If the "AHRS MISCOMP" error message comes up then the Autopilot will stay ON and work properly if the reason is only the Turn Coordinator (which is only used for comparison) – if you have Release 2 of the A/P software. WITHOUT R2 the Autopilot will disconnect.
But if you have the "AHRS MISCOMP" message on the ground then you cannt engage the A/P and have to hand fly. Recycling the Avionics switch can sometimes help in this case.
WIthout "Release 2" the A/P will go offline in flight
What does it do?
All I know (I have a SR22 G2 with the DFC90)
The TURN COORDINATOR only delivers data for comparison. A WORN OUT Turn Coordinator will make a "AHRS MISCOMP" message come up, meaning (in 90% of the cases) that the TC has to be overhauled. My PFD sometimes shows "AHRS MISCOMP" after 60 degree steepturns or other aggressive airwork. I will let the TC be overhauled soon.
Even with the "AHRS MISCOMP" message the A/P will stay on and fully functional if "Release 2" is installed. If the "AHRS MISCOMP" comes up on the ground, the A/P cannot be used and the aircraft must be handflown.
WIthout "Release 2" the A/P will go offline in flight – which is a very good reason (among others) to upgrade to R 2.
I have of course read all the manuals and quite honest might not remember all the details over time. Good point to look at it again.
There is nothing in the POH or AP manual that gives me that kind of information and to be honest, I have no real interest in them.
Actually there is, both in the S-TEC 55X manual and DFC90 manual you find detailed descriptions how they work. Imagine you're in IMC and your turn coordinator is causing problems. Proper systems knowledge will tell you that the S-TEC can no longer be trusted because it is driven by the TC and the DFC90 will probably turn off with an ADHRS miscompare alert because it uses the TC to monitor the ADHRS information it uses as primary input. Autopilots can have a few very nasty failure modes, also of the slow and steady nature.
I would think that detailed systems knowledge is more important in the Cirrus than with any other 4 seater because it has by far the most complex system and it is equipped to take you into situations where equipment failure can bite you.
A friend of mine did an emergency landing in his SR20 last week after a complete failure of the electric system. He was very surprised and worried by the extreme forces he had to apply to control the aircraft, he thought the electric issue (which started the usual way, first one device, then the next, then all) was "propagating" to the control surfaces. Well, the reason is simple: the Cirrus only has electric trim, there is no mechanical trim wheel. Had he spent more time studying the systems, then he would have been more relaxed without the fear of his elevator being stuck.
During climbout, the voltage fluctuated between 23.6V and 28.7V, when the decision for diversion was taken, the voltage was 24.4V with 0.1V/min downward trend, during approach it fell to 12.1V where measurements stopped.
SUST found a few causes, but one of them was that the pilot apparently didn't really understand the electrical system of his plane.
I think a reasonable level of systems knowledge is important.
But I suspect the SR22 crash could have been avoided rather more easily. For example, what was his bus voltage at the time he thought he was running on battery alone? If it was say 23.5 then yes you have lost all battery charging and you need to get down ASAP unless the conditions are CAVOK everywhere of relevance. If it was say 28.0 then at least one alternator is working fine and you are good to continue. Obviously an SR22 will have a voltmeter - somewhere... If you don't know stuff at this level then you should not be flying a C150, never mind an SR22.
I don't think a pilot needs to know where the autopilot gets its pitch data from, but he ought to know which systems depend on what. For example my heading gyro is on BUS1 so if I pull the CB on that, I will lose everything that depends on the heading: the EHSI(s), the HDG mode of the autopilot (ROL will still work, and you still have altitude hold, so hugely better than nothing), any overlays of TCAS or stormscope data on your GPS map will be disabled, etc.
I read the manual and POH where it is specified how to use it. I am proficient in using the AP and can disable it or pull the fuse. I know of quite a few pilots who fly the Cirrus and have not a clue to how the autopilot works. I have no clue to how the wiring is. There is nothing in the POH or AP manual that gives me that kind of information and to be honest, I have no real interest in them. Maybe the SR22 accident in Zurich will make me think twice?
I am not expert in this stuff, except that I have used both.
Hmm, this reminds me of the SR22 accident in Zurich! I would think that a pilot should very well know where the autopilot takes its input from and which failure modes exist...
The STEC-55 is not stable in turbulent weather. The DFC90 is solid. Why, I don't know. They say that the DFC90 takes its signal from the AHRS module in the PFD while the STEC-55 not. I am not expert in this stuff, except that I have used both.
