What is the Critical Mach Number?

• A. The speed at which the entire wing becomes supersonic.
• B. The speed at which the trailing edge becomes sonic.
• C. The speed at which the airflow over any portion of the upper wing surface becomes supersonic in level flight.
• D. The speed at which the aircraft reaches stall buffet.

Answer: (C)

The Critical Mach Number is the speed at which airflow over any portion of the upper wing surface first reaches Mach 1 in level flight. As you accelerate, the flow speeds up on the wing, and a sonic point forms where local Mach equals 1. That point marks the onset of compressibility effects and shock waves, causing a rapid increase in drag and changes in handling. It’s a property of the wing and flight condition, not the entire wing suddenly becoming supersonic. It’s also not tied to the trailing edge or to stall buffet; those are different phenomena. The concept helps explain why performance changes sharply as you approach transonic speeds.