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Different Airspeeds

The airspeed indicator in our aircraft really only measures pressure – not speed. Air flows into the front of the pitot tube and the faster we are flying, the higher the pressure measured. However, measuring speed in this way can lead to some errors so we have a few different airspeeds to consider:

Indicated Airspeed (IAS)

Indicated airspeed (IAS) is the speed shown on the airspeed indicator. It is a pressure measurement that comes from comparing the airflow going into the pitot tube with the outside static pressure.

Calibrated Airspeed (CAS)

The indicated airspeed suffers from:

Instrument Error
Minor inaccuracies due to the construction of the airspeed indicator and friction
Position Error
Errors when the airflow around the pitot tube is disturbed, usually due to the positioning of the pitot tube or flap & landing gear extension

When IAS is corrected for both instrument & position errors, we get Calibrated Airspeed (CAS).

A high angle of attack can prevent the smooth flow of air into the pitot tube and cause position errors

True Airspeed (TAS)

The airspeed indicator (ASI) is calibrated for the International Standard Atmosphere (ISA) density. Any changes in temperature or pressure will cause a change in density, which leads to density errors.

Since density reduces at higher altitude, the airflow going into the pitot tube will have a lower pressure. The lower pressure causes the airspeed indicator to display a slower speed.

When CAS is corrected for these density errors, we get True Airspeed (TAS).

TAS is the actual speed the aircraft is flying through the air. In fact, TAS is the only speed we talk about, the rest of the airspeeds are all pressure measurements.

Indicated Airspeed (IAS) is the airspeed read directly from the airspeed indicator (ASI)

Calibrated Airspeed (CAS) is the indicated airspeed corrected for instrument & position errors

True Airspeed (TAS) is the calibrated airspeed corrected for density errors