Stabilizer Trim Alert

[Fly By]

A recent in-flight incident concerning a hopelessly jammed stabilizer trim on a K model prompted increased attention to an obscure Mooney Service Instruction (M20-88), and most likely a new Service Bulletin will be released soon.  I have written an article that will deal with this subject in more detail in forthcoming issues of the MAPA Log and The Mooney Flyer but wanted to get the word out to the troops sooner so am posting an excerpt here.  This problem may occur on K and earlier model Mooneys but here are some generalized procedures you may find useful for stab trim issues in any Mooney.

Stabilizer Trim Problem Actions:

Jammed Stabilizer—full nose up:  The faster the plane goes the more forward elevator pressure will be required, so slow down as much as possible because this force can be considerable.  Lower the flaps (takeoff or landing setting as appropriate), fly with the gear up, and use the least power needed… i.e. no need to climb quickly unless you have obstacle or terrain issues.  Try to break the jammed trim loose by “shocking” the trim wheel smartly with as much nose down force (front of trim wheel towards the floor) as you can, but don’t forget to fly the plane.  Declare an emergency!  Pick a suitable airport (a longer runway is better).  Configure normally for landing and fly a stabilized approach in the 1.2 to 1.3 V_SO speed range.  A slightly steeper than normal glideslope will also help.  Slowly retard the throttle when landing is assured, flare, and touchdown normally.

Jammed Stabilizer—mid position:  Not as critical as the full nose up jam previously discussed, but it will still get your attention.  It could be caused by a mechanical failure, a foreign object (e.g. rag, pen, flashlight, seat belt) getting lodged in the mechanism, or ice.  Turn off Elevator Trim and Autopilot.  Apply a moderate amount of manual trim in the direction opposite to that which caused the jam in the first place and consider jostling the plane in an attempt to dislodge foreign objects.  (Don’t get overly aggressive—remember the Air Alaska MD-80 that tried to fix a stab trim problem rather than landing.)  Land as soon as practical.  If the out-of-trim condition is causing problems controlling the aircraft declare an emergency.  Consider a no-flap landing (add 10 knots to V_REF) if the trim is jammed nose down.

Runaway Trim:  Grab the trim wheel to stop the motion.  The trim motor clutch will slip allowing you to restrain the wheel.  Turn off the Elevator Trim switch and disconnect the Autopilot.  Some airplanes have a Trim circuit breaker you can pull.  Use manual trim.

Be sure always to check the trim position prior to takeoff.  Improperly set nose trim can make pitch control very challenging.  Excess nose down trim will make rotation difficult and can result in porpoising on the runway and a possible prop strike.  Excess nose up trim will make it difficult to keep climb attitude and airspeed under control.

You can check your Mooney’s trim system yourself to see if it exhibits any tendency to stick at the limits.  On the ground, run the trim to the full nose up position and then back slightly to see if there is any tendency to bind at the stop.  Then run it to the stop again and give it “a little extra” with the manual trim wheel to see if it exhibits any tendency to stick.  Check with your A&P or MSC if you have any concerns about your trim system.

Lee Fox

 

[Yetti]

trying to understand this.   what is the clutch force that the trim servo breaks over at?   what is the ratio on the trim wheel chain?   I see the scoring on the the two surfaces. It does not seem like a lot of force to break it away from the other surface.

 

You mention mid trim stick.  What is causing that?

[Fly By]

The picture is NOT of the clutch trim but rather the two limit nuts that contact and can jam to the point that it requires vice grips to separate them!  A mid-position trim jam, as the article says, is usually caused by a foreign object or substance.

[Amelia]

Kudos and my deepest gratitude to Fly By /Lee for his persistence in figuring out what went wrong, and even more, what to do about it. Y'all pay attention, now. You now know what I truly wish I had known a few weeks ago. Somebody else suggested that "unloading the wing," i.e. a steep turn, would have taken the strong nose-up pressure off, such that I might have been able to keep the nose on the horizon by myself. I don't know... But as it was, I am glad I wasn't solo!!

[Yetti]

56 minutes ago, Fly By said:

The picture is NOT of the clutch trim but rather the two limit nuts that contact and can jam to the point that it requires vice grips to separate them!  A mid-position trim jam, as the article says, is usually caused by a foreign object or substance.

