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) 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.
The indicated airspeed suffers from:
When IAS is corrected for both instrument & position errors, we get Calibrated Airspeed (CAS).
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.