When does the critical Mach number occur?

The critical Mach number is defined as the lowest Mach number at which airflow over any part of the aircraft reaches the speed of sound, which is Mach 1.0. This condition typically occurs at a point on the aircraft, often over the wing, where local airflow compression effects begin to manifest. This is significant because once the critical Mach number is exceeded, drag increases sharply due to shockwaves forming, and the aircraft may experience various aerodynamic changes.

When an aircraft reaches its critical Mach number, it is a pivotal phase in flight as it indicates the approach to transonic speeds, where compressibility effects become pronounced. Understanding this limit is crucial in aircraft design and operational safety to manage stability and performance during flight.

The other options do not accurately describe the critical Mach number. Stalling refers to a loss of lift rather than a specific Mach condition. A Mach number of 0.75 is below the critical level for most aircraft and does not signify the transition to supersonic airflow. The effectiveness of the elevator is related to control surface performance and stall characteristics, rather than a direct correlation to the critical Mach number.