In the real world, an airfield has several different distances — Page 387, Lesson 486

All right, let's pick this up right where the idea of the field limit mass comes from. We've been talking about the accelerate-stop distance required, and the key condition we need is that this accelerate-stop distance required must be within the accelerate-stop distance available. That's the fundamental safety check. Now, here's the clever part, and the reason we only have one graph to work with. In the real world, an airfield has several different distances. You have the take-off run available, the take-off distance available, and the accelerate-stop distance available. These can all be different lengths. But the graph we use to calculate field limit mass assumes they are all the same length. Why? For simplicity. The graph just assumes that the take-off run available, the take-off distance available, and the accelerate-stop distance available are all identical, even though in reality the take-off distance available and the accelerate-stop distance available may be longer. Because of that assumption, no stopways or clearways are accounted for in this graph. A stopway is an area beyond the take-off run available that the aircraft can use to stop in an aborted take-off, and a clearway is an area beyond the runway that the aircraft can use for the initial climb. Neither of those exists in this graph's world. Now, when the take-off distance available and the accelerate-stop distance available are the same length, 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 the practical instruction for using the graph is this: make sure you only enter the take-off run available as the length of field available. That's the only distance you put into the graph. Let me show you what this looks like. This is a typical balanced field length graph. It's exactly like 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 the graph with. But, of course, an airfield has many distances, such as the TODA and the ASDA. TODA is the take-off distance available, and ASDA is 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 you have an unbalanced field, where those distances are not all the same, you don't use this graph directly. For unbalanced fields, 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 take-off decision speed, the speed at which you commit to taking off or abort. So when the field isn't balanced, you adjust V1 to make the distances work out. So the whole picture is: the graph gives you a simple, single-entry calculation based on a balanced field, and the take-off run available is your entry. If your field isn't balanced, you step out of the graph and use the V1 adjustment procedure instead.