General Principles - Landing — Page 284, Lesson 354

Let's pick up with the next factor that shapes landing performance: runway slope. I want you to think about the weight of the aeroplane. Weight always acts straight down, towards the centre of the earth. But when the runway is sloped, that weight gets split into components. One component presses the wheels into the runway, and another component acts along the longitudinal axis of the aeroplane — that's the fore-and-aft axis, running from nose to tail. Here's the key: on an upslope, that component of weight acts backwards, in the same direction as drag. It's literally pulling the aeroplane back down the slope. That adds to the deceleration, so the landing distance decreases. On a downslope, the weight component acts forwards, in the direction of thrust. It's adding to the forward force, so it reduces the deceleration and increases the landing distance. Now, a rough rule of thumb to quantify this quickly: assume that for every 1% slope, the landing distance is affected by 5%, or a factor of 1.05. So a 1% upslope shortens the landing distance by about 5%, and a 1% downslope lengthens it by about 5%. There's a special case you need to know about: some airfields have slopes so significant that landing is only possible in one direction. These are called unidirectional runways. Treat them with extra caution. Always check the current wind velocity, because you may be forced to land with a tailwind, which is something you'd normally want to avoid. Now let's move to runway surface. Most landing performance graphs assume a paved, hard surface. If the runway isn't like that, you need to understand the effect and apply corrections. Take grass, for example. Many small airfields have grass runways. The grass increases the drag on the wheels — this is called impingement drag, and it helps decelerate the aeroplane. But here's the catch: grass severely reduces the wheel friction compared to a paved runway. That means the brakes can't retard the wheel efficiently. If you brake too hard, the wheel will lock, and the wheel friction with the runway reduces even further. So the overall effect is that grass runways increase the landing distance. In light general aviation aeroplanes, that increase is about 15% compared to a paved surface. But for your professional career, most landings will be on hard paved runways. Still, you need to know these corrections because you might encounter a grass strip, and the performance figures you plan with must reflect the actual surface.