Airspeed Calculator — Formula
## Indicated vs True Airspeed
Indicated airspeed (IAS) is what the pitot-static system reads, calibrated for sea-level standard density. As altitude increases and air density decreases, the true airspeed (TAS) becomes higher than the indicated reading.
### Simplified Relationship
**TAS = IAS / sqrt(sigma)**
where sigma is the density ratio (local density divided by sea-level standard density). This approximation is valid for low subsonic speeds where compressibility effects are negligible.
Indicated airspeed (IAS) is what the pitot-static system reads, calibrated for sea-level standard density. As altitude increases and air density decreases, the true airspeed (TAS) becomes higher than the indicated reading.
### Simplified Relationship
**TAS = IAS / sqrt(sigma)**
where sigma is the density ratio (local density divided by sea-level standard density). This approximation is valid for low subsonic speeds where compressibility effects are negligible.
Esempio Risolto
An aircraft reads 250 KIAS at 10,000 ft pressure altitude with OAT of -5°C.
- ISA temperature at 10,000 ft = 15 - 1.981 × 10 = -4.81°C
- Temperature ratio component = 288.15 / 268.15 = 1.0746
- Pressure ratio component = (1 - 0.000006875 × 10000)^4.2561 = 0.6877
- Density ratio sigma = 1.0746 × 0.6877 = 0.7392
- TAS = 250 / sqrt(0.7392) = 250 / 0.8598 = 290.8 knots