General Principles - Take-off — Page 160, Lesson 184

Let's talk about wind and runway slope, because these are the two big environmental factors that change how much runway you actually need to get airborne. First, wind. The key idea is that wind changes your ground speed for a given true airspeed. True airspeed is how fast the aeroplane is moving through the air — that's what the wing needs to generate lift. Ground speed is how fast the aeroplane is moving over the runway surface. These are different, and wind is what separates them. A headwind blows against you. So for any given true airspeed, a headwind reduces your ground speed. Let me give you the example from the book: with a headwind of 20 knots and a true airspeed at take-off safety speed of 120 knots, your ground speed is only 100 knots. Since you only need to accelerate to 100 knots over the ground, you need less runway to get there. That's why a headwind reduces take-off distance. There's a second benefit too — a headwind also increases the angle of your initial climb, which further reduces the distance required. This is exactly why pilots always aim to take off into wind. A tailwind does the opposite. It increases your ground speed for a given true airspeed, so you need more runway to reach take-off speed, and the take-off distance increases. Now here's a really important regulatory point. The regulations for all classes of aircraft require that when you calculate take-off distance, you assume no more than 50% of the headwind component, and no less than 150% of the tailwind component. Let me unpack that. If the forecast says 10 knots headwind, you're only allowed to count 5 knots of it in your distance calculation. Why? Because winds can vary — it would be unwise to plan a distance-limited take-off counting on a full 10 knots of headwind if, at the actual moment of take-off, the wind is weaker. If you'd planned on that full headwind and it wasn't there, you wouldn't complete the take-off within the available distance. So the regulation builds in a safety margin. Conversely, for a tailwind, you assume 150% of it — you plan for a stronger tailwind than forecast, again as a safety margin. Now, most performance manuals and operating handbooks already have these wind rules built into their take-off graphs or tables. So in practice, you just use the forecast wind, and the graph automatically corrects the take-off distance for the regulation. There's a useful note here. For any headwind, the distance required to take off will be less than the calculated distance, because only half the headwind is allowed for. Equally, for any tailwind, the distance required will be less than calculated, because a stronger tailwind is allowed for. And if the wind is a 90° crosswind — blowing straight across the runway — the distance required is the same as the distance calculated for zero wind component. Now let's move to runway slope. If the runway slopes, a component of the aeroplane's weight acts along the longitudinal axis — that's the fore-and-aft axis of the aeroplane. This component either augments thrust or augments drag, which increases or decreases the accelerating force. The amount of weight doing this is called either "weight apparent thrust" or "weight apparent drag." You calculate it by multiplying the force of weight by the sine of the angle of the runway slope. Let me show you what that looks like on a downslope. On a downhill slope, a proportion of the weight acts in the direction of thrust. That increases the accelerating force and reduces the take-off distance. On an uphill slope, the weight component acts against you — it reduces the accelerating force and increases the take-off distance. So to tie it together: headwind and downhill slope both help you — they reduce take-off distance. Tailwind and uphill slope both hurt you — they increase take-off distance. And the regulations force you to be conservative about how much help you count on from the wind.