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

Let's start with the core idea of this section: the "Balanced V1" and what it means for your take-off planning. I want you to picture a graph with speed along the horizontal axis. On that graph we plot two curves. One curve is the accelerate-stop distance required — that's the total runway you need if you accelerate to a certain speed and then have to abort the take-off and stop. The other curve is the take-off distance required — the runway you need if you commit to becoming airborne at that same speed. Now, here's the key. As you increase V1, the accelerate-stop distance goes up, because you're committing to a higher speed before you decide to stop, so you need more room to stop. But the take-off distance goes down, because a higher V1 means you're rotating later and using more of the runway to accelerate, so the actual airborne distance is shorter. Where those two curves cross — that intersection point — is special. That speed is called the "Idealized V1", and it's also called the "Balanced V1". Why balanced? Because at that exact speed, the take-off distance required equals the accelerate-stop distance required. And that's the best V1 to use, because it requires the least total field length — the least amount of runway overall. Now, what happens if you move away from that intersection? Let's say you increase V1 above the balanced value. The accelerate-stop distance increases, the take-off distance decreases, but the total field required — the sum of the two — increases. Conversely, if you reduce V1 below the intersection, the accelerate-stop distance decreases, the take-off distance increases, and again the total field required increases. So either way, moving away from the balanced point costs you more runway. That's the whole point of the graph — it shows you why the balanced V1 is optimal. Now let's move to the factors that affect V1. The principle is simple: anything that changes either of those two curves — the accelerate-stop distance or the take-off distance — will shift V1. But there's a more practical way to think about it. In every aeroplane flight manual, and in CAP 698, there are tables and graphs that let you calculate V1 for any given day. Specifically, on pages 18 and 19 of section 4 in CAP 698, there's a table with columns labelled "A", "B", and so on, listing the three V speeds — V1, V2, and VR — against aeroplane mass. Let's look at the effect of mass. Under column A, for V1: if the aeroplane mass is 50,000 kg, V1 is 129 knots. If you increase the mass to 65,000 kg, V1 increases to 151 knots. So the relationship is direct — increasing mass increases V1. That makes sense physically: a heavier aeroplane needs more speed to generate the lift and control authority required for a safe take-off, so your decision speed goes up. So to summarise what we've covered: the balanced V1 is the speed where take-off distance equals accelerate-stop distance, giving you the minimum field length. Deviating from it in either direction increases the total field required. And mass is one factor that shifts V1 — more mass, higher V1.