Definition
The critical Mach number (MCR) is the lowest free-stream Mach number at which the airflow over any part of the airplane first reaches the speed of sound (Mach 1.0), even though the airplane itself is still flying below the speed of sound. Above MCR, localized supersonic flow forms over the wing or other surfaces, producing shock waves that can cause buffet, drag rise, and loss of control effectiveness.
Plain English
It is the speed at which some of the air flowing over the airplane first hits the speed of sound, even though the airplane itself is still flying slower than sound. Past this speed, shock waves start forming on the wing and the airplane begins to behave very differently.
Context Anchor
Seen in high-speed flight and speed margin discussions, especially when learning why an airplane can have high-speed limits before the whole airplane reaches the speed of sound.
Derivation
From Ernst Mach, the Austrian physicist who studied the speed of sound. "Critical" here means the threshold point — the speed at which something important first happens. So MCR is the threshold Mach number at which sonic flow first appears somewhere on the airplane.
Why Pilots Care
Crossing this speed causes a sudden rise in drag and possible control problems, so pilots must stay below it for safe operation.
Grounding Statement
Air speeds up as it flows over the curved upper surface of a wing. So even when the airplane is flying at, say, Mach 0.78, the air over the wing's upper surface might already be moving at Mach 1.0 — and that's the critical Mach number for that airplane.
Intuition Check
Critical does not mean the airplane is already failing or out of control. Here it means the threshold where the first part of the airflow reaches the speed of sound and high-speed effects can begin.
Example Sentence 1
The pilot kept cruise speed below the airplane's MCR to avoid the onset of shock-induced buffet.
Example Sentence 2
Buffeting started as the jet approached its MCR, signaling the start of shock waves.