Re: Honeywell KFC-225 autopilot - what could cause this failure?

From: Peter (nospam_at_nospam9876.com)
Date: 06/12/04 

Date: Sat, 12 Jun 2004 09:18:16 +0100

Joerg <notthisjoergsch@removethispacbell.net> wrote

>Hi Peter,
>
>The mpeg didn't play for me but just a few thoughts:
>
>Since they say they couldn't reproduce it on the ground, could this be a
>noise issue? Possible sources would be the radio, a transponder
>transmission, engine ignition, maybe a motor driven trim, fuel equalizer
>pump if you have one. Just think about other stuff on your aircraft that
>produces either RF or commutation noise and comes on automatically and
>possibly unnoticed by the pilot.
>
>I have seen such noise become a real problem with composite structures.
>
>Regards, Joerg
>
>http://www.analogconsultants.com

Yes Joerg, very possibly. However I have done 250 hrs in this
aircraft, using the autopilot in much the same way whenever I have
used it, perhaps 30% of those hours. It has gone for as much as (!!)
50 airborne hours between failures, so if it was noise I would have
expected it to happen a lot more often.

I do have EMC issues, e.g. VHF transmission causes the oil pressure to
go to zero, but that is common on this type and variant (TB20GT) and
the reason for that is fairly obvious: a poorly filtered input on the
diff amplifier on the pressure transducer input.

It is possible that autopilot failures correlate to proximity of
ground radar... The wiring is of generally very high standard though;
I've been doing hardware/software design for 25 years and can see some
things...

The bottom line is whether servos should burn out, no matter what. The
KFC-225 has only one processor (I have the schematics) and the
software didn't appear to crash outright in any of the failures.

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