Definition
Airflow moving faster than the local speed of sound (Mach 1.0). In supersonic flow, pressure changes cannot travel upstream against the airflow, so the air ahead of the airplane gets no warning of its approach, and shock waves form where the flow abruptly slows or changes direction.
Plain English
Air flowing past part of the airplane faster than sound travels through that air. Once the flow goes supersonic, it behaves very differently from normal subsonic airflow, and shock waves appear.
Context Anchor
Seen in discussions of transonic airflow, especially around wings and control surfaces at high speeds.
Derivation
From Latin 'super' meaning 'above' and 'sonic' from 'sonus' meaning 'sound.' So 'supersonic' literally means 'above sound speed' — the air is moving faster than sound can travel through it.
Why Pilots Care
Supersonic flow over parts of an aircraft creates shock waves that sharply increase drag, reduce control effectiveness, and require specific design features to manage safely.
Grounding Statement
Picture air rushing over the top of a wing fast enough that pressure signals can't push back upstream — at that point a shock wave forms where the flow has to slow down again.
Intuition Check
Supersonic flow does not always mean the entire airplane is supersonic. It can mean only a local area of airflow over the airplane has gone faster than the speed of sound.
Example Sentence 1
As the airplane approached its critical Mach number, pockets of supersonic flow began to form on the upper surface of the wing.
Example Sentence 2
The test pilot noted the onset of supersonic flow as the Mach meter passed 1.0.