Which statement best describes IAS, CAS, TAS and their use in performance planning?

• A. IAS is the instrument reading corrected for position error; CAS corrects for instrument and installation errors; TAS accounts for density; performance planning primarily uses IAS (and TAS for cruise planning).
• B. IAS is the instrument reading corrected for density; CAS corrects for position error; TAS accounts for instrument errors; planning uses TAS.
• C. IAS is the speed read from the airspeed indicator; CAS and TAS are not used in planning.
• D. IAS is the velocity of the aircraft relative to the air; CAS and TAS are identical.

Answer: (A)

The concept being tested is how IAS, CAS and TAS relate to what you’re actually flying through and how they’re used in planning.

Indicated Airspeed is what the airspeed indicator shows, but it isn’t corrected for instrument or installation errors. Because of pitot-system and installation factors, IAS can be off from the airplane’s true performance.

Calibrated Airspeed takes IAS and corrects it for those instrument and installation (including position) errors. It gives a standardized speed that better represents the airspeed the airplane would display if the system were perfectly calibrated.

True Airspeed goes a step further: it takes CAS and corrects for air density variations (altitude and temperature). TAS is the actual speed of the airplane through the air, which changes with altitude and temperature even if CAS stays the same.

In performance planning, you typically use calibrated airspeed as the basis for performance charts and limits, and you convert to true airspeed for cruise planning to estimate time and distance in the actual flight conditions.