Class A - Landing — Page 470, Lesson 586

Right, let's get into Class A landing performance. This is where we figure out the heaviest we can be when we touch down, and it's governed by three separate limits. The maximum mass for landing is the lesser of three values: the landing climb limit mass, the field length limit mass, and the structural limit mass. Let's unpack each of those. The landing climb limit mass is the maximum mass at which the aircraft can still achieve the required landing climb performance. The field length limit mass is the maximum mass at which the landing distance available is sufficient for the required landing distance. And the structural limit mass is simply the certified maximum weight the landing gear and structure are designed to handle on touchdown. We take the most restrictive of the three — the smallest number — and that's our absolute ceiling for landing weight. Now, the heart of this is the landing climb requirement, which is defined in CS-25.119. This is a certification standard, and it demands a climb gradient of not less than 3.2% with all engines operating. Let me break down the conditions attached to that gradient, because they're very specific. First, all engines are operating at the power available 8 seconds after initiation of movement of the thrust control from the minimum flight idle to the take-off position. So this isn't instant thrust — it's the power you'd actually have 8 seconds after you slam the levers forward. Second, the aircraft is in the landing configuration — gear down, flaps in landing position. Third, we're at the aerodrome altitude and the ambient temperature expected at the time of landing. And fourth, the climb is conducted at a speed of VREF. Now, VREF itself has its own definitions. It must be not less than VMCL — that's the minimum control speed in the landing configuration, the speed below which you can't maintain directional control with the critical engine failed. It must also be not less than 1.23 times VSR0 — that's 1.23 times the reference stall speed in the landing configuration, giving you a safety margin above the stall. And finally, VREF must provide the manoeuvring capability specified in CS-25.143(h), which is the certification requirement for adequate control authority during the approach and landing. So that's the all-engines-operating case. But we also have to consider the discontinued approach climb, which is the one-engine-inoperative case, covered by CS-25.121(d). Here the required climb gradient depends on the number of engines: 2.1% for a 2-engine aircraft, 2.4% for a 3-engine aircraft, and 2.7% for a 4-engine aircraft. The more engines you have, the more climb performance you're expected to retain with one failed. The conditions here are different. The critical engine is inoperative — that's the engine whose failure gives the worst performance. The remaining engines are at go-around thrust. The landing gear is retracted. The flaps are in the approach configuration, but there's a catch: this is only allowed provided that the approach flap stall speed, VSR, does not exceed 110% of the landing flap VSR. So the approach flap setting can't be so much less effective than the landing flap setting that it significantly raises the stall speed. We're also at aerodrome altitude and ambient temperature, flying at the normal approach speed but not greater than 1.4 times VSR. And this is all at maximum landing weight. So this is the worst-case go-around: one engine dead, gear up, flaps in approach, at max weight, at altitude and temperature. Now, here's the key takeaway: the more limiting of the landing climb requirement and the approach gradient requirement will determine the maximum mass for altitude and temperature at the landing aerodrome. In other words, we check both the all-engines 3.2% case and the one-engine-inoperative case, and whichever one gives us the lower allowable mass is the one that governs. Figure 18.1 shows a typical presentation of this data — you'd see the mass plotted against altitude and temperature, with the limiting curve shown. Finally, there's a separate operational requirement from EU-OPS 1.510 for the discontinued approach instrument climb. For instrument approaches with decision heights below 200 feet, the operator must verify that the approach mass of the aeroplane — which is the take-off mass minus the fuel expected to be consumed in flight — allows a missed approach gradient of climb of at least 2.5%, or the published gradient, whichever is the greater. This is with the critical engine failed and with the speed and configuration used for go-around. So even on a low-visibility approach, if you have to go around with an engine out, you must be able to climb away at that gradient. So to tie it all together: landing mass is capped by three limits — climb, field length, and structure. The climb limit itself has two certification cases — all engines at 3.2% and one engine out at 2.1%, 2.4%, or 2.7% depending on engine count. And then there's the operational EU-OPS requirement for low decision-height approaches. The most restrictive of all of these is what we plan to.