Mach tuck the speed of sound and control authority both increase. To prevent Mach stall from progressing, the pilot should keep the airspeed below the type's critical Mach number by reducing throttle, extending speed brakes, and if possible, extending the landing gear. A number of design techniques are used to counter the effects of Mach tuck. On both conventional tailplane and canard foreplane configurations, the horizontal stabiliser may be made large and powerful enough to correct the large trim changes associated with Mach tuck. In place of the conventional elevator control surface, the whole stabiliser may be made moveable or "all-flying", sometimes
Critical Mach number not designed to fly at or above the critical Mach number, the shock waves that form in the airflow over the wing and tailplane are sufficient to stall the wing, render the control surfaces ineffective, or lead to loss of control of the aircraft (such as Mach tuck, when shock waves in the airflow over the elevator send the aircraft into an uncontrollable dive). These problematic phenomena appearing at or above the critical Mach number became known as compressibility. Compressibility led to a number of accidents involving high-speed military and experimental aircraft in the 1930s and 1940s. Although unknown at