It could have ended differently---a lot differently

[donkaye, MCFI]

Last week we were at 4500' off the California coast by Santa Cruz.  We were just starting Commercial training.  Before doing any of the new Commercial maneuvers, I start by doing slow flight, then Power off and Power on Stalls.  The airplane, a 231, has been upgraded to 2 G5s, and the GFC 500 autopilot.  As such, when operating outside of the ESP (Electronic Stability Protection) envelope the ESP needs to be disabled in the G5. Additionally, the AP circuit breaker needs to be pulled because, if the airplane is operated outside the envelope with the AP off for more than 10 seconds, the AP will automatically turn on and engage the servos in LVL mode.  I hadn't pulled the circuit breaker, but we had slowed to just above the stall and, unbeknownst to me, my student had used the electric trim to trim up fully.  Pulling the CB disabled the electric trim.  First, we found that the stall warning switch was inoperative, but continued the exercise anyway.  Power was added to maintain altitude on the backside of the power curve and the nose was lowered to increase speed and lower the AOA.  To relieve yoke pressure trim needed to be adjusted down.  With the electric trim off the student tried to trim down with the trim wheel.  It wouldn't budge.  The student said that he couldn't control the pitch with the yoke, the force was too strong.  At that point I added my control input to the yoke.  The force was so strong that with both of us locking our arms straight out we couldn't overcome the pitch up force.  I tried to unlock the trim  wheel with both hands--unsuccessfully.  In all of my flying I have never had a flight where the outcome was in doubt.  But for a few seconds that's exactly what I thought.  Knowing that the 231 is somewhat nose heavy and on final at 75 knots with low power setting on a 3° slope the plane was controllable, I pulled the power.  That did relieve some of the control pressure.  I tried to unlock the trim again with the trim wheel.  No dice.  It was totally locked up.  We were on flight following with NORCAL, and I was about to declare an emergency, when I decided to turn the AP on and see if the power of the trim servo would unlock the trim.  I pushed in the CB and waited while the system went through its PFT.  It took a few seconds, but ultimately finished successfully.  I had the student try the electric trim---and it worked.  The trim became unlocked.  We had dodged a bullet.

I said, "Let's head back to San Jose.  We're done for the day".  I got no argument.

Epilogue:  I told the student that until we found out the reason for our issue, we wouldn't be flying the plane.  The maintenance shop was able to recreate the problem.  I called Paul Kortopates and related the issue.  He told me that a Service Instruction had been issued on December 14, 2016, SIM20-88A Stabilizer trim stop screw Modification.  I've attached it here.  It applys to the M20J, M20K, M20L, and M20M.  It was to be done within 100 hours.  On this airplane it apparently had not been done, so the force of the electric trim on trimming up for slow flight had pushed the trim wheel past the stop and locked it up.

I'm posting this because for those owning aircraft to which it applies, you should confirm that it has been complied with.  Otherwise, you might be put into an unrecoverable situation the likes of which we were lucky enough to have overcome.

SIM20-88A Stabilizer trim stop screw Modification.pdf

[Sue Bon]

Yikes! Thanks for sharing this. I'm glad you are ok.

[Ibra]

Thanks for sharing !

 

If there is anything I would need to remember is when flying out of trim is to "reduce power" (I used to fly C177 without auto-trim and it's AP used to disconnect in turbulence, leaving one with unpleasant gym session, at least the trim wheel was working not locked or runs away, still unpleasant).

Edited December 23, 2025 by Ibra

[Mooneymite]

Perhaps that SI should be an AD?

[Jeff Uphoff]

Interesting that it applies to M20L and M20M aircraft--but not to the M20R. It appears from the M20M serial numbers that this was fixed in production fairly early on in that model's run--and thus before the introduction of the M20R?

--Up.

[Z W]

Glad you got it safely on the ground. Thank you for posting this.

I was not aware of it and the Service Instruction has not been completed on my plane.

Until I can get it done, I think I'll run the trim all the way nose up with the electric trim on the ground and see if it jams. I've never experienced it, but I make it a point to never run full nose up trim on landing to avoid excessive forces during a go-around, so I'm not sure I've ever done it. I'd hate to find out it happens for the first time in the air like you and your student did.

