Post crash analysis- did the failed servo cause the crash?

[Frank v B]

A member crashed an airplane and found an aileron servo that would not work properly.  It skipped when it was deflected a little to one side. Typical symptom of broken gear teeth.
I did not witness the crash.  It would be good to know the plane's attitude when it mated with Mother Earth.

Question- did the servo break cause the crash or was it due to the impact?

Took the servo apart:
Two of the black output gear (attached to the servo arm) teeth were broken.  One of the smaller white gear teeth was broken as well.  See photo.  The broken teeth are in the middle.  The sharp end of the screws point to the missing teeth.

Why is it dangerous?
Two things can happen when teeth are broken in flight...based on personal experience:
1) the servo will not deflect the flying surface.  When it is an aileron servo, you usually still have one good servo in the other wing.  The plane will still fly but with sluggish control.
2) dangerous.  When the servo starts to deflect near the broken gear, the pressure on the control surface will make the output gear to skip and lose neutral, sometimes significantly.  The next deflection can cause the control servo to doubly deflect.

Note the first photo where the pieces were embedded in the white grease.  If the teeth broke because of the crash, I would expect the black output gear pieces to be embedded beside each other.
The teeth were found on opposite ends of the white gear which means they broke when the gears were turning...in flight.

My opinion: I believe the gears broke in flight and caused the control problems.

Coming to a future episode of "Mayday", the crash investigation TV show.

Frank

[msatin]

It's interesting that both the output gear and the the smaller white gear, both had broken teeth.
Would one losing teeth cause the other to lose them as well?

[msatin]

Perhaps RealFlight (RC Flight Simulator) should include various types of failures into the their sim

[ppalumbo]

Perhaps RealFlight (RC Flight Simulator) should include various types of failures into the their sim

Thats a really good idea Mark!!  It would go a long way in evolving piloting skills.... and would be super fun without the cost   

[Frank v B]

Mark, I like your idea on the failures randomly programmed into RealFlight.

After 30 years of flying, the following should be programmed in:
- elevator coming off after a violent maneuver.  Probability 1 in 100 flights (had it happen once)
- engine cutting out 5 seconds after take-off.  Prob 1 in 20 for electric.  1 in 10 flights for IC.
- aileron unhinging 1 in 200 flights. (had it happen twice).   One caused a violent crash.
- servo failure 1 in 100 per surface (aileron, elevator, rudder).  (happened once-vibration cut through the elevator servo wire)
- broken stab half (had it happen!)
- prop flying off the motor on take-off or randomly after going to full throttle for vertical. (Happened twice).
- battery falling out/battery failure (had it happen twice!). 1 in 200
- wheel coming off on take-off and landing. Had it happen three times.
- retracts getting stuck up (belly land) or down (but not locked).
- servo horn/aileron horn breaking in flight. 1 in 300
- stabilization accidentally turning on or off.  Caused an Apprentice fly-away.
- mid-air collision.  Had it happen twice (one chewed up my rudder from behind, one sheared off the top of my fin/rudder).

Added from posts:
-  flight battery coming loose and sliding rearward (tail heavy CG issue).
- accidental electric motor reversing in mid-flight.  Yes, it happened!
-
-
-
-
-

You can add your own....  Please!  I will update this post with your ideas to make the final list and forward it to Horizon Hobbies. Without these items they should be forced to call it "near (or mostly) RealFlight". Truth in advertising.

Neat idea.

Frank

[GuyOReilly]

Change in CG location due to battery pack not secure.
Guy

[msatin]

Accidental reversing the esc in mid air

[Knightlite]

Taking off with reversed servos after a transmitter swap.

[pmackenzie]

Perhaps RealFlight (RC Flight Simulator) should include various types of failures into the their sim

FWIW, it already does do this

[msatin]

Thanks Pat - Never noticed that