Definition
Flight at speeds below the speed of sound, where airflow over all parts of the aircraft remains slower than the local speed of sound. In practice, subsonic flight is generally considered to occur below approximately Mach 0.75, where no part of the airflow has accelerated to sonic speed.
Plain English
Flying slower than the speed of sound, with the air moving over the aircraft also staying slower than the speed of sound.
Context Anchor
Seen in aerodynamics discussions, aircraft performance descriptions, and comparisons between normal airplane flight and higher-speed flight near or above the speed of sound.
Derivation
From Latin sub- meaning 'below' and sonus meaning 'sound.' Literally 'below sound' — that is, slower than the speed at which sound travels.
Why Pilots Care
Staying subsonic keeps handling predictable and prevents the rapid drag rise and control changes that begin at critical Mach.
Grounding Statement
If the sound from the aircraft can travel ahead through the air faster than the aircraft itself is moving, the aircraft is in subsonic flight.
Intuition Check
Subsonic does not mean “slow.” It means slower than the local speed of sound, which can still be very fast.
Example Sentence 1
Most piston-engine aircraft operate entirely in subsonic flight, well below Mach 0.5.
Example Sentence 2
The pilot reduced power to keep the jet in subsonic flight while descending through the transition altitude.