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KFC 150 problem today


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Today, my KFC 150 failed. After I did the required self-test, I pushed the FD button and it worked normally. When I pushed the AP button, the AP did not engage. The AP light did not illuminate. I shut down the system several times but same problem each time I tried to engage the AP.  Finally after 10 or so attempts it worked normally.  After it worked normally I shut it down and engaged it several more times.  If worked normally each time.  Maybe a bad AP switch?

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It could be a bad trim servo, the trim could be turned off, it could be a bad auto pilot disconnect switch, there’s actually like three switches in that gang on the yoke

Edited by jetdriven
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2 hours ago, Glen Davis said:

Today, my KFC 150 failed. After I did the required self-test, I pushed the FD button and it worked normally. When I pushed the AP button, the AP did not engage. The AP light did not illuminate. I shut down the system several times but same problem each time I tried to engage the AP.  Finally after 10 or so attempts it worked normally.  After it worked normally I shut it down and engaged it several more times.  If worked normally each time.  Maybe a bad AP switch?

@Bob WeberCall Bob Weber. It will be worth the consulting fee.  webairconsulting.com

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1 hour ago, jetdriven said:

It could be a bad trim servo, the trim could be turned off, it could be a bad auto pilot disconnect switch, there’s actually like three switches in that gang on the yoke

Two things to test:

--Is your trim wheel easy or difficult to turn? Many years ago, I used to have similar symptoms to yours in very cold weather; if I worked the trim wheel back and forth I could get the autopilot test to succeed. (You should hope this is your problem, as it is a relatively cheap fix.)

--Is your split trim switch on your yoke out of line (one half pushed in, the other half not)?

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Couple of things to note when the system is doing the PFT test.  

  • Closely watch the trim annunciation during the PFT,  the trim annunciator should flash 4 times.  This indicates the unit is sending 2 up pulses and 2 down pulses to the trim servo and looking for feedback.
  • If you have no trim flashes this normally indicates that the unit is not seeing the regulated trim voltage from the trim servo.
  • After a successful test the system will flash the AP annunciation approximately 12 times.
  • If you don't see a flashing AP annunciation after the PFT, the system could be stuck in the PFT mode.  There are some relays used internally to the unit for a attitude ramp test.  If these relays fail the unit will not be able to finish the PFT.

As others have mentioned always note the position of the MET switch on the yoke, specifically the left side of the split rocker.  As the MET switch assemblies age the return springs can get weak or break and will not fully return the switch to a neutral position.  If the computer sees that the command for MET is present it will not engage even if it has passed PFT.  There are some other issues with the KFC-150 that will cause a PFT test fail like capacitor leakage but it is limited to a manufacture date range of the unit (1993-1998).

Edited by Jake@BevanAviation
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