Altimatic IIIB altitude hold

[John Moore]

My Altimatic works perfectly with the following exception. At higher altidudes with four up and the C of G further aft than normal, it has a tendency to "hunt" up and down slightly. Not a big problem as I seldom fly in that configuration, and it hasn't got worse, but recently I discovered I could replicate the problem at any altitude by slowing down to about 120 kts. It holds perfectly as the speed bleeds the back and auto trims, but then starts to pitch up and down quite markedly - more so than in the previously described situation. Kicking out the altitude hold gets rid of the problem - it doesn't occur in pitch mode.

The unit works perfectly in every other configuration. It holds altitude very accurately during turns and speed variations. Problem seems to relate to angle of attack. "Hunting" in altitude hold mode with two previous autopilots (Alt IIIB and Century IV) has been cured by overhauling the pitch servo, but not this time - no change whatsoever.

Any thoughts?

John Moore

[David Pyle]

Try disengaging the electric trim if you have that. In CIII/IV's it seems to make the pitch/altitude component less sensitive.

[Zach Grant L1011jock]

Call the guys at Muncie and ask their opinion. Ask for Rocky in avionics.
-Zach

[Kristin Winter]

Check control cable tensions and the bridle cable tensions. If one of those is loose, it could exhibit what you describe.

[John Moore]

Thanks for the responses. Much appreciated as always. I flew to Canberra today and picked up a load of ice on both legs (unusual in Australia but it does happen, and around Canberra is our major problem area due to the higher terrain, at least by our standards). At 10,000 I was back to 120 - 130 indicated at times with downdraughts and the alttitude hold was rock solid. With the ice load that would have been a reasonably high angle of attack. The A/P worked perfectly in every mode.

The pitch and aileron cable tensions were checked when the pitch servo was extracted from under the floor. Pitch was fine, aileron loose (new balance cable last year). Don't know when bridle cables were last checked.

If it ain't broke..... Of course the next flight could be quite different!

John Moore

[ics-12766]

Please be very careful of autopilot use in icing conditions...

[steen]

Amen to Zach.
I can't say enough good things about Rockie at Muncie Aviation (Indiana).
If you have a problem with an Altimatic, he has the answer.
Steen

[John Moore]

It does look like the servo overhaul has solved the problem. I did 10 hours on a big trip over the weekend with four up and luggage. These were the conditions that used to induce the problem. Alt hold worked perfectly.

Note your comments, Rich, and agree. I usually hand fly in icing conditions but left the AP engaged on this occasion to see what it would do, but was watching very carefully.

John Moore

[ics-12766]

Glad you got it fixed, John