As altitude increases, the indicated airspeed at which a given airplane stalls in a particular configuration will

• A. Remain the same
• B. Increase
• C. Decrease
• D. Vary unpredictably

Answer: (A)

Stall is determined by a wing reaching its maximum lift coefficient at a specific angle of attack. For a given airplane weight and configuration, that same angle of attack occurs at the same lift, so the stall condition is basically the same AoA no matter how high you are. The indicated airspeed is a measure of dynamic pressure, which depends on both speed and air density. As altitude increases, air density drops, so the true airspeed needed to reach the same dynamic pressure increases. But the indicated airspeed is tied to that dynamic pressure as if it were at sea level density, which cancels out the density change. Put another way, TAS rises with altitude to compensate for thinner air, while IAS remains tied to the same dynamic pressure a sea-level-calibrated indicator compares to, so the stall IAS stays the same. Therefore, the indicated stall speed does not change with altitude, assuming weight and configuration are constant.