by Jan Zumwalt
rue airspeed (TAS; also KTAS, for knots true airspeed) of an aircraft is the
speed of the aircraft relative to the airmass in which it is flying. The true
airspeed is necessary for accurate navigation of an aircraft. TAS is the true
measure of aircraft performance in cruise, thus listed in aircraft specs,
manuals, performance comparisons, pilot reports, and every situation when
actual performance needs to be measured. It is the speed normally listed on
the flight plan, also used in flight planning, before considering the effects
At sea level in the International Standard Atmosphere (ISA) and at low speeds
where air compressibility is negligible, IAS corresponds to 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 ASI will indicate less than TAS when the
air density decreases due to a change in altitude or air temperature.
Above approximately 100 knots, the compressibility of air introduces error
and begins to differ significantly from TAS. To overcome this Mach speed may
be used. Mach incorporates the above data including the compressibility
factor. Large or high performance aircraft use a Air Data Computer to perform
this calculation in real time and display the TAS reading directly on the
EFIS (electrionic flight information computer).
Since temperature variations are of a smaller influence, the ASI error can
be roughly estimated as indicating about 2% less than TAS per 1,000ft of
altitude above sea level. For example, an aircraft flying at 15,000ft in the
international standard atmosphere with an IAS of 100kt, is actually flying
at 126kt TAS.
Note: standard pressure is 29.92126, if you're trying to test for zero.