We didn't find a correct way to manage dynamically manual autopilot altitude arming.
Currently, the sim always arms altitude so it's not possible to have a VS, FLC, or PITCH mode that doesn't switch to ALT at the selected altitude.
There is a parameter "no_alt_capture_in_vs_mode" in the aircraft config file but nothing to do that dynamically so implementing a manual arming in an aircraft is a nightmare.
Using slots is not working too because the active slot value is always copied into the "AUTOPILOT ALTITUDE LOCK VAR" slot 0 variable.
The workaround we have found is to set a very high value (ex 99000) in the "AUTOPILOT ALTITUDE LOCK VAR" but many users complain because their HUD or other aircraft instrument displays 99000 as the currently selected altitude in this case.
We have also noticed that the "AUTOPILOT ALTITUDE ARM" sim variable is never set.
Maybe a K event that disables auto arming could help or any other solution allowing dynamic manual arming.
The Asobo KAP140 ARM button is not implemented, probably because of this situation.
Thanks for your feedback about that. This is currently a blocking situation for many aircraft.