Definition
The flight Mach number at which airflow over some part of an aircraft first reaches the speed of sound, even though the aircraft itself is still flying below the speed of sound. Above this Mach number, shock waves begin to form on the aircraft, causing increased drag and possible control problems.
Plain English
The speed at which air flowing over the wings or body of an aircraft first goes supersonic, even though the aircraft itself is still flying slower than the speed of sound. It marks the point where the aircraft starts to feel the effects of approaching the sound barrier.
Context Anchor
Seen in high-speed aerodynamics, propeller blade discussions, and aircraft performance limits.
Derivation
Mach number is named after Ernst Mach, an Austrian physicist who studied airflow and shock waves. 'Critical' here means the threshold point — the speed at which something important first happens, in this case the first appearance of supersonic airflow over the aircraft.
Why Pilots Care
Crossing this speed causes shock waves, a sharp rise in drag, and possible loss of control effectiveness.
Analogy
Think of it like one small part of a river current hitting a rough, fast-water point before the whole river looks turbulent. The local change can matter even if the whole aircraft is not yet moving at the speed of sound.
Intuition Check
Do not read “critical” as meaning the airplane is already in an emergency. Here it means an important threshold where the airflow first reaches the speed of sound at one spot.
Example Sentence 1
The pilot avoided exceeding the critical Mach number to prevent shock-induced buffeting on the wings.
Example Sentence 2
Maintenance records noted the aircraft's critical Mach number to confirm it stayed within safe operating limits during high-speed tests.