Other Airspeeds
From IAS, the following speeds can also be calculated:
- convert CAS to equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude); EAS is used by aircraft engineers and some very high-altitude flying aircraft such as the U-2 and the SR-71;
- convert EAS to true airspeed (TAS) by allowing for differences in density altitude.
On large jet aircraft the IAS is by far the most important speed indicator, which is used at take-offs, approaches and landings. The TAS is not imperative on modern aircraft and does not start showing anything until the KTAS is 200 knots. Though, at cruise altitude and cruise speed the TAS indicator is best instrument to use, since the IAS indicator doesn't show correct KIAS (above approx. 280-300 knots above 20.000 feet the IAS indicates too low KIAS speed)
Modern jet airliners also include ground speed (GS) and MACH indicator. Ground speed shows the actual speed that the aircraft uses compared to the ground. This is usually connected to a GPS or similar system. Ground speed is just a pilot aid to estimate if the flight is on time, behind schedule or after schedule. It is not used for start and landing purposes, since the imperative speed for a flying aircraft always is the speed against the wind.
The MACH-meter is, on subsonic aircraft, a warning indicator. Subsonic aircraft must not fly faster than a specific percentage of the speed of sound (at present conditions, which varies by altitude and air density). Usually passenger airliners do not fly faster than around 85% of speed of sound, or MACH 0.85. On supersonic aircraft, like the Concorde and military fighters the MACH-meter instead is the main speed instrument with the exception of take-offs and landings.
Some aircraft also have a Taxi speed indicator for use on the ground. Since the IAS often starts at around 40-50 knots (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground. Its range is around 0 to 50 knots.
Read more about this topic: Indicated Airspeed