In unaccelerated flight, when are the four forces acting on an airplane in equilibrium?

• A. During takeoff
• B. During accelerated flight
• C. During unaccelerated flight
• D. During steady descent

Answer: (C)

Equilibrium of the four forces means zero net force on the airplane, so no acceleration. In unaccelerated flight the aircraft flies at constant speed and maintains its altitude and direction, which requires lift to balance weight and thrust to balance drag. With these pairs balanced, the vector sum of all forces is zero, so the airplane’s velocity doesn’t change. Takeoff, climb, or any accelerated flight involves a net force and a change in speed or altitude, while a descent generally has lift less than weight (or other force imbalances) resulting in acceleration as the aircraft moves downward.