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Performance & Navigation Topic 8.

What is VMCG?


This speed relates to takeoff in a multi-engine aircraft, and is the abbreviation for “minimum control indicated airspeed - ground”. It is found with aircraft flaps and trim in takeoff configuration, critical engine suddenly failed, most unfavourable C of G (rear), and takeoff power on remaining engine(s). With thrust set at high values needed for takeoff, failure of one engine will cause the aircraft to swing (yaw) toward the dead engine. This is particularly marked when considering large twin engine aircraft that feature wing mounted engines, more so than for those aircraft with fuselage mounted engines, where the engine thrust lines are closer together, and asymmetrics less of a problem.

Vmcg is the minimum speed at which the pilot can maintain directional control of the aircraft with one engine suddenly becoming inoperative during the takeoff roll, with the use of aerodynamic controls only. Rudder effect is the only steering control assumed available to counter asymmetric yaw when finding Vmcg speed for the aircraft type under consideration. It is assumed the aircraft has just been rotated off the runway when the engine fails, and the nose wheel is no longer in contact with the runway, but that the main wheels are still on the ground. Obviously we would need to ensure we did not rotate the aircraft off the ground until we had achieved or exceeded Vmcg, otherwise we risk departing the runway. The larger the rudder surface area and deflection range available, the greater the ability to counter the asymmetric yaw at any given speed, and lower Vmcg speed. Larger fin area will also assist here on both counts. Another consideration effecting Vmcg is the position of the center of gravity (C of G). A rearward centre of gravity produces a more adverse yawing couple, and reduces the leverage force that can be provided by the rudder to counter the yaw. Conversely, a forward C of G will provide better yaw control, and a lower Vmcg.


The thrust setting for takeoff also has an effect on Vmcg, such that high power settings increase Vmcg values, as more asymmetric yaw follows an engine failure. De-rated (low thrust setting) takeoffs reduce the asymmetric yaw, and with it Vmcg speeds. We would prefer a very low Vmcg speed, as this gives us greater asymmetric control. See summary of factors that reduce Vmcg values.
Fig 1. Rearward C of G, less leverage, less control.
Factors that reduce Vmcg values
  • Density altitude .
  • Reduced thrust takeoff.
  • Large tail fin area.
  • Large rudder area/large rudder deflection available.
  • Forward C of G.
  • Short distance from engine to fuselage.


Fig 2. Forward C of G, greater leverage, more control.

These texts form part of the ATPL Performance and Loading course produced by Rob Avery. It is presently available as a self study course in paper version,  via the internet or on CD ROM, anywhere in the world. It is focussed on the knowledge required to pass the CASA ATPL examination. The course includes assignments with answers provided. More information can be found that Avfacts web site.

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Marty says ... "Goodbye to GA".

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