What happens to controllability when Vmc is exceeded?

When Vmc, or minimum control speed, is exceeded, controllability decreases significantly. Vmc is the airspeed at which a multi-engine aircraft can maintain controlled flight with one engine inoperative. Exceeding Vmc usually means that the airspeed has increased to a point where the effectiveness of the control surfaces is altered due to changes in the airflow around the aircraft.

When the control surfaces are affected in this way, especially in the case of yawing moments induced by differential thrust from the operational engine, it becomes increasingly challenging to maintain directional control. Without sufficient rudder authority, the aircraft may begin to yaw uncontrollably towards the inoperative engine, which can lead to a loss of control if not corrected promptly.

This loss of controllability is particularly critical during scenarios such as takeoff and climb, where maintaining directional control is essential for safety. Pilots need to be diligent in recognizing and avoiding flight operations beyond Vmc to ensure safe handling of the aircraft.