IAS vs CAS vs TAS
- IAS — What your airspeed indicator reads. Affected by instrument and position error.
- CAS — IAS corrected for instrument and position error. For most GA aircraft, CAS is approximately equal to IAS.
- TAS — CAS corrected for non-standard temperature and pressure. TAS increases roughly 2% per 1,000 ft of altitude.
Understanding IAS, CAS, and TAS
Your airspeed indicator measures dynamic pressure — the difference between pitot (total) pressure and static pressure. That reading is Indicated Airspeed (IAS). It is what matters for aerodynamics: stall speed, maneuvering speed, and flap limits are all referenced to IAS.
Calibrated Airspeed (CAS) corrects IAS for instrument and position error. For most GA aircraft at typical speeds, CAS is nearly identical to IAS — the correction is usually under 2 knots.
True Airspeed (TAS) corrects CAS for non-standard air density. As you climb, air density decreases, so TAS increases relative to CAS — roughly 2% per 1,000 feet. TAS is what matters for flight planning, ground speed calculations, and performance charts.