General Principles - Landing — Page 280, Lesson 342

Let's start with the landing distance required. That's the combined length of two sections: the airborne section and the ground run, which is also called the landing roll. So the landing distance required is the total distance your aeroplane needs, from the moment it crosses the threshold at 50 feet until it comes to a complete stop on the runway. Now, as a pilot, your job is to make sure that the landing distance required does not exceed the landing distance available. That's the fundamental safety check for every landing. So what is the landing distance available, or LDA? This is a formal definition, so let's take it piece by piece. The landing distance available 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. Let me unpack that. The starting point is the point on the runway surface directly below where you begin your landing — and you have to consider the obstructions in your approach path, meaning you need a clear path to get down to that point. The ending point is the nearest point in the direction of landing where the runway surface can no longer safely bear the weight of the aeroplane, or where there's an obstacle that 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. These thresholds are not always at the end 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 might be shorter than the full physical runway length. You'll recall that the landing distance starts at 50 feet. That point, the 50-foot point, must be directly above the threshold. And landing on the threshold itself is not the aim of the landing — you aim to touch down beyond it, not on it. Now let's move to the forces at play. To understand landing performance, we analyse the forces acting on the aeroplane and how they might be modified throughout the landing. The first force is 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 you have to respect. Now let's consider lift. While lift helps balance weight when 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 the landing performance. Why? Because producing lift will reduce the load placed on the wheels, and therefore decrease the braking effect. Less weight on the wheels means less friction available for braking, so your stopping distance increases. That's why, in large commercial aeroplanes, once the main wheels are on the ground, the aim is to reduce lift — typically by spoilers — to keep the aeroplane firmly planted on the runway for maximum braking effectiveness. Let me show you the landing distance available on a diagram. And here's an illustration of a jet engine during reverse thrust mode, which is another way we modify forces during the landing roll. So to summarise what we've covered: the landing distance required is the airborne section plus the ground run. The landing distance available is the runway length from one threshold to another, possibly with displaced thresholds. Weight is less at landing than take-off, but there's a maximum structural landing mass. And lift, while helpful in the air, is detrimental on the ground because it reduces wheel load and braking effect.