The tendency of an airplane to pitch up and down over time when you disturb the stick. Phugoids come in three flavors:
- Stable (Tending to stay at a fixed amplitude.)
- Unstable (Tending to get larger and larger as time goes on. Eventually resulting in a loss of control if the pilot doesn't intervene.)
- Damped (Tending towards smooth horizontal flight on its own.)
Passenger planes should be designed with stable or damped phugoids. This makes for a smooth ride. Fighters will often have unstable phugoids. This allows the pilot to change their attitude quite rapidly without the plane trying to seek straight and level.