Now, notice something important about the runway itself — Page 336, Lesson 412

Let’s pick this up right where the landing distance graph starts to do its real work. I want you to pay particular attention to the power and flap settings shown at the top of the graph. Those settings are the configuration the aeroplane is in for the entire landing calculation — the thrust you’re carrying and the flap position. They are fixed assumptions, and you must read them off the top of the graph before you do anything else, because the whole graph is built around that configuration. Now, notice something important about the runway itself. This graph assumes a runway which is paved, level and dry. That’s the baseline. If the runway conditions required for a given calculation are different to those specified — say it’s wet, or sloping, or not paved — then corrections will need to be made to the values this graph gives you. We saw those correction factors earlier in the chapter, so you’ll be applying them on top of what the graph produces. Let me walk you through the structure of the graph, because it’s a carpet-type graph and each carpet handles one variable. The left hand carpet involves the variations in temperature and pressure altitude. That part accounts for the effect of air density on the landing distance. Remember, warmer air and higher pressure altitude both reduce air density, which changes how the aeroplane performs — and this carpet is where that effect is captured. The middle carpet accounts for the effect of the mass. That’s your landing weight, and it shifts the distance you need. To the right of that carpet is the wind correction carpet. Now, look carefully at the differences in the slope of the headwind and tailwind lines. They are not parallel — and that’s deliberate. The steeper and shallower slopes mean the 150% and 50% wind rules have been applied. That’s a rule of thumb: a headwind component reduces landing distance by about 150% of the wind effect, and a tailwind increases it by about 50% of the wind effect. The differing slopes on the graph are exactly those rules built in geometrically. The last carpet on the far right of the graph is labelled “obstacle height”. Now, there is no obstacle as such — but you will recall that the landing starts at a height of 50 ft above the landing surface. So that carpet is accounting for the fact that your landing distance is measured from the point where you cross the threshold at 50 feet, not from the moment the wheels touch. The obstacle height carpet lets you adjust for that 50-foot screen height. Now, the best way to get comfortable with this is to follow through the example that has been carried out for you on the graph itself. Trace the lines step by step — from the temperature and pressure altitude carpet, across the mass carpet, through the wind correction, and out to the obstacle height carpet. That worked example is your guide to using the graph correctly. And there’s more help: at the bottom of page 9 of section 2 in CAP 698, you’ll find another example you can work through yourself. Use the examples — they are there to help you. And if you need practice working through the graphs, use the questions at the end of the chapter. So the key points to hold onto: fixed power and flap settings at the top, a paved level dry runway baseline with corrections if conditions differ, and four carpets — air density on the left, mass in the middle, wind with the 150% and 50% rules, and obstacle height on the far right tied to that 50-foot landing start.