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DFC 90 Roll channel failure

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Ellis Brigman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ellis Brigman Quote  Post ReplyReply Direct Link To This Post Topic: DFC 90 Roll channel failure
    Posted: 09 Dec 2010 at 3:05pm
This is the situation:  A prior to take off check of the DFC90 indicated normal mode annunciator lights in HDG and ALt modes.  The HDG bug on PFD was solid but when the bug was moved left or right the yoke would not track. The VS and ALT functions were normal in that the yoke and elevator trim were moved to a climb position.  Aileron and Elevator Trim was set in the appropriate settings and the flight controls were free and correct.  VMC conditions prevailed and a normal takeoff and climb was accomplished. When the DFC90 was engaged during climb the flight control response was the same as indicated in the ground check; VS and ALT functions were active but the roll function was not.  The Fight Director was functional and showing the proper attitude to fly either in HDG or GPSS modes but the aircraft had to be manually controlled to maintain heading. The autopilot and trim circuit breakers were cycled but no change was observed. There were no warning annunciations illuminated and the system acted as if everything was functional.  Troubleshooting determined that the aileron trim was inop but that manual control was easily accomplished by overiding the spring pressure. The DFC90 could be engaged and utilized for altitude control, climbs and descents, and the Flight Director could be used for track guidance while manually controlling the ailerons.  Maintenance revealed that the relays were functional but the aileron trim motor was inop and the shaft could not be moved by hand.
Question/Comment:   It appears as though there is no warning if the trim motor is not functional. The explanation in the Pilot Guide "Reaching Servo Limits Can Cause
Uncommanded Rolls" seems to be different situation than the one encountered.  In any event, in this situation, had the weather been IMC and a ground check not  conducted; a situational awareness situation would have occurred at a critical time after takeoff. This emphasizes the need for pre-takeoff checks and troubleshooting.
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AviJake View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviJake Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2010 at 5:13pm
We agree.  Scenarios like this are exactly the reason why we emphasize the Pre-flight check on page 2-3 of the DFC90 Pilot Guide and the accompanying note on page 2-4. 

As Ellis notes, this was not a failure of the DFC90 autopilot but instead, a failure of the aircraft roll trim system.  We believe the same set of conditions in a STEC-55X equipped aircraft would have resulted in the same indications to the pilot.   In summary, the DFC90 behaved per design in this situation.
Steve Jacobson
sjacobson@avidyne.com
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jorgea View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jorgea Quote  Post ReplyReply Direct Link To This Post Posted: 02 Sep 2012 at 9:16pm
Are you yet experiencing these same technical problems of late?




Edited by AviJake - 03 Sep 2012 at 8:36am
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AviJake View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviJake Quote  Post ReplyReply Direct Link To This Post Posted: 03 Sep 2012 at 8:38am
If you are asking Avidyne, the answer is no, we have not been hearing of people experiencing what Ellis saw.
Steve Jacobson
sjacobson@avidyne.com
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