Minimum control speeds
The minimum control speed (VMC) of an aircraft (specifically an airplane) is a V-speed that specifies the calibrated airspeed below which directional or lateral control of the 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. Indeed, 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. These are all included in the aircraft flight manual of all multi-engine aircraft. When design engineers are sizing an airplane's vertical tail and flight control surfaces, they have to take into account the effect this will have on the airplane's minimum control speeds.
Minimum control speeds are typically established by flight tests as part of an aircraft certification process. They provide a guide to the pilot in the safe operation of the aircraft.
When an engine on a multi-engine aircraft fails, the thrust distribution on the aircraft becomes asymmetrical, resulting in a yawing moment in the direction of the failed engine. A sideslip develops, causing the total drag of the aircraft to increase considerably, resulting in a drop in the aircraft's rate of climb. The rudder, and to a certain extent the ailerons via the use of bank angle, are the only aerodynamic controls available to the pilot to counteract the asymmetrical thrust yawing moment.
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