We've already covered aerodynamic drag, so let's focus on the other two — Page 280, Lesson 349

Let me walk you through the drag forces during the landing roll, because this is where the physics of stopping an aeroplane really comes together. We've got three drag forces at work during landing: aerodynamic drag, wheel drag, and brake drag. We've already covered aerodynamic drag, so let's focus on the other two. First, wheel drag. As the aeroplane slows down, the lift reduces, which means more of the aeroplane's weight is transferred onto the wheels. That increased load on the tyres increases the friction between the tyres and the runway, which in turn increases the wheel drag. So wheel drag increases throughout the landing roll, and it reaches its maximum value just before the aeroplane comes to rest. That's a key point — the maximum wheel drag happens right at the very end, just before stopping. Now, brake drag. This is by far the most important and the most effective of the various drag forces during landing, because it provides the greatest retarding force. But here's the critical relationship: brake drag is only effective if there is sufficient wheel drag, or wheel friction, between the tyres and the runway. If wheel drag is low, brake drag will also be low. So the brakes are effective only if there's sufficient friction between the tyres and the runway. Here's the practical consequence. During the early part of the landing run, there's not much load on the wheels, and therefore not much wheel friction. So brake drag is consequently ineffective in slowing down the aeroplane. But as the lift reduces and more weight is placed on the wheels, brake drag does become more effective. So brake drag increases as the landing roll progresses. This concept explains something really important for pilots: you need to destroy lift as soon as possible after touchdown, so that the braking action can be at its peak effectiveness early on in the landing. That's why we use spoilers and lift dumpers immediately on touchdown — we want the weight on the wheels quickly so the brakes can do their job. Now, in most large commercial aeroplanes, the braking action may not actually be carried out by the pilots. Instead, it can be carried out by a highly effective automatic anti-skid braking system. This system can be set to low, medium, or high braking levels — or levels 1 through to 3 — and it's especially important to use it when landing on contaminated runways, like wet or icy surfaces. So the chain is: destroy lift → weight on wheels → wheel friction increases → brake drag becomes effective → the anti-skid system manages the braking at the level you've selected. That figure shows a jet engine in reverse thrust mode, which is another retarding force we'll look at next.