Imagine a runway with a 2% downslope — Page 379, Lesson 468

We’re now into the fine-tuning of V1 — the corrections and the two speeds that can override the ideal V1. Let’s start with the slope and wind corrections, because those are the numbers you’ll actually apply. Imagine a runway with a 2% downslope. With an aeroplane mass of 70,000 kg, V1 must be reduced by 3 knots. Now flip it: a 2% upslope, same mass, and V1 must be increased by 4 knots. So the rule is simple — downslopes reduce V1, upslopes increase V1. Why does that make sense? On a downslope, gravity is pulling you forward, so you accelerate faster and need less runway to reach a given speed — you can afford a lower V1. On an upslope, gravity is working against you, so you need more runway and a higher V1. The right-hand side of the table is the wind correction. Take a 15-knot tailwind with a mass of 70,000 kg — V1 must be reduced by 3 knots. Now a 40-knot headwind, same mass — V1 must be increased by 1 knot. So the rule again: tailwinds reduce V1, headwinds increase V1. A tailwind helps you accelerate, so you need less runway, hence a lower V1. A headwind slows your acceleration, so you need more runway and a higher V1. Now, those are the main factors. But there are two other influences that may or may not change V1 — and these are the speeds VMCG and VMBE. Let me define each precisely. First, VMCG — we touched on it earlier. It’s the minimum control speed on the ground. The key constraint from CAP 698, section 4, page 2, is this: V1 must not be less than VMCG, and not greater than VR, and not greater than VMBE. So V1 is sandwiched — it must lie between VMCG on the low side, and VR and VMBE on the high side. Depending on the values of those speeds, they may push V1 higher or lower than the ideal V1 we calculated from the main factors. Now VMBE — Maximum Brake Energy Speed. This is the second speed that can influence V1. From CAP 698, section 4, page 3: VMBE is the maximum speed on the ground from which an aeroplane can safely stop within the energy capabilities of the brakes. Let me unpack that. If you reject the take-off at a speed higher than VMBE, and you apply maximum braking force, the brakes would not be able to safely bring the aeroplane to a stop — regardless of how much runway is left. The brakes would most probably catch fire, melt, and/or disintegrate. So VMBE is a hard limit on how fast you can be going and still stop safely on the brakes alone. You do need to be aware of the factors that control VMBE, but the good news is that most manuals — and CAP 698 itself — have a VMBE graph or table with all the variables on it. That graph is on page 15 of section 4. If you want to see the effect of a variable, say mass, you simply work through the graph twice with two different masses. In this case, the heavier mass has reduced VMBE. The variables that affect VMBE are pressure altitude and ambient air temperature — and the excerpt cuts off there, but those are the two named so far. So to tie it together: V1 is not just a number you read off a table. It’s corrected for slope and wind, and then it must be checked against VMCG, VR, and VMBE. If the ideal V1 falls outside that envelope, the actual V1 gets pushed to stay within it — and VMBE in particular protects you from a rejected take-off that would destroy the brakes.