DFC90 and Reported Roll Oscillations on Cirrus
Printed From: Avidyne
Category: Avidyne General
Forum Name: DFC90/DFC100 Digital Autopilot
Forum Description: Topics on the Avidyne DFC Digital Autopilots
URL: http://forums.avidyne.com/forum_posts.asp?TID=117
Printed Date: 21 Nov 2024 at 7:59pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com
Topic: DFC90 and Reported Roll Oscillations on Cirrus
Posted By: AviJake
Subject: DFC90 and Reported Roll Oscillations on Cirrus
Date Posted: 13 Nov 2010 at 11:47am
One of the two most common observations following DFC90 installations involve some manner of roll oscillation (the other most common observation are AHRS-TC Miscompares which has its own topic thread). The typical report notes that in lateral modes, especially HDG mode, the autopilot may gently s-turn or oscillate around the bugged heading. The other complaint talks about overshoots or undershoots in trying to acquire a new heading or course.
As of this post date, we're aware of 17 aircraft that have exhibited this behavior following a DFC90 installation. In all 17 cases that we've been involved with so far, 100% (17 of 17) were resolved by repairing or replacing the roll trim motor/cartridge. The symptoms totally disappear when this maintenance action is performed.
The DFC90 Installation Manual has a troubleshooting section for installers including identifying a test for determining if the roll trim motor is out of spec. Typical out-of-spec conditions are the result of built up carbon deposits in the brushed DC motor and can be detected by measuring the voltage required to start the motors turning. There is a pass/fail voltage spec in the DFC90 Installation Manual but the symptoms of roll oscillations and consistent overshoots/undershoots in a turn almost always point to the root cause too.
Steve Jacobson
VP Product Management
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Steve Jacobson
sjacobson@avidyne.com
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Replies:
Posted By: AviJake
Date Posted: 25 Apr 2011 at 5:38pm
25 Apr 2011 Update: We continue to receive reports of post-DFC90 install roll oscillations. Batting percentage remains at 100% in that all troubleshot aircraft were positively root caused to the roll trim motor being out of spec. The out of spec condition has almost always been of the high start-up voltage type and one or two have been found to have loose bearings or loose mounting. In every known case of diagnosed roll oscillating aircraft, when the roll axis trim motor was repaired or replaced, the observed roll oscillations cease.
We do have several new datapoints on the definition of out-of-spec start up voltage. The value that Avidyne publishes as known to be out of spec is 3.0 VDC. We know for a fact that at 3 or above, the airplane will experience roll oscillations and/or course overshoots. It is widely understood that Cirrus cites 2.5 VDC as the spec'd value, though we don't have any reliable written support of that. It has just been learned that the actual motor vendor cites 2.0 VDC as the maximum expected start up voltage before performance can degrade and is observable. We are working to try and get that value in writing.
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Steve Jacobson
sjacobson@avidyne.com
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Posted By: AviJake
Date Posted: 30 Apr 2011 at 7:44am
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30 Apr 2011 Update: There are now a few instances being reported where the roll axis motors were indeed out of spec, but only by a little. Cleaning or replacing those motors did help some/a lot but for a few folks, the problem was also caused in part by the aileron rudder interconnect bungees. In at least one report, you could actually see the dead zone when trying to activate the ailerons. As soon as the bungees were adjusted per the Cirrus service manual, the problem was resolved and roll performance was returned to nominal.
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Steve Jacobson
sjacobson@avidyne.com
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Posted By: AviJake
Date Posted: 22 Feb 2012 at 6:26pm
22 Feb 2012 Update: in the ensuing 10 months since the last post on this topic, we've become aware of about another 20 Cirrus aircraft with similar roll anomalies when flying in coupled flight with the DFC90 or DFC100. In every case that I'm aware of, the source of the sub-par performance in the roll axis was traced to bad bungees. This is the very first thing our Tech Support crew recommend looking at on phone calls now.
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Steve Jacobson
sjacobson@avidyne.com
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