Hello Mike, strange thing with this is I tried an AI take off earlier on one of the maps mentioned and this time they successfully managed to get off the field without ramming themselves into the ground, not sure if I have removed something that may have been causing the problem or added something that cured it, wtf, always happens after you post a problem. ???
Anyway, it was happening with a mix of aircraft modded and stock.
Each of the flights was done in QMB on existing missions copied from my 4.11 Modact, essentially when I was adding a new aircraft to try it this was the simplest way rather than creating new missions.
One aircraft with a problem was the BE2c, the AI just could not get it off a runway, with others it was as though they could not quite get to take off velocity, they almost lumbered begrugingly into the sky. Jets both stock and modded are a particular problem, they overheat very quickly and even if they get airborne they tend to return to earth rather quickly as the engines give up, usually they are on fire.
I tend to test everything with AI to see if anything will cause problems for mission building, if you fly the aircraft yourself the problems do not appear as bad.
In an un-modded 4.12.1 there was none of the behaviour that is why I thought maybe the ModAct was at fault but if you have not changed AI behaviour in any way I guess it will remain a mystery.
Sorry I cannot be more helpfull with this, I posted really to see if others were experiencing similar problems, obviously Walter is seeing some of those I mentioned.
Wishing you all the very best, Pete.