Airspeed indicator (ASI)

a) ASI is a differential pressure gauge that measures dynamic pressure, the difference between static and total, or ram, pressure caused by movement of the aircraft through the air

b) Corrugated phospor-bronze aneroid, or diaphragm, receives ram pressure from the pitot tube, while the sealed instrument case receives static pressure

c) Indicated airspeed (IAS) is the uncorrected airspeed shown on the ASI dial

d) Calibrated airspeed (CAS) is IAC corrected for instrument and position errors; POH provides CAS for IAS with various flap and landing gear configurations

e) Equivalent air speed (EAS) is CAS corrected for compression of air in the pitot tube (same as CAS in standard atmosphere at sea level; less than CAS at higher altitudes and faster airspeeds)

f) True airspeed (TAS) is CAS corrected for nonstandard temperature and pressure

i) TAS = CAS in standard atmosphere at sea level
ii) Some true airspeed indicators have temperature-compensated aneroid bellows
iii) Others have a knob requiring pilot input to align outside air temperature (OAT) with pressure altitude so that TAS is then indicated