Refer to figure.
VMCG is the minimum speed on the ground at which the take-off can be safely continued, when the critical engine suddenly becomes inoperative with the remaining engine(s) at take-off thrust.
V1 is defined as being the maximum speed at which the pilot must take the first action in order to stop the airplane within the remaining accelerate-stop distance.
V1 cannot be allowed to be less than VMCG because engine failure below VMCG means the airplane is uncontrollable and the definition of V1 is that the take-off can be continued following engine failure.
Rotation speed, VR, is the speed at which the pilot initiates action to raise the nose gear off the ground, with the intention of becoming airborne. VR may not be less then decision speed V1.
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