Definition
A long-period, lightly damped pitch oscillation in which an aircraft slowly trades airspeed for altitude and altitude for airspeed at nearly constant angle of attack. The nose rises and falls gradually over many seconds while the aircraft climbs slightly, slows, then descends and speeds up, repeating in a smooth wave-like motion.
Plain English
A slow, gentle up-and-down motion where the airplane keeps trading speed for height and back again, climbing a little as it slows down and descending as it speeds up. The cycle is long — usually 30 seconds or more — and the pilot has time to correct it.
Context Anchor
Seen in aircraft stability discussions, especially when learning how an airplane behaves after a disturbance such as a gust or a control input.
Derivation
From the Greek 'phyge,' meaning 'flight' or 'fleeing.' The term was coined by the British aerodynamicist F.W. Lanchester in the early 1900s to describe this slow flying motion. Knowing the root helps you remember it refers to the aircraft's gradual flight path, not a rapid pitch wobble.
Why Pilots Care
Excessive phugoid motion can reduce passenger comfort, mask other stability issues, and require gentle pilot corrections to dampen.
Analogy
It is like a ball rolling gently over a series of smooth hills: it slows as it goes up, then speeds up as it comes down. In the airplane, the “hill” is the changing flight path through the air.
Grounding Statement
Picture trimming the aircraft, letting go of the controls, and watching the nose slowly rise as speed bleeds off, then drop as speed builds back up — over and over, like a gentle long wave.
Intuition Check
Do not think of phugoid oscillations as quick shaking or flutter. They are usually slow changes in speed and altitude over several seconds or longer.
Example Sentence 1
After trimming for cruise and releasing the controls, the instructor demonstrated a phugoid oscillation by letting the nose rise and fall on its own.
Example Sentence 2
The check pilot noted the aircraft’s phugoid oscillations were well damped and within normal limits for the type.