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

Let's pick up with the climb limit brake release mass. This is the next mass we have to consider after the field limit mass, and it's a big one. The climb limit mass is sometimes called the Weight Altitude Temperature limit, or the Mass Altitude Temperature limit — abbreviated to the WAT or MAT limit. So you'll see "WAT limit" or "MAT limit" used interchangeably. The idea is that this mass is a function of weight, altitude, and temperature. Now, what does this mass actually mean? The climb limit mass is the maximum mass that will enable the aeroplane to achieve a certain minimum climb performance. That minimum climb performance is defined as the most severe of the climb gradient requirements. And the most severe climb gradient requirement is 2.4% — we'll cover that in detail later. Here's the key logic. If the aeroplane's mass were greater than the climb limit mass, the aeroplane might still be able to climb — it would still get airborne and go up — but it would not achieve the minimum air gradients that the authorities have laid down. In other words, it would not meet the climb requirements. So this is a regulatory performance limit, not just a physical one. One important point: these gradients are air gradients, and because they're air gradients, they are unaffected by wind. So wind doesn't change this limit. The graph for calculating this mass is Figure 4.5, which is on page 11 of section 4 in CAP 698. And Figure 14.12 in our book shows a typical presentation of the climb limited take-off mass — that's the same graph you'd find in CAP 698 on page 11 of section 4. Now let's move on to the tyre speed limit mass. This one is about heat. There is naturally resistance between the wheel and the runway. As the wheel rotates, that resistance generates heat. The greater the wheel speed, and/or the greater the load on the wheel, the greater the heat generated. Too much heat is dangerous for two reasons. First, it can disintegrate the tyre. Second, it can expand the air within the tyre and over-pressurize it. That over-pressurization can result in a tyre blow out. Modern tyres do have fusible plugs to help prevent this — those are designed to melt and release pressure before a blowout — but the limit is still there. So there is a maximum ground speed and a maximum mass that the wheels can be subjected to. The maximum ground speed the tyre will experience is at VLOF — that's the lift-off speed, the speed at which the aeroplane actually leaves the runway. As a result, tyre speed limits are designed to be greater than or equal to the fastest VLOF. So the tyre is rated to survive the worst-case lift-off speed. For most medium range jets, the maximum tyre speed limit is set at 195 knots, which is about 225 miles per hour. Figure 14.13 shows a typical presentation of the tyre speed limited take-off mass graph — that's found in CAP 698 on page 13 of section 4. So to summarise where we are: we've got the field limit mass, then the climb limit mass — the WAT or MAT limit — which is about meeting the minimum climb gradient of 2.4%, and now the tyre speed limit mass, which is about not overheating the tyres at VLOF. Each of these gives you a different maximum mass, and the take-off performance is limited by the most restrictive of them all.