We're moving into a new chapter now — General Principles of Landing — Page 273, Lesson 338

We're moving into a new chapter now — General Principles of Landing. This is where we take everything about performance and bring it down to the ground, literally. Let's start with the very first concept, which is the landing distance itself. The landing distance is the total horizontal distance an aeroplane travels from the moment it crosses the runway threshold — that's the start of the runway — until it comes to a complete stop. That's the fundamental definition. It includes the airborne phase, the flare, the touchdown, and the braking roll. Now, there's a critical distinction we need to make right away, and it's between landing distance and landing distance available. The landing distance available, which we abbreviate as LDA, is a declared runway length. It's the length of runway that is actually usable and suitable for landing. In short, the landing distance available is the length of runway from one threshold to another. That's the official definition. So the LDA is a physical property of the runway — it's what the airport declares as available. The landing distance is what your aeroplane actually needs. And for safety, the landing distance required must never exceed the landing distance available. Let me show you what that looks like on a runway diagram. Now, the next concept is lift and weight during the landing phase. As the aeroplane slows down, the lift it generates decreases because lift is proportional to the square of the airspeed. As speed decays, lift decays. Meanwhile, weight remains constant. So the aeroplane settles onto the runway as the lift can no longer support the weight. That's the basic physics of the touchdown. Then we have reverse thrust. This is a means of deceleration where the engine's thrust is redirected forward — that is, opposite to the direction of travel — to help slow the aeroplane down. It's one of the primary deceleration devices, along with the wheel brakes and aerodynamic drag. And that brings us to drag. During the landing roll, drag acts to decelerate the aeroplane. There are two main components: parasite drag, which increases with speed, and induced drag, which is related to lift generation. On the ground, with the aeroplane in the landing configuration — flaps extended, landing gear down — the drag is significant and helps slow the aeroplane. Now, the landing distance formula. This is the core relationship. The landing distance is determined by the balance of forces during the landing roll. The deceleration comes from the brakes, the reverse thrust, and the drag, all working against the inertia of the aeroplane. The formula essentially relates the work done by these decelerating forces to the kinetic energy that must be dissipated. The heavier the aeroplane, the more kinetic energy there is to shed, and the longer the landing distance. Then we have the effect of variable factors on landing distance. Several things change the landing distance: the aeroplane's weight, the wind — a headwind reduces landing distance, a tailwind increases it — the runway surface condition, the runway slope, the air density, and the use of reverse thrust and braking. Each of these factors shifts the landing distance one way or the other. Next is hydroplaning. This is a critical safety concept. Hydroplaning occurs when a layer of water builds up between the tyres and the runway surface, so the tyres lose contact with the runway and ride on the water film. When that happens, the brakes become ineffective because there's no friction between the tyre and the runway. The aeroplane essentially skates on the water. This dramatically increases the landing distance and can lead to loss of directional control. And that leads to the landing technique on slippery runways. When the runway is wet or contaminated, the pilot must adjust the technique. The key is to avoid heavy braking early in the roll, because that can induce hydroplaning. Instead, the pilot should allow the aeroplane to slow down naturally through drag, apply brakes gently and progressively, and use reverse thrust early and aggressively, because reverse thrust is not affected by runway slipperiness — it works through the engines, not through tyre friction. Finally, we have microbursts and windshear. A microburst is a localized column of sinking air that spreads out violently when it hits the ground, producing strong windshear. Windshear is a sudden change in wind speed or direction over a short distance. During landing, a microburst can cause a sudden loss of headwind, which reduces lift and increases the rate of descent — a very dangerous situation close to the ground. The pilot must be ready to react with maximum thrust and a go-around. So that's the structure of this chapter: landing distance, LDA, lift and weight, reverse thrust, drag, the landing distance formula, variable factors, hydroplaning, slippery runway technique, and microbursts and windshear. We'll work through each of these in detail.