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

Let’s pick up right where the physics of the take-off run gets interesting: the wind. I want you to think of the take-off distance as the distance the aeroplane actually travels along the ground to reach its take-off airspeed. The key here is the difference between airspeed and ground speed. For any given true airspeed, the wind changes the true ground speed. A headwind reduces the ground speed at the required take-off airspeed. Let me give you the example from the text: with a headwind of 20 knots and a true airspeed for the take-off safety speed of 120 knots, the ground speed is only 100 knots. Because the aeroplane only has to accelerate to a ground speed of 100 knots, it needs less distance to do so. There’s a second benefit too: a headwind also increases the angle of the initial climb, which further reduces the required distance. So the net effect is that headwinds reduce the take-off distance, and that is exactly why pilots always aim to take off into wind. Now, a tailwind does the opposite. It increases the ground speed and therefore increases the take-off distance. Simple enough, but here is where the regulations come in, and this is a professional point you must remember. 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 a tailwind component. Let me unpack that. If you have a 10-knot headwind, you are only allowed to count 5 knots of it in your performance calculation. Why? Because the reported wind might vary during the actual take-off. If you planned for a full 10-knot headwind and the wind dropped to less than that at the moment of take-off, the aeroplane would not be able to complete the take-off within the available distance. So the regulation builds in a safety margin. For a tailwind, it is the opposite: you must assume a stronger tailwind than reported, at least 150% of it, again to cover variations. Now, a very practical point: most aeroplane performance manuals and operating handbooks already have these wind rules factored into their take-off graphs or tables. So in practice, you simply use the forecast wind, and the graph or table automatically corrects the take-off distance to account for the regulation. There is a note in the text that I want you to hold onto. 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, because a stronger tailwind is allowed for. And here is the special case: if the wind is a 90° crosswind, the distance required to take off will be the same as the distance calculated for zero wind component. So a pure crosswind has no effect on take-off distance. Now let’s move to runway slope, and this is where the geometry comes in. If the runway is sloping, a component of the weight will act along the longitudinal axis of the aeroplane. That component will either augment thrust or augment drag, which will increase or decrease the accelerating force. The amount of weight that is augmenting either thrust or drag is given a specific name: it is called either “weight apparent thrust” or “weight apparent drag.” You can calculate it by multiplying the force of weight by the sine of the angle of the runway slope. So if the runway slopes down, a proportion of the weight acts in the direction of thrust, as you can see in Figure 2.12. A downhill slope will increase the accelerating force and reduce the take-off distance. An uphill slope will reduce the accelerating force and increase the take-off distance. So the rule of thumb is: downhill helps you get airborne sooner, uphill makes you work harder and need more runway.