As an example, let us assume that this distance is 1950 metres — Page 372, Lesson 457

I want to walk you through how the manufacturer arrives at the certified net take-off run required, and then we’ll look at the net accelerate-stop distance required. This is the heart of how we decide whether a runway is long enough for a given take-off. Let’s start with the take-off run. The manufacturer calculates three separate distances, and I’ll give you a concrete example so the numbers stick. The first distance is for all engines operating on a dry runway. Let’s assume that comes out to 1950 metres. The second distance is for one power unit inoperative on a wet runway. That’s defined as the horizontal distance from the brake release point — we call that the BRP — to the point at which the aeroplane is 15 feet above the take-off surface. And that 15-foot height must be achieved in a manner consistent with the attainment of V2 by 35 feet, assuming the critical power unit fails at VEF. Let me unpack that. The brake release point is exactly where you release the brakes to start the take-off roll. V2 is the take-off safety speed, the speed at which you climb after an engine failure. VEF is the engine failure speed — the speed at which we assume the critical engine fails. So this second distance is measured from the brake release point to where the aeroplane reaches 15 feet above the runway, and the climb profile must be such that you’ll reach V2 by the time you’re 35 feet high. Let’s assume this second distance is 2001 metres. The third distance is the accelerate-stop distance on a dry runway — we’ll define that properly in a moment, but for now assume it comes out to 2009 metres. Once the manufacturer has calculated all three, the greatest of the three is published as the certified net take-off run required. In our example, that’s 2009 metres — the largest of 1950, 2001, and 2009. In the exam you’ll be given various distances and you must be able to pick out which one is selected as the net take-off run required. The rule is simple: it’s always the greatest of the three. Now let’s move to the net accelerate-stop distance required. This is the distance you need to accelerate and then stop if you reject the take-off. On a wet runway, the accelerate-stop distance is the greatest of three things. The first is with all engines operating: the sum of the distance to accelerate from the brake release point to the highest speed reached during the rejected take-off, assuming the pilot takes the first action to reject at the V1 for take-off from a wet runway, plus a distance equivalent to 2 seconds at that same V1, and then decelerate to a full stop on a wet hard surface. So you accelerate, you reach V1, the pilot starts rejecting, and you add 2 seconds’ worth of travel at V1 before you begin decelerating to a full stop. The second is with one engine inoperative: the sum of the distance to accelerate from the brake release point to the highest speed reached during the rejected take-off, assuming the critical engine fails at VEF and the pilot takes the first action to reject at the V1 for take-off from a wet runway with all engines operating, and then decelerate to a full stop on a wet hard surface with one engine inoperative, plus a distance equivalent to 2 seconds at the V1 for take-off from a wet runway. Notice the difference — here the engine fails at VEF, and the deceleration happens with one engine inoperative, which means less braking effectiveness. The third is simply the accelerate-stop distance on a dry runway. So the net accelerate-stop distance required on a wet runway is the greatest of those three: all engines operating, one engine inoperative, and the dry-runway accelerate-stop distance. Let me tie this together. For the take-off run, we take the greatest of three distances — all engines dry, one engine inoperative wet, and dry accelerate-stop. For the accelerate-stop distance on a wet runway, we again take the greatest of three — all engines operating, one engine inoperative, and the dry accelerate-stop distance. The key skill for the exam is identifying which of the given distances is the greatest, because that’s the one that gets published as the required distance.