Definition
Airflow in which the air is moving slower than the speed of sound at every point around the aircraft. In aerodynamics, flow is generally treated as subsonic when the local airspeed stays below Mach 1, and the air behaves as if it is essentially incompressible at the speeds typical of light aircraft.
Plain English
The air flowing over the aircraft is moving slower than the speed of sound everywhere around it. This is the normal flow regime for almost all general aviation flying.
Context Anchor
Seen in aerodynamics discussions when comparing normal low-speed flight with flight near or above the speed of sound.
Derivation
‘Sub-’ comes from Latin meaning ‘below’ or ‘under.’ ‘Sonic’ comes from Latin sonus, meaning ‘sound.’ Together: ‘below the speed of sound.’ Knowing the root makes the related terms (transonic, supersonic) easier to keep straight.
Why Pilots Care
It determines how lift and drag behave on the wing during everyday flight without supersonic complications.
Grounding Statement
If the air moving over the aircraft is slower than sound can move through that same air, the flow is subsonic.
Intuition Check
Subsonic does not mean “slow” in an everyday sense. It means slower than the local speed of sound, even if the aircraft is still moving very fast.
Example Sentence 1
Light training aircraft operate entirely in subsonic flow, so pilots don’t have to worry about shock waves forming on the wing.
Example Sentence 2
The pilot maintains subsonic flow by keeping indicated airspeed well below the critical Mach number.