There has been a lot of forum and Avidyne Tech Support traffic regarding DFC90-Turn Coordinator Miscompares.
As a reminder, we kept the blind mounted turn coordinators in the DFC90 modified airplanes for several reasons. The principal motivation for keeping the TC was to deal with the concern of slow-over detections. For good reasons, it is somewhere between difficult and impossible to certify an autopilot system in which the same attitude source drives both the display and the autopilot. The theory is that a slow roll error would be hard to detect on the display and that the autopilot would blindly follow that incorrect attitude. By keeping the turn coordinator involved with the system, the slow-over concern is monitored by an independent source, thus mitigating the concern.
Avidyne chose to force a disconnect of the autopilot in the event a AHRS-Turn Coordinator miscompare is experienced. We felt that this disconnect behavior will truly get the attention of the pilot and provide some motivation to manually determine the health of the PFD attitude (compare to other PFD indications, compare to standbys, compare to outside view if VMC) before re-engaging the autopilot and relying on the PFD attitude as your unmonitored autopilot source.
Two important points to remember:
1. The output of the turn coordinator is very coarse. It is not a precision instrument and therefore, the AHRS-TC comparator is not an excessively tight or small difference allowance.
2. The AHRS-TC comparator is only running in the roll axis.
As posted in a different topic in this same forum, the turn coordinators have a high failure rate. Many DFC90 customers point out that they did not seem to have any turn coordinator problem before the DFC90 installation but now they seem to get an unexpected number of AHRS-TC Miscompares and the accompanying autopilot disconnect.
Our experience to date is that every time a TC is replaced, the observed problem of Miscompares disappears.
So why the unexpected frequency of Miscompares for some customers when they thought they had healthy TCs prior to the DFC90 installation?
We are paying very careful attention to this issue and have the following understanding:
+ The replacement rate of TCs in DFC-equipped aircraft is tracking the manufacturer supplied failure rates.
+ The TC has a range of operation - it required a minimum spin-up RPM to allow a S-TEC 55 "AP Ready" state but that was not a sure sign of perfect TC health.
+ A decaying TC (but not sufficiently decayed to prevent the initial minimum spin-up RPM) showed few externally visible signs of decay to the pilot in a Cirrus.
+ Now that the output voltage of the TC is being constantly monitored in a DFC90 equipped, there is heightened awareness of the potential decay which appears to the pilot to now suddenly have a problem after DFC90 install.
+ Unless there is an obvious roll error in the PFD, a replacement TC has resolved the AHRS-TC Miscompare in every case that we are aware of.
Avidyne remains extremely interested in customer experience with these miscompares and encourages anyone who experiences one to ensure they or their service center communicate these occurrences and useful details to Avidyne Technical Support. Every report does immediately get forwarded to the DFC Product Manager who is monitoring this closely.
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Steve Jacobson
sjacobson@avidyne.com
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