Let’s pick this up right where the graph’s layout left off — Page 302, Lesson 377

Let’s pick this up right where the graph’s layout left off. We’ve already seen the three carpets — temperature/pressure altitude, mass, and wind — plus the obstacle-height scale. Now I want to walk you through the actual calculation, because that’s where all those pieces come together. First, a critical point about the speeds. The graph was built using specific reference speeds, and a pilot must adhere to these accurately, because they are the speeds which have been used to construct this graph. If a pilot were to deviate from these speeds, the required aircraft performance would not be achieved. So the whole take-off distance chart is only valid if you fly the speeds it was designed around. Now let’s work the example, following the red line in Figure 7.3. We start with temperature: assume 15°C at an airfield 4000 ft above mean sea level. So from the 15°C point on the temperature axis, move upwards until you reach the 4000 ft pressure altitude line. Then move right until you meet the first reference line. That first reference line is the boundary between the temperature/pressure-altitude carpet and the mass carpet. The next variable is mass. In our example, assume a mass of 3400 lb. From that first reference line, move down along the sloping guidelines until you reach 3400 lb. Those sloping guidelines are what carry you through the mass carpet. From this point, go horizontally to the right until you hit the next reference line. The next variable is wind. Assume a 10 knot headwind. Now notice the slopes of the headwind and tailwind lines are different. That difference is deliberate — it means the 150% tailwind and 50% headwind rules, which you learnt about in the general principles for take-off lesson, have already been applied to the graph. So travel down the headwind line until you reach 10 knots headwind, then move horizontally right once more, up to the last reference line. And here’s the key reminder: the take-off is not complete until the aeroplane has reached the 50 ft screen height. So from that last reference line, move up the guidelines to the end — that final climb represents the aeroplane climbing to the screen height. The take-off distance in our example is approximately 2300 ft, measured from brake release to the 50 ft screen. So the full sequence is: temperature up to pressure altitude, right to the first reference line, down the mass guidelines to 3400 lb, right to the next reference line, down the headwind line to 10 knots, right to the last reference line, then up the guidelines to the end. And the answer, 2300 ft, is the total take-off distance from brake release to the 50 ft screen.