Single-engine Class B Aircraft - Take-off — Page 302, Lesson 376

Let’s pick this up right where the take-off distance correction factors live, because that’s the heart of what we’re about to apply. I’m talking about the single-engine Class B aeroplane, and we’re working through the take-off performance data as it appears in CAP 698. First, the slope correction. At the top of page 2, section 2, under the table, point “d” tells you exactly what to do when the runway has a slope. If you have an upslope — that is, the runway rises as you roll — you must increase your take-off distance by 5%, or multiply by a factor of 1.05, for every 1% of upslope. So a 2% upslope means you multiply your take-off distance by 1.10, and so on. That’s a mandatory correction. But here’s the subtle part, and I want you to remember this because it’s a classic trap. The book states that “no factorization is permitted for downslope.” That means if the runway slopes downwards — the aeroplane is rolling downhill — you apply no correction factor at all. You do not reduce the take-off distance. Why? Because a downslope will decrease the take-off distance. Gravity is helping you accelerate, so the distance naturally shortens. By refusing to factor that benefit in, you build a little extra safety margin into your calculation. You plan for the worse case, even though the physics is on your side. Now, how is this data actually presented? The take-off distance required is usually given in graphical form. If you look at Figure 2.1 on page 3 of section 2 in CAP 698, you’ll see the graph. The title of that graph tells you it’s for calculating the gross take-off distance with no flaps selected. That’s a critical qualifier — if you’re using a different flap setting, you may have to consult another graph entirely. You can’t just reuse this one. Look at the associated conditions at the top of that figure, and pay particular attention to the power and flap settings. The graph assumes a runway that is paved, level, and dry. Those are the baseline conditions. If your actual runway differs from any of those — say it’s grass, or wet, or sloped — then you must apply corrections to the values the graph gives you. Those are the correction factors we just touched on. There’s also a small box in the middle of the graph that highlights the rotation speed and the screen height speed for different weights. Rotation speed is the speed at which you rotate the aeroplane to lift off, and screen height speed is the speed at the point where you clear the specified screen height — the imaginary obstacle at the end of the take-off distance. You must adhere to these speeds accurately, because they are the exact speeds that were used to construct this graph. If you fly a different rotation speed, the graph’s distances no longer apply to your situation. So the takeaway here is a chain: the graph gives you a gross distance under ideal, specified conditions — paved, level, dry, no flaps, specific power. You then correct that distance for any deviation from those conditions, using the factors we discussed, including the slope rule where upslope is factored but downslope is ignored. And you fly the speeds the graph was built on, or the whole calculation falls apart.