Which of the following is a possible compressibility effect when crossing critical Mach?

• A. Control Difficulties
• B. Drag Decreases
• C. Flight Calmness at High Speed
• D. Increased Stability

Answer: (A)

Crossing the critical Mach introduces compressibility effects because parts of the airflow over the airplane reach Mach 1, creating shock waves on surfaces like the wings. These shocks change the pressure distribution suddenly and can cause boundary-layer separation, buffet, and a loss of effective control authority on the surfaces you rely on for maneuvering. In practice, that means the aircraft can become harder to control as speeds move through the transonic range.

Drag does not decrease when these effects occur; it actually increases due to wave drag from the shocks. Flight can feel less calm and stable because the shocks move and the flow can buffet, altering lift and pitching moments. Increased stability is not typically observed in this regime; the handling often becomes more challenging.

So the most consistent compressibility effect when crossing critical Mach is control difficulties.