252 cowl flap motor shearing pins

[Oldguy]

My friends 252 has started shearing the pin in the cowl flap motor somewhat regularly (every 2-3 months). The alignment on it has been checked, and nothing seems out of whack. His plane is used in his business and regularly gets 400-500 hours a year put on it, so having it down for any amount of time is a difficulty. Any suggestions on additional items to check would be greatly welcome. TIA

[gsxrpilot]

I had the same problem... Check the micro-switches to ensure they are shutting the motor off as or just before it reaches the stops. Usually just adjusting the micro-switches will do it. I also noticed that the pins installed were too short. they weren't actually shearing off, but slipping out of place. So longer pins helped as well.

The motor is super powerful and if it doesn't stop when suppose to it will shear those pins easily.

[kortopates]

Also check the fit of the rollpin that goes through both the shaft of the motor and the overlying control arm bracket - sorry not sure what to call that except that it fits over the motor shaft and attached with 2 rollpins.  The motor shaft will probably be fine, but if the rollpin holes in the overlying control arm is enlarged then that will lead to premature failure of the roll pin(s) as the pin rotates back and forth against the hole sides. It so it should be replaced with a new one to get a tight tolerance fit again. The two rollpins are 90 degrees apart so that you don't loose cowl flaps with just one pin failure. Check them both. But my guess is that to minimize his down time, no one has disassembled the assembly enough to do a full inspection and find the issue. Being in a really back tight spot makes it very difficult to see what's going on without pulling things out.  

BTW, the current spec on the motors went to the next size up in roll pin diameter making them much stouter and thus harder to break. So getting the motor overhauled to the current spec is another way to attack the issue. Going to the next size up allows drilling out the smaller hoes on the attaching bracket and making it like new with tight tolerance holes again without replacing it. That savings almost pays for the electric motor overhaul.

Edited November 6, 2018 by kortopates

[gsxrpilot]

Yeah what @kortopates said. The motor should be out of the plane and on the bench to work on this. You can also pull the screws on the end of the motor where the shaft is. That will allow easy access to drill, measure, fit, etc, new roll pins. Just don't turn the motor over with the end off and spill the gears and guts out of it. 

As the pilot, I do my part by not relying solely on the microswitches to stop the motor. I'll run the cowl flaps closed and turn the switch off just as it reaches the limit... if I can. Same when opening them. It certainly can't hurt.

[Oldguy]

Thanks, Paul ( @kortopates ) and Paul ( @gsxrpilot ).

I will pass your suggestions to him and his mechanic.

[gsxrpilot]

He could also call JD at SWTA. JD did all the work on my 252 and knows the cowl flap motor intimately. JD would be happy to answer any questions.

[Jsavage3]

Yep, I had this happen on my 252 about 6 months after I bought it.  A limit switch failed (smashed flat like a pancake) and then the motor shaft sheared.  I had the motor O/H’d by the manufacturer and I replaced all the limit switches too.  Works like a champ now, but...I made two small red dots (lady’s nail polish) on the indicator, one at the upper limit and the other at the lower limit, positioned just right such that if I return the switch to its center (off) position abeam the appropriate red dot then a limit switch doesn’t get touched or almost touched...I.e. I become the limit switch and the actual limit switch is there to back me up... Works great, lasts long time!  Good luck with your repair and I hope you’re able to get it fixed once and for all.

[larryb]

Don’t forget to lube the cowl flap linkage every time the lower cowl is off. With the manual actuator in my J I knew when it got stiff. With the motorized version it is harder to know when it gets stiff. Just a quick shot of triflow.