FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightMedium
A CFI is briefing a client on aircraft stability concepts. The client asks about an airplane that, after being disturbed from pitch equilibrium, initially returns toward the trimmed attitude but overshoots and continues oscillating with increasing amplitude over several minutes. This airplane exhibits:
- Anegative static stability and positive dynamic stability
- Bpositive static stability and negative dynamic stability
- Cpositive static stability and positive dynamic stability
- Dneutral static stability and neutral dynamic stability
Show answer & explanationAnswer & explanation
Correct answer: B. positive static stability and negative dynamic stability
Static stability describes the initial tendency after a disturbance; because the airplane initially moves back toward equilibrium, static stability is positive. Dynamic stability describes the oscillation over time; because the oscillations grow larger instead of damping out, dynamic stability is negative.
Why the other options are wrong
- A. Negative static stability would mean the airplane initially moves further away from trim, which contradicts the scenario.
- C. Positive dynamic stability requires oscillations to decrease over time, which is not the case here.
- D. Neutral stability implies neither returning to nor diverging from equilibrium, which does not match the increasing oscillations.
Static vs Dynamic Stability
Static stability is the initial tendency of an aircraft to return to equilibrium after a disturbance; dynamic stability describes whether the resulting oscillations increase, decrease, or remain constant over time.
- Positive static stability: initial tendency to return to equilibrium
- Positive dynamic stability: oscillations decrease over time (damped)
- Negative dynamic stability: oscillations increase over time (divergent)
- An airplane can have positive static stability but negative dynamic stability
Memory trick: 'Static = first move, Dynamic = the whole dance'