Class A Aircraft - Take-off — Page 379, Lesson 460

Let's start with the take-off distance required, because that's the foundation of everything we do on the runway. The take-off distance required is not a single number. It is the greatest of three distances, and we must always plan for the worst case. Let me walk you through each one. First, all engines operating. This is the horizontal distance the aeroplane travels, with every engine running, to reach a screen height of 35 feet. Then we multiply that distance by 1.15. So we take the actual distance to 35 feet and add 15% on top of it. That 1.15 factor is our safety margin for the all-engines-operating case. Second, one engine inoperative on a dry runway. Here we assume the critical power unit fails at VEF. VEF is the engine failure speed — the speed at which we assume the critical engine fails. The distance is measured from the BRP, the brake release point, to the point where the aeroplane attains 35 feet. This is on a dry, hard surface. Third, one engine inoperative on a wet runway. Same idea, but now the screen height drops to 15 feet instead of 35. The distance is from the BRP to the point where the aeroplane attains 15 feet, assuming the critical power unit fails at VEF on a wet or contaminated hard surface. And this must be achieved in a manner consistent with the achievement of V2 by 35 feet. V2 is the take-off safety speed — the speed at which we climb after the engine failure. Now, why the difference in screen height? The note explains it clearly. The reduction from 35 feet to 15 feet is to help reduce the take-off mass penalties that a wet runway will undoubtedly cause. A wet runway gives us less friction, so the aeroplane accelerates more slowly and we need more distance. If we demanded 35 feet on a wet runway, the take-off mass would have to be reduced so much that it would be a serious penalty. Dropping the screen height to 15 feet eases that penalty. Now let's move to the V speeds, starting with VMCG. VMCG stands for ground minimum control speed. CAP 698 describes it as 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. Let me explain what that actually means. When an engine fails, the remaining engine or engines still generate thrust. That thrust pushes the aeroplane's nose to one side — it yaws away from the live engine. The amount of yaw depends on how much thrust the live engine is producing. More thrust from the live engine means more yaw. To counteract that yaw, we use the ailerons and the rudder to steer the aeroplane in the right direction. But here's the key point: when the aeroplane is on the ground, the rudder is only effective if we have enough airflow over it. Below a certain speed, the rudder simply cannot generate enough force to overcome the yaw from the live engine. That speed is VMCG. So VMCG is the minimum speed at which, if the critical engine fails, we can still maintain directional control on the ground using the rudder and ailerons, with the remaining engines at take-off thrust. Below VMCG, we cannot safely continue the take-off — we must abort. Let me show you a graph that illustrates the ideal position of V1, because V1 is the decision speed — the speed at which we decide whether to continue or abort the take-off. That graph shows how V1 sits at the intersection point of the accelerate-stop and accelerate-go distances. It's the balance point between stopping safely and continuing the take-off. Now, one important thing to note: the excerpt cuts off mid-sentence at "when the aeroplane is on the ground, yo..." — that's where the text stops. But the key concept is clear: on the ground, our control authority is limited by speed. Below VMCG, we don't have enough rudder authority to counter the asymmetric thrust, so we cannot continue. Let me summarise what we've covered. The take-off distance required is the greatest of three distances: all engines operating to 35 feet times 1.15, one engine inoperative on dry runway to 35 feet, and one engine inoperative on wet runway to 15 feet. The wet runway screen height is lower to reduce mass penalties. And VMCG is the minimum ground speed for safe continuation after a critical engine failure, limited by our ability to control yaw with rudder and ailerons. That's the core of what we need for take-off performance. Do you have any questions on these concepts before we move on?