I get that it is the trim indicator that runs along the screw.   What I was asking is how much torque can the servo apply to get the indicator bar to jam against the limit?  What are the servos that are utilized in this application?

[Yetti]

Probably not the same servos, but there seems to be maintenance that needs to be done to BK servos   http://www.ntsb.gov/safety/safety-recs/recletters/A86_132_134.pdf

[Fly By]

I do not have the torque figures you request.  I am only stating empirically that this has reportedly happened to quite a few Mooneys.  D-Max states that he sees several a year and has had the trim jam on him in flight.  He was only able to free it up by shocking it with great force.  The plane I inspected would not release after the incident under any circumstances using the trim wheel, both hands, and both knees firmly planted on the seats.  After the trim was freed up the trim servo was able to be restrained easily when held between two fingers.  I cannot explain how it generated enough force to produce the affinity between the two nut surfaces that required two vice grips to separate, but I can testify that it certainly did. 

[Yetti]

My google fu says these pitch servos are interesting 

KS-270A PITCH SERVO BRIDAL ( BRIDAL CABLE, 2 CABLE CLAMPS )...P/N...065-0059-01

If I had a K I would consider not using the electric trim

[Guest]

I'm sorry but I'm still confused.  In what flight regime would you end up trimming to the point the trim would jamb either full nose up or full nose down?  In both of my E models I would land with anywhere between take off and full flaps.  Full flap landings required use of near full nose up trim, but I never had one jamb. Neither had electric trim.

In the case of Amelia, I still don't know if it was an uncommanded run away trim, or the trim ran on after use in flight. Maybe I missed it, but I re read the entire thread looking for an answer.  

I can't imagine a situation in flight where I would need/ use full nose up trim, other than landing.

For  those with electric trim, or an autopilot, who does regular bench checking of the slip clutches? 

Clarence

[Yetti]

Best I have read is that it was an uncommanded runaway. 

[Fly By]

The K model is more nose-heavy than the other earlier models.  Full nose up trim is frequently used during a normal landing.  From the pictures it can be seen that there was frequent contact between the stop limit nuts over the years. 

Here are three possible scenarios when you might discover an untimely jammed stab trim:

1. A last-second go-around
2. A runaway trim that is not arrested until it hits the stops

3. Failing to reset the trim before takeoff

[Oldguy]

11 hours ago, M20Doc said:

For  those with electric trim, or an autopilot, who does regular bench checking of the slip clutches? 

Clarence

Does pre-flight checking count? Maybe I'm just a geek, but I run through the pre-flight check of my KAP 150 as it is described in the FM Supplement. Takes a couple of extra minutes, but it's just something to make sure everything is working like it should. Was not doing that until I went to the Mooney Summit in 2014 and heard DMAX talk about reading your POH at least annually to learn something new about your aircraft. The A/P check was the thing I took away from my first re-read of my POH.

[Yetti]

Also It looks like the threads are buggered about 7 thread from the rear stop. Can we get a better picture of that?     Also reading the original post it seems like the plane went to the shop for trim issues several times prior to the incident.

I think the trim indicator guide block needs to be taken off and threads checked.  It would be good to know their condition

[PeytonM]

I posted my own experience with runaway trim (nose down) during landing phase in a J model in early October, likely due to an electrical short in the yolk-mounted trim switch. 

 

[Fly By]

19 minutes ago, Yetti said:

Also It looks like the threads are buggered about 7 thread from the rear stop. Can we get a better picture of that?     Also reading the original post it seems like the plane went to the shop for trim issues several times prior to the incident.

I think the trim indicator guide block needs to be taken off and threads checked.  It would be good to know their condition

Threads are fine.  You're just seeing some loose junk that did not clean off completely for the picture.  Yes, the plane had previous inspections and lube done by two unrelated A&Ps. Worked fine after that... for a while!  See Mooney Service Instruction MS20-88 for the approved "cure."  This matter has been escalated at Mooney and a new Service Bulletin is expected to be released soon.

[Bob - S50]

I would not suggest running the trim stop to stop.  If I remember right that was part of the finding in the Alaska MD80 accident.  The practice had been to run the trim stop to stop to make sure they had full range available.  However, when the trim hit the stops it put a lot of stress on the threads in the traveler nut.  Over time it stripped the treads and allowed the nut to slide along the jack screw which resulted in a loss of control.  If you want to check your trim, I would stop a bit short of the stop.

