Definition
The airspeed at which an aircraft in a power-off glide is in a steady, balanced descent — lift, weight, thrust (zero), and drag are in equilibrium, producing a constant airspeed and a constant rate of descent along a fixed glide path angle.
Plain English
The single airspeed at which a gliding aircraft settles into a steady descent that doesn’t speed up, slow down, or change angle. Hold this speed and the airplane glides smoothly at a constant rate.
Context Anchor
Seen when studying descents and glides, especially how an airplane behaves after power is reduced or removed.
Derivation
‘Equilibrium’ comes from the Latin aequilibrium, from aequus (equal) and libra (balance, scales). It describes a state where opposing forces balance out — here, the forces acting on the gliding aircraft are in balance, so the speed and descent angle stay steady.
Why Pilots Care
It is the speed that gives maximum glide distance after an engine failure.
Analogy
It is like coasting a bicycle down a hill. At first it speeds up, but if the hill and wind resistance stay the same, it can settle at a steady speed.
Grounding Statement
In a glide, the airplane accelerates only until the downhill pull of weight is matched by air resistance.
Intuition Check
Do not assume equilibrium glide speed means “best glide speed.” It means the speed where the forces are balanced for that particular glide condition.
Example Sentence 1
After the engine quit, he pitched for the equilibrium glide speed and trimmed the airplane to hold it hands-off while he searched for a landing spot.
Example Sentence 2
Maintaining equilibrium glide speed allowed the aircraft to cover the greatest distance over the ground during the descent.