Definition
An engine air inlet duct whose internal shape can be changed in flight to control the speed and pressure of the air delivered to a turbine engine. By moving ramps, spikes, or doors inside the duct, the inlet slows supersonic air to subsonic speed before it reaches the compressor and maintains efficient airflow across a wide range of flight speeds.
Plain English
An engine air intake that can change its own shape while the aircraft is flying, so the air entering the engine is always at the right speed and pressure no matter how fast the aircraft is going.
Context Anchor
Seen in turbine-engine and high-performance aircraft discussions, especially where the engine inlet must work well over a wide speed range.
Derivation
Variable means changeable. Geometry comes from Greek roots meaning 'measurement of shape.' Together, variable-geometry means 'changeable shape.' The duct physically reshapes itself, which is exactly what the name describes.
Why Pilots Care
Proper adjustment prevents engine surge or stall and maintains thrust and fuel efficiency at both subsonic and supersonic speeds.
Analogy
It is like an adjustable vent in a car or building: the opening changes position to control how air flows through it.
Intuition Check
Variable-geometry does not mean the whole duct moves around the aircraft. It means parts of the duct change position, changing the shape or size of the path the air follows.
Example Sentence 1
The aircraft's variable-geometry air inlet duct adjusts automatically as airspeed changes from subsonic to supersonic flight.
Example Sentence 2
Maintenance confirmed the variable-geometry air inlet duct was operating correctly before the next supersonic mission.