Indicated vs Calibrated Airspeed (IAS vs CAS)

Indicated Airspeed (IAS) is the direct, uncorrected reading taken straight from the aircraft's airspeed indicator. It is driven by the dynamic pressure derived from the pitot-static system. Because the pitot tube and static ports are subject to aerodynamic disruptions based on the aircraft's angle of attack and flap configuration, IAS is inherently flawed.

Calibrated Airspeed (CAS) is IAS corrected for these installation and position errors. Manufacturers determine these errors during flight testing and provide a correction table in the Pilot's Operating Handbook (POH). At high speeds and clean configurations, the difference between IAS and CAS is often negligible, but at low speeds and high angles of attack (such as during landing approaches), the difference can be significant.

Understanding the difference is critical for safety and exams. Stalling speeds and flap operating speeds are technically based on CAS, even though the pilot flies referencing the IAS gauge.