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Old 1st Aug 2008, 15:24   #1 (permalink)
 
Join Date: Mar 2005
Location: UK
Posts: 508
KAP 140 autopilot problem?

Our club aircraft are all fitted with a CAP 140 autopilot. For some reason I keep getting the red (P) symbol after the system test, indicating that the pitch mode has failed. Strangely, by turning off power to the avionics and re-starting the pre flight test seems to cure the problem.

I would have put it down to a faulty unit, but it's happened to me on all 3 of our club aircraft, which leads me to believe I must be doing something wrong, but what? I make sure that the Avionics master is OFF until the engine is stabilized and the Alternator is online. I don't touch the trim wheel until the test is complete... what else am I doing wrong, or not doing?

Any ideas?
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Old 1st Aug 2008, 15:28   #2 (permalink)
 
Join Date: Jun 2003
Location: UK
Age: 96
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Have you spoken to an avionics shop that deals with Honeywell kit?

The self test on Honeywell autopilots is deeply dodgy and the AP will pass the self test even with a roll or pitch servo completely burnt out. Pitch trim problems are usually detected (just as well).

If you have three planes which all do this, you need to have it looked at. It sounds like a dodgy installation.
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Old 2nd Aug 2008, 02:26   #3 (permalink)
 
Join Date: Dec 2001
Location: FL, USA
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I know my answer is not going to help anybody...
but there is a reason as to why they are called K(r)AP 140-ies.
If they work, they work fine and are easy to use, however unusual amount of pitch trim failures in a lot of different airframes.
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Old 2nd Aug 2008, 07:05   #4 (permalink)
 
Join Date: Sep 2006
Location: Aylesbury,Bucks
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I have just had my pitch servo replaced for a second time (first was a service bulletin requirement) - and the aircraft is less than 3 years old! Shortly before it was knackered I had a similar story - occasional diagnostics failure - power cycle - ok.

I did also have a similar error when the G1000 was upgraded incorrectly and stopped the ADC talking to the KAP140 - but that was permanent error.
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Old 2nd Aug 2008, 09:34   #5 (permalink)
 
Join Date: Mar 2005
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If in doubt, consult the manual...

I managed to get a copy of the KAP140 Pilot's Guide from our club. It does actually mention this phenomenon.

Quote:
NOTE: Following the preflight test, the red P warning on the face of the autopilot may illuminate indicating that the pitch axis cannot be engaged. This condition should be temporary, lasting no more than 30 seconds. The P will extinguish and normal operation will be available.
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Old 2nd Aug 2008, 15:19   #6 (permalink)
 
Join Date: Jul 2002
Location: UK
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Ah, 'RTFM' wins again.
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Old 2nd Aug 2008, 15:47   #7 (permalink)
 
Join Date: Jun 2003
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I would like to see an explanation for this behaviour. After the power-up test, the autopilot should be ready for use, not messing about doing other things. What plane is this?
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Old 2nd Aug 2008, 18:47   #8 (permalink)
 
Join Date: Mar 2005
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Quote:
I would like to see an explanation for this behaviour. After the power-up test, the autopilot should be ready for use, not messing about doing other things. What plane is this?
It's a PA28-181 Archer III. Just to clarify, the text was taken from the actual Bendix/King® User Manual, not a guide written by the club.

There's a copy of it online HERE (page 8)
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Old 2nd Aug 2008, 21:19   #9 (permalink)
 
Join Date: Jun 2003
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OK, thank you. An interesting artefact on B/King autopilots which don't have a pitch trim servo, I suppose...
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Old 25th Feb 2009, 18:33   #10 (permalink)
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Join Date: Feb 2009
Location: Ohio
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KAP 140 Trim Fail

I am jumping onto your thread a long time after it concluded. I just bought a '98 Archer III with a KAP 140. I get the trim fail (PT) message about every fifth flight, right after takeoff. So the electric trim is out until my next flight. I hadn't thought of shutting off the avionics until you mentioned it. Had the trim rigging checked and lubricated, but no improvement.

Wondering if you learned anymore on the topic. gcrandall@glipoolproducts.com

Thanks.
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Old 25th Feb 2009, 20:11   #11 (permalink)
 
Join Date: Jun 2003
Location: UK
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You can get weird pitch trim failures as a result of the autopilot disconnect switch (the one with the red button) being faulty.

This switch has two sets of normally-closed contacts. One set tells the AP you have pressed it. The other set interrupts the power to the servos, causing their clutches to drop out, so you can hand fly the plane - while you are depressing the red button.

This switch is too flimsy for the current drawn by the servos, these being inductive loads which cause a lot of arcing on the contacts. I took one apart yesterday and there wasn't a lot left of the contacts - after 700hrs of flight.

Doing the AP diagnostics (laptop running hyperterminal, into the AP diagnostic port) reveals nothing because that contact is always OK.

Check your emails/PMs Gary.
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