General Principles - Landing — Page 280, Lesson 342

Let's start with the landing distance required. That's the combined length of two distinct phases: the airborne section and the ground run, which is also called the landing roll. So the moment you cross the threshold at 50 feet, you're in the airborne section, and that continues until the wheels touch. Then the ground run begins, and that carries on until the aeroplane comes to a complete stop. Add those two together, and you have the landing distance required. Now, the pilot's job is to ensure that the landing distance required does not exceed the landing distance available. That's the fundamental safety check for every landing. Let me define the landing distance available precisely, because the wording matters. It is the distance from the point on the surface of the aerodrome above which the aeroplane can commence its landing, having regard to the obstructions in its approach path, to the nearest point in the direction of landing at which the surface of the aerodrome is incapable of bearing the weight of the aeroplane under normal operating conditions, or at which there is an obstacle capable of affecting the safety of the aeroplane. That's a dense definition, so let me unpack it. The landing distance available starts at the point on the runway surface directly below where you can begin your landing — and that point is the threshold. The definition mentions obstructions in the approach path because those determine where you can actually start descending. The landing distance available ends at the nearest point where the runway surface can no longer safely bear the aeroplane's weight, or where an obstacle could affect safety. In short, the landing distance available is the length of runway from one threshold to another. Now, here's an important nuance: thresholds are not always at the physical ends of the runway. Sometimes there are displaced thresholds, which are some way in from the end of the paved surface. So the usable landing distance may be shorter than the full paved length. And you'll recall that the landing distance starts at 50 feet. That 50-foot point must be directly above the threshold. Landing on the threshold itself is not the aim of the landing — you aim to touch down beyond it, within the runway. Let me show you this visually. Now let's move to the forces acting on the aeroplane during landing, starting with weight. Weight acts vertically downwards towards the centre of the earth, from the centre of gravity. During flight, weight is mainly balanced by lift. But once the aeroplane is on the ground, weight is balanced by the reaction of the ground acting up through the wheels on the undercarriage. Here's a key point: the weight of the aeroplane on landing will be less than at take-off, because fuel has been consumed during the flight. However, there is a maximum structural landing mass which must not be exceeded. That's a hard limit — exceed it and you risk structural damage on touchdown. Now let's consider lift. While lift helps balance weight in the air, once the aeroplane is on the ground, lift is no longer required. In fact, during the landing roll, lift is detrimental to landing performance. Here's why: producing lift will reduce the load placed on the wheels, and therefore decrease the braking effect. Think about it — brakes work through friction between the tyres and the runway, and that friction depends on the downward force on the wheels. If lift is still being generated, it's effectively unloading the wheels, so the brakes have less grip. That's why large commercial aeroplanes, once the main wheels are on the ground, will do everything they can to kill the lift — spoilers, flaps, and so on. Let me show you how a jet engine can help in this phase. So to summarise what we've covered: the landing distance required is the airborne section plus the ground run, and it must not exceed the landing distance available, which runs from threshold to threshold, possibly with displaced thresholds. Weight acts down from the centre of gravity, balanced by ground reaction through the wheels once on the ground, and it's less than at take-off due to fuel burn, but limited by the maximum structural landing mass. And lift, while helpful in the air, is detrimental on the ground because it reduces wheel load and therefore braking effectiveness.