Indicated airspeed (IAS) - compressible fluid


Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator (ASI) on an aircraft, driven by the pitot-static system. It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.

Indicated airspeed measured by pitot-tube can be expressed by following equation delivered from Bernoulli’s equation. The compression effects can be corrected by use of Poisson constant. This compensation corresponds to Equivalent airspeed and expressed as shown.

Related formulas


uIndicated airspeed (m/s)
γPoisson constant (1.4 for air) (dimensionless)
psstatic pressure (pa)
ρfluid density (kg/m3)
ptstagnation or total pressure (pa)