TruTrak Roll Servo Locked Up

So there I was, taking off from Airventure 2019 Wednesday morning and suddenly the ailerons did not want to work. I pushed left-right hard and it seemed to be ok. But . . then the AP had no roll control.

Back home in an hour the culprit was the servo. It locked up and sheared the screw designed for this failure mode.

Root cause: Sitting on the ground the sweltering hot and high humidity from Friday to Wednesday at OSH, there was liquid in the stepper motor. The PM rotor and stator had light corrosion and filled the air gap. I removed the motor and could not turn it with the gear held in a vice! Light sanding with some 320 sandpaper, wiped clean and it can be rotated with the slippery rotor shaft, a quite low torque to be sure.

I have never seen this type of failure posted, and since it was only this one time my plane has every been outside overnight or in the rain, I thought it was noteworthy to warn others of this possibility.

Check on the ground and don't take off like I did.




The shear screw that saved my life. Smart people at TruTrak, thanks.


Rust on the stator, rotor was the same.

Bill,

Given where the servo is located it makes me wonder how it could get soaking wet?

rapid_ascent said:

Bill,

Given where the servo is located it makes me wonder how it could get soaking wet?

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I am supposing that either the humidity, condensation, or 4.5" of rain created mist and blew backwards through the pushrod hole to then create a super humid environment when parked. One storm blew from the tail, not sure about the others, I was in the tent asleep.

Pictures added. The external rust spot indicates liquid on the exterior, but not sure how it got inside the stepper, but it did.

Pictures above.

A scary looking servo. I'm a little confused though. Are you saying that the roll control was locked on the ground, with the A/P off, prior to departure (i.e., a seized servo)? I would guess you didn't have a flight control check in your checklist or you departed on the A/P?? Both seem like bad ideas. Not an attack, you just caught my attention with the "check on the ground" statement, but I'm not sure what I would check. I depart A/P off and I do a controls check as part of my pre-departure checklist. I would hope that that would catch a seized motor if I had one.

I've seen this before with a servo that sat on the shelf for a year or so before it was installed.

AndyRV7 said:

A scary looking servo. I'm a little confused though. Are you saying that the roll control was locked on the ground, with the A/P off, prior to departure (i.e., a seized servo)? I would guess you didn't have a flight control check in your checklist or you departed on the A/P?? Both seem like bad ideas. Not an attack, you just caught my attention with the "check on the ground" statement, but I'm not sure what I would check. I depart A/P off and I do a controls check as part of my pre-departure checklist. I would hope that that would catch a seized motor if I had one.

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Guilty: I can not say that an aileron check was done once I was strapped in. Elevator, yes, rudder yes. The AP was off for take off and not engaged until at 1300 ft and 150 deg heading. I am not sure precisely when it locked, but believe it was that way when the gust locks were removed. The break-away force for the shear screw was not that high. Clearly higher than flight forces which caught my attention, the screw is 1/16" dia brass at the shear plane. I think it would be noticed, but not necessarily after the screw is broken.

Excuse: This is on my checklist and after getting help to get the plane in position I was going though the checklist and a follow-me guide arrive rapidly and was waving me to follow. The taxi to the runway was fast. I got distracted. It won't happen again. It was my first OSH.

Walt said:

I've seen this before with a servo that sat on the shelf for a year or so before it was installed.

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This one did sit but was always inside a climate controlled area and had always rotated freely. It is being shipped back for upgrading/overhaul. I am hoping that the later stepper motor has internal corrosion protection, or is sealed. If anyone still has the blue stepper motor in operation, they should do some investigation.

Walt, I saw you taxi into HBC, and sorry I did not get over to say hello. Next time!

Bill, thanks for being forthright. While I am good on my control checks (my wife finds it cute I still say 'aileron waves hello' from time to time) I had become lax in my autopilot operational checks in my pre-takeoff flows. Good reminder.

A heavy rain on top of wing allows water to seep in at the wing skin seam on the top of main spar. water follows top spar web and drips onto servo.

