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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