What happens to the true airspeed (TAS) as air density decreases?

As air density decreases, the true airspeed (TAS) increases due to less aerodynamic resistance. True airspeed is the speed of an aircraft relative to the surrounding undisturbed air and is influenced by various factors including aircraft weight, lift requirements, and drag.

When the air becomes less dense, typically due to higher altitude or higher temperatures, the aircraft experiences reduced drag because there are fewer air molecules for the aircraft to push against. This reduction in drag allows the aircraft to achieve a higher TAS for the same power output. Although the indicated airspeed (IAS) may not reflect this increase in speed accurately because it is based on dynamic pressure, the TAS reflects the actual speed of the aircraft through the air, which is ultimately higher in less dense conditions.

Thus, in lower density air, the aircraft can move more efficiently, resulting in an increase in true airspeed as compared to sea level conditions, where air density is higher and offers more resistance to the aircraft.