I had to seal 2 RV's in the past due to this. Some construction irregularities allow the drips to follow an alternate path that misses the servo altogether and in some cases the paint/primer seals the seam well enough. Sometimes they run for years with some moisture getting on the servo only to fail at a later date.


On one airplane I had trouble with a drip getting onto the servo connector plug and corroding the terminals but nothing was getting into the servo motor.


A place that I now inspect more diligently.

Weasel said:

A heavy rain on top of wing allows water to seep in at the wing skin seam on the top of main spar. water follows top spar web and drips onto servo.

I had to seal 2 RV's in the past due to this. Some construction irregularities allow the drips to follow an alternate path that misses the servo altogether and in some cases the paint/primer seals the seam well enough. Sometimes they run for years with some moisture getting on the servo only to fail at a later date.


On one airplane I had trouble with a drip getting onto the servo connector plug and corroding the terminals but nothing was getting into the servo motor.

A place that I now inspect more diligently.

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I had the same thing happen to me. 3 days of rain sitting on the ramp while on a trip to Jacksonville FL. The servo was locked, opened up the access panel drips along the skin seam dripping right on top of the servo. Flew it home, took the servo out, made sure I could reproduce it with a hose. It took a while and a lot of water. Sealed it from inside the wing, tested with the hose again. I have not had a problem since.

I?m just curious, what did you use to seal the inside of the wing.

Give us a shout

Hey, Andrew here...

Thanks for posting as a heads up to make sure that you always double check your controls. We have seen some motors corrode like that, but not very often. That reason is exactly why we have the shear screw. You are exactly right, it doesn't take too much force to shear the screw. We have LOTS more cases of screws that shear when we wish they wouldn't, but I am glad that they are there for this exact reason. We want to keep you safe!

Give me a shout, we will get you a new servo. From the pictures, I see that your servo is pretty old. The early servos had corrosion protection added by us. Everything for that last 8 years or so has corrosion protection from the motor manufacturer.

Glad you are safe.

thanks!

TruTrakAndrew said:

Hey, Andrew here...

Thanks for posting as a heads up to make sure that you always double check your controls. We have seen some motors corrode like that, but not very often. That reason is exactly why we have the shear screw. You are exactly right, it doesn't take too much force to shear the screw. We have LOTS more cases of screws that shear when we wish they wouldn't, but I am glad that they are there for this exact reason. We want to keep you safe!

Give me a shout, we will get you a new servo. From the pictures, I see that your servo is pretty old. The early servos had corrosion protection added by us. Everything for that last 8 years or so has corrosion protection from the motor manufacturer.

Glad you are safe.

thanks!

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Outstanding! Give me a call 30957353 six eight and we can get it rolling . . . I don't know how to get in touch, precisely.

bhassel said:

I?m just curious, what did you use to seal the inside of the wing.

Click to expand...

DAP Dynaflex 230

BillL said:

Guilty: I can not say that an aileron check was done once I was strapped in. Elevator, yes, rudder yes. The AP was off for take off and not engaged until at 1300 ft and 150 deg heading. I am not sure precisely when it locked, but believe it was that way when the gust locks were removed. The break-away force for the shear screw was not that high. Clearly higher than flight forces which caught my attention, the screw is 1/16" dia brass at the shear plane. I think it would be noticed, but not necessarily after the screw is broken.

Excuse: This is on my checklist and after getting help to get the plane in position I was going though the checklist and a follow-me guide arrive rapidly and was waving me to follow. The taxi to the runway was fast. I got distracted. It won't happen again. It was my first OSH.

Click to expand...

Bill, I hesitated to post above because I didn't want you to feel like I was being critical. Your post got my attention though and I wanted to understand more so I could make sure I wasn't missing something in my own routine. Thanks for all the honest info!!

For what it's worth, I must have a clutch in my servo. It is one of the two choices, apparently. I mindlessly tried to make a turn with the A/P on and thought I sheared the screw. TruTrak told me I had to find out if I had a screw or clutch before they sent me a fix for the screw. Next time I flew, the A/P was working normally so I never bothered to access the servo for confirmation.

Andy