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DFC90 - Sudden "Autopilot Disconnect" in Cruise

Three days ago I flew to Berlin from Munich to get a 50 h inspection for my SR22. Flying IFR in FL90 the first 45 minutes were uneventful when suddenly ... "beep beep beep...Autopilot disconnected""... First I thought I had accidentally touched the trim (which in this installation immediately disconnect the A/P) - but then it happened again after 15 minutes, again after 5, 3, 2 1... minites and then it disconnected right away when I swithed it on.

I tried all different modes (GPSS, NAV, HDG) but the only thing that worked for the remainder of the flight was the Flight Director, so I flew the the rest uncoupled with the FD+GPSS

Bizarre: When I decided to make another flight later in the afternoon I flew for 90 minutes around Berlin and EVERYTHING WORKED. No matter what I did, turns, intercepts, alt changes ... all perfect

During the 50 h inspection i sent Avidyne an eMail and the suggested it might have to do with the rack, so we pulled it out (all looking good, I mean it is 3 months old!) and reinstalled it. The technician measured the voltage on the roll servo when the A/P was running, max was about 3.5 volts and he said that this is fine.

When I told Avidyne that i had (a versy short) "Servo Limit" message on the PFD a while ago they said that it might be the servo and that I would probably have to change the roll servo.

On the flight back today, IFR FL100, VMC, same thing: After 45 minutes it disconnected, then again, and again... I hand flew for 5 minutes, tried again ... and it worked without a flaw for the last 50 minutes ...

Anybody have an idea? What I really don't want to do is buy a Roll Servo for (probably) 2000 dollars only to find out that this was not the reason. I bet the Avidyne Warranty will not cover that ;-))

Anybody have an idea?

I think you have to read the error log of the DFC90 to know what external event made it disconnect.

Unfortunately to my knowledge only Avidyne can do it and they need the AP to be sent to the US. You should demand the error log to be read out at your dealer's premises, the currently approach is not acceptable. And replacing components at will and wasting hours of troubleshooting because they can't provide a basic diagnostics feature is a shame.

I haven't had an issue so far but I'd run into the same problem as you if I did.

If you had a roll servo failure, the AP would probably not disconnect if the plane was actually flying with wings level.

But a pitch servo failure can cause the trim to run for too long, and cert requirements are a TRIM IN MOTION message after ~ 10 secs and a disconnect after ~15 secs.

But if you had no messages and just the disconnect, it could be anything...

The PFD should have an error log which should give you a clue.

I don't know about the STEC servos but the King servos have a "Valid" signal coming back to the computer and if this goes out of range, it will disconnect the AP.

The servos should be easy to bench test, or even test in situ, but if the problem is intermittent you may never find it conventionally. Could it be temperature dependent?

Shoreham EGKA, United Kingdom

Thanks, you two ...

well temperature was only -3 or -4 C.... and what speaks against that is that it worked for the rest of the flight under the same circumstances...

The pitch servo was also checked at the Service Center, but was ok. It was AVIDYNE who suggested it must be the roll servo. ...

I wish it wouldn't work AT ALL. I knew exactly how the Service Center would react when I told them that it "worked later"...

Maybe a loaned Autopilot would be an idea. Then I know that it's not the controller itself. Avidyne says the A/P is not temperature sensitive but it only happened after the first 45 minutes of flight....

Of course I could try to change the servo (found out: $ 850 plus european customs, taxes) ...

In COPA I read that "many roll servos" died after the DFC90 installation, but "many" is somewhat anecodotal (and I don't use homeopathy either ;-))

I saw nothing like this. As you are under warranty Avidyne should send you a swap unit while they investigate the log. If that unit has the same problem, that will also be diagnostic. And my servos were running in -40 degree OATs.

EGTK Oxford

In COPA I read that "many roll servos" died after the DFC90 installation, but "many" is somewhat anecodotal (and I don't use homeopathy either ;-))

That sounds a bit scary because that is a part of the massive cockup which Honeywell did with the KFC225 - they have some weird software in the control loop which thrashes the servos (both roll and pitch ones) continuously, and wears out the motor commutators through arcing.

Shoreham EGKA, United Kingdom

JasonC, mine worked PERFECTLY for the first 45 hours!

I had ONE "TC MISCOMPARE" message o the second or third flight (which means that the internally installed Turn Coordinator sent data to the DFC90 which the A/P did not like when he compared it to the Attitude data from the PFD. But that went away and the message never came up again (I think that it maybe had to do with the 2 minths in the shop when I bought the plane)

I should have tested it in the descent with ALT OFF when it was bad towards Berlin on Sunday. If it is the pitch servo then it should have stayed on in that phase where it wouldn't stay on longer than 10 seconds anymore ...

BTW, the "TRIM IN MOTION" is not abnormal with the DFC90. You get it every now and then. This AP does all of the pitch work using the pitch trim servo, it only drives the pitch servo in case it has to perform a massive correction (e.g. flaps deployed). The SR20 doesn't have a pitch servo at all.

Peter, the death of many roll servos (if correct at all!) could have to do with the much higher precisison and the monitoring of the servos by the digital autopilot ... at least i could imagine that. Yes, all that stuff is scary.

MAYBE, after all, it is a good message to learn NOW, before I start taking it into heavy IMC: DO NOT TRUST ANY OF THAT STUFF

achimha, correct, only the SR22 has a pitch servo. If you put a DFC90 into the SR20 it will use the pitch trim.

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