Class A Aircraft - Take-off — Page 395, Lesson 487

Let's pick up right where the field limit mass left off, because that's the foundation. The field limit mass is the maximum mass you can have and still physically get the aeroplane off the ground and stop it if you have to abort, within the runway available. But that's not the only limit we have to respect. There are several more, and the next one I want to walk you through is the climb limit brake release mass. Now, the name tells you a lot. It's the mass at the moment of brake release — that's the start of the take-off roll — that is limited by the climb requirement. The graph we use to calculate this mass is Figure 4.5, which you'll find on page 11 of section 4 in CAP 698. And you'll often hear this called by a couple of different names. It's sometimes referred to as the Weight Altitude Temperature limit, abbreviated to the WAT limit, or the Mass Altitude Temperature limit, the MAT limit. Same thing, just different words for the same concept. But before we work through the graph, let's make sure we actually understand what this mass means, because it's a subtle idea. The climb limit mass is the maximum mass that will enable the aeroplane to achieve a certain minimum climb performance. So it's not about getting airborne — it's about what happens after you're airborne. The key phrase there is "minimum climb performance," and that minimum is defined as the most severe of the climb gradient requirements. And I want to flag this number for you now: that most severe climb gradient requirement is 2.4%. We'll cover exactly where that comes from later, but for now, hold onto it. Here's the important logic. If your mass were greater than the climb limit mass, the aeroplane might still be able to climb. It's not that it physically can't get off the ground. But it will not achieve the minimum air gradients that the authorities have laid down. In other words, it would not meet the climb requirements that the regulations demand. So the climb limit mass is a regulatory performance limit, not a physical one. And one more crucial point about these gradients: they are air gradients, and because they're air gradients, they are unaffected by wind. That's a distinction you'll want to remember — wind doesn't change an air gradient. Figure 14.12 shows a typical presentation of the climb limited take-off mass, and again, that's found in CAP 698 on page 11 of section 4. Now let's move on to the next limit, and this one is a bit more physical and intuitive: the tyre speed limit mass. Let's think about why a tyre speed limit even exists. Naturally, there's resistance between the wheel and the runway. As the wheel rotates, that resistance generates heat. And here's the relationship to hold onto: the greater the wheel speed and/or the greater the load on the wheel, the greater the heat generated. So heat is driven by both speed and load. Why does too much heat matter? Two reasons. First, too much heat can disintegrate the tyre — literally break it apart. Second, it can expand the air within the tyre and over-pressurize it. That's dangerous because it can result in a tyre blow out. Now, modern tyres do have fusible plugs fitted to help prevent that blow out — those are designed to melt and release pressure before things get catastrophic — but the limit is still there for a reason. So because of all that, there is a maximum ground speed and a maximum mass that the wheels can be subjected to. And here's the key detail: the maximum ground speed that the tyre will experience will be at VLOF. VLOF is the lift-off speed — the speed at which the aeroplane actually leaves the runway. That's when the wheels are spinning fastest. As a result, tyre speed limits are designed to be greater than or equal to the fastest VLOF. So the tyre limit is set so it never becomes the binding constraint at the moment of rotation. For most medium range jets, the maximum tyre speed limit is set at 195 knots, which is about 225 miles per hour. So that's your number to remember: 195 knots. Figure 14.13 shows a typical presentation of the tyre speed limited take-off mass graph, found in CAP 698 on page 13 of section 4. So now you've got three limits stacking up: the field limit mass, the climb limit mass — the WAT or MAT limit — and the tyre speed limit mass. Each one is a separate ceiling on how heavy you can be for take-off, and the one that bites is the lowest of them. That's the logic we're building toward.