FAA Ground School Practice Test

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

1. During takeoff

2. During unaccelerated flight

3. During landing

4. During climbing flight

The correct answer is: During unaccelerated flight

The four forces that act on an airplane—thrust, drag, lift, and weight—are in equilibrium during unaccelerated flight. This state of equilibrium occurs when the lift generated by the wings equals the weight of the airplane, and the thrust produced by the engines equals the drag acting against the aircraft. In unaccelerated flight, the airplane maintains a constant altitude and velocity, indicating a balance among these forces. For instance, if the airplane is cruising at a steady speed without climbing or descending, the conditions are met for dynamic stability. Understanding this equilibrium is crucial for pilots, as it allows for smoother and more controlled flight operations. During takeoff, landing, or climbing flight, the forces are typically not in equilibrium. Takeoff and landing involve significant changes in lift and thrust as the aircraft accelerates or decelerates, and climbing flight generally requires a greater lift to overcome weight while thrust typically exceeds drag to enable ascent. Therefore, it is during unaccelerated flight that the forces are balanced and in equilibrium.