Changes

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.

Most of the times the instrument designers are not completely stupid, so the constant density hard coded into the ASI is not a random number. It is actually the average density of air at sea level, which the airfields and airports are usually not far away from. Therefore, at low levels where the density is quite close to the sea level density, the ASI is reasonably accurate. The problem most commonly arises when aeroplanes are flown at high altitudes: as we have discussed in the atmosphere section, the density of the air decreases with altitude. Therefore, the higher the altitude, the larger the difference between the IAS and the TAS. For example, a jet airliner that operates at FL360 can read an IAS of 280kts, but the TAS is actually around 450kts. For flying in wave lift which can take a glider to high altitudes, this is an important point to understand: you will travel faster than the ASI tells you.