How does density altitude affect true airspeed?

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.

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.