I'm confident I could maneuver and land at 80-90 knots or so with flaps down and full up trim. The plane likes to fly like that just fine, though not at full power. But it could certainly catch one by surprise at high power settings.

I also have a G5 and GFC500, and for stalls, steep turns, and unusual attitude recoveries, I have been holding down the autopilot disconnect button to disable ESP. That is what is recommended in Garmin's manual for the autopilot (below). I don't think I would like having the circuit breaker pulled, and electric trim disabled, during ordinary training, though practicing with it pulled might be a good exercise as well.

A question - how could an owner get notified of these service instructions and any new ADs or SBs? I have been reading Mooneyspace every morning since before 2016. If it was discussed here I missed it. I've tried hard to be involved and pro-active in maintenance. I've taken it to Mooney specialized shops who have not said anything about it. If there's a better way I'd like to know.

[Hank]

Didn't Mimi post about this happening to her Marvin the Mooney, with a shaky but successful landing? I don't remember her username . . . It was a handful of years ago, maybe '14 or '15?

[skykrawler]

Service instructions found here:

https://mooney.com/contact-2/

Personally I was aware of the stepped nut mod, but not replacing the trim screw on the J.  I doubt it is still available - and if so, at some stunning price.

An Airworthiness Directive could ground tens if not hundreds of aircraft.

I would add that the maintenance manual description for adjusting the chain tension is not particularly good.    Loosening the four screws on the gearbox to adjust the tension is virtually impossible without removing the assembly from the airframe - there are things in the way.

[Mooneymite]

52 minutes ago, Z W said:

A question - how could an owner get notified of these service instructions and any new ADs or SBs? I have been reading Mooneyspace every morning since before 2016. If it was discussed here I missed it. I've tried hard to be involved and pro-active in maintenance. I've taken it to Mooney specialized shops who have not said anything about it. If there's a better way I'd like to know.

The FAA notifies the public of Airworthiness Directives (ADs) through the Federal Register, where they publish notices of proposed rulemaking and final rules. Additionally, individuals can subscribe to receive email notifications about new ADs relevant to their aircraft type and make.

[takair]

That’s pretty scary. Had a version of this happen recently in an J and has come across the bulletin as well. In our case it unlocked.
 

Don….easy to Monday morning quarterback so not questioning just looking to learn….did you consider adding some bank to basically get the nose down and get some speed?  Years ago I worked on an incident on a business jet where the crew had a similar event for different reasons (for which I had to find a solution which became an AD). They temporarily recovered by using variable bank angle. You obviously need some altitude and a plan B at some point. It is also not very intuitive but may buy some time. Might this have helped?

[Slick Nick]

Why pull circuit breakers in flight instead of following the manufacturer’s procedure for disabling the system during maneuvers?

[201Mooniac]

Don, Glad to hear you were able to get it unjammed and land safely.  I'll need to get Mark to check on that for me as my plane is in for annual right now.  Great to know the AP worked but I'm curious if there is any chance it made the situation any worse when disabled.  I know the servo should be easily overpowered.  I have run my trim full nose up and nose down when on the ground and never had any issues, not sure if air loads have an impact but I might ask you to go for a flight where we test controlability at both extremes just so I know what to expect.

[Max Clark]

Thanks for sharing. I can't imagine how intense that was, or how long those few moments in the plane felt. Glad this story ended with you being the one writing about it.

[flyboy0681]

10 hours ago, donkaye, MCFI said:

First, we found that the stall warning switch was inoperative

 

Wasn't the stall warning checked during the pre-flight? 

Glad you're here to tell the tale.

[donkaye, MCFI]

1 hour ago, Slick Nick said:

Why pull circuit breakers in flight instead of following the manufacturer’s procedure for disabling the system during maneuvers?

The ESP was disabled on the G5, but, while I may be wrong, that doesn't prevent the GFC 500 from turning back on after 10 seconds of being out of the envelope in slow flight.

[donkaye, MCFI]

40 minutes ago, flyboy0681 said:

Wasn't the stall warning checked during the pre-flight? 

Glad you're here to tell the tale.

Yes, the student should have checked it.  We did check it after the fact.  The switch checked good, the module has been sent in for overhaul.

