Single-engine Class B - Landing — Page 336, Lesson 408

Let's walk through this landing-mass rule for a single-engine Class B aeroplane. This is the heart of how we decide, before we even take off, what mass the aeroplane may be despatched at — and it all hinges on two assumptions. First, the two despatch assumptions. The first one is that the aeroplane will land in still air — that is, zero wind. The second is that the aeroplane will land on the most likely runway to be assigned, considering the probable wind speed and direction. We use both of these together to pick the despatch mass. Let me show you with the example in Figure 10.3. Look at the column labelled "STILL AIR MASS". Here, runway 04 and runway 22 both allow a maximum landing mass of 1500 kg. But runway 31 and runway 13 are much shorter runways, so they allow a maximum landing mass of only 1000 kg. So in still air, the most favourable runway is either 04 or 22, because both allow that higher 1500 kg. Those two masses are highlighted in red. Now the second assumption — the aeroplane will land on the most likely runway considering the probable wind. Look at the second column, "FORECAST WIND MASS". Runway 04 has a strong headwind for landing. A headwind reduces your groundspeed on approach, so it lets you land at a much greater mass than in still air. In this example, that headwind has increased the maximum landing mass to 1750 kg. Runway 22, however, has a tailwind, which increases your groundspeed and therefore decreases the maximum landing mass — down to 800 kg. Runway 31 and runway 13 both have a full crosswind, so their maximum landing mass stays the same as for still air — 1000 kg. So under the second assumption, the most likely runway is 04, with a maximum landing mass of 1750 kg, highlighted in red. Now here's the critical part — what do we actually use as the despatch mass? If we despatched the aeroplane at 1750 kg, and then on arrival the wind was less than 15 knots, the aeroplane would not be able to land. That headwind benefit would disappear. So we must despatch at 1500 kg. Despatching at 1500 kg means that, no matter what wind conditions prevail at the destination, the aeroplane will have a mass that allows it to land. That's the safety principle — we take the lower of the two, so we're never caught overweight on arrival. So to summarise: the first assumption gives us 1500 kg on runway 04 or 22; the second assumption gives us 1750 kg on runway 04. The despatch mass is the conservative 1500 kg, because that guarantees we can land regardless of the wind that actually greets us.