Which factors primarily cause TAS and IAS to differ?

• A. Altitude, temperature, and air density
• B. Wind direction
• C. Time of day
• D. Aircraft color

Answer: (A)

The difference between TAS and IAS comes from how each speed is defined in relation to air density. IAS is basically a measure of dynamic pressure the air presents to the aircraft, calibrated for standard sea‑level density. TAS is the actual speed through the air mass. When air density changes, the same dynamic pressure corresponds to different true speeds.

Altitude lowers air density, so at a higher altitude you must move faster through the thinner air to produce the same dynamic pressure, making TAS exceed IAS. Temperature also affects density: warmer air is less dense than cooler air, which similarly raises TAS for a given IAS. In cooler, denser air the gap shrinks, bringing TAS closer to IAS.

Choices like wind direction, time of day, or aircraft color don’t set this relationship; wind affects ground speed, not the dynamic pressure reading, and the other factors don’t directly alter how IAS and TAS relate. So the primary factors are altitude, temperature, and air density.