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

Right, let’s pick this up with the wet runway take-off distances. We’ve already worked out the dry runway case, and now we’re adding the wet runway complication. The key idea is that the manufacturer calculates three separate distances, and the greatest of the three becomes the certified net take-off run required. Let me give you the three distances we’re comparing. First, all engines operating on a dry runway — that’s the baseline. Second, all engines operating on a wet runway. Third, one power unit inoperative on a wet runway. Let’s take the second one first. All engines operating, wet runway. The distance is measured horizontally from the brake release point — that’s the BRP, the exact spot where the brakes are released and the take-off roll begins — to the point where the aeroplane is 15 feet above the take-off surface. And that 15-foot height must be achieved in a manner consistent with attaining V2 by 35 feet. So the climb path is constrained: you must be able to reach V2, the take-off safety speed, by the time you’re 35 feet high. The 15-foot point is just an intermediate marker on that same climb path. Now the third distance. One power unit inoperative, wet runway. Same horizontal measurement from BRP to the point where the aeroplane is 15 feet above the surface, again consistent with attaining V2 by 35 feet. But here we assume the critical power unit fails at VEF — that’s the engine failure speed, the speed at which the critical engine is assumed to fail for performance calculations. So the whole distance is computed assuming you lose that critical engine at VEF. Now, in our example, let’s say the all-engines-operating wet runway distance is 1950 metres. And the one-engine-inoperative wet runway distance is 2001 metres. The dry runway distance we already had — let’s say 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 dry runway case wins. In the exam, you’ll be given various distances and you must be able to identify which one is selected as the net take-off run required. It’s always the greatest of the three. Now let’s move to the net accelerate-stop distance required. This is the distance needed to accelerate and then stop if the take-off is rejected. On a wet runway, the accelerate-stop distance is the greatest of three values. First, all engines operating. The sum of two distances: the distance to accelerate from BRP to the highest speed reached during the rejected take-off, assuming the pilot takes the first action to reject at V1 for take-off from a wet runway, and then decelerate to a full stop on a wet hard surface. Plus a distance equivalent to 2 seconds at that same V1. Second, one engine inoperative. The sum of the distances to accelerate from BRP 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 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 again a distance equivalent to 2 seconds at the V1 for take-off from a wet runway. Third, the accelerate-stop distance on a dry runway. So the wet runway accelerate-stop distance is the greatest of those three: all engines operating, one engine inoperative, and the dry runway value. The 2-second allowance is added to account for the time between the decision to reject and the actual first action — that’s the recognition and reaction time built into the calculation. Let me make sure you’ve got the distinction clear. The net take-off run required is about getting airborne — the greatest of the three take-off distances. The net accelerate-stop distance required is about stopping if you reject — the greatest of the three stopping distances. Both use the same logic: take the worst case, the greatest of the calculated values. One more thing to note: in the one-engine-inoperative accelerate-stop case, the rejection is assumed to happen at V1 for take-off from a wet runway with all engines operating. So even though the engine has failed at VEF, the pilot’s decision speed is still the wet runway V1 as if all engines were working. That’s a subtle but important detail — the V1 used for the rejection decision is the all-engines-operating wet runway V1. So to summarise: for the net take-off run required, compare the three take-off distances and pick the greatest. For the net accelerate-stop distance required, compare the three stopping distances and pick the greatest. In both cases, the wet runway calculations include the 2-second allowance and the specific assumptions about engine failure at VEF and rejection at V1.