Which statement about lift at higher altitude is true?

Which statement about lift at higher altitude is true?

• A. Lift does not depend on altitude.
• B. A higher altitude requires a higher angle of attack to produce the same lift.
• C. To generate the same lift at higher altitude, fly at a higher true airspeed for any given angle of attack.
• D. Lift always increases with altitude.

Answer: (C)

Lift is driven by dynamic pressure, which is proportional to the air density times velocity squared: L ~ 0.5 * rho * V^2 * S * Cl. As you climb, air density rho decreases. If you keep the angle of attack constant, Cl stays about the same for that AOA, so the reduced rho lowers lift. To keep producing the same lift at higher altitude, you must increase the true airspeed to raise the dynamic pressure enough to offset the thinner air. That’s why the statement about needing a higher true airspeed for the same lift at higher altitude is the best choice. Lift isn’t independent of altitude, so that option isn’t correct. You can compensate for thinner air by increasing speed; it isn’t universally needed to increase angle of attack to maintain lift. And lift does not always increase with altitude—the density drop generally reduces lift unless speed or angle of attack is increased.

Lift is driven by dynamic pressure, which is proportional to the air density times velocity squared: L ~ 0.5 * rho * V^2 * S * Cl. As you climb, air density rho decreases. If you keep the angle of attack constant, Cl stays about the same for that AOA, so the reduced rho lowers lift. To keep producing the same lift at higher altitude, you must increase the true airspeed to raise the dynamic pressure enough to offset the thinner air. That’s why the statement about needing a higher true airspeed for the same lift at higher altitude is the best choice.

Lift isn’t independent of altitude, so that option isn’t correct. You can compensate for thinner air by increasing speed; it isn’t universally needed to increase angle of attack to maintain lift. And lift does not always increase with altitude—the density drop generally reduces lift unless speed or angle of attack is increased.