We present the common conventions for qualifying airspeed:
- indicated airspeed = IAS
- calibrated airspeed = CAS
- equivalent airspeed EAS
- true airspeed = TAS
- ground speed = GS
Indicated airspeed – IAS
The pitot-static system comprises one or more pitot probes facing the in-coming air flow to measure pitot pressure (also called stagnation, total or ram pressure) and one or more static ports to measure the static pressure in the air flow.
The airspeed is derived from the difference between the ram air pressure from the pitot tube, or stagnation pressure, and the static pressure. An airspeed indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure.
The static pressure measurement is subject to error due to the inability to place the static ports at positions where the pressure is true static pressure at all airspeeds and attitudes. The correction for this error is the position error correction (PEC) and varies for different aircraft and airspeeds.
Further errors of 10% or more are common if the airplane is flown in “uncoordinated” flight.
Indicated airspeed is the starting point for all other calculations.
Calibrated airspeed - CAS
During clean flight, position and instrument errors are usually small. That means that the CAS value is nearly equal to the IAS.
This really only comes into play when the flaps are down. What happens when the flaps are down? The angle of incidence of the pitot tube changes. Rather than having the pitot tube aligned perfectly with the airflow, the center of lift is moved aft (how much depends on the type of flaps) and the pitot tube pitches slightly down.
Equivalent airspeed - EAS
At standard sea level pressure, calibrated airspeed and equivalent airspeed are equal. Up to about 200 knots CAS and 10,000 feet the difference is negligible, but at higher speeds and altitudes CAS must be corrected for compressibility error to determine the EAS.
It really only comes into play with very high performance airplanes.
But for your own information, a pitot tube has a limit to how much air it can take in. If you fly faster than it can accept the air, then it will show an IAS that is less than what you would expect.
True airspeed – TAS
It is the speed listed on the flight plan, also used in flight planning, before considering the effects of wind.
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, the IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The airspeed indicator will indicate less than TAS when the air density decreases due to a change in altitude or air temperature.
For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an E6B flight calculator or its equivalent.
- For low speeds, the data required are static air temperature, pressure altitude and IAS
- Above approximately 100 knots, the compressibility error rises significantly and TAS must be calculated by the Mach speed.
Modern aircraft instruments use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS.
TAS = True airspeed
a0 = Speed of sound at standard sea level = 661.478 KT
M = Mach number
T = Temperature in Kelvin
T0 = Standard sea level temperature (288.15 Kelvins)
One approximation is when temperature is at standard value (T=T0):
Since temperature variations are of a smaller influence, the airspeed indicator error can be roughly estimated as indicating about 2% less than TAS per 1,000ft of altitude above sea level (approximation for altitudes below 12000ft):
Be aware that calculation is one of the possible approximations.
Example, an aircraft flying at 15,000ft with an IAS of 100kt:
At high speed (240KT<IAS<400KT), another approximation can be:
Ground speed – GS
GS = Ground speed Vw = Wind speed vector TAS = True airspeed
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DATE OF SUBMISSION
- 00:19, 14 May 2021
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