Inlet air density is determined by which pair of variables?

• A. Altitude of the aircraft, Ambient temperature
• B. Humidity and wind speed
• C. Engine RPM and fuel flow
• D. Airspeed and pressure

Answer: (A)

Air density entering the engine is set by the ambient conditions through the ideal gas law, so it hinges on pressure and temperature in the free stream. The most influential factors are altitude, which governs the ambient static pressure (and how temperature varies with height), and the actual ambient temperature. Warmer air is less dense, and lower pressure at higher altitudes also reduces density. Humidity can tweak density slightly, since moist air is a bit less dense than dry air at the same pressure and temperature, but its effect is small for typical engine calculations. Wind speed, engine RPM, or fuel flow don’t change the intrinsic density of the air themselves; they affect flow or operation, not the density property of the inlet air. Therefore, the pair that determines inlet air density is altitude and ambient temperature.