General Principles - Landing — Page 284, Lesson 356

Let’s pick up right where the landing distance discussion leaves off, because now we hit the single most dangerous variable in the whole landing performance story: runway contamination. I want you to picture the runway surface itself. If it’s covered, partially or fully, by standing water, snow, slush, or ice, we call those contaminants. And the moment any of those are present, you must pay special attention to what they do to your landing distance, because they change the physics of stopping in two very specific ways. The first effect is impingement drag. Think of it like this: as the wheels roll through water or slush, the substance physically strikes the landing gear and the tyres, and that impact creates a drag force. It’s the same principle as the drag grass created on the aeroplane during the landing roll — the substance is pushing back against the aircraft as it moves through it. So impingement drag actually helps slow you down a little. But here’s the critical part — the second effect is far more important, and it works against you. These contaminants substantially reduce the friction between the wheel and the runway. That friction is what lets the brakes grip the surface and retard the wheel. If friction drops, the wheel cannot be retarded efficiently by the brakes. The brakes simply can’t do their job because there’s nothing for the tyre to bite into. Now follow the chain of cause and effect: reduced friction leads to reduced braking action. And reduced braking action means any contamination — water, snow, slush, or ice — will significantly increase your landing distance. That’s the headline. A contaminated runway is a longer landing runway, and you must plan for it. 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 strong grip. But on wet, slippery, or icy runways, that coefficient can fall to less than 0.2. So you can lose roughly three-quarters or more of your available braking friction. That’s a massive loss. And because of that lack of effective braking on slippery surfaces, the other stopping forces suddenly become much more important. Aerodynamic drag and reverse thrust now carry the burden of bringing the aeroplane to a stop. In fact, on flooded or icy runways, reverse thrust accounts for 80% of the deceleration force. So on a contaminated runway, you are no longer relying on the brakes as your primary stopper — you’re relying on reverse thrust and the air itself. I want to make sure you hold onto the contrast here. On dry ground, brakes dominate. On contaminated ground, the brakes are nearly useless, and reverse thrust becomes the dominant deceleration force — 80% of it on flooded or icy runways. That single shift in responsibility is why contamination so dramatically stretches your landing distance, and why you must always factor it into your performance calculations.