Definition
Airflow around an airplane in which some regions of the flow are subsonic (slower than the speed of sound) and other regions are supersonic (faster than the speed of sound) at the same time. This mixed-flow condition typically occurs as the airplane approaches the speed of sound, when air accelerating over the wing's upper surface reaches supersonic speed even though the airplane itself is still flying below the speed of sound.
Plain English
Air flowing around the airplane is moving slower than sound in some places and faster than sound in others, all at once. It happens in the speed range where the airplane is getting close to, at, or just past the speed of sound.
Context Anchor
Seen in high-speed flight discussions, especially when studying speed margins, critical Mach number, shock waves, and airflow patterns over the wing.
Derivation
From Latin trans- meaning 'across' and sonic meaning 'of sound.' Transonic literally means 'across the sound barrier' -- the speed range where the airplane is crossing from purely subsonic flow into supersonic flow, with both types present at once.
Why Pilots Care
Produces sharp drag rise, buffet, and reduced control effectiveness that can limit safe operation near the speed of sound.
Grounding Statement
Picture air speeding up as it curves over the top of the wing -- even when the airplane is flying below the speed of sound, that accelerated air over the wing can already be supersonic, while the air under the wing is still subsonic.
Intuition Check
Transonic flow does not mean the entire airplane is flying faster than sound. It means the airflow around parts of the airplane is near the speed of sound, with slower-than-sound and faster-than-sound areas existing at the same time.
Example Sentence 1
As the jet accelerated past its critical Mach number, transonic flow developed over the wing and the crew felt the first signs of Mach buffet.
Example Sentence 2
The pilot noted the onset of transonic flow through a slight buffet before reducing thrust.