Mach tuck occurs when?

• A. The center of pressure moves forward, causing a pitch-up moment.
• B. It occurs only during takeoff.
• C. It is caused by engine surge.
• D. The center of pressure moves aft, causing a pitch-down moment; air flow separation may make elevator ineffective.

Answer: (D)

Mach tuck is a high-speed, compressibility-driven effect where the pressure distribution shifts so the center of pressure moves toward the tail as airspeed nears the transonic range. That aft shift creates a nose-down pitching moment. At the same time, shock-induced flow separation can develop on the tail or elevator, reducing elevator effectiveness and making it harder to counter that nose-down tendency. Together, these factors can lead to an uncomfortable or dangerous pitch-down behavior if not managed. This is why the description of the center of pressure moving aft, causing a pitch-down moment, with possible elevator inefficacy due to flow separation, best captures Mach tuck. It’s not about takeoff conditions or engine surge, and the relevant effect is an aft shift, not a forward one.