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

Hi Peter,

Looking at the kfc225 computer unit schematic, there is just one CPU
(68hc16) and no watchdog....


No watchdog? Ouch. How did they ever get this certified?

Today I've discovered that once the kfc-225 is in level flight and
holding altitude, it doesn't use the gray code data from the encoding
altimeter - it uses its own internal encoding air pressure sensor.
This rules out the altimeter problem, and narrows it down to the
computer unit, or very few other things like the input from the
altimeter subscale pot (which it still uses).


Maybe that airpressure sensor has a problem. Either EMI or maybe the
mounting location isn't as good as for the regular altimeter. Can the
KFC-225 altitude annunciator be used to see if the unit's altimeter goes
on the fritz?

The problem is that the ability of any ground based engineers to
diagnose the product is very limited, due to the really dumb firmware.


Well, they could blast it with RF in a screened room. It is standard
procedure for any med, AV or other critical gear before type cert. I
have found pretty much any EMI problem in the screen room provided I
could have the room for at least a day. But they have to test to more
than the usual 10V/m field strength.

And I still think Socata should do a courtesy fix on that oil pressure
gauge and take care of the paperwork.

Regards, Joerg

http://www.analogconsultants.com

Hi Peter,

I agree; DC servo brushes do last a very long time; generally years of
continuous operation. There was nothing wrong with the brushes in that
roll servo; it was the power amp which went up in smoke...


Looks like a "suboptimal" power amp design. Electronics should not blow
just because of excessive actuation. I guess with all the regs you
aren't allowed to replace the busted part yourself but have to let the
service folks sell you a refurb or new version plus labor.

BTW, the clamp ferrite cores I mentioned the other day might still be pretty useful to diagnose an EMI problem. You can't leave them in there during flight but they are really handy to try out things on the ground, running the engine, keying the mike and so on.


Regards, Joerg

http://www.analogconsultants.com

Hi Peter,

No watchdog? Ouch. How did they ever get this certified?



Easy, general aviation autopilots are permitted to fail at any time,
without a warning, in any way whatsoever.


Wow. They'd never let us get away with that in medical electronics. Even
after a defibrillator hit many systems must come back to normal
operation within a prescribed time frame.

The servo clutches are supposed to be always possible to overpower,
and the pitch *trim* subsystem is supposed to warn the pilot if the
trim has been running out of control (because an excessively out of
trim condition might require more yoke force to overpower than a pilot
can physically manage). But I think those are the only certification
requirements for TSO.


I knew about the servo clutches, without override the airplane would
probably become uncontrollable if the AP fails to disengage for some
reason. But I still can't believe that the amplifiers blow.

Regards, Joerg

http://www.analogconsultants.com

"Dan Luke" wrote in message
...
"Joerg" wrote:
Wouldn't the pilot feel if the auto pilot issued lots
of servo repositionings? I am not a pilot but I could
imagine that would make for a pretty uncomfortable
flight. At least for the passengers.

Hmm, good question. Still, it might be possible that rapid, very small
repositions could be imperceptible. I was just taking a wild shot at
this one.
--
I read somewhere that the F-16 is so unstable that a human
can't fly it without the computer control. I'd think that
means it makes corrections faster than a human can, but it's
making corrections to a known airframe, I guess. I'd think
they'd want the response time of an autopilot to be fairly
slow so it doesn't do abrupt stuff - but then it really
wouldn't be able to handle weather.

Cheers!
Rich

Rich Grise wrote:

"Dan Luke" wrote in message
...
"Joerg" wrote:
Wouldn't the pilot feel if the auto pilot issued lots
of servo repositionings? I am not a pilot but I could
imagine that would make for a pretty uncomfortable
flight. At least for the passengers.

Hmm, good question. Still, it might be possible that rapid, very small
repositions could be imperceptible. I was just taking a wild shot at
this one.
--
I read somewhere that the F-16 is so unstable that a human
can't fly it without the computer control. I'd think that
means it makes corrections faster than a human can, but it's
making corrections to a known airframe, I guess. I'd think
they'd want the response time of an autopilot to be fairly
slow so it doesn't do abrupt stuff - but then it really
wouldn't be able to handle weather.

Cheers!
Rich

Yes, this is true of all modern fighters.

If you watch movies of a takeoff or landing, you will notice the ailerons and
horizontal stabilizers fluttering all over the place in what is pretty much
straight flight.

Hi Peter,

Looks like a "suboptimal" power amp design. Electronics should not blow
just because of excessive actuation. I guess with all the regs you
aren't allowed to replace the busted part yourself but have to let the
service folks sell you a refurb or new version plus labor.



That's correct; also I have not been able to find the schematic of the
KFC225 servos anywhere. I have found out that there is no service
manual as such. One could design an exact functional replica easily
enough but frankly I have more pressing things to do


Would be a nice biz opportunity though if the legal side is properly
covered. If this problem is widespread people would be willing to pay a
premium for a servo that doesn't give up. You don't have to design it
yourself.

I remember a muffler maker in Europe who created an exhaust set for a
popular car that wouldn't corrode out in just a few years. It cost a lot
more but still the muffler replacement sales of the car manufacturer
then dropped substantially.

BTW, the clamp ferrite cores I mentioned the other day might still be pretty useful to diagnose an EMI problem. You can't leave them in there during flight but they are really handy to try out things on the ground, running the engine, keying the mike and so on.



Yes, I could try placing some on the wires close to the oil pressure
gauge amplifier; that would not require any paperwork.


The clamp-on versions (just for ground tests) are often available at
Radio Shack. But usually only the large ones that still have the toolbox
type metal drawer shelves. Then when it works you could use real cores
but this often means re-doing at least one connector unless you select a
very large core (they come up to two inches O.D.).

Regards, Joerg

http://www.analogconsultants.com

Julie wrote:

Rich Grise wrote:

"Dan Luke" wrote in message
...

"Joerg" wrote:

Wouldn't the pilot feel if the auto pilot issued lots
of servo repositionings? I am not a pilot but I could
imagine that would make for a pretty uncomfortable
flight. At least for the passengers.

Hmm, good question. Still, it might be possible that rapid, very small
repositions could be imperceptible. I was just taking a wild shot at
this one.
--

I read somewhere that the F-16 is so unstable that a human
can't fly it without the computer control. I'd think that
means it makes corrections faster than a human can, but it's
making corrections to a known airframe, I guess. I'd think
they'd want the response time of an autopilot to be fairly
slow so it doesn't do abrupt stuff - but then it really
wouldn't be able to handle weather.

Cheers!
Rich


Yes, this is true of all modern fighters.

If you watch movies of a takeoff or landing, you will notice the ailerons and
horizontal stabilizers fluttering all over the place in what is pretty much
straight flight.

But it is only straight and level flight because of all of the control
surface "fluttering!"

Matt