Definition
Flight at speeds in the range where airflow over the aircraft is partly subsonic and partly supersonic, typically Mach 0.75 to Mach 1.20. In this range, shock waves begin forming on parts of the airframe even though the aircraft itself is still flying below the speed of sound.
Plain English
The speed range close to the speed of sound, where some air flowing over the aircraft is moving slower than sound and some is moving faster. It is the messy middle zone between normal flight and full supersonic flight.
Context Anchor
Seen in high-speed aerodynamics, wing design, swept-wing aircraft, and discussions of drag and control changes near the speed of sound.
Derivation
From Latin trans- meaning 'across' or 'through,' combined with 'sonic' from sonus meaning 'sound.' The word literally means 'crossing through sound' — the speed range where the aircraft is transitioning across the sound barrier rather than fully below or above it.
Why Pilots Care
This regime produces rapid drag rise, buffeting, and altered control response that must be managed for safe operation.
Grounding Statement
Picture an aircraft flying so fast that air over one part of the wing is still below sound speed while air over another part has already reached sound speed.
Intuition Check
Transonic does not mean only exactly at the speed of sound. It means the transition range near the speed of sound, where mixed airflow conditions can exist around the aircraft.
Example Sentence 1
Swept wings were developed largely to delay the drag rise that occurs during transonic flight.
Example Sentence 2
Design teams evaluate transonic flight behavior to ensure the aircraft remains controllable during the transition to supersonic speeds.