We're now into the heart of take-off performance — the field length… — Page 372, Lesson 455

We're now into the heart of take-off performance — the field length requirements. Let me set the scene, because this is where the regulations start to bite. The core idea is a set of hard limits. For any take-off, the accelerate-stop distance must not exceed the accelerate-stop distance available. The take-off distance must not exceed the take-off distance available. And the take-off run must not exceed the take-off run available. Those are three separate checks, and each one has its own "available" figure from the aerodrome and its own "required" figure we calculate from the aircraft's performance. To comply with the regulations, we must use a single V1 speed — that's the decision speed, the one speed at which, if an engine fails, you can either stop or continue. We'll examine V1 later. And we must account for the aerodrome conditions — things like runway surface, slope, wind, and temperature. Now, to determine the maximum permissible mass for take-off, we have to consider a whole set of limits. Let me list them, because each one is a separate ceiling on how heavy we can be: - The aerodrome distances available — that gives us the Field Limit Mass. - The climb requirements — that's the Climb Limit Mass. - Obstacle clearance — the Obstacle Limit Mass. - Brake energy limitations — that's VMBE, the maximum brake energy speed. - Tyre speed limitations — the Tyre Speed Limit Mass. - Runway strength limitation — that's ACN/PCN, the Aircraft Classification Number versus Pavement Classification Number. - And finally, the maximum structural mass. The take-off mass we actually use is the lowest of all of these. Each one is a separate constraint, and the limiting one wins. Now let's get into how these distances are actually calculated — the field length requirements. The key word here is "net." The authorities have built in safety margins, and the net distances are what we use for certification. These distances are defined in CS-25 — that's the European certification specification for large aeroplanes — and 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 commit these to memory — CAP 698 explains them suitably, and they're listed over the page and in CAP 698. This is a reference document, not a memory test. So let's look at the Net Take-off Run Required. This applies when the take-off distance includes a clearway — that's an area beyond the runway that's clear of obstacles, which you can use in the take-off distance calculation but not in the take-off run. When a clearway is involved, the take-off run is the greatest of two things. The first is with all power units operating, on both dry and wet runways. Here's the calculation: take the total of the gross distance from the start of the take-off run to the point at which VLOF is reached — VLOF is the lift-off speed, the speed at which the aeroplane first becomes airborne. Then add one half of the gross distance from VLOF to the point at which the aeroplane reaches 35 feet. That whole total is then factored by 1.15 to obtain the net TORR — the net take-off run required. Let me give you the worked example. Suppose the distance from brake release to halfway between VLOF and 35 feet is 1747 metres. We multiply that by 1.15. So 1747 metres times 1.15 gives us 2009 metres. That's the net take-off run required for the all-engines-operating case. The second case is 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 at which 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 performance. So you see, the take-off run required is the greater of these two — the all-engines-operating case with its 1.15 factor, or the engine-failure case. And that's the figure we compare against the take-off run available. Let me just make sure the picture is clear. The 35-foot height is the screen height — the obstacle clearance height we must reach. The 1.15 factor is the safety margin the authorities have built in. And the "net" distance is the certified, factored distance we use for compliance. That's the discipline of this whole subject — every distance we use has a safety margin baked in, and we must never exceed what the aerodrome actually provides.