DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

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Gearle
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DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Gearle »

Hello all,
Avionics mechanic says:
“ the trim servo takes ships power and makes 16 volts to drive servo. That 16 volts goes through trim switch as part of self test, if AP doesn't see TRIM voltage it fails. I believe that trim switch has reached the point it doesn't pass the voltage,causing the failure.”

Has anybody else had this problem?
Has anybody else been able to fix their switch? The switches are very expensive if you can find it…

Thank you ahead of time!
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Charles K
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Re: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Charles K »

I think it is a very common switch used in many older planes.
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Re: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Charles K »

For what it is worth - I never really use it - I like my manual trim wheel.
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Rich
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Re: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Rich »

The wiring diagram does not support this analysis. Could you elaborate on the symptoms? Does the AP fail its preflight checks or does it balk at altitude hold only?
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: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Rich »

In reading the original post a different way, maybe it has to do with one of the three switches being shorted when none should be. This could happen if one of the two rockers is stuck. All three switches are normally open and when operated they connect to ground.
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: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Gearle »

Rich wrote: Tue Apr 13, 2021 10:29 pm The wiring diagram does not support this analysis. Could you elaborate on the symptoms? Does the AP fail its preflight checks or does it balk at altitude hold only?
Hello - passes preflight tests (well it allows the AP to be enabled) but will not hold an altitude. yes i use the manual trim wheel all the time. but the switch has been problematic for a long time. you have to really push it to get it to actuate. i just wonder if it can be taken apart and cleaned the old fashioned way with a pencil eraser to remove varnish...
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Re: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Rich »

Gearle wrote: Wed Apr 14, 2021 4:32 pm
Rich wrote: Tue Apr 13, 2021 10:29 pm The wiring diagram does not support this analysis. Could you elaborate on the symptoms? Does the AP fail its preflight checks or does it balk at altitude hold only?
Hello - passes preflight tests (well it allows the AP to be enabled) but will not hold an altitude. yes i use the manual trim wheel all the time. but the switch has been problematic for a long time. you have to really push it to get it to actuate. i just wonder if it can be taken apart and cleaned the old fashioned way with a pencil eraser to remove varnish...
Does it go into ALT mode but fails to hold or does it refuse to go into ALT mode?

I have had a somewhat analogous problem in my motorcycle. The brake (front handle and rear pedal) and clutch handle each have an additional microswitch which is there to cancel the cruise control setting. The front brake switch has been problematic, in that some mung or other would get in there and prevent the CC from arming. I opened it up and sprayed electrical contact cleaner (NOT WD40) inside, which cured the problem. You might give this a try, which is consistent with your suggestion. This front brake switch was recently replaced under a safety recall, as problems with the contacts/actuating plunger had been shown to cause problems with the the ABS.

I also had a problem when the rear brake pedal needed lubrication, causing the pedal to hang in an intermediate position, causing the same problem as with the front switch.
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: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Rich »

Here is the diagram of the switches on the pilot stick, showing the wiring of the switches. In the interest of zooming in, it does not show where the wiring goes from there. But the wires from terminals 1, 2 and 3 are connected to the flight computer (FC) box directly. Terminal 4 is connected to ground.

Note that all 3 switches are shown normally open. This means that when your fingers are off the two rockers just having some munge on the contacts themselves wouldn't likely produce a problem with other functions. However, if one of the contacts was somehow being held closed (i.e. grounded) I could see the FC interpreting this as conflicting signals and override the ALT function, since it involves both pitch and pitch trim servos. As difficult as they are to move, if the trim switches do work manually I'd be inclined to suspect the ARM switch is somehow being held closed. Goop accrued over the years could account for this and it could even be between the base of the operator and the top of the stick, external to the inner workings.
Trim Switch Diagram.jpg
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: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Gearle »

Rich wrote: Wed Apr 14, 2021 5:43 pm Here is the diagram of the switches on the pilot stick, showing the wiring of the switches. In the interest of zooming in, it does not show where the wiring goes from there. But the wires from terminals 1, 2 and 3 are connected to the flight computer (FC) box directly. Terminal 4 is connected to ground.

Note that all 3 switches are shown normally open. This means that when your fingers are off the two rockers just having some munge on the contacts themselves wouldn't likely produce a problem with other functions. However, if one of the contacts was somehow being held closed (i.e. grounded) I could see the FC interpreting this as conflicting signals and override the ALT function, since it involves both pitch and pitch trim servos. As difficult as they are to move, if the trim switches do work manually I'd be inclined to suspect the ARM switch is somehow being held closed. Goop accrued over the years could account for this and it could even be between the base of the operator and the top of the stick, external to the inner workings.

Trim Switch Diagram.jpg
Yes this is probably what has happened to the switch but it’s just a guess until I can take a look at it. The mechanic thinks he has found a similar switch in a Baron FYI.

To the other question about whether it engages, the AP engages, does not fail while engaged, but will not hold altitude - like it can’t control the servo. So it is a strange issue.
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Re: DA40/G KAP140 altitude hold fail due to Electric trim yoke switch

Post by Rich »

When the AP is in control, it is operating in one of two altitude modes: ALT hold or VS maintain (even when ALT is "armed", it's in VS). In both modes the pitch and pitch trim servos are both involved. Some things worth trying (if changing/cleaning switch doesn't work):

1. While in stable VS (I assume that's working) at 500 FPM or so, kill the AP and see if the plane is out of pitch trim. It shouldn't be.
2. Set VS to 0 FPM and see if it holds altitude reasonably well. It might drift a bit if it's kind of turbulent. Retry 1, above.

I assume this isn't an altitude preselect mode issue, which I have. (Mine holds altitude, it just doesn't hold the right altitude unless I lie about baro setting :tap: )
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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