Phugoid

A phugoid is a movement phenomenon that describes a short-period oscillation. In the context of aviation, a phugoid refers to the repetitive changes in altitude, speed, and pitch that an aircraft undergoes when the throttle is changed.

Consider an aircraft flying in trimmed, stable, flat flight. If the throttle is reduced, the airspeed will drop, resulting in reduced lift from the wings. The aircrafts nose will tend to drop. This will cause the aircraft to decrease altitude and increase airspeed. Increased airspeed promotes increased lift from the wings, causing the nose to rise again. The aircraft will increase altitude while it decreases airspeed. This process may repeat one or more times, but generally reduces in severity with each repetition.

Subjectively, this phenomenon can be counterintuitive and surprising to new pilots. Practically, changes in throttle are accompanied by elevator inputs to quickly subdue the phugoid.

For a dramatic example of phugoids, read about the United Airlines Flight 232 incident, where an engine failure caused total hydraulic system failure. The crew flew the aircraft with throttle only. Suppressing the phugoid tendency was particularly difficult.