Electric Trim Warning Light - Audible Warning Issue

[Ksaunders]

My yoke electric trim switch is working just fine as well as the Altitude up/down on my auto pilot.  Problem is while I'm

flying with Auto-pilot activated; my Trim warning annunciator light and audible warning is going off intermittently on my KFC 200 auto pilot warning display.  The only way to make it stop is to disengage the auto pilot then re-engage.  Sometime this corrects the issue and sometime it does not.  Sometimes I get the audible and flashing warning for a few minutes then it stops flashing and beeping, other times it won't quit unless I deactivate the auto pilot then re-activate.  It does not matter if it is smooth air or turbulent.  It will happen both on the ground and in the air.    

 

I'm personally thinking it may be something with the aircraft electric trim on/off panel mount switch.  Anyone have any thoughts/ideas/similiar experience with this?  It's not causing any issue with my flight but it's disturbing to my passengers.  I also have my bi-annual IFR flight review this summer and probably don't want this little distraction to contend with along with the fun of all that!

 

Appreciate your advice.

 

K. Saunders

ksaunder@prattindustries.com

N23MD

 

 

 

 

[OR75]

Question for you:

when you disengage the A/P (or the altitude hold only), is the aircraft trimmed properly ? (if it is trimmed correctly, you should not have an abrupt up or down change in the aircraft attitude)

[kortopates]

If the electric trim is still working when you AP disconnects, it’s hard to imagine it being an electric trim rocker switch issue, but if the electric trim is intermittent, sure that could be your cause.

 

 

 

Otherwise the first thing I would check is the buss voltage at the time of disconnect. The King AP are very sensitive to voltage and if you were idling on the ground below the alternator coming on RPM and have a weak battery, the AP would be the first electrical device to complain. Or if in cruise and you have everything turned on, including incandescent landing lights, pitot heat etc., with older high wattage devices you could be exceeding your alternators output in the 12v 60amp systems and quickly see symptoms with a weak battery resulting in the AP disconnecting. But in each case, the low battery light should be flashing. Yet since you mentioned this happens in cruise, if it was due to a low battery/buss voltage I am sure you'd notice the low battery annunciator flashing - so this probably isn't the cause. In which case you could have a loose wire between the AI or HSI and the AP or worse. But suggest verifying you have normal buss voltage at the time it disconnects to make sure its an AP problem versus an electrical issue. If not another thing to look at is your HSI when it happens, a loss of valid compass which will be accompanied by HDG flag, will cause a AP disconnect if in heading mode (but the AP will work in FD & Alt modes), and of course it could be a failure of the flight control system.

 

.

[Ksaunders]

Truly appreciate your replies.   

 

When the trim light flashes and the audible alert sounds and I disconnect the auto pilot, the plane is in perfect trim.  It's also happening in cruise with your normal equipment running (strobes, GPS, panel, etc) - not any high strain events.  My battery seems to be very strong.  The times it has gone off momentairly on the ground, it has been during taxi without the auto pilot being on.  When this happened, I would cycle the electric trim panel switch and it would seem to re-set the warning light and tone.     

 

I have also recently had my electric trim switch re-soldered when it acted up a few moths ago but it is working fine now.  It took us two times and we ultimately had to extend the trim switch connecting wires and re-solder the connections to get the trim switch to work properly.  Now the electric trim switch works perfect but I'm getting the electric trim light and tone.    

 

When I asked my Avionics guy the question about the trim warning light and tones, he replied, "It could be your panel switch or the trim pitch servo may need to be cleaned".   I trust him and really appreciate his service.  Was just looking to learn a little on my own and maybe avoid a 1/2 day of not solving the problem myself. 

 

 

  

[OR75]

I was thinking it is the trim servo. It obviously has a trim sensor that may need tuning. But it sounds like the sensor is doing its job of timing.

[N201MKTurbo]

I think the auto pilot monitors the current to the pitch servo, when the current goes over a threshold ti lights the light. The current draw of the servo is directly proportional to the amount of force being exerted by the motor.

 

If there is to much friction in the pitch system or servo then the motor will go over the threshold during normal operation.

 

I would first lube the pitch system including the yoke shafts and make sure it operates smothly. After that I would have the pitch servo overhauled.

[kortopates]

Let us know what your shop finds out to be the problem. I am betting its an electrical or connection issue vice a trim servo since its maintaining trim till it disconnects. But there must be lots of different failure modes.