Which problem can arise when speeds exceed the critical Mach number, as a compressibility effect?

• A. Control Difficulties
• B. Noisy Cabin Environment
• C. Improved Fuel Economy at High Speeds
• D. Longer Takeoff Run

Answer: (A)

Exceeding the critical Mach number brings compressibility effects into play, with shock waves forming on the airframe as local flow goes sonic. Those shocks can cause boundary-layer separation and a big rise in wave drag, and, crucially, they disrupt the airflow over control surfaces. When control surfaces are affected by these disturbances, their effectiveness is reduced, making the aircraft harder to control—especially in maneuvering or at high speeds. The other options don’t describe the primary consequence of transonic compressibility: cabin noise, better fuel economy at high speeds, or a longer takeoff run aren’t the direct outcomes of crossing the critical Mach threshold.