True airspeed

True airspeed is the actual speed of an aircraft through the airmass in which it flys. Under zero wind conditions this is equal to the speed over the ground. Under wind conditions an estimation of the wind is used to make a windspeed vector calculation that computes an estimated ground speed from the true air speed.

Aircraft display an indicated airspeed on an instrument called an airspeed indicator. Indicated airspeed will differ from true airspeed at air densities other than some reference density. Air density is affected by temperature, moisture content, and altitude. Indicated airspeed is usefull in aircraft operation as the aircraft will always stall at the same indicated airspeed, regardless of its true airspeed, and below the speed of sound, maximum structural speeds are determined by indicated airspeed. Proper navigation via dead reckoning (without constant ground reference) requires the use of true airspeed and wind corrections.