KAP-140 course/heading drift

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ahmebane
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KAP-140 course/heading drift

Post by ahmebane »

I have a 2003 steam-gauge DA40 with the King KCS-55A HSI, 430W, 530W, and KAP-140.

Ive noticed that when the AP is engaged, whether in HDG or NAV mode, it will fly along just fine for a while (10+ minutes), then start drifting to the right of course or heading. Recently, I noticed this while tracking to a GPS waypoint. The plane had turned slightly to the right & was heading off course. I turned the heading bug to a heading appropriate to recapture the course, switched to HDG mode, hit NAV, which armed it. The plane rolled sharply to the left, continuing well past a std rate turn. After it hit about 35 degrees of bank, I switched it off & hand flew.

Ive seen similar behavior in HDG mode, and it doesn't seem to matter whether its tracking a localizer, VOR, or the GPS.

Before the shop starts replacing parts, any theories? I believe the AP is driven by the turn coordinator, correct? Could it be a loose connection there or a flaky-in-some-way TC? Any thoughts on troubleshooting?

One additional data point: the rudder trim is slightly off, pointing the nose to the right. (I need to apply slight left rudder for perfectly coordinated flight). When the AP is engaged, it flies continuously with the left wing slightly down to compensate. Could this possibly be causing the servo clutch (if that's the right term) to start slipping & result in this behavior? And yes, Ive been trying to get the shop to adjust the trim , but its somehow not making it to the top of their to-do list. :(

TIA.
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Rich
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Re: KAP-140 course/heading drift

Post by Rich »

This sounds similar to a problem I had some years back (2010 - IMC flights to/from Boundary Bay, BC and back). It turned out to be a connector at the back of the KI525A had come partly loose. Accessible by removing the top of the instrument panel, it was easily detectable. I corrected the root cause which was a screw securing the connector had vibrated loose, though it was still in place. Never happened again.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP-140 course/heading drift

Post by Lance Murray »

If not loose like above post try re-racking the autopilot, also disconnect and reconnect the turn coordinator.

The turn coordinator is many times the culprit. How is the performance in turns before the drift occurs? Sluggish? Are the bearings louder than normal?
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ahmebane
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Re: KAP-140 course/heading drift

Post by ahmebane »

Cant hear the bearings - plane is way too loud. From what I can tell the TC works fine. I'll have the shop check the TC connections. I find that when it goes bad, the AP overcorrects - either overbanks past std rate or just turns too sharply for a minor correction.
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Re: KAP-140 course/heading drift

Post by Rich »

ahmebane wrote: Fri Apr 30, 2021 5:41 pm Cant hear the bearings - plane is way too loud. From what I can tell the TC works fine. I'll have the shop check the TC connections. I find that when it goes bad, the AP overcorrects - either overbanks past std rate or just turns too sharply for a minor correction.
On the ground, turn on the master and listen. To avoid noise being masked by the AI, pull its breaker (in the essential bus group).
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP-140 course/heading drift

Post by ahmebane »

Yeah, that makes sense - duh!

The shop did fix the trim today, btw. I dont think it was the famous corked nosewheel issue, as it was very consistent flight to flight. Just added a touch of trim & now the plane flies straight, level, coordinated.
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Re: KAP-140 course/heading drift

Post by Rich »

I'll describe a particular behavior I experienced in 2 different 1990's-era C-172's w/ KAP140 and the same TC:

Flying level in cruise in NAV mode all is fine. Suddenly the plane decides to make a fairly radical turn to the left. It gets a bit off course, so it comes level and then a slight right turn back for a few seconds, then the left turn reappears. It keeps doing this, with the plane slowly getting off course to the left. I kill the AP and hand fly. It came to be attributed to the TC signal to the AP.

In the incident I quoted in my DA40, it was different. More like you describe.

My TC is the last iron gyro in the plane. There appear to be no non-mechanical-gyro replacements in existence. Thankfully, mine continues to work just fine after almost 2 decades.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP-140 course/heading drift

Post by Steve »

Rich wrote: Fri Apr 30, 2021 11:09 pmMy TC is the last iron gyro in the plane. There appear to be no non-mechanical-gyro replacements in existence. Thankfully, mine continues to work just fine after almost 2 decades.
Now you've done it! I predict it will now fail within 20 hours... ;)



...I hope not. Mine is also original to the aircraft.

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Re: KAP-140 course/heading drift

Post by Rich »

Steve wrote: Fri Apr 30, 2021 11:48 pm
Rich wrote: Fri Apr 30, 2021 11:09 pmMy TC is the last iron gyro in the plane. There appear to be no non-mechanical-gyro replacements in existence. Thankfully, mine continues to work just fine after almost 2 decades.
Now you've done it! I predict it will now fail within 20 hours... ;)



...I hope not. Mine is also original to the aircraft.

Steve
Thou shalt not screw with that which is not broken. 8-)
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP-140 course/heading drift

Post by nathanda40 »

ahmebane wrote: Wed Apr 28, 2021 12:18 am I have a 2003 steam-gauge DA40 with the King KCS-55A HSI, 430W, 530W, and KAP-140.

Ive noticed that when the AP is engaged, whether in HDG or NAV mode, it will fly along just fine for a while (10+ minutes), then start drifting to the right of course or heading. Recently, I noticed this while tracking to a GPS waypoint. The plane had turned slightly to the right & was heading off course. I turned the heading bug to a heading appropriate to recapture the course, switched to HDG mode, hit NAV, which armed it. The plane rolled sharply to the left, continuing well past a std rate turn. After it hit about 35 degrees of bank, I switched it off & hand flew.

Ive seen similar behavior in HDG mode, and it doesn't seem to matter whether its tracking a localizer, VOR, or the GPS.

Before the shop starts replacing parts, any theories? I believe the AP is driven by the turn coordinator, correct? Could it be a loose connection there or a flaky-in-some-way TC? Any thoughts on troubleshooting?

One additional data point: the rudder trim is slightly off, pointing the nose to the right. (I need to apply slight left rudder for perfectly coordinated flight). When the AP is engaged, it flies continuously with the left wing slightly down to compensate. Could this possibly be causing the servo clutch (if that's the right term) to start slipping & result in this behavior? And yes, Ive been trying to get the shop to adjust the trim , but its somehow not making it to the top of their to-do list. :(

TIA.
i basically joined this forum because our 2002 steam da40 was having an issue where it makes sudden turns to the left in hdg/nav mode. sometimes it also overbanks. quickly clicking the cws seems to stop the problem and the airplane will level off and return to the proper heading before doing it again

i had our turn coordinator (which was starting to get wonky) replaced and it mostly fixed the issue, but it would still happen in turbulence. our hsi was just tested and found to be drifting beyond manufacturer tolerances, so we are having it repaired. i'm not convinced that is the problem, as an issue with the hsi doesn't explain the overbanking. hoping that it starts working again, but at least now i am down to the KC140 or the servos

i say all of this because it sort of sounds like your issue. uncommanded turns, overbanking, etc... based on what you have said, i would be quite suspicious of the turn coordinator and go from there.
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