First, VGO — Page 379, Lesson 463

I want to walk you through the decision speed V1 — the heart of take-off performance. Let's start with the two speeds that define its extremes. First, VGO. That's the lowest decision speed from which a continued take-off is possible within the take-off distance available. In plain terms: if the engine fails at or above VGO, you still have enough runway ahead to keep going, accelerate, and get airborne within the distance you have. Second, VSTOP. That's the highest decision speed from which the aeroplane can stop within the accelerate-stop distance available. So if the engine fails at or below VSTOP, you can still bring the aircraft to a halt on the runway you have left. Now here's the key idea: VGO and VSTOP are the two extremes of V1. V1 sits somewhere between them. Below V1, you abort. Above V1, you continue. And the rules that pin down exactly where V1 sits are laid out in CAP 698, section 4, page 2, right alongside the V1 definition. Let me give you those four constraints precisely. Rule one: V1 may not be less than VEF plus the speed gained with the critical engine inoperative for the time between engine failure and the point at which the pilot applies the first means of retardation. Let me unpack that. VEF is the engine failure speed — the speed at which the critical engine is assumed to fail. Between that moment and the moment you actually apply the first braking or retardation, the aeroplane keeps accelerating on the remaining engine. So V1 has to be at least VEF plus that gained speed. You can't set V1 so low that you'd be committed to a decision before you've even recognised the failure and reacted. Rule two: V1 must not exceed VR — the rotation speed. You can't decide to continue past the point where you'd be rotating for take-off. Rule three: V1 must not exceed VMBE — the maximum brake energy speed. Beyond that speed, the brakes can't absorb the energy of stopping without overheating. Rule four: V1 must not be less than VMCG — the minimum control speed on the ground. Below that, you couldn't maintain directional control with the critical engine failed. Now, the logic of the decision. If the engine fails before V1, the decision is to abort the take-off. Why? Because with only one engine operating, there's insufficient take-off distance left to accelerate the aeroplane to the screen height — that's the height you must reach by the end of the take-off distance. You simply can't make it. If the engine fails after V1, the decision is to continue. Why? Because the aeroplane is travelling too fast to be able to stop within the remaining accelerate-stop distance available. You'd run off the end trying to stop. So V1 is the balancing point — the last moment you can safely stop, and the first moment you can safely go. To see how V1 is actually derived, we need a graph. Let me bring that up for you. This is Figure 14.1. It plots the take-off distance required and the accelerate-stop distance required, both based on a varying engine failure speed. The take-off distance required increases as the engine failure speed gets later — because you have less runway left to accelerate to screen height. The accelerate-stop distance required also increases with engine failure speed — because you're going faster when you try to stop. The ideal V1 sits at the intersection of those two curves, where the take-off distance required and the accelerate-stop distance required are balanced. That intersection is the sweet spot — the speed that gives you the shortest balanced field length. So to summarise what we've built: VGO and VSTOP are the extremes, the four CAP 698 rules constrain V1 between them, the decision logic flips at V1, and the graph shows how the two distance curves intersect to give you the ideal V1. That's the complete picture of how V1 is derived.