General Principles - Landing — Page 280, Lesson 348

We're starting the landing phase of the performance chapter now. Let's look at the forces that actually stop the aeroplane once it's on the runway. I want to walk you through the two big families of drag that do the work: aerodynamic drag and wheel and brake drag. First, aerodynamic drag. There are two forms of it that matter here: induced drag and parasite drag. We're focusing on parasite drag right now. Parasite drag is a function of two things: the aeroplane's forward-facing cross-sectional area — which is more accurately known as form drag — and the aeroplane's forward speed. So the bigger the frontal area and the faster you're going, the more parasite drag you get. Now, here's the key for landing. The landing configuration is set up so that the flaps and slats are fully extended. That significantly increases the form drag, and therefore significantly increases parasite drag. Then, once the aeroplane touches down and the spoilers and speed brakes are deployed, parasite drag increases even further. But here's the catch: as the speed rapidly decays after touchdown, so does the parasite drag. It eventually decreases to zero once the aeroplane reaches a full stop. So to summarise the aerodynamic picture: the total aerodynamic drag — induced plus parasite — is very high during the early part of the landing, but very soon after touchdown it decays rapidly, because speed is falling off so quickly. Now let's move to the second family: wheel drag and brake drag. These are two distinct friction forces. Wheel drag is the friction force between the wheel and the runway, and also with the wheel bearings. Brake drag, on the other hand, is the friction force between the brake discs and the brake pads. Here's the important sequencing. Wheel drag comes into play as soon as the aeroplane touches down on the runway. But — and this is the subtle part — friction is a function of the force pushing two surfaces together. Because there is still a lot of lift being generated during the initial part of the landing run, the wheel load is small, and therefore the wheel drag is also small. As speed reduces, and as lift is destroyed by the spoilers, the wheel load increases, which in turn increases the wheel drag. So wheel drag increases throughout the landing roll and will reach a maximum value just before... well, that's where the passage cuts off, but you can see the trend: it builds as weight transfers onto the wheels. Let me show you the runway geometry that frames all of this. That figure shows the landing distance available — in short, the length of runway from one threshold to another. That's the physical distance you have to work with, and all these drag forces are what you use to stop within it. And there's one more tool in the stopping kit. That's an illustration of a jet engine during reverse thrust mode. Pilots need to recognise that the process of getting the engine into reverse thrust is a significant additional stopping force, on top of the aerodynamic, wheel, and brake drag we've just covered. So the full stopping picture on landing is: high aerodynamic drag early, decaying fast; wheel drag building as lift is destroyed and weight comes onto the wheels; brake drag from the discs and pads; and reverse thrust as an extra contributor. Each one has its own timing and its own peak, and understanding that sequence is what lets you manage the landing roll properly.