Altitude Hold - doesn't

[ridgeback68]

Having a small problem with the Altitude Hold auto pilot mode.

 

Whenever I engage the Altitude Hold my altitude slowly decays. I can see the flight path vector slowly drop below the reference altitude poles and the altimeter unwinds. This is especially nasty at low level or if I'm heads down as I turn my jet into a glowing fireball.

 

It's doesn't matter what altitude, it isn't airspeed related, and happens over flat or hilly terrain. I believe the autopilot uses barometric altitude and my QFE is correct as well.

 

I've RTFM but am at a loss to figure out what I'm doing wrong. Any thoughts or help would be much appreciated. Thanks in advance.

[fjacobsen]

I think stick input as well as excessive trim will override the autopilot.

 

It seems important to properly trim the aircraft for level flight before enganging the autopilot altitude hold.

[Corrigan]

Altitude hold should trim the plane itself.

[Gladman]

I haven't seen as you describe but if you change the baro pressure your aircraft certainly gains/looses the alt hold setting. It'll still be in alt hold but higher or lower than your setting.

I've always trimmed first but I'd bet that if it doesn't hold due to baro change then it doesn't for trim issues.

[ridgeback68]

Many thanks to all who have replied. It doesn't seem to be a common issue so it must be something I'm doing wrong or something that's screwed up from my end.

 

I think stick input as well as excessive trim will override the autopilot.

 

I've tried trimming her up before engaged the Altitude Hold and it doesn't seem to make a difference. I'll do some more testing tonight and will re-calibrate my controller in case it's doing something it shouldn't.

 

 

Altitude hold should trim the plane itself.

 

That's what I thought. Manual says the autopilot should override the manual trim but I'm still getting this slow and gradual nose down effect every time I engage Alt Hold.

 

 

...if you change the baro pressure your aircraft certainly gains/looses the alt hold setting. I've always trimmed first but I'd bet that if it doesn't hold due to baro change then it doesn't for trim issues.

 

Changes in the barometric pressure has no effect. Even I'm nose high when I engage Alt Hold she slowly and gracefully ends up in a nose down attitude until I fly into the dirt! Might also do a recording of it so you guys can actually see what's happening.

[Corrigan]

What about stick calibration? Do ctrl+enter to see if you're commanding a dive.

[ridgeback68]

What about stick calibration? Do ctrl+enter to see if you're commanding a dive.

Yeah. I was going to re-calibrate but wasn't aware of the cntl + enter thing for control axis window. Will be the first thing I do when I get home. Thanks!

[Corrigan]

An even better bet might be to temporarily unmap the pitch axis and see if it still happens.

[ridgeback68]

Problem solved. Definitely a hardware issue. The axis window showed a very slight nose down pitch when the stick was centered. Mapped a small deadzone in and now flying straight and level in Altitude Hold.

 

Thanks to Corrigan and all for your time and help. Very much appreciated.