When altitude or air temperature increase the density of air decreases and so true airspeed increases. This is because there is less air to put up resistance against the aircraft moving forward so the aircraft moves faster through the air. ... Pilots also use true airspeed when performing dead reckoning or pilotage.

Why does true airspeed increase with altitude?

For a given power setting, True Airspeed increases with altitude because there is less drag due to the air being less dense. Aircraft are more efficient at high altitude because of this simple fact.

How true airspeed varies due to altitude and temperature?

How Much Does True Aispeed Change With Altitude? On average, true airspeed increases about 2% per 1,000' of increase in altitude, but the actual change depends on temperature and pressure.