Class A Aircraft - Take-off — Page 379, Lesson 465

Let's start with the heart of this whole topic — the graph in Figure 14.1, which shows the ideal position of V1. V1 is the decision speed. Up to V1, you can still abort the take-off and stop safely on the remaining runway. Above V1, you are committed — you must continue the take-off even if an engine fails. So V1 is the critical balance point between "stop" and "go." Now, look at the graph. There are two curves plotted against V1. One is the take-off distance required — that's the runway length you need to get airborne. The other is the accelerate-stop distance required — that's the runway length you need to bring the aeroplane to a complete stop if you abort. As V1 changes, these two distances change in opposite directions. Here's the key insight. At the intersection point of the two curves, you get the best V1 to use. Why? Because at that speed, the total field required — the runway you need — is at its minimum. That intersection V1 is sometimes called the "Idealized V1." It's also called the "Balanced V1," because at that speed the take-off distance required equals the accelerate-stop distance required. The two required distances are balanced. Now, what happens if you move away from that intersection? Let's say V1 is increased — you decide to commit to the take-off later, at a higher speed. The accelerate-stop distance required increases, because you're travelling faster when you finally decide to stop, so you need more runway to stop. The take-off distance required decreases, because you're already moving faster when you rotate, so you need less runway to get airborne. But here's the catch — the total field required increases. You need more runway overall. Now reverse it. Reduce V1 below the intersection. The accelerate-stop distance decreases — you decide to stop earlier, so you need less runway to stop. The take-off distance required increases — you're rotating at a lower speed, so you need more runway to get airborne. And again, the total field required increases. So in both directions, moving away from the balanced point costs you runway. The balanced V1 is the sweet spot. Now, what factors can influence V1? In essence, anything that changes either of those two curves — the accelerate-stop distance or the take-off distance — will affect V1. But there's a simpler way to think about it. Every aeroplane flight manual contains tables or graphs that let you calculate the correct V1 for any given day. In CAP 698, section 4, pages 18 and 19, you'll find such tables. Let's look at page 18, the second table from the top. You'll see columns labelled "A," "B," and so on. This table lists the three V speeds — V1, V2, and Vr — against the aeroplane mass. So we can use it to see the effect of mass on V1. Look under column A, under V1. If the aeroplane mass is 50,000 kg, V1 is 129 knots. Increase the mass to 65,000 kg, and V1 rises to 151 knots. So the relationship is clear: increasing mass increases V1. A heavier aeroplane needs a higher decision speed, because it needs more runway to accelerate and stop, and the balance point shifts accordingly. That's the core of V1 — the balanced decision speed, and how mass shifts it.