Vmca is defined as the minimum speed, whilst in the air, that directional control can be maintained with one engine inoperative (critical engine on two engine aerolanes), operating engine(s) at takeoff power and a maximum of 5 degrees of bank towards the good engine(s).
When an engine, on a multi engine aircraft, fails or is inoperative in flight, the asymmetric thrust creates a yawing moment which causes the aircraft to turn towards the failed engine. Rudder is used to counteract this yaw and adverse roll effects are negated by use of the ailerons. There is a minimum speed at which, by using full rudder deflection, it is possible maintain a constant heading with level wings. This speed is reduced if the aircraft is banked towards the live engine(s). The speed that corresponds to full rudder deflection in combination with a 5 degree bank angle is defined, by regulations, as the minimum control speed and is referred to as Vmca (Velocity of Minimum Control in the Air).
For a given size of vertical tail with rudder, at speeds below Vmca, the generated rudder side force is not large enough to counteract the asymmetrical thrust. As a consequence, heading cannot be maintained at speeds below Vmca with the critical engine failed and the other engine(s) operating at maximum thrust. Vmca was calculated (assumed) by the design engineer when determining the size of the vertical tail. Whilst a large tail will result in a reduced Vmca, it will also have adverse consequences in terms of weight, production costs and hangar accessability. A small vertical tail can mitigate those issues but will result in an increase in Vmca. Regulations prohibit Vmca values greater than 1.2 Vs.
Vmca is a manufacturer determined speed which will be published in the Aircraft Flight Manual (AFM). The speed is mathematically calculated during aircraft design and verified as part of the certification test flight process. Conditions and criteria for determination of Vmca include the following:
- maximum permissibile power on all operating engines
- critical engine windmilling
- full rudder deflection or some nominated amount of pedal force, whichever occurs first
- 5 degrees of bank away from failed engine
- flaps in takeoff position
- landing gear retracted
- the most unfavorable center of gravity (full aft)
Vmca / Derated Thrust Relationship
To maintain directional control with an inoperative engine, the rudder must be deflected to counteract the adverse yaw. The force that can generated by the rudder is dependent upon the size of the rudder, the amount that the rudder can be deflected and the speed of the airflow across the rudder surface.
In the case of an aircraft fitted with engines that can be derated for takeoff, the reduction in thrust will result in a corresponding reduction in the amount of yaw induced should an engine fail. As the rudder size and deflection capability remain constant, the amount of force required to counter that yaw can be generated at a lower airspeed than would be case during a full thrust takeoff. This results in a reduction in Vmca.