Single-engine Class B - Landing — Page 327, Lesson 404

Let’s pick this up right where the numbers matter most. We’ve just worked out that the gross landing distance for this light single is 1200 ft, and the regulation demands we land within 70% of the landing distance available. So we multiply 1200 by 1.43 — that’s the factor that turns gross into the required runway length — and we get 1716 ft. That 1716 ft is the net performance, the one-in-a-million worst-case scenario. The gross is 1200 ft, but net is always worse than gross, and it’s the figure the regulation actually cares about. So the rule, which you’ll find in CAP 698, Section 2, top of page 9, states that the landing distance, measured from a screen height of 50 ft, must not exceed 70% of the landing distance available. If the aeroplane can’t come to a full stop within that length, you have two levers to pull: select a higher flap setting, or reduce the mass of the aeroplane. Both shorten the landing distance. Now, the regulation doesn’t just let you use any number. CS-23 tells us exactly what must be accounted for when calculating that gross landing distance. Let me walk you through each one, because each is a real, physical effect on how far you’ll roll. First, the pressure altitude at the aerodrome. Higher pressure altitude means thinner air, which means higher true airspeed for the same indicated speed, so you land longer. Second, standard temperature — the calculation assumes a standard day, so if it’s hotter, you’ll need more distance. Third, the runway surface conditions and the type of runway surface — a wet, icy, or grass runway gives you less friction, so you stop longer. Fourth, the runway slope — landing downhill increases distance, uphill reduces it. Fifth, and this one is precise: you must account for not more than 50% of the reported headwind component, or not less than 150% of the reported tailwind component. A headwind helps you stop, but the regulation only lets you credit half of what’s reported. A tailwind hurts you, and the regulation forces you to assume 150% of what’s reported — a safety margin against the wind shifting. Finally, the despatch rules for scheduled or planned landing calculations, per EU-OPS 1.550 (c). So the whole picture: gross is the ideal, net is the worst case, and the 70% rule is the bridge between them. Every factor in that list is there to make sure the net figure is genuinely conservative.