FAA Ground School Practice Test

What determines the longitudinal stability of an airplane?

1. The shape of the wing

2. The location of the CG with respect to the center of lift

3. The weight of the aircraft

4. The size of the elevators

The correct answer is: The location of the CG with respect to the center of lift

The longitudinal stability of an airplane is primarily determined by the location of the center of gravity (CG) with respect to the center of lift. When the CG is ahead of the center of lift, the aircraft tends to pitch down, leading to a natural stability that helps maintain a level flight attitude. Conversely, if the CG is located behind the center of lift, the airplane can become unstable, resulting in a tendency to pitch up or down without pilot input, making it more difficult to control. This relationship ensures that when the airplane experiences perturbations, such as turbulence or changes in thrust, the stability and control mechanisms of the aircraft have a greater chance of returning the aircraft to its desired flight path. Therefore, the positioning of the CG is crucial for achieving a balance that facilitates manageable handling characteristics throughout the flight envelope.