Private Pilot Private Exam

When are the four forces that act on an airplane in equilibrium?

1. During takeoff.

2. During unaccelerated flight.

3. During high-speed flight.

4. During descent.

The correct answer is: During unaccelerated flight.

The four forces acting on an airplane—lift, weight, thrust, and drag—are in equilibrium during unaccelerated flight. This state occurs when the forces are balanced, meaning that the lift generated by the wings is equal to the weight of the aircraft, and the thrust produced by the engines is equal to the drag acting against the aircraft’s forward motion. In this condition, the airplane maintains a constant altitude and speed, demonstrating stable and level flight without any acceleration. During takeoff, the forces are not in equilibrium because the aircraft is increasing its speed to generate sufficient lift to become airborne, resulting in an imbalance between thrust and drag and between lift and weight until it takes off. In high-speed flight, the airplane is often experiencing exceeding lifting surfaces' limits, which can affect stability and do not maintain equilibrium. Similarly, during descent, while the airplane might be losing altitude, the forces may not be balanced since thrust and drag may also vary according to the descent rate and configuration of the aircraft. Thus, unaccelerated flight is the only time when all four forces are balanced and in equilibrium.