Longitudinal dynamic instability in an airplane can be identified by which of the following?

Longitudinal dynamic instability in an airplane can be identified by which of the following?

• A. Pitch oscillations becoming progressively steeper
• B. Pitch oscillations damping out quickly
• C. Bank angle returning to initial value
• D. Altitude remaining constant during pitch motion

Answer: (A)

Longitudinal dynamic stability is about how the airplane’s pitch motion behaves over time after a disturbance. If anything perturbs the nose, a dynamically stable aircraft will have that pitch motion damped out and the attitude returns toward the original state. When pitch oscillations become progressively steeper, the oscillations grow in amplitude instead of damping, showing negative damping in the pitch axis. That growing response is the hallmark of longitudinal dynamic instability. The other scenarios don’t indicate instability: rapid damping means positive dynamic stability; a return of bank angle to its initial value concerns roll stability, not pitch; and altitude staying constant during pitch motion doesn’t describe the time-domain behavior that defines dynamic stability.

Longitudinal dynamic stability is about how the airplane’s pitch motion behaves over time after a disturbance. If anything perturbs the nose, a dynamically stable aircraft will have that pitch motion damped out and the attitude returns toward the original state. When pitch oscillations become progressively steeper, the oscillations grow in amplitude instead of damping, showing negative damping in the pitch axis. That growing response is the hallmark of longitudinal dynamic instability.

The other scenarios don’t indicate instability: rapid damping means positive dynamic stability; a return of bank angle to its initial value concerns roll stability, not pitch; and altitude staying constant during pitch motion doesn’t describe the time-domain behavior that defines dynamic stability.