Autopilot altitude hold has odd behaviour

[dzubot]

I have two dynon 10" touch with roll/pitch advanced autopilot and have some weird things happening. I do aerial surveying and require to hold just my altitude so I can just focus on banking the airplane. When I initially select altitude bug on the autopilot control box it automatically turns on a roll option as well as altitude, is this normal? Why can't I just turn on altitude hold? I have to select either track or nav then click it again to get autopilot to just hold altitude so I can control bank.

Second issue, sometimes when I select altitude hold it doesn't hold the altitude I have entered, I have to hit the alt up/down button, it's like it gets confused if I select altitude hold when I'm at the altitude I want it to hold. It'll sometimes descend or ascend on its own until I tell i to go back down to 4000'. Again is this normal? Shouldn't I just be able to select altitude and it'll hold the altitude I have entered?

 

[dynonsupport]

Turning on both modes is normal. 99% of the time that the autopilot is used across the fleet, the pilot wants both roll and pitch engaged, so doing this saves button presses overall. It's also the way basically every integrated autopilot works. In fact, most autopilots won't allow only one axis on.

You could pull the breaker for the roll servo if you didn't want it to do this.

As for alt hold, it matters how you enter the mode. Was the flight director on? What was the first button you hit (AP, ALT, VS)? What altitude were you at and where was your bug?

You may find simplified mode does more of what you want. It always holds the selected altitude. Advanced mode is all about sequencing, and the altitude bug is just a place if it happens to intercept it, but you need to program it how to intercept.

 

[dzubot]

Thanks for reply and detailed explanation how it works. I will try your suggestion to use non advanced autopilot and see if it works for my needs better.

I first tune the altitude bug to my altitude I want, which is usually the same altitude that I am at, then hit the"Alt" button, and usually track or nav button also since if I don't it'll turn on roll and start rolling the airplane. After this I hit "AP" button and sometimes the altitude will work okay, other times it'll start ascending or decending from my current altitude. For example I'll be at 4000' and I tune the altitude bug to 4000' since I need it to hold this, i never know what the plane will do once I turn on the AP. In most cases it'll start descending and I have to hit the climb "up" button to get it to stay at 4000'.

 

[Raymo]

Z-Air,

If using the AP advanced mode, it is working as designed. You enter an altitude that you plan for climb or decent, then hit an up or down button to initiate the action.

If in simple mode, it will do what you want.