Check Your Autopilot Servo Arm Screws!

[sneedrv-6]

If someone could tell me (or give me a spruce link) what size nuts are used on both sides of the shaft it would help me out a bunch. The guy that installed my linkage did not put the nuts on. These would be the nuts on the fore and aft side of the tube that is female threaded.

Thanks,

Jason

[dynonsupport]

Originally Posted by Sam Buchanan

The Trio servo drive system is similar in design (but much beefier in execution) to the old Navaid servo which no doubt has the safest failure mode of all the current servos. Another safety feature of the Trio system is how the autopilot will automatically disengage once the aircraft starts moving if you inadvertently leave it engaged following a pre-flight check.

The TruTrak and Dynon servos use stepper motors that can always be overpowered, no matter what the control head tells them to do. No clutch needed, no power removal required. They're both safe systems, but I fail to see why the Trio system is superior in the safety area. The Dynon and TT servos only have 2 moving parts, and mechanical simplicity means a lot when trying to analyze failure modes. On top of that, the Dynon servos have a hardware disconnect of power to the motor whenever the disconnect stick is pressed. So even if everything in the system is yelling "MOVE" to the servo, including the processor inside the servo, if you press the disconnect button, the disconnect signal goes right to the last hardware step in the servo and fully disconnects it.

The Dynon system disengages when it gets airspeed as well.

[Skyflyer]

First of all kudos for keeping your head. I have been asking myself if I could have done the same.

My question is this. Had you ever removed and reinstalled the #8 pan head screw that attaches the control arm to the servo? The screw is ?secured? with blue Loctite when delivered. Once broken loose, it should be cleaned and new Loctite applied before reassembly to reestablish the thread lock.

[rbibb]

I do not have an autopilot installed but have been following this thread for its educational value.

In following the "safety wire vs. not saftefy wire the nut" discussion it occured to me that perhaps a small bracket could be riveted to the control horn such that, if the nut came loose and the arm fell it could not jam the controls. That way you have all the inherent safety of shear pin override plus prevention of catostophic failure mode associated with jam of servo arm in primarly control path.

I don't know if the geometry would allow it and it would entail removing elevator control horn from aircraft and careful check for clearance of said bracket but it might be worth considering.

[cleve_thompson]

I was in the process of ordering screws with drilled heads so I could safety wire my TruTrak servos. However, after reading comments from Dynon support, I decided to contact TruTrak support by email through the Forum. I recieved this reply within an hour!!! Now that is support!

I thought before we all crawl back in the tail, maybe we better wait!

Here is TruTrak's reply:

dynonsupport is correct. I suppose I will need to make a post confirming his/their warning about using the safety wire. It could invariably cause a dangerous situation.

When we have a solution that we feel comfortable with, I will make a post on here and we'll put some info on our website.

However, please don't be worried about your servos. As I stated in my post, we have over 10,000 servos in the field operating right now. This is the only one we know of that has happened. Most likely, the arm was removed at some point in its service and threadlocker was not reapplied.

Thanks Cleve!
__________________
Lucas
Trutrak Tech Support

[4kilo]

Before my incident, I did not remove, modify or alter either servo arm screw.

I never did find the screw from the elevator servo. After the incident, I removed the screw from the aileron servo. It had no threadlocker on it whatsoever. If, "the arm was removed at some point in its service and threadlocker was not reapplied," it was done before the servo was shipped to me.

Pat

[sneedrv-6]

I removed the screw on my servo 3 days ago, there was no thread locker at all on it. I added some and screwed it back in. Would be nice to get to the bottom of this. My servo is pretty new, I would say it was shipped to the guys doing my install about 3 months ago.

Jason

[Janekom]

Well done Pat!
Just another view talking specifically about installations on the RV7. Looking at the picture in post #38 it is clear that if one uses a long enough rod this will and can never happen. At least this is the way we have installed it. The rod is about 4 times the length of the servo arm.

[David-aviator]

Great job flying the airplane, Pat. You instinctively did what is written in some flight manuals, if a control is jammed - apply pressure until it breaks loose.

Can't comment on the screw issue except it sounds like someone failed to apply the locktite. I will be checking the servo arms although I am using an old Navaid devices autopilot.

Thanks for a great write up.

[Skyflyer]

Look carefully at the screw before you decide there is no Loctite present. It is dark blue in color and may be hard to see. I removed mine to check just how well secured it was and initially thought there was no thread locker present, however, on closer inspection I found it quite plainly there. (Do not attempt to remove the brass sheer screw, it is also inserted with Loctite and will break. It does not contribute to securing the servo arm, which slips over the head of the screw.)