Professional Curiousity - Runaway Elevator Trim
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Join Date: Jul 1999
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Forgot to mention that, running the trim to both ends of travel as part of the preflight is a smart move for two reasons, check indication and that there are no restrictions.
Trim is a "control", full and free it needs to be.
While I'm at it the Navajo elevator system has a "bob weight" attached at the working end.
It is there to correct a pitch stability problem they didn't get right at the original design stage and it got worse when it was stretched to a Chieftain.
It is why the the controls generally flop to the full down position, or they feel "loaded" when you pull back when the aircraft is stationary. That is normal and part of the damping effect.
Its fairly well attached but given the age and serviceability of the average type nowadays it is something you need to watch out for. If it has or does become detached expect some lively flying characteristics, it will not necessarily be uncontrollable, but if you are overweight or at the extreme ends of the C of G envelope who knows, nobody else does either.
Trim is a "control", full and free it needs to be.
While I'm at it the Navajo elevator system has a "bob weight" attached at the working end.
It is there to correct a pitch stability problem they didn't get right at the original design stage and it got worse when it was stretched to a Chieftain.
It is why the the controls generally flop to the full down position, or they feel "loaded" when you pull back when the aircraft is stationary. That is normal and part of the damping effect.
Its fairly well attached but given the age and serviceability of the average type nowadays it is something you need to watch out for. If it has or does become detached expect some lively flying characteristics, it will not necessarily be uncontrollable, but if you are overweight or at the extreme ends of the C of G envelope who knows, nobody else does either.
Join Date: Mar 2005
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Hope this helps answer the original question to this thread.
When the Chieftains' auto pilot is engaged, in alt hold or pitch mode, it must adjust the elevator trim automaticaly (auto trim) to keep the loads on the elevator servo to a minimum.
To auto trim the system senses the amount of electrical power required for the elevator servo to hold the elevator in a position corresponding to the required nose attitude of the aircraft.... with me so far?
A bit like hand flying the aircraft when the elevator is out of trim.... your arm gets tired.
The auto trim system senses the load on the elevator servo, then as if by magic, adjusts the elevator electric trim to reduce the load. Just like manually trimming in the climb
. You may be excused for thinking that if the elevator servo is electrical, how can it get tired!
Well, if you remember the auto pilot test you did before you took off, you may remember engaging the auto pilot and testing that you can "manually override" the auto pilot motor. This happens because there is a clutch in the system that disengages at a pre-set control force.
Now back to the auto trim......If the system doesn't auto trim, the clutch on the elevator servo would suddenly disengage and a very sharp nose pitch up or down would occur. In the case of a Runaway Trim with the auto pilot engaged. Simply the device that senses the load on the elevator servo has failed and commands the electric trim to run full up or down. Eventually the elevator servo cannot resist the loads on the elevator any longer, due to the out of trim situation, and lets the elevator go. resulting in a violent nose pitch up or down.
This happened to me once on a night freight run....scared the living SH1T out of me.
As for an electric trim runaway without the auto pilot engaged. This is simply a stuck relay or control circuit causing the trim motor to run in one direction continuously. Operating the trim switch in the opposite direction will trip the circuit breaker for the system and you can continue using manual trim (unless in a PA60)
When the Chieftains' auto pilot is engaged, in alt hold or pitch mode, it must adjust the elevator trim automaticaly (auto trim) to keep the loads on the elevator servo to a minimum.
To auto trim the system senses the amount of electrical power required for the elevator servo to hold the elevator in a position corresponding to the required nose attitude of the aircraft.... with me so far?
A bit like hand flying the aircraft when the elevator is out of trim.... your arm gets tired. The auto trim system senses the load on the elevator servo, then as if by magic, adjusts the elevator electric trim to reduce the load. Just like manually trimming in the climb
. You may be excused for thinking that if the elevator servo is electrical, how can it get tired!Well, if you remember the auto pilot test you did before you took off, you may remember engaging the auto pilot and testing that you can "manually override" the auto pilot motor. This happens because there is a clutch in the system that disengages at a pre-set control force.
Now back to the auto trim......If the system doesn't auto trim, the clutch on the elevator servo would suddenly disengage and a very sharp nose pitch up or down would occur. In the case of a Runaway Trim with the auto pilot engaged. Simply the device that senses the load on the elevator servo has failed and commands the electric trim to run full up or down. Eventually the elevator servo cannot resist the loads on the elevator any longer, due to the out of trim situation, and lets the elevator go. resulting in a violent nose pitch up or down.
This happened to me once on a night freight run....scared the living SH1T out of me.
As for an electric trim runaway without the auto pilot engaged. This is simply a stuck relay or control circuit causing the trim motor to run in one direction continuously. Operating the trim switch in the opposite direction will trip the circuit breaker for the system and you can continue using manual trim (unless in a PA60)
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Thank you
JARULE, a most insightful explanation.
Thank you for you time and comments.
AirSic
and thank you to all the others..it appears that this little issue crops up all too often...glad you are all still with us!
Thank you for you time and comments.
AirSic
and thank you to all the others..it appears that this little issue crops up all too often...glad you are all still with us!
