AP servo mechanically locked

[rjcthree]

Preflighting today found my ailerons locked in place. All power to the A/C was off. Troubleshooting led to finding the AP roll servo mechanically locked. Disconnecting the servo arm from the aileron belcrank resumed normal operation of the ailerons. Servo itself remained locked until power was turned on. At that point the servo seemed to act normally, no error codes. I removed the servo. Since removing the servo, I can?t replicate the mechanical locking. It?s going back to Dynon.

Has anyone else experienced mechanical locking of a servo without power? I should note that the AP was used on the previous flight to this one, and functioned normally.

[az_gila]

Could the linkage have moved (bumped) into an over-center condition?

[rjcthree]

Gil, the travel limiting bracket was in place. The servo locked in about the neutral position of both ailerons and servos. Part of me wonders if the limit bracket screws may be path of the question, but I used the Dynon provided screws and spaced with a washer. I?m sending it as installed to Dynon.

I?m worried they won?t find root cause.

[RV10inOz]

I will bet, a small chocky frog, that your servo has been corroded internally from condensation build up, and has seized.

It has happened before, and on ROLL.

Make sure you alert this to Dynon's AP engineer Lawrence Doan.

I am not sure how you stop it. These days they have a metallic sticker covering the hole in the end shield of the motor to stop drips getting in, but condensation is hard to stop.

All that you can do is put a drain hole in to let moisture out.

Having done Beta Testing for Dynon in the past, this was not part of the test plan, but I did participate

[rjcthree]

David, corrosion is possible. My hanger isn’t the best when it comes to rain intrusion, and while my airplane doesn’t get wet, it’s humid, especially with the temp swings we’re having day to night. The servo was manufactured in 2010 if I’m reading the motor date code correctly, and it has no sticker over the motor shaft hole. I will contact Dynon as you suggest.

Update: servo Locked up overnight sitting in my kitchen in a box. I didn’t imagine the whole thing. Last evening, it moved freely, this morning locked tight. There is the smallest amount of gear lash in the arm, suggesting the output shaft is free, but beyond that.....

On the other hand it was kinda nice hand flying it this smorning, not having the temptation to use the AP.

[Ironflight]

Originally Posted by rjcthree

David, corrosion is possible. My hanger isn?t the best when it comes to rain intrusion, and while my airplane doesn?t get wet, it?s humid, especially with the temp swings we?re having day to night. The servo was manufactured in 2010 if I?m reading the motor date code correctly, and it has no sticker over the motor shaft hole. I will contact Dynon as you suggest.

Update: servo Locked up overnight sitting in my kitchen in a box. I didn?t imagine the whole thing. Last evening, it moved freely, this morning locked tight. The is the smallest amount of gear lash in the arm, suggesting the output shaft is free, but beyond that.....

On the other hand it was kinda nice hand flying it this smorning, not having the temptation to use the AP.

Just a suggestion - when you pack it up, put labels and signs and notes that are imp0ossible for the ?unpacker? at Dynon to ignore that say ?don?t move the output arm until on a test bench?. Since you?ve managed to recreate the problem, wouldn?t it be nice for the technician to see it that way, and not have someone in the mail room grab it and ?unbind it? without thinking......

Curious about the root cause for this!

Paul

[rjcthree]

Paul, good suggestion. I see cardboard surgery in my future! It beats cleaning the garage.

David, what in the great googly-moogly is a chocky frog? I?m leaping to a Harry Potter reference?

[Jesse]

THis type of situation is exactly why these servos have sheer screws. I haven?t seen this before on a Dynon servo, but I have with another brand, although not a tight lock up, just significant drag. Whoever at Dynon gives you the RMA, make sure hey know the symptoms and make sure they note in the RMA notes that it sometimes does and sometimes doesn?t lock up.

[rjcthree]

I can’t tell you exactly what the breakaway torque was, but while it was still installed I opted to remove the push rod from the servo arm with a torque wrench. While not perfect in technicqe, as it’s a click type, it’s calibrated and the removal torque was greater than the 25 in-lb setting, and the servo did not move. The other end connecting to the bell crank was already free. I admit I didn’t lean on it too hard while it was still connected. I wasn’t going to fly it that way even if it broke loose.

It’s boxed, pushrod protected from bumping.

[RV10inOz]

https://www.cadbury.com.au/products/...ry/freddo.aspx