Class A - Landing — Page 470, Lesson 586

Let’s start with the big picture. When we talk about landing performance for a Class A aeroplane, the very first thing we do is establish the maximum mass at which we are allowed to land. That maximum landing mass is not a single number — it is the lesser of three separate limits. I want you to hold that word “lesser” because it is the whole point. We compute three candidate masses, and the smallest one wins. The three candidates are: the landing climb limit mass, which is the maximum mass that still lets the aeroplane meet its landing climb requirements; the field length limit mass, which is set by how much runway we actually have; and the structural limit mass, which is what the airframe itself can physically withstand on touchdown. So the certified maximum landing mass is whichever of those three is the most restrictive. Now let’s dig into the first of those, the landing climb requirement. This is governed by CS-25.119, and it applies with all engines operating. The rule demands a climb gradient of not less than 3.2%. A gradient of 3.2% means that for every 100 units of horizontal distance, the aeroplane must be able to gain at least 3.2 units of height. That is the bare minimum climb performance we must demonstrate. But that gradient is only valid under a very specific set of conditions, and I want you to memorise each one because they all feed into the certification test. First, all engines are operating at the power available 8 seconds after the pilot initiates movement of the thrust control from the minimum flight idle position to the take-off position. So we are not talking about instant full power — we are talking about the power that exists eight seconds after the throttles start moving. Second, the aeroplane is in the landing configuration, meaning the gear is down and the flaps are set for landing. Third, we are at the aerodrome altitude — that is the elevation of the landing field. Fourth, we use the ambient temperature expected at the time of landing. And fifth, we climb at a speed called VREF. Now, VREF is the reference landing speed, and it is not a free choice. The regulations impose three constraints on it. VREF must be not less than VMCL — that is the minimum control speed in the landing configuration, the slowest speed at which you can still maintain directional control. VREF must also be not less than 1.23 times VSR0 — and VSR0 is the reference stall speed in the landing configuration, so 1.23 times that stall speed gives you a safety margin above the stall. And finally, VREF must provide the manoeuvring capability specified in CS-25.143(h), which is the handling and manoeuvrability requirement. So VREF is the lowest speed that satisfies all three of those conditions at once. Now, there is a second, separate climb requirement, and this one is for a discontinued approach — that is, when you have to abandon the approach and climb away. This is governed by CS-25.121(d), and here the critical difference is that one engine is inoperative. The required climb gradient depends on how many engines the aeroplane has. For a 2-engine aeroplane, the gradient must be not less than 2.1%. For a 3-engine aeroplane, it is 2.4%. And for a 4-engine aeroplane, it is 2.7%. Notice the pattern — the more engines you have, the steeper the required gradient, because losing one engine out of four is a smaller proportional loss of thrust. The conditions for this discontinued approach climb are also specific. The critical engine is inoperative — that is the engine whose failure would have the most adverse effect on performance and handling. The remaining engines are at go-around thrust, which is the maximum thrust used for a go-around. The landing gear is retracted. The flaps are in the approach configuration, and there is a subtle condition here: the approach flap stall speed, VSR, must not exceed 110% of the landing flap stall speed. That is a way of ensuring the approach flap setting is not too different from the landing flap setting. We are at aerodrome altitude and ambient temperature, just like before. The speed is the normal approach speed, but it must not be greater than 1.4 times VSR. And we are at maximum landing weight — so this is the worst-case weight for the climb. Now here is the key interaction. 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. So we do not just take one of them — we take whichever is more restrictive, and that becomes the governing limit. Figure 18.1 shows a typical presentation of this data, and I want you to look at it because it shows how these two requirements combine into a single limiting curve. Finally, there is a third requirement, and this one comes from a different regulation — EU-OPS 1.510 — for a discontinued approach instrument climb. This applies specifically to instrument approaches with decision heights below 200 feet. A decision height is the altitude at which you must decide whether to continue the approach or go around. For these low approaches, the operator must verify that the approach mass of the aeroplane — and note, that is the mass at the start of the approach, taking into account the take-off mass and the fuel expected to be consumed in flight — allows a missed approach gradient of climb. The conditions are: the critical engine failed, and the speed and configuration are those used for go-around. The required gradient is at least 2.5%, or the published gradient for that particular approach, whichever is the greater. So if the published procedure demands a steeper gradient than 2.5%, you must meet the published one. So to tie it all together: the maximum landing mass is the lesser of three limits — landing climb, field length, and structural. The landing climb limit itself is governed by the more limiting of the all-engines-operating 3.2% gradient and the one-engine-inoperative discontinued approach gradient. And for low instrument approaches, there is an additional verification that your approach mass lets you climb away at 2.5% or the published gradient, whichever is greater. That is the complete landing mass picture.