Locked up servo

[pksgwfac6v]

I'M CONCERNED. The pitch servo (SV32) from one flight to the next was found locked in position. The previous flight it was fine; preflight for the next flight one week later reveled a locked servo (and it occurred during a pre-buy inspection too). Thinking something had jammed the elevator, I did a inspection of the canard, push-pull control rods and stick mechanism and found nothing lodged. Disconnecting the pitch servo and the elevator travel was normal. After the aforementioned inspection I applied a fair amount of steady force to the stick and the servo would not budge. I'm sure I was below the threshold of the shear pin force but what concerned me was this: in pitch a Cozy has a short control rod from the stick to the elevator; no bell crank, just a short throw of abt 18 inches. To overcome the shear pin would, I'm guessing, be a hard jabbing control input (?). However in roll, with the servo mounted on the firewall, there is a long torque tube from the stick to the fire wall with a universal mid way, then a bell crank in the wing root and another torque tube half wing span to the aileron with a universal joint in this as well. My concern is this: Trying to overcome a frozen servo in roll with a hard input to release the shear pin would result in just a wind-up of the torque tubes and possible hitting the stick stops before any real force is generated at the SV 32 arm. This I'm afraid would just be a locked up issue in roll. Catastrophic results. What would cause the servo to lock up from one flight to the next just static on the ground? These servos are numbered 2983 and 2984. 2984, the offending servo, was just replaced by Dynon. What happened? This concern is spreading through our hanger group with servos of like serial numbers.

 

[dynonsupport]

We've seen this twice before. Both times, the servo was full of rust, and had clearly been exposed directly to signifigant water. The metals that need to be used to make motors tend to be very ferrous, and thus rust easily when directly exposed to water. To be clear, this doesn't just happen with very humid air, it is direct liquid infiltration.

Given that rust is a slow process, this issue never happens in flight. As long as the controls move every now and then, they won't sieze. It's only once it stops and sits motionless on the ground for a few days that the issue can occur.

At the same time, if your install does not let you safely break the shear pin when the servo is frozen, then we would encourage you to review your install and see what can be done to improve this situation. While we have never heard of a servo freezing in flight, the shear pin is there to protect aginst this in a very robust way, and we hope all installs allow this protection to be functional.

Please send us a email or give us a call to get that servo sent in so we can go a full tear down anlaysis on it and make sure there is not another kind of failure that has occured. We can of course repair the servo for you as well.

 

[pksgwfac6v]

Dynon, thanks for the reply. The servo was already exchanged under a RMA order two weeks ago when the problem was first discovered. Steve sent an updated s/n and it has since been installed. I do live in south Florida where humidity is a problem and I understand that the armature is very close to the housing thereby creating the potential for this internal corrosion to lock-up a step motor. I noticed there is a hole in the top of the servo, should this be covered (metal tape, etc.)? Also, the non offending servo on the firewall has traces of rust on the outside of the housing, like the paint was too thin. If this condition appeared on the outside (humidity), what's on the inside? Washing or rain does not get into the servo(s) compartment. Canard builders, rethink the roll servo installation.
Again, thanks for the reply. Sure would like to know what happened to mine.

 

[Dynon]

The servos aren't hemtically sealed, and shouldn't generally be modified to try to make them more water-resistant than they already are. In other words, if the servo actually gets wet, no amount of protecting it is going to make practical difference. But more-over, if you're getting actual rusting, the servo is seeing more than just regular humidity (even severely humid climates won't cause rusting of the servos). Can you send pictures of the remaining servo directly to our support team at support at dynonavionics dot com (with a link to this thread)? We'll be researching what happened with your returned servo and will get back to you directly.

 

[bruceh]

Reviving an older thread brought up by the Search feature. This describes my recent experience with my Roll servo. On the ground parked in my hangar, I was working on the airplane (RV-9A) and as I was walking around the wing, I put my hand on the aileron and found it completely locked in place. The plane had been flown in the preceding week and the A/P was used with no issues. My attempts to move the aileron were met with resistance and then none, so I figured the shear screw had just been broken off by me moving the aileron. I powered up the Dynon Skyview EFIS and went into the Setup menu and in the HW calibration menu I could see that moving the stick left and right was not showing any movement on the servo positions. Tonight I got up into the right wing and took out the servo. It did shear the screw, and the servo is completely locked up. I'll be calling Dynon in the morning to get it shipped back for service/replacement. The plane is hangared in SoCal, so it has lived it's 4+ years in a very dry climate and non-humid environment. The servo has not gotten wet or rained on, so I'm wondering if it is something other than corrosion that would cause the servo to lock up tight when it is not even turned on. Leading up to this event, I did experience a brief "roll servo offline" message on the EFIS several weeks earlier. The servo was offline for a few minutes (I was not using the AP in that phase of my flight), then the servo came back online by itself and seemed to be working normally on a couple of flights after that event.

 

[William_Ince]

I'M CONCERNED. The pitch servo (SV32) from one flight to the next was found locked in position. . . . . What would cause the servo to lock up from one flight to the next just static on the ground? These servos are numbered 2983 and 2984. 2984, the offending servo, was just replaced by Dynon. What happened? This concern is spreading through our hanger group with servos of like serial numbers.

After you receive the servo back from Dynon, please provide us with a follow up message on what Dynon found.
Very interested.

 

[bruceh]

Got the servo back from Dynon today. It is a remanufactured servo unit with a different serial number. No explanation on what they found.

 

[djones]

Bruce,
We found the servo had moisture in it at one time or another and the motor had seized due to rust on the armature. The circuit board also had visible corrosion and water stains. We elected to replace rather than repair the servo. If you have any questions, feel free to give me a call to discuss it.

 

[bruceh]

Interesting. I know for a fact that the AP servo has never been in any kind of moisture. I live in SoCal (very dry 99% of the time and we don't really have any humidity), the plane has been hangared, the servo is inside of the wing mounted against the spar where it would be above any sort of standing/dripping water, and I have never really flown the airplane in any sort of significant rain. Visible corrosion and water stains seem like quite a stretch. Nothing on the outside of the servo unit looked like it had any corrosion or water damage. Anyway, the new unit is working fine and I'm happy about that. I just don't think there is anything that would cause what you are describing, nor do I think I would need to address with the new unit. I'm hoping that the internals of the new unit are going to stay corrosion free. Does Dynon recommend any sort of recurring lubrication of the servo? In past annual inspections, I've only lubricated the external rod ends in the controls.

 

[Roger_Hirschbein]

After my wife noticed our aircraft wasn't holding course, I suspected a broken shear screw. I ordered a replacement and today it was installed. I noticed that the original screw broke and left a slight shard on the edge of the capstan. I filed it smooth and installed the new screw. I believe that turbulence with the autopilot engaged is what caused the screw to fail. (Correct me if this is wrong.)The rough edge on the broken screw allowed enough drag on the capstan that made it seem like perhaps the settings in the software were off enough to allow the plane to wander off course and then slowly try to correct the opposite direction. I would suggest that anybody who has had a working unit first try checking under the control arm for this condition before undertaking any major action with the servo.