Let’s pick this up right where the table left off — Page 379, Lesson 468

Let’s pick this up right where the table left off. We were looking at the corrections for runway slope and wind, and I want to make sure you see the pattern clearly, because it’s going to show up again and again in your performance work. First, the slope correction. If the runway has a downslope of 2%, and the aeroplane mass is 70,000 kilograms, then V1 has to be reduced by 3 knots. Now flip it: if the runway has an upslope of 2%, with the same mass, V1 must be increased by 4 knots. So the rule is simple and worth memorising: downslopes reduce V1, upslopes increase V1. The reason is intuitive once you think about it — a downslope helps you accelerate, so you can afford to commit to the take-off a little later, which lowers V1. An upslope fights your acceleration, so you need to be going faster before you commit, which raises V1. Now the right-hand side of that same table is the wind correction. Take a 15-knot tailwind with a mass of 70,000 kilograms — V1 has to be reduced by 3 knots. But if you have a headwind of 40 knots for the same mass, V1 has to increase by 1 knot. So the rule there is: tailwinds reduce V1, headwinds increase V1. Again, think about it — a tailwind pushes you along, so you reach your decision speed sooner, and you can lower V1. A headwind slows your ground acceleration, so you need a higher V1 to make sure you still have enough runway to stop or continue. Now, we’ve covered the main factors that affect V1. But there are other influences that may or may not change its value. Two in particular are the speeds VMCG and VMBE. Let me introduce those properly, because they act as constraints on V1. In CAP 698, on page 2 of section 4, you’ll find the definition of V1. But more importantly, at the end of that paragraph, it says V1 must not be less than VMCG, and not greater than VR, and not greater than VMBE. So V1 is boxed in — it has to sit 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 speed we’ve been calculating. So V1 isn’t just a free choice; it’s constrained by these other speeds. Let’s look at VMBE specifically. VMBE stands for Maximum Brake Energy Speed. It represents 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 simply cannot 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 based on the brakes’ ability to absorb energy, not on runway length. You do need to be aware of the factors that control VMBE, but the good news is that most manuals — and CAP 698 itself — provide a VMBE graph or table with all the variables and factors 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 using two different masses. In that example, the heavier mass reduces VMBE. The variables that affect VMBE are pressure altitude and ambient air temperature — and we’ll pick up right there with those next.