. Their filing states that their test pilot was able to recover from a full down servo excursion in 400'. The ATC. "Snitch" will go off in a 300' deviation in altitude when IFR and there is another aircraft in the vicinity. It took me 13 months before a FAA senior engineer agreed with me and Bendix/King, that a 400' exclusion because of a runaway pitch servo was not actionable, and dropped the potential violation they attempted.

[DXB]

Thanks to PeytonM for posting his experience - a useful discussion, and  I'm really glad his malfunction didn't cause a more serious mishap.   I just ordered some of those colored breaker labels for my A/P etc. 

As an aside, when I got my C, it came with a rickety aftermarket electric trim that was added based on an obscure one off STC a long while back.  What was striking is that it made turning the trim wheel by hand almost impossible.  This made me fear for the day the motor burns out, and one of the first things I did was have it torn out.  

[markejackson02]

I guess this is one of the upsides to not having an autopilot