General Principles - Landing — Page 284, Lesson 354

Let’s pick up with the next factor that shapes landing performance: the slope of the runway. This matters because slope changes how the component of weight acts along the aeroplane’s longitudinal axis — that’s the fore-and-aft line running from nose to tail. Think about it this way. Weight always acts straight down toward the centre of the earth. But when the runway tilts, that vertical weight vector splits into components. On an upslope, part of the weight acts backwards along the longitudinal axis — in the same direction as drag. That backward component adds to the deceleration, so the aeroplane slows down more quickly, and the landing distance decreases. On a downslope, the weight component now acts in the direction of thrust — forward. That adds to the forward force, so it reduces deceleration, and the landing distance increases. So upslope shortens your landing roll; downslope lengthens it. There’s a rough rule of thumb to quantify this quickly. For every 1% of slope, the landing distance is affected by 5% — that’s a factor of 1.05. So a 2% upslope would shorten the distance by about 10%, and a 2% downslope would lengthen it by about 10%. It’s a quick mental check, not a precise calculation. Now, some airfields have slopes steep enough that landing is only possible in one direction. These are called unidirectional runways. Treat them with extra caution. Always check that the current wind velocity still allows a safe landing, because you may be forced to land with a tailwind — and a tailwind, combined with a downslope, is a double penalty on your landing distance. Next, the runway surface itself. Most landing performance graphs assume a paved hard surface. If the runway isn’t like that, you need to understand the effect on landing distance and apply corrections. Take grass. Many small airfields have grass runways. 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 cannot retard the wheel efficiently. If you brake too hard, the wheel will lock, and once the wheel locks, the wheel friction with the runway reduces even further — so you lose braking effectiveness just when you need it. The grass also has to be pushed out of the way as the wheel advances, which adds to the resistance. 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 in your professional career, most landings will be on hard paved runways — still, you need to know the correction when you don’t have one.