Critical Mach Number is defined as:

• A. The maximum airspeed that an aircraft can achieve
• B. The airspeed where the airflow is between subsonic and transonic
• C. The airspeed used for Critical Point calculations
• D. The aircraft speed at which airflow becomes sonic over any part of the aircraft

Answer: (D)

The key idea is that Critical Mach Number is the free‑stream speed at which some part of the airflow over the aircraft first reaches the speed of sound. Once any portion of the flow becomes sonic, even if most of the flow remains subsonic, shock waves can form and transonic effects begin. This onset is what defines Mcrit, and it signals the beginning of a shift in the airplane’s aerodynamic behavior, including a rapid rise in drag and changes in lift.

Mcrit depends on how the air is accelerated by the aircraft’s surface, so geometry matters (camber, thickness, leading-edge radius) as well as flight conditions (angle of attack, temperature, Reynolds number). The speed of sound itself varies with temperature, so Mcrit isn’t a fixed speed in knots or mph but a Mach number tied to the current conditions.