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KFC225 pitch oscillation, alternate static error, and static tube internal diameter

This started recently


I thought… oh bugger this is another pitch servo, with the tachometer wire having come off, as detailed here. The loss of tacho feedback increases the loop gain 2-3 times and makes the plane unstable in pitch, especially in certain conditions (loading, airspeed, altitude), with pretty big pitch excursions. But this one happens only in ALT mode. Switching to PIT mode removes the problem.

The only currently working avionics engineer I know of on EuroGA is @garryIAE but this is clearly not a common problem. It isn’t the pitch servo, but could be the KEA130A encoding altimeter, or the KC225’s internal barometer (not sure if that is used in PIT mode).

Interestingly, and only just once for about 1 second and never again, I saw the GTX330 show a silly FL, around FL560. This happened before and it was the KEA130A’s encoder which was faulty. But I am not seeing this, while the oscillation is happening. It could still be the KEA130A though, perhaps, if the problem was with the LSB wire rather than the MSB wire which causes the FL5xx issue.

The flight director (later in the video) shows movement there…

Administrator
Shoreham EGKA, United Kingdom

Does the altitude change a lot – can you show a video of the VSI and altimeter ?
The oscillation is very small….

EDLN, Germany

Altitude changes about 10ft – barely visible on the altimeter. Frequency of the order of 0.1Hz.

Administrator
Shoreham EGKA, United Kingdom

That level of oscillation could be caused by slight friction in the control system (a tight pulley or bearing) or a bit of wear causing excessive backlash. It is less likely to be an autopilot computer problem.

Avionics geek.
Fairoaks. EGTF

However, why would it go away in PIT mode? In that mode, the KFC225 is holding the pitch angle, derived from its internal accelerometer. That mode is still exercising the entire pitch control loop.

I think it has to be something related to the altitude input, in ALT mode.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

or the KC225’s internal barometer

That would have been my guess. Or an issue with (the respective part of) the static system? A bit of water in line?

Peter wrote:

LSB wire rather than the MSB

There is no LSB or MSB wire with Gray encoding. I once had a broken wire which resulted in altitude values all over the place. However, with the KFC150 ALT mode only depends on the internal barometer, so this still worked. But I don’t know about the KFC225.

EDFM (Mannheim), Germany

Moisture in your static lines? Is there a drain?

NeilC
EGPT, LMML

Good point; I will check the static drain.

AFAIK the ALT mode works by

  • using the gray code input for the capture
  • using the internal baro for ALT hold after capture
  • using the gray code as a very slow background process to integrate out any steady state baro drift

I do have a spare KC225 so could slide that in. Unfortunately this issue is very hard to reproduce. I don’t think it happens at low levels e.g. 5000ft. My next trip is Elba+Greece

Administrator
Shoreham EGKA, United Kingdom

Had these pitch oscillations also on my C41 turned out it was a intermittent loose wire from nav/com causing voltage jumps influencing the F/D AP loop. Are your AP
bus voltages stable?

EBST

terbang wrote:

There is no LSB or MSB wire with Gray encoding. I once had a broken wire which resulted in altitude values all over the place. However, with the KFC150 ALT mode only depends on the internal barometer, so this still worked. But I don’t know about the KFC225.

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.

ESKC (Uppsala/Sundbro), Sweden
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