Class A - Additional Take-off Procedures — Page 421, Lesson 512

I want to walk you through a really elegant procedure today — the take-off with increased V2 speed. This is one of the Class A additional take-off procedures, and it's a clever way to rescue a take-off that would otherwise be severely mass-limited. First, let's set the scene. This procedure is used when the performance limited mass is the climb limit mass. Let me unpack that. Every aeroplane has a maximum take-off mass, but it's often not the structural limit that bites — it's performance. You'll have a field limit mass, which is what the runway length will allow, and a climb limit mass, which is what the climb performance will allow. The performance limited mass is always the lower of the two. So if the climb limit mass is the lower one, that means the climb performance is poor and is severely restricting the potential mass of the aeroplane. That's exactly the situation this procedure is designed for. Now, before we go further, I need to make sure you understand a critical distinction. In the event of an engine failure, the initial climb-out speed is V2. But here's the key point: V2 is not the best climb angle speed. V2 is considerably slower than the best angle of climb speed, which is called VX. Let me give you a concrete feel for that. For a typical new generation 737, VX is 80 knots faster than V2. So if you climb out at V2, you get a climb angle much less than if you climbed out at VX. That's the whole problem — V2 gives you a shallow climb, and that's what's killing your climb performance. So what does the improved climb procedure aim to do? It aims to increase V2 to be closer to VX. That will greatly enhance the climb performance. The logic is simple: if V2 is the problem because it's too slow, push it up toward VX, and your climb angle improves dramatically. Let me illustrate this with a worked example, because that's the clearest way to see it. Suppose the field limit mass is 61,000 kg and the climb limit mass is 52,000 kg. The performance limited mass is always the lower mass, so the mass for take-off must be 52,000 kg. Now, that's a shame, because the runway can allow a far greater mass — 61,000 kg. So taking off with only 52,000 kg means there would be a significant proportion of the runway left unused. And that leftover runway is the clue to the solution. Here's the trick. With all that excess runway, it would be possible to stay on the runway for longer during the take-off to build up more speed. That extra time on the ground lets you accelerate to a higher speed before you rotate. This ensures that at rotation and at the screen height, a faster V2 will be reached. And because that faster V2 is much closer to VX, the climb performance significantly improves. Let me make sure you've got the sequence straight, because this is the heart of the procedure. You have excess runway. You use it to stay on the ground longer and build more speed. You rotate at a higher speed, so at the screen height — that's the height at the end of the runway where you must have reached a certain climb capability — you're already flying at a V2 that's closer to VX. And that's what gives you the improved climb angle. Now, I want you to look at the sample data for a runway with 2 mm contamination. That's Figure 15.3. This shows you the kind of data you'd work with in practice — the numbers for a contaminated runway, which is exactly the sort of situation where climb performance might be the limiting factor. And there's also Figure 15.2, which shows the procedure itself — the sequence of steps you'd follow. So the key takeaways I want you to hold onto: this procedure is for when climb limit mass is the performance limited mass. V2 is not the best climb angle speed — VX is, and it's much faster. The procedure increases V2 toward VX by using excess runway to build more speed before rotation. And the payoff is a significantly improved climb performance. That's the increased V2 take-off procedure.