Definition
A long-period, lightly damped oscillation in which an aircraft flying with fixed elevator trades altitude for airspeed and back again, climbing and slowing, then descending and accelerating, in a slow porpoising motion. Angle of attack remains nearly constant throughout the cycle; pitch attitude and airspeed vary together as the aircraft exchanges potential and kinetic energy.
Plain English
A slow, gentle up-and-down motion the aircraft makes on its own, where it climbs while losing speed, then dives while gaining speed, repeating over and over before settling out.
Context Anchor
Encountered in aircraft stability discussions, flight training after pitch or trim changes, and explanations of how an airplane returns toward steady flight after being disturbed.
Derivation
From the Greek 'phyge,' meaning flight or fleeing. The term was coined by British aerodynamicist F. W. Lanchester in the early 1900s to describe this flight-path oscillation. The Greek root reminds you the motion is about the path of flight itself, not the aircraft's rotation about its own axes.
Why Pilots Care
Recognizing this mode helps pilots understand why an aircraft may slowly oscillate in speed and altitude after a disturbance and how to damp the motion if needed.
Analogy
Like a marble rolling back and forth in a shallow bowl: it speeds up at the bottom, slows at the top, and gradually settles in the middle.
Grounding Statement
Picture the nose rising slightly, the airplane slowing and climbing, then the nose lowering as the airplane speeds up and descends, with the cycle repeating slowly.
Intuition Check
Do not think of phugoid oscillation as a rapid vibration or stall warning. It is a slow pitching and speed-changing cycle, usually spread over several seconds or more.
Example Sentence 1
After trimming for cruise and releasing the controls, the instructor pointed out the gentle phugoid oscillation as the aircraft slowly climbed, slowed, then descended and sped up again.
Example Sentence 2
The pilot applied gentle forward pressure at the right moment to damp the phugoid and restore steady cruise flight.