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

Let’s pick this up right where the landing-mass logic starts to bite. We’re looking at the landing case for a single-engine Class B aeroplane, and the whole point here is the two despatch assumptions that decide how heavy we’re allowed to be when we take off. First, the still-air assumption. We assume zero wind, and we look at the “STILL AIR MASS” column in Figure 10.3. Runway 04 and runway 22 both allow a maximum landing mass of 1500 kg. Runway 31 and runway 13 are much shorter runways, so they only allow 1000 kg. So in still air, the most favourable runway is either 04 or 22, both at 1500 kg — those two masses are highlighted in red in the figure. Now the second despatch assumption. This one says the aeroplane will actually land on the most likely runway to be assigned, considering the probable wind speed and direction. So we move to the “FORECAST WIND MASS” column. Runway 04 has a strong headwind for landing, and a headwind lets you land at a greater mass — in this example it raises the maximum landing mass to 1750 kg. Runway 22 has a tailwind, which 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 still air — 1000 kg. So under the second assumption, the runway most likely to be assigned is runway 04, at 1750 kg, and that mass is highlighted in red. So the first assumption says pick runway 04 or 22 for a still-air maximum of 1500 kg. The second assumption says pick runway 04 for 1750 kg. Now here’s the crucial part — what do we actually despatch at? If we despatched at 1750 kg, and then on arrival the wind was less than 15 knots, the aeroplane would not be able to land. That headwind that gave us the extra margin would be gone. So we must despatch at 1500 kg. That way, no matter what wind conditions prevail at the destination, the aeroplane will have a mass that allows it to land. That’s the safe despatch mass — the still-air figure, not the forecast-wind figure.