Sheared screw and auto-trim interaction.

[vlittle]

This is a lessons learned.

I had a shear screw fail in my roll servo.  The underlying cause was the castellated nut was not properly torqued, so the screw was taking all of the load.  This was a replacement (rebuilt) servo from Dynon received as a swap for a non-Skyview compatible unit.

Even though the screw sheared, the autopilot continued to fly most of the route.  My first indication of a problem was that my trim indicator was wonky. In fact, I noticed this for some time, and attributed it to a faulty trim spring, but kept my eye on it.

The second indication was when a significant course change was followed by a non-compliance of the aircraft!  I noticed no servo force in roll, but manually steering and engaging the a/p on a new course worked ok, and small course changes worked fine. Turns out that it was my autotrim system flying the aircraft.

On the ground, I located the sheared screw and have sent an email request to support for a replacement.

I learned several years ago to always monitor the autopilot when reaching a waypoint.  I've had strange operation from other brands as well.  One caveat: this is not the Dynon autotrim system, but the one I designed for Makerplane.

Vern

 

[n456ts]

The underlying cause was the castellated nut was not properly torqued, so the screw was taking all of the load.  This was a replacement (rebuilt) servo from Dynon received as a swap for a non-Skyview compatible unit.

If I understand everything correctly, there was nothing wrong with the torque of the nut. The shear screw SHOULD take the load. If you tighten the castle nut, you are effectively disabling the shear screw. So, don't tighten that down. Leave it the way Dynon set it up.

 

[vlittle]

The underlying cause was the castellated nut was not properly torqued, so the screw was taking all of the load.  This was a replacement (rebuilt) servo from Dynon received as a swap for a non-Skyview compatible unit.

If I understand everything correctly, there was nothing wrong with the torque of the nut.  The shear screw SHOULD take the load.  If you tighten the castle nut, you are effectively disabling the shear screw.  So, don't tighten that down.  Leave it the way Dynon set it up. 

Spec is 4.5 in-lb max, it seemed looser which may have been wear on the plastic washer after the screw let go.

I have adjusted the mechanical advantage of my linkage to reduce the torque requirement. However, I have never gotten a slip warning from the servo or even a trim warning so I am not sure of the root cause. Harmon Rocket, light in roll forces.

 

[vlittle]

A question for Dynon: is there any way your servos can detect a sheared shear screw? Even as a pre-takeoff checklist procedure... Lock the servos and feel the clutching? I know the AP can be engaged on the ground, but it usually results in running the controls to the stops. Maybe a test mode?

Of course, we don't want the test mode to shear the shear screws....

Vern

 

[Dynon]

We don't have an automated way / test mode to detect it currently, but engaging on the ground and ensuring that the AP can hold the servos would do it (you wouldn't need to get to slip force if you're familiar with the normal "strength" of the AP motors' hold)

 

[vlittle]

We don't have an automated way / test mode to detect it currently, but engaging on the ground and ensuring that the AP can hold the servos would do it (you wouldn't need to get to slip force if you're familiar with the normal "strength" of the AP motors' hold)

That works for preflight. For in-flight, if you integrate correction steps and account for slips, you can infer the position of the servo shaft and determine that the screw has sheared and raise an alarm.

In fact, I can probably put this into my Autotrim controller code.... A symptom of a sheared screw would be detected in the serial data stream. Unpaid work, yuck! It would be better to have Dynon do it.

 

[vlittle]

I received my new shear screw and cotter pin form Dynon, thanks! Everything working fine now.
My trim indicator is now stable. It's been wonky for a while which indicates that my roll servo may have had a sheared screw for some time.

New pre-takeoff control check procedure includes an autopilot servo engagement check.