[Fly By]

Thanks for clarifying.  I was not suggesting that this be done every flight but only run to the stops periodically as a maintenance check until MS20-88 or subsequent Service Bulletin has been accomplished to see if there is any tendency to bind.. and on the ground.

[DVA]

12 minutes ago, Bob - S50 said:

I would not suggest running the trim stop to stop.  If I remember right that was part of the finding in the Alaska MD80 accident.  The practice had been to run the trim stop to stop to make sure they had full range available.  However, when the trim hit the stops it put a lot of stress on the threads in the traveler nut.  Over time it stripped the treads and allowed the nut to slide along the jack screw which resulted in a loss of control.  If you want to check your trim, I would stop a bit short of the stop.

I agree, but with one caveat: You should occasionally take the trim to the stops, but do it by hand. The electric trim is designed to limit the torque by slipping at the stops, but there is no need to beat the ends of the trim system to death.

You want “feel” for any binding or roughness, also looking for any areas of the travel that get progressively tighter then get loose again - in essence any unevenness warrants a closer look.  If you don’t run the trim through it full travel occasionally you will never discover issues before they come issues, you also do not spread lubricants evenly and that can in and of itself cause unnecessary problems.

As proof, I had this happen in flight on a rental a few years ago. (Not a Mooney!) Apparently I was the first one to run the trim front to back and discovered the badly frayed cable that passed the annual for a few years.  It snapped on climb-out as the A/P was trimming for pitch. Plane felt funny so I reached down to trim manually and found this in my hand. Happy to tell the story of the flight if anyone is interested. (and yes, that’s a chopped off tip of my index finger from a horrible and disturbing freak accident involving an angry snow-cone machine in the early 70’s)

 

[Fly By]

Mooney just released Service Bulletin M20-325 pertaining to the stabilizer trim on J, K, L, & M model Mooneys.  I am told it may be a few days before it is up on the Mooney Support site, but your MSC should have received it.  It closely follows the guidance in Service Instruction M20-88 which is currently posted.

[Guest]

8 hours ago, Bob - S50 said:

I would not suggest running the trim stop to stop.  If I remember right that was part of the finding in the Alaska MD80 accident.  The practice had been to run the trim stop to stop to make sure they had full range available.  However, when the trim hit the stops it put a lot of stress on the threads in the traveler nut.  Over time it stripped the treads and allowed the nut to slide along the jack screw which resulted in a loss of control.  If you want to check your trim, I would stop a bit short of the stop.

I thought that the Alaska MD 80 trim jack screw failed due to lack of lubrication.  The airline continued to stretch the lubrication interval until it failed in flight.  At the time of failure the threads were almost completely gone.

Clarence

[Guest]

7 hours ago, Fly By said:

Thanks for clarifying.  I was not suggesting that this be done every flight but only run to the stops periodically as a maintenance check until MS20-88 or subsequent Service Bulletin has been accomplished to see if there is any tendency to bind.. and on the ground.

Given enough force by manually turning the trim wheel or by using the electric trim with a sticking clutch any airplane is capable of having a jambed trim system.

The applicable maintenance manual lists the clutch slip values for electric trim systems.

Clarence

[cliffy]

Not to lessen the seriousness of the situation BUT-

manual gear, manual flaps, manual trim? All you electrified guys always have problems. Progress marches on?

[carusoam]

I'm taking it as a reminder...

I trim pretty nose high during the landing process.  I have found putting a hand on the trim is feed back enough to know that the trim is actually moving.  Now my hand will be there to detect if the trim stops as well...

I don't usually hit the stop with my current loading.  Adding a larger person up front might do it though...

Best regards,

-a-

[Tommy]

Trimming full nose up for landing means in my J landing without pulling back on the stick. Don't know if I like the idea at all. I normally trim up half way between Take Off and Full Nose Up (one thing that I really appreciate is the visual indicator of both trim and flap positions on a Mooney). Only a small back pressure on the stick needed for flaring (And bit of down force for go-arounds)

[cliffy]

I'm not quite sure what is meant by "trim nose high during landing"?

I always trim for correct speed on short final and just hand flare for landing. 

Have done it that way in everything I've  ever from from Cubs to 757s.