Because the ASI assumes that the air density is a constant, if we fly under any condition where the actual density of air is not the same as the value used in the ASI(\(\rho_{air} \neq \rho_{ASI}\)), the airspeed that the ASI reads will not be the '''true airspeed (TAS)'''. TAS is the airspeed of an aeroplane if examined in a kinematic sense, i.e. it will be the speed of the aeroplane you see if you remain stationary relative to the air.