Many Cirrus pilots have seen it – the yellow “AHRS Miscompare” message on the PFD. What does it mean?
The Avidyne DFC90 autopilot needs a second reference for attitude and for this it uses a blind mounted Turn Coordinator. It “compares” the data from the PFD and the Turn Coordinator. Most Cirrus pilots are not even aware of the existence of the blind mounted TC – until they get the “AHRS Miscompare” message for the first time. The main problem is that Avidyne set the tolerance for the TC too tight. This means that even TCs that are still good will produce the message.
If the blind mounted TC (it is located behind the copilot’s side kickplate) is worn out it will occasionally happen that the autopilot receives “miscomparing” attitude data from the PFD and the Turn Coordinator. With the first software version of the DFC90 (“R1”) the autopilot would disconnect when the message came up and it could not be engaged again. If it happened on the ground you could try to restart the PFD and avionics and hope it would work but if the message came up again you could not use the autopilot.
This changed with the introduction of the “Release 2” software version. From now on if the “AHRS Miscompare” came up in flight you could still use the autopilot for the remainder of the flight, it would use the PFD data. You can still not engage the autopilot if the message comes up on the ground. “R2” has one other feature the pilot should be aware of: Other than with R1 the autopilot cannot be used on “Alternator 2 or Battery 2” meaning that if your ALT 1 fails you can fly on A/P as long as your BAT 1 still works but will have to fly by hand once Bat 1 is drained. There is a workaround for this some US pilots have done, so that the A/P can be used on ALT 2/ BAT 2 but it is not clear if the rewiring is legal (I will check and report). In real life BAT 1 should have enough capacity after an ALT 1 failure to get you on the ground in most cases.
Typically the “AHRS Miscompare” message comes up (in my airplane) when the airplane has not flown for some time and if I do a steep turn or other more extreme maneuvers. Since I have the “R2” software in the DFC90 the message has no consequence and the A/P will work fine until I land. If i then restart the electrical system and the avionics the message is gone.
If the message comes up more often then the remote TC has to be overhauled or replaced. My avionics shops did the overhaul (of the bearings inside the TC) while I went to lunch some years ago, but most shops don’t have the capability and can only offer an exchange (around $/€ 2500 for a new TC). You can also get yourself a second used TC and get it overhauled. A used 1394T100-14RB (look for this model! There are cheaper ones but they will not have the Autopilot interface) will cost around $/€ 500 and it will be another 3-500 to get it overhauled. But then you also have a spare!
Summary:
- With the R1 software version the DFC90 autopilot will disengage in flight and cannot be used again. If you are in VMC conditions you can try to reset the electrical system and the avionics. Many times it will work again. If you are on the ground the A/P self test will not be completed and you cannot use the autopilot until the message is gone or until the TC is repaired.
- With the R2 software version the A/P will keep on working until you land but you will not be able to use it for the next flight if the message does not disappear at the next start-up. But in most cases the message will disappear at the next start-up and you can fly.
- With the R2 Software version of the DFC90 the autopilot will only announce “AHRS Miscompare” but the autopilot will keep working without limitation. But if the message come up on the ground the AP self-test will fail and the AP will not announce “READY”. You can try to restart the electrical system, many times it will work.
