Private Pilot License (PPL) Aerodynamics Practice Exam

When are the four forces acting on an airplane in equilibrium?

1. During takeoff

2. During unaccelerated flight

3. During descent

4. During landing

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 lift generated by the wings matches the weight of the airplane, and the thrust produced by the engines equals the drag opposing the flight. In this scenario, there is no acceleration in any direction, meaning the airplane can maintain a steady altitude and velocity. During takeoff, the aircraft is accelerating down the runway, so thrust exceeds drag to overcome inertia, and lift is increasing to counteract weight. Similarly, during descent, if the aircraft is in a controlled glide, the forces may not be balanced because the aircraft is intentionally descending, and the pilot typically has to manage the relationship between these forces. When landing, the forces are also not in equilibrium as the airplane decelerates and descends, necessitating changes in thrust and lift. In contrast, unaccelerated flight represents a balanced condition where the aircraft's movement remains stable without changes in speed or altitude, confirming that the correct scenario for force equilibrium is during unaccelerated flight.