I have added this precise air density formula to make ballistics calculator work. After quick check I can say that it gives proper value unlike default function with simple array of values instead of real life calculations that was replaced in 2014.
As example, air density at 1500m (8*C base temp) is 1,06144106993015 given by this function, 1.061919 by random online calculator in old function, at 5000ft density is 0.928
Another check - at 4572m = 0,794604829807298 15000 ft = 0.793
which is pretty close. Looks like wrong density values near to surface was accounted by original flight model, or hovering code. As result - helicopter lift too small at surface and fine at ceiling, or fine at surface and too strong above ceiling. Total headache.
BANITA, I can confirm game crash because of texture unload. It is not related with changes of this topic, but with Havoc instruments fix.