If the altitude increases, what happens to true airspeed (TAS) if air density decreases?

As altitude increases, the air density decreases, leading to a reduction in the mass of air molecules present in any given volume of air. True airspeed (TAS) is the actual speed of the aircraft relative to the air mass through which it is flying. While the indicated airspeed (IAS) is affected by air density, TAS is related to how an aircraft performs in the thinner air found at higher altitudes.

In thinner air, an aircraft can achieve a higher TAS for a given power setting, as there is less drag from the surrounding air. The engines of an aircraft produce the same thrust regardless of altitude, but the aircraft can fly faster because of lower aerodynamic drag in the less dense air. Consequently, with an increase in altitude and a decrease in air density, the true airspeed increases.

This relationship demonstrates the importance of understanding how altitude affects performance metrics like TAS, which is critical for flight planning and understanding aircraft capabilities at different flight levels.