First, the accelerate‑stop distance must not exceed the accelerate‑stop… — Page 372, Lesson 455

Let’s start with the big picture, because this is the heart of take‑off performance. When we certify an aeroplane for take‑off, the regulations demand that three distances never exceed what the runway actually gives us. First, the accelerate‑stop distance must not exceed the accelerate‑stop distance available. Second, the take‑off distance must not exceed the take‑off distance available. And third, the take‑off run must not exceed the take‑off run available. So we have three pairs — required versus available — and every one of them has to fit inside the physical runway. To make that work, we use a single V1 speed. V1 is the decision speed — the speed at which, if an engine fails, you can either continue the take‑off or abort it, and still stay within the runway limits. We’ll examine V1 in detail later, but for now, understand that the whole distance‑matching exercise hinges on picking one V1. And we also have to account for the aerodrome conditions — things like runway surface, slope, wind, and temperature. Now, to find the maximum permissible mass for take‑off, we don’t just take one number. We have to consider a whole set of limits, and the actual take‑off mass is the lowest of all of them. Let me list them, because each one is a separate constraint. There’s the aerodrome distances available, which gives us the Field Limit Mass. There’s the climb requirements, giving the Climb Limit Mass. There’s obstacle clearance, giving the Obstacle Limit Mass. There’s brake energy limitations, which is tied to VMBE — that’s the maximum brake energy speed, the speed beyond which the brakes can’t absorb the energy of an aborted take‑off without overheating. Then there’s tyre speed limitations, giving the Tyre Speed Limit Mass. There’s runway strength limitation, expressed as ACN/PCN — Aircraft Classification Number over Pavement Classification Number, which tells us whether the runway can bear the aeroplane’s weight. And finally, the maximum structural mass. So the take‑off mass is the smallest of these seven limits. Now let’s get into how those distances are actually calculated — the take‑off run, the take‑off distance, and the accelerate‑stop distance. The key word here is “net.” The authorities build in safety margins, so the distances we use for certification are net distances, not the raw gross distances. These definitions come from CS‑25, which is the European certification specification for large aeroplanes. They cover two cases: take‑off with all engines operating, and take‑off with engine failure. And they cover both dry and wet runways. You’ll find abbreviated versions of these requirements in CAP 698, on page 7 of section 4, under paragraph 2.1.2. Don’t try to memorise them — CAP 698 lays them out clearly, and they’re also listed over the page. Let’s look at the first one: Net Take‑off Run Required, or TORR. This applies when the take‑off distance includes a clearway. A clearway is an area beyond the runway, clear of obstacles, that you can use for the initial climb. When a clearway is involved, the take‑off run is the greatest of two values. The first value is for all power units operating, on both dry and wet runways. Here’s the calculation. You take the gross distance from the start of the take‑off run to the point where VLOF is reached. VLOF is the lift‑off speed — the speed at which the aeroplane first becomes airborne. Then you add one half of the gross distance from VLOF to the point where the aeroplane reaches 35 feet. So you’re going from brake release to VLOF, then adding half the distance from VLOF up to 35 feet. Then you factor that whole total by 1.15. That 1.15 factor is the safety margin — it increases the distance by 15 percent to give you the net TORR. Let me give you the example from the text. Suppose the distance from brake release to halfway between VLOF and 35 feet is 1747 metres. That’s the total we just described — the distance to VLOF plus half the distance from VLOF to 35 feet. We multiply 1747 by 1.15, and that gives us 2009 metres. So the net take‑off run required is 2009 metres. The second value is for one power unit inoperative, on a dry runway. Here, the take‑off run is the horizontal distance from the brake release point — the BRP — to a point equidistant between VLOF and the point where the aeroplane reaches 35 feet, with the critical power unit inoperative. The critical power unit is the one whose failure has the most adverse effect on the aeroplane’s performance — typically the one that creates the most asymmetric thrust. So with that engine failed, you measure from brake release to a point that’s halfway between VLOF and 35 feet. So the take‑off run required is the greater of those two — the all‑engines‑operating value factored by 1.15, or the one‑engine‑inoperative value on a dry runway. And remember, this whole calculation only applies when the take‑off distance includes a clearway. Let me show you what this looks like on the ground. — this figure illustrates how grass runways increase the take‑off distance compared to hard surfaces, which is one of those aerodrome conditions we have to account for. And here’s the obstacle clearance climb profile. — this shows the climb path when there’s no cloud, which ties into the Obstacle Limit Mass we mentioned earlier. So to sum up where we are: we have three distance limits — accelerate‑stop, take‑off distance, and take‑off run — each with a required value that must fit inside the available value. We pick a single V1 to make that work. And we determine the maximum take‑off mass by taking the lowest of seven limits, from field length through to structural mass. Then we calculate the net take‑off run required, using the 1.15 factor for all engines, or the critical‑engine‑inoperative distance on a dry runway, whichever is greater.