Factors Affecting Vmc

FAR 23.149 states that Vmc cannot exceed 1.2 Vs1. To determine the actual speed of Vmc, the aircraft is tested under the following conditions:

Maximum takeoff weight
Most aft allowed CG
Critical engine inoperative
Operative engine at maximum takeoff power
Inoperative engine propeller assuming natural position (windmilling unless equipped with autofeather)
Flaps in takeoff position
Landing gear retracted
Not in ground effect
Not more than 5 degrees of bank into the good engine

If any of these factors are changed, the speed at which directional control cannot be maintained will also change. Here is a summary of what factors affect directional control. Note that directional control and performance are not the same thing – many factors oppose each other in this respect.

Factor	Increase Control	Decrease Control	Effect on Performance
Weight	Higher weight (assumes 3-5 degrees bank angle)	Lower weight (assumes 3-5 degrees bank angle)	Higher weight decreases performance
Center of Gravity	Forward CG (lengthens arm between CG and rudder)	Aft CG (shortens arm between CG and rudder)	Aft CG increases performance
Propeller	Feathered	Windmilling	Decreased performance with windmilling propeller
Landing Gear	Extended	Retracted	Increased performance with retracted gear
Atmospheric Conditions	High density altitude	Low density altitude	High density altitude decreases performance
Bank	Approx. 3 degrees of bank to establish zero sideslip	No bank	Up to 5 degrees will increase performance; additional bank adds drag
Power	Less power	More power	More power increases performance
Engine	Non-critical engine inoperative	Critical engine inoperative	Critical engine inoperative decreases performance

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