Definition
The range of airspeeds, roughly Mach 0.75 to Mach 1.20, in which airflow over different parts of the aircraft is partly subsonic (below the speed of sound) and partly supersonic (above the speed of sound) at the same time. This mixed flow produces shock waves on the airframe and can cause buffeting, control difficulties, and changes in stability.
Plain English
The speed range where some air flowing over the airplane is moving slower than the speed of sound and some is moving faster than the speed of sound at the same time. It is the messy zone between fully subsonic flight and fully supersonic flight.
Context Anchor
Seen in high-speed airplane performance, jet operations, and discussions of airflow changes near the speed of sound.
Derivation
From Latin trans- meaning 'across' or 'through,' combined with 'sonic' (relating to sound). 'Transonic' literally means 'crossing through the speed of sound,' which fits this range where the aircraft is partly below and partly above it.
Why Pilots Care
Aircraft in this range experience a sharp rise in drag, reduced control effectiveness, and the need for specific handling techniques.
Grounding Statement
Picture a fast jet nearing the speed of sound: the air over one part of the wing may already be faster than sound while air elsewhere is still slower.
Intuition Check
Transonic does not mean simply “supersonic.” It means the mixed region near the speed of sound, where both slower-than-sound and faster-than-sound airflow can exist around the airplane.
Example Sentence 1
As the jet accelerated into the transonic range, the crew watched for the slight buffet that signals shock wave formation on the wings.
Example Sentence 2
High-performance aircraft require careful handling when operating in transonic conditions.