At sea level in the International Standard Atmosphere (ISA), and at slow speeds where air compressibility is negligible, IAS corresponds to True Air Speed (TAS). When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance.

The Air Speed Indicator (ASI) will indicate less than TAS when the air density decreases due to increase in altitude or temperature. For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using a flight calculator (E6B also known as a Dalton Computer, or its equivalent).

For slow speeds, the data required are static air temperature, pressure altitude and IAS (or CAS for more precision). Above approximately 100 knots and with altitude increase, the density error rises significantly and, therefore, at higher speeds and altitudes (typically above FL250) TAS differs quite significantly from IAS. For speeds where TAS is above 300 kts, the airspeed is referenced to the Mach number. Mach incorporates the above data including the compressibility factor.

Since TAS increases as altitude increases with constant IAS, a very simple rule of thumb is to add 2% to the IAS for every 1 000 ft of altitude:

• 2% x 8000 ft/1000 ft = 16% increase in TAS
• 16% x 150 kt = 24 kt increase in TAS
• 150 kt (IAS) + 24 kt = 174 kt (TAS)