First, the setup — Page 302, Lesson 377

Let’s pick up right where the graph’s layout leaves off, because now we’re going to actually fly through it with a worked example. I want you to see how every one of those carpets — temperature, pressure altitude, mass, wind, and obstacle height — gets used in sequence to produce a single number: the take-off distance. First, the setup. We assume a temperature of 15°C at an airfield that is 4000 ft above mean sea level. So on the left-hand carpet, you start at 15°C on the temperature axis and move straight upwards until you hit the 4000 ft pressure altitude line. That’s your first intersection. Then you move horizontally to the right until you meet the first reference line. That reference line is just a vertical guide — a place where you change direction to pick up the next variable. The next variable is mass. In our example, we assume a mass of 3400 lb. From that first reference line, you move down along the sloping guidelines until you reach 3400 lb. Notice those guidelines are sloped, not vertical — that’s because the mass effect isn’t a simple straight drop; the slope encodes how mass changes the distance. From that point, you go horizontally to the right again until you hit the next reference line. Now the wind. We assume a 10 knot headwind. Here’s where I want you to really look at the slopes of the headwind and tailwind lines — they are different. That difference is not accidental. It’s there because the 150% tailwind and 50% headwind rules have already been applied to the graph. So when you travel down the headwind line until you reach 10 knots headwind, you’re following a line that already has that 50% factor built into its slope. Then you move horizontally right once more, up to the last reference line. And that last reference line brings us to the obstacle height carpet. Remember, the take-off is not complete until the aeroplane has reached the 50 ft screen height. There’s no physical obstacle at the end of the runway, but the take-off distance is measured from brake release all the way to that 50 ft screen. So from this point on the graph, you move up the guidelines to the end — that final upward leg accounts for the climb to screen height. In our example, the take-off distance comes out to approximately 2300 ft from brake release to the 50 ft screen. Now, one critical point I want to hammer home before we move on: the speeds used to construct this graph are fixed, and a pilot must adhere to them accurately. If you deviate from those speeds, the required aircraft performance will not be achieved. The graph is built on those specific speeds — change them, and the whole calculation falls apart. So the take-off distance you read off is only valid if you fly the speeds the graph was built around. Let me also tie the structure back together, because it’s easy to get lost in the steps. The left-hand carpet handles temperature and pressure altitude — that’s the effect of air density on take-off distance. The middle carpet handles mass. To the right of that is the wind correction carpet, with its distinct headwind and tailwind slopes. And the far right carpet is labelled “obstacle height” — which, as we said, is really the 50 ft screen height. So the full sequence for any calculation is: temperature and pressure altitude first, then mass, then wind, then screen height. Each step moves you through a reference line to the next variable, and the final reading is your take-off distance. In our example, that’s 2300 ft. That’s the complete worked example. Take a moment to trace it yourself on the graph — start at 15°C, up to 4000 ft, right to the first reference line, down to 3400 lb, right to the next reference line, down the headwind line to 10 knots, right to the last reference line, and up to the end. That’s your 2300 ft.