I altered the air density because the helo's couldn't hover. I found that someone had gone in and divided the air density by 1.3 for all of the helo's so I corrected it back to 1.0. 30% lower and they couldn't hover, the air density equated to about 20,000 ft altitude. If the change I made increases the ultimate ceiling greater than it should be, then I would guess the 'get_air_density' command isn't set up correctly.

It's located in \aphavoc\source\misc\airspeed.c

float get_air_density (float altitude)
{
float
m = 0.0289644, // physics constants
r = 8.31447,
l = 0.0065,
p0 = 101325, // standard air pressure, Pa
t0 = 288.15, // standard temperature, K
t = t0 - l * altitude, // actual temperature, K
p = p0 * exp(- (G * m * altitude) / (r * t0)), // air pressure, Pa
air_density = p * m / (r * t); // kg/m^3

return air_density;
}