Planes (tested on vanilla A-10 with big modpack)
1. in Alt/hdg mode you have oscillations that got damped in too long time, imo. They so big at start that mode just disables itself.
2. Alt hold again does not always enables. when enables it has oscillations again, that disables it right from the start.

But all in all it is a good work, I never believed it could be done in Arma.

Also, if I to be permitted to suggest you to take up DCS terminology and autopilot modes? ^)

The issue with yaw-to-death on littlebirds has been resolved along with the error you posted.
Altitude Hold's collective control has been rasied a little to a more reasonable amount.
Position Hold remains unchanged as real position hold only holds your position. It does not work like Auto-Hover does in the game.

I adjusted the max pitch angles for fixed wing's altitude holds. It seems BIS's A-10 has a nearly 5 degree AoA which made low speed altitude hold stop itself. You still can't be going too slow with alt hold but now should work with BIS's A-10 at speeds over 250km/h.

As far as the oscilliations, that is the autopilot getting comfy (except the raging little bird one) and is a result of how far off you were from where the autopilot tries to get you.

Thank you.
Some comments, if you perrmit.
First heli mode is basically Level flight now. Btw, I can turn it on the ground with zero speed, but not within 0..70 km/h. ^)
Second Heli mode is looks like Pitch/Roll/Alt hold, but it is losing pitch gradually to about -5 while in flight.
Third mode is ok.
Plane modes right now working ok. I only recommend to rename them accordingly - Roll/Alt hold, Level hold, Pitch/Roll hold.
Also second mode can be deactivated too easy and activates only very close to level flight. This is workaround to reduce oscillations, I believe?

But very nice work, thank you again.

The first heli mode (Attitude/Velocity) will hold your speed and that speeds direction. It goes level at the start as your speed when you engage it is the reference speed. You will find if you go hands off the heli will maintain that speed and direction of flight.
I have had no problems turning it on at speeds from 10km/h to 275km/h.

Second mode (Altitude) will hold your altitude in MSL (Mean Sea Level) by using only the collective. As a result of the collective not being perfectly above the center of mass, the helicopter does pitch but the altitude will be maintained.

Plane mode names are taken from their real-life names as used in the A-10C.
The Alt/Hdg deactivates if you are 10 degress off from level flight on the bank or 7.5 degrees from level on pitch.

Direction - yes, speed - not exactly, it reduces gradually to about -10 km/h from speed you enter the mode with, stabilizes after that.

Ok, fair point. But you need to rename first two heli modes, because they just both Altitude hold.

Love the idea of this and I like implementation of CBA keys so I can use this with my programmable joystick, however it's not working at all for me in Vanilla planes nor helicopters. The addAction changes so I know it's turning the action text on and off. CBA keys is working. I don't get any errors. But I don't feel the aircraft steering or maintaining altitude nor doing any of the things it's supposed to be doing. Not sure what I'm doing wrong. I'm guessing it's a conflict with another mod. I'll test further and report back.

Possible improvement would be a debug mode so we can see what it's trying to do to test if it's working or not. It's possible I'm just too far outside of the envelope of the AP. However, I see the addAction change when I make a large change in the controls. So I know that part is working.
Also, a sound when activated and deactivated would definitely help.

Thanks for letting me know you are having problems. Do you know if it is every autopilot for both rotary and fixed-wings or just specific autopilots from either? Fixed wing autopilots do not have a standby mode like helicopters so if it is an issue with fixed wings it is an error in my code that I must've overlooked.

I had considered a debug mode so Ill consider adding it with the patch to fix these issues (provided its not a mod conflict.) And by the way, it does make a click noise of the switch whenever an autopilot is activated or turned off for any reason, maybe a volume problem?

Looking forward to hearing your followup so I can begin correcting the issues! :)

For jets/planes... altitude hold also does heading hold when you have a separate option for altitude/heading. They both do the same thing.
Helicopters... only velocity hold works and it locks in altitude and heading, not velocity. No other options work on helos.
I tested these on vanilla aircraft as well as Firewill jets and RHS choppers.