Staring approximately one year ago pressing the TEST button on the KFC 150 / KC192 would intermittently result in the test not initiating. On some occasions later during the flight pressing the test button would result in the test proceeding and completing, after which the autopilot seems to work fine (to the extent \I have used it). I have searched this forum and have also read the KFC related material on www.peter2000.co.uk – a mine of information but the specific failure mode I am experiencing does not seem to be referred to. The optimist in me hopes it might simply be a defective test button. Does anyone have a similar experience or any other ideas as to what the cause might be or how the cause may be determined?
If the button seems dead, it might just be that.
I would open up the unit on the bench and check with a meter if the switch is dodgy.
Finally had the opportunity to do this with help of a sympathetic maintenance engineer. The test switch shows perfect function every time it is depressed. When in the aircraft the AP test commences and completes fine about 10% to 15% of the time during check list run through and on some occasions if depressed during flight will start and complete self test. I can see nothing in the manual that describes and specific configuration or mode the AP and associated components should be placed in prior to initiating the AP test. I have read elsewhere that the P1 control column A/P DESEL switch can cause problems – I presume this is a momentary push to connect so the implication being it can remain on?
If anyone has any other ideas or suggestions of a UK based workshop with appropriate expertise much appreciated.
I did for a short while get excited about the Aerocruise 230 drop in replacement until I learned about another parties dire experience on this forum. Are there alternative options?
You should check A/P disconnect switch as well as electrical trim switch(es). What aircraft is the case?
Socata TB20 GT. Currently in Annual so your suggested check hopefully not too difficult – thank you.
Steven_P wrote:
Socata TB20 GT. Currently in Annual so your suggested check hopefully not too difficult – thank you.
Than this thread is relevant for you with GFC500 as the best candidate. I guess it’s around 20k or more these days.
So upon inspecting the switch it was noted the unit is based upon three key printed circuit boards all interconnected via pin headers. Given these had been in situ for some 20 years it was decided to cycle the connection of each a few times prior to reassembly. Everything looked fine otherwise and the standard of assembly given the products age was impressive. To the surprise of maintenance and my delight the AP now initiates test every time the test button is depressed. As far as all other features could be tested in the hanger everything else also seemed to function.
Excellent, and not wholly surprising 
