Vmcgis defined as the minimum calibrated airspeed, during the takeoff run at which, if the critical engine suddenly fails, it is possible to maintain directional control using only aerodynamic controls.
Vmcg must not be greater than V1.
Vmcg is a design certification speed which is established by the manufacturer during the certification process. It is the minimum speed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued using normal piloting skill. Assuming that the path of the airplane, accelerating with all engines operating, is along the centerline of the runway, under standard Vmcg criteria, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. Manufacturers may be able to nominate a deviation value other than 30' provided that the value is published within the flight and performance manuals for the aircraft. Deviation values of less than 30' can be particularily useful when considering operations from narrow runways.
The conditions under which the certification tests are conducted include:
- a bare and dry runway
- calm winds (no crosswind)
- nose wheel steering disconnected to simulate a slippery runway
Vmcg must be established with the airplane in each takeoff configuration or, at the option of the manufacturer, in the most critical takeoff configuration. The critical engine must be subject to a sudden failure. In addition, the following criteria must be met:
- maximum available takeoff power or thrust on the operating engines
- the most unfavorable center of gravity
- the airplane trimmed for takeoff the most unfavorable weight in the range of takeoff weights.
Vmcg / V1 Relationship
By definition, V1 is the maximum speed at which a rejected takeoff can be initiated in the event of an emergency. V1 is also the minimum speed at which a pilot can continue takeoff following an engine failure. During acceleration for takeoff, an aircraft which has not yet attained Vmcg cannot maintain directional control in the event of the failure of the critical engine and the takeoff must be rejected. As a consequence, V1 cannot be less than Vmcg.
Vmcg / Derated Thrust Relationship
Vmcg is affected by the amount of thrust available and will vary with pressure altitude and temperature. The reduced thrust available at higher altitudes or under temperature conditions above the engine flat rated temperature will result in a reduction of Vmcg. The capability to derate the engine thrust limit also has an effect.
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 Vmcg and the potential for a reduction in V1.