Vmca
VMCA (Minimum Control Speed - Airborne) is a term from aviation and describes the speed at which the aerodynamic steerability of an aircraft in level flight is maintained in the event of an engine failure (critical engine inoperative). In EASA CS-23 and EASA CS-25, aerodynamic steerability is defined as the situation of an aircraft whose inclination during flight and engine failure is not more than 5 °. The maximum certified take-off mass, the most unfavorable center of gravity and the maximum take-off thrust on the engines are assumed. If the VMCA is not reached, the controllability of the aircraft is no longer guaranteed. The aerodynamic steerability during take-off and engine failure must be guaranteed with a maximum deviation from the runway centerline of 9.1 m (30 ft). The maximum certified take-off mass and a maximum unfavorable center of gravity are assumed.
Center of gravity
The position of the center of gravity on the longitudinal axis of the aircraft is essential for determining the VMCA. If the center of gravity is far forward, the VMCA can be lower than with a center of gravity further back. The reason for this is the fact that with the front center of gravity the lever arm between the rudder of the tail unit and the center of gravity is greater than with the rear center of gravity and thus a greater or equal moment (moment = force × lever arm) can arise at lower speed. Because only one engine is active, the yaw (yoke torque) of the active engine must be aerodynamically balanced by the rudder. The higher the engine power of the still functioning engine, the higher the aerodynamic forces on the rudder to compensate.
Density height
With increasing density altitude, the VMCA decreases, as the engine performance decreases due to the decrease in air density and therefore less force has to be applied to the rudder to correct the flight direction in the event of an engine failure.