How does density altitude affect true airspeed?

• A. TAS increases with density altitude for the same IAS due to thinner air.
• B. TAS decreases with density altitude for the same IAS.
• C. TAS remains constant regardless of density altitude.
• D. TAS becomes unpredictable at high density altitude.

Answer: (B)

Density altitude describes thinner air. True airspeed is how fast the airplane is moving through that air mass. When the air is less dense, the same indicated airspeed corresponds to a smaller dynamic pressure unless the true speed is higher. Since dynamic pressure q equals 1/2 ρ V^2, lowering the density ρ means you must increase V to keep q the same. So, as density altitude rises, true airspeed increases for a given IAS. A handy way to think about it is that a drop in density by a factor requires a square-root increase in TAS to maintain the same dynamic pressure; the result is that you’re moving faster through thinner air than your instruments indicate.