Lose an engine on the A320 and the live engine tries to turn the aircraft. See how big that yawing moment is, why the rudder can only hold it above a certain speed, and how certification turns this into VMCG, VMCA and VMCL and into the limits on V1, VR and V2.
One engine pushes, the other only drags. The yawing moment is the live engine’s thrust times its distance from the centreline, plus the windmilling drag of the failed engine on the other side. The rudder answers with a side force on the fin, and that force grows with the square of the speed. Centre the beta target with the rudder pedal.
Fail an engine on the take-off roll. The pilot recognises it after about a second and applies full rudder, without nose-wheel steering, as in certification. VMCG is the lowest failure speed from which the aircraft is back on a heading parallel to the centreline without drifting more than 30 ft sideways. In the air the same logic gives VMCA, flown with no more than 5° of bank toward the live engine.
Engine failure at take-off thrust, rudder only, no nose-wheel steering. The track may not deviate more than 30 ft from the centreline before the heading is parallel again. V1 may not be lower than VMCG.
Critical engine suddenly inoperative, take-off thrust on the other, most adverse CG, take-off configuration, out of ground effect, straight flight with no more than 5° of bank toward the live engine. VR must be at least 1.05 VMCA and V2 at least 1.10 VMCA.
The same idea in the landing configuration, with go-around thrust on the live engine. The reference approach speed may not be lower than VMCL, so flying VLS or above keeps you clear of it during a one-engine approach and go-around.
Teaching model of an A320-size aircraft with typical values. It is calibrated so that take-off thrust at sea level, ISA, gives VMCG ≈ 107 kt and VMCA ≈ 109 kt, the same values the performance lab uses. Your aircraft’s figures are in its flight manual and performance software.
Typical Airbus technique for teaching. The exact calls, altitudes and actions are in your operator’s FCOM, FCTM and SOPs.
Teaching model of an A320-size aircraft. Thrust, arms, rudder authority and the yaw dynamics are simplified and use typical values; they are not FCOM or certification data. VMCG, VMCA, VMCL and the take-off speeds for your aircraft come from its flight manual and performance software.