Now, here is the clever part — Page 387, Lesson 486

Let’s pick this up right where the idea of the field limit mass comes from. I want to walk you through what it means for the accelerate-stop distance required to be within the accelerate-stop distance available, and why the graph we use only gives you one distance to work with. First, the key phrase: the accelerate-stop distance required must be within the accelerate-stop distance available. The accelerate-stop distance required is the distance the aircraft needs to accelerate to a decision speed, then reject the take-off and brake to a full stop. The accelerate-stop distance available, or ASDA, is the physical runway plus any stopway that is certified for that manoeuvre. So the rule is simple: what the aircraft needs must fit inside what the airfield offers. Now, here is the clever part. Even though an airfield has several different distances — the take-off run available, the take-off distance available, and the accelerate-stop distance available — the graph we use assumes they are all the same length. Why? For simplicity. The graph assumes that the take-off run available, the take-off distance available, and the accelerate-stop distance available are identical, even though in reality the take-off distance available and the accelerate-stop distance available may be longer. That means no stopways and no clearways are accounted for in this graph. A stopway is an area beyond the runway that can be used for stopping, and a clearway is an area beyond the runway that can be used for the initial climb. Neither is considered here. When the take-off distance available and the accelerate-stop distance available are the same, we describe the field as being balanced. And in this balanced case, the balanced field length also happens to be the same length as the take-off run available, precisely because there are no stopways or clearways. So when you use this graph, you must only enter the take-off run available as the length of field available. That is the single distance you plug in. Let me show you what a typical balanced field length graph looks like — it is Figure 14.11 in our material, and it is exactly the same as the one in CAP 698 on page 9 of section 4. Notice at the bottom of the graph there is only one field distance to enter. But, of course, an airfield has many distances, such as the TODA — that is the take-off distance available — and the ASDA, the accelerate-stop distance available. Because there is only one distance to enter into the graph, it must be the balanced field length. The introduction to the graph is at the top of page 7 of section 4 in CAP 698, and it reiterates that the graph assumes a balanced field. So if your field is unbalanced — meaning the take-off distance available and the accelerate-stop distance available are not the same — you do not use this graph directly. Instead, you use the information under paragraph 2.5.1 on page 16 of section 4. That latter information is specifically for adjusting V1 when the field is unbalanced. V1 is the decision speed — the speed up to which you can reject the take-off and still stop on the remaining runway, and beyond which you must continue the take-off. So the whole picture is this: the balanced field graph gives you one distance to enter, it assumes all three distances are equal, it ignores stopways and clearways, and if your field is not balanced, you adjust V1 using the separate procedure. That is the core of how we determine the field limit mass from this graph.