Are your AP bus voltages stable?
AFAIK, yes.
There are no LSB or MSB wires in the strict sense, but of the two leftmost bits (D2 and D4), D4 is zero below FL 308 and D2 is zero below FL 628. So any problems with one of these wires is likely to cause a much larger error than problems with the other wires — particularly at lower levels.
I am wondering if there is a loose wire on one of the bits which just by accident only causes a small change. Probably not though, because I fly at all sorts of levels from 1000ft to FL200 and something like that ought to show up on gray code…
Another possibility could your KI256 gyro bearings getting worn out causing too much friction hence the control loop get’s amplified. I think voltage or friction..:-). Was your TKS system switched on? Honestly in my case I never would have thought that a power surge not noticeable whatsoever, as everything else worked fine only the main avionics bus screw was not 100% tightened correctly or came loose could have such an impact. There was some arcing visible on the main bus wire after investigation. With the engine vibration tough everything seemed normal…. Curious to see the correct fault analysis in your case..
BTW when the failure happened intermediately the AP worked fine in ATT hold but lost it in Altitude hold, which made me believe it was the computer….of course A C41 is not KFC225
Peter can you see the pitch change on the horizon or is it too small to notice ?
It is very visible on the horizon, yes.
I have found something very interesting:
The pitch oscillation stops immediately when you open the alternate static vent (which goes to the cockpit).
So it must be a blockage in the static pipe after all. That’s a bugger; much of that pipe is inaccessible without extensive dismantling. I might have to run another one…
One cannot fly with alternate static because (a) the displayed altitude is some 200ft too high and (b) the displayed airspeed is some 15kt too high which could be quite dangerous.
The way I found this was by noticing that when (during the oscillation) the plane climbed, the altimeter actually went down (about 5ft). So there was a lag of maybe 2 seconds. That has to be a blockage. I have a video which I will try to process.
Is it possible to disconnect the instruments and blow compressed air through it? Not suggesting, just curious.
Yes it should be. It might work. It would also blow out any water which is not draining through the drain.
It won’t work if the pipe is pinched, but I am pretty familiar with the innards of the plane and I know where that pipe lies, and would be surprised if it was pinched.
That’s one of the most important things to learn in IR theory, what happens if the static port freezes and how you recognise it. It’s not common knowledge that it can get blocked by other means as well. well done…
I well knew the altitude error, but not the airspeed one. It is obvious really…
it’s what schools call ‚transfer knowledge‘ :) It takes experience to get the clue. I wouldn’t have got it.
