Right, let's pick this up — Page 379, Lesson 463

Right, let's pick this up. We've just defined the decision speed V1, and now we're looking at the two speeds that form its absolute boundaries. These are VGO and VSTOP. VGO is the lowest decision speed from which a continued take-off is possible within the take-off distance available. So, if you're at or above VGO when the engine fails, you have enough runway left to keep going, get airborne, and reach the screen height. Below VGO, you simply don't have the distance to complete the take-off on one engine. VSTOP is the highest decision speed from which the aeroplane can stop within the accelerate-stop distance available. That's the distance you need to accelerate to that speed and then bring the aircraft to a complete halt. If you're at or below VSTOP when the engine fails, you can stop in time. Above VSTOP, you're going too fast to stop before the runway ends. Now, here's the key relationship: these two speeds are the extremes of V1. V1 must sit somewhere between them. If V1 were below VGO, you'd be committing to a take-off you can't complete. If V1 were above VSTOP, you'd be committing to a stop you can't achieve. So V1 is the balancing point between those two limits. Now, there are specific rules governing where V1 can actually be set. These are laid out in CAP 698, section 4, page 2, right alongside the V1 definition. Let me walk you through each constraint. First, 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 speed at which the critical engine is assumed to fail. Between that failure and the moment you actually apply the brakes or other retardation, the aircraft keeps accelerating on the remaining engine. So V1 has to be at least VEF plus that extra speed gained during the recognition and reaction time. Otherwise, you'd be making a decision before you've even had time to react. Second, V1 must not exceed VR. VR is the rotation speed — the speed at which you rotate the aircraft to lift off. You can't have a decision speed higher than your rotation speed, because you'd be deciding to continue after you've already started rotating. Third, V1 must not exceed VMBE. VMBE is the maximum brake energy speed — the highest speed from which the brakes can absorb the energy of stopping without overheating and failing. If V1 were above VMBE, the brakes couldn't handle the stop. Fourth, V1 must not be less than VMCG. VMCG is the minimum control speed on the ground — the lowest speed at which you can maintain directional control with the critical engine inoperative, using rudder alone. Below VMCG, you can't control the aircraft on the ground with one engine out. Now, let's think about the decision logic. 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. You're still slow enough to stop, but too slow to safely continue. If the engine fails after V1, the decision is to continue the take-off. Why? Because the aeroplane is travelling too fast to be able to stop within the remaining accelerate-stop distance available. You're too fast to stop, but you still have enough runway to get airborne. To understand how V1 is actually derived, we need to look at a graph. This is Figure 14.1. It plots the take-off distance required and the accelerate-stop distance required, based on a varying engine failure speed. As the engine failure speed increases, the take-off distance required increases — because you're committing to continue from a higher speed. And the accelerate-stop distance required also increases — because you're stopping from a higher speed. The ideal V1 is where these two curves intersect, giving you the balance between being able to stop and being able to continue. So, to summarise: VGO is your lower bound for continuing, VSTOP is your upper bound for stopping, and V1 sits between them, constrained by VEF, VR, VMBE, and VMCG. The graph shows you how those distances grow with engine failure speed, and the intersection is where V1 lives.