It's been a long time since density altitude appeared in JSBSim, so I've replaced it with a system called "ADC", which stands for Air Data Computer. So far all the ADC does is take air density (rho) as an input and looks up a corresponding altitude, resulting in a density altitude figure. This value is then used by the turbine engine thrust lookup tables. This allows the turbine engine output to be effected by non-standard air temperature.
Airplanes modified so far are the F-105, A-3D, A-7E and F-4C.