General Principles - Landing — Page 284, Lesson 356

Let’s pick up right where the landing roll leaves off, because the next thing that can change everything is the state of the runway surface itself. I’m talking about contamination. If the runway is covered, partially or fully, by contaminants — and the four we care about are standing water, snow, slush, or ice — then you must pay special attention to the effect they have on landing distance. These substances do two main things, and I want you to hold both in your head because they work together. The first effect is that they create what we call impingement drag. You’ll remember we met impingement drag when we talked about grass — it’s the drag caused by the tyres physically pushing through or against the surface material. So standing water, snow, slush, or ice all add that same kind of resistance. But here’s the more important second effect: these substances substantially reduce the friction between the wheel and the runway. That means the wheel cannot be retarded efficiently by the brakes. Think about what braking relies on — the friction between tyre and surface. If that friction collapses, the brakes simply can’t do their job. So because of the reduced friction, and therefore the reduced braking action, any contamination of the runway due to water, snow, slush, or ice will significantly increase the landing distance. Now let me give you the numbers, because this is where it gets stark. On a dry runway, the braking coefficient of friction is typically between 0.8 and 1.0. That’s a high friction value — the brakes are very effective. But on wet, slippery, or icy runways, that braking coefficient of friction can fall to less than 0.2. So you’re going from roughly 0.8–1.0 down to under 0.2 — a massive drop in the ability of the brakes to stop you. Because of that lack of effective braking on slippery surfaces, the other stopping forces become much more important. Specifically, aerodynamic drag and reverse thrust now carry the burden of bringing the aeroplane to a stop. And here’s the striking figure: on flooded or icy runways, reverse thrust accounts for 80% of the deceleration force. So on a normal dry runway, the brakes dominate. On a flooded or icy runway, the brakes are nearly useless, and reverse thrust becomes the primary stopping force — four out of every five units of deceleration come from reverse thrust. Let me show you this graphically. Here’s a graph showing the variations in drag through the landing roll — you can see how the drag forces change as the aeroplane slows. And here’s the expanded landing distance formula, which lets you see how a change in one of the variables affects the total distance. So the key takeaway for you as a professional pilot: contamination isn’t just a small penalty — it’s a fundamental shift in which forces stop the aeroplane. On a dry runway, brakes do the work. On a contaminated runway, you’re relying on reverse thrust and aerodynamic drag, and the landing distance grows significantly. That’s why you must always account for runway condition when you calculate your landing performance.