Under ISA conditions, descending above the tropopause at constant Mach and aircraft mass, the lift coefficient will:

• A. Increase
• B. Remain constant
• C. First increase then decrease
• D. Decrease

Answer: (D)

The main idea is that lift is proportional to dynamic pressure times wing area and lift coefficient: L = q S CL, and for a given weight L must equal W. With constant Mach, the dynamic pressure at a given altitude is q = 0.5 ρ V^2, and V scales with the speed of sound a (V = M a). Since a^2 is proportional to temperature, q for constant Mach becomes proportional to ρ T (q ∝ ρ T).

Descending from well above the tropopause toward lower altitudes, air density rises very quickly while the temperature falls. The density increase dominates the change in V, so the dynamic pressure q increases. To keep the lift equal to weight with the same wing area, the lift coefficient must drop. Therefore, the lift coefficient decreases as you descend.

In short: as you descend at constant Mach and mass, dynamic pressure increases due to higher density, so CL must decrease to maintain the same lift.