How does indicated airspeed relate to true airspeed as altitude increases?

• A. For a given IAS, TAS increases with altitude due to lower air density; performance charts use IAS as the reference for lift and drag.
• B. For a given IAS, TAS decreases with altitude due to thinner air.
• C. IAS equals TAS.
• D. TAS is independent of altitude.

Answer: (A)

Indicated airspeed is a measure tied to dynamic pressure in the airflow, which is proportional to 0.5 times air density (rho) times the true airspeed squared (V^2). True airspeed is the actual speed of the airplane through the air. As you climb, air density decreases. To keep the same dynamic pressure—and thus the same indicated airspeed—the aircraft must move faster through the thinner air. Therefore, for a given IAS, true airspeed increases with altitude. This is also why performance charts are referenced to IAS: lift and drag depend on dynamic pressure, not on TAS directly, making IAS a consistent reference across different densities. The other statements don’t match how dynamic pressure and density interact, so TAS does not decrease with altitude for a fixed IAS, IAS does not equal TAS, and TAS is not independent of altitude.