[takair]

Don. You might also have them check the stab rigging. There is a recent thread on the topic and a large number I. The fleet are finding this, especially after autopilot work. In the cases described, it was biased down, but no reason it can’t be up. I would have thought that even with full up trim it could be overcome if rigged properly. Perhaps wishful thinking that this was a certification standard. Anyway, easy enough to confirm it is not out of rig. 

[kortopates]

Didn't Mimi post about this happening to her Marvin the Mooney, with a shaky but successful landing? I don't remember her username . . . It was a handful of years ago, maybe '14 or '15?

Exactly right. Successful in that no one was injured but her 231 was totaled and why she has an Ovation now. Your recollection of how long ago matches mine. Amelia was her user name i believe.
I had always considered this SB but i had never experienced the jamming yet; so i hadn’t installed it. Well neither had she experienced any jamming till that last flight in her 231. So her unfortunate experience inspired me to install it. It was only a couple hours of my time during one of my annuals back then. It has nothing to do with what kind of AP you have but her BK KFC-200 was unable to unjam it for her.
Hopefully this event that Don experienced will encourage more Mooney owners without it to get it installed - you just never know when it could jam otherwise.


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[PT20J]

The auto engagement of the autopilot it level mode is part of ESP, so it won't engage if you disable ESP.

I usually disable ESP for VFR flights. I'm unlikely to lose control VFR and I don't like the automation taking uncommanded action. But, I always have it on if flying IFR.

With the latest G3X software versions, you can disable it flight by holding the AP DISC switch on the yoke for 5 seconds. The GI 275 and G5 might do that also -- I don't know.

[PT20J]

LASAR used to maintain this spreadsheet. But, it hasn't been published for several years so it may not be current.

SB_chart.xls

[varlajo]

2 hours ago, donkaye, MCFI said:

The ESP was disabled on the G5, but, while I may be wrong, that doesn't prevent the GFC 500 from turning back on after 10 seconds of being out of the envelope in slow flight.

Don, in my setup disabled ESP does prevent GFC500 from reengaging. 

[Will.iam]

Jam stabilizer is a classic emergency procedure that we would practice in the tanker as well as any Boeing product when you have a horizontal stabilizer that moves like our Mooney horizontal stabilizer does, you have this potential problem. as referenced earlier if it is full nose up yes you can Bank to the left or the right to help alleviate some of the flight loads that you’re putting on the horizontal stabilizer when you are jabbing the elevator in the opposite direction trying to control your flight. We used to call it the spiral of Life— roll it over about 30° to 40° so that you are not pushing so hard on the yoke. You’ve got to get the yolk pressure off to be able to move that horizontal stabilizer, jack screw. People vastly underestimate how much force that’s trim motor is under just moving it under flight air loads. It’s not near as easy as your hand does on the ground, but that motor is under tremendous amounts of force when the airplane is out of trim.  the closer you can alleviate the elevator from applying additional load on the horizontal stab the less wear and tear on the whole system you’ll be producing.  one of the reasons why I never used an electric trim and I manually trim on my wheel is so that I know how close I’m getting to that stop so I don’t jam it into the top of the Jack crew and also because my KFC switch is un obtainium and I want that to last as long as possible not using it is one way of doing that. Try sometimes to manually trim your aircraft while you got it out of trim, especially if you’re pulling back with a ton of elevator force you will see that that wheel is damn near impossible to move of course, if it’s on the ground with no air load, it’s gonna be very easy to move. Another technique was to unload the aircraft i. E. Let go of the yoke while it zooms up as you frantically turn the trim wheel, but the spiral of life is safer. Also don’t forget if you are way nose up and slow anyway lower the flaps that will put you closer to in trim and get off the throttle thrust makes it worse just like jamming the throttle full on a go around. Way to destabilizing just add enough power for a 1000ft climb. 

[LANCECASPER]

9 hours ago, Hank said:

Didn't Mimi post about this happening to her Marvin the Mooney, with a shaky but successful landing? I don't remember her username . . . It was a handful of years ago, maybe '14 or '15?

@Amelia 2016

 

[LANCECASPER]

2 hours ago, kortopates said:

Exactly right. Successful in that no one was injured but her 231 was totaled and why she has an Ovation now. Your recollection of how long ago matches mine.

Close, it's an Eagle

[IvanP]

Thanks for posting this, Don. Glad you got it down safely.