VMC stands for "Minimum Control Speed". It is a V-speed that specifies the calibrated airspeed below which directional or lateral control of a multi-engine aircraft can no longer be maintained, after the failure of one or more engines. The VMC only applies if at least one engine is still operative, and will depend on the stage of flight. Multiple VMCs have to be calculated for landing, air travel, and ground travel, and there are more still for aircraft with four or more engines. VMCs are included in the aircraft flight manual of all multi-engine aircraft. Aviation regulations define several different VMCs and require design engineers to size the vertical tail and the aerodynamic flight controls of the aircraft to comply with these regulations.
It is important to note that VMC can also refer to "Visual Meteorological Conditions" in aviation. VMC in this context refers to the meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling equal to or better than specified minima. It is an aviation flight category in which visual flight rules (VFR) flight is permitted, meaning conditions in which pilots have sufficient visibility to fly the aircraft maintaining visual separation from terrain and other aircraft.
