FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
After being displaced from trimmed level flight by a gust, an airplane exhibits a slow, long-period oscillation in pitch attitude and airspeed that gradually decreases in amplitude until the airplane returns to its trimmed condition. This behavior demonstrates which characteristic?
- ANegative static stability
- BPositive dynamic stability
- CNeutral static stability
- DNegative dynamic stability
Show answer & explanationAnswer & explanation
Correct answer: B. Positive dynamic stability
A long-period, gradually damping oscillation (the phugoid) after a disturbance is the classic sign of positive dynamic stability: the airplane's corrective tendencies eventually damp out the oscillation and return it to equilibrium over time.
Why the other options are wrong
- A. Negative static stability would mean the airplane continues to diverge from equilibrium immediately after displacement, not oscillate back.
- C. Neutral static stability would produce oscillations of constant amplitude that never decrease.
- D. Negative dynamic stability would show oscillations that increase in amplitude over time, not decrease.
Dynamic Stability (Phugoid)
The tendency of an aircraft's oscillations following a disturbance to increase, decrease, or remain constant over time.
- Positive dynamic stability = oscillations dampen out
- Negative dynamic stability = oscillations grow (divergent)
- Neutral dynamic stability = oscillations stay constant
- Phugoid is a slow pitch/airspeed oscillation trading altitude for airspeed
Memory trick: Static asks 'does it try to return?' Dynamic asks 'does it actually settle down over time?'