General Principles - Landing — Page 289, Lesson 359

I want to walk you through the three types of hydroplaning that affect landing performance, because this is one of the most safety-critical subjects in the whole performance syllabus. We've already covered the landing distance and the factors that affect it, and now we're getting into what happens when the runway is wet or contaminated — because that's where the real danger lies. Let me start with the first type, which is called dynamic hydroplaning. This is the one you've probably heard of. It happens when a tyre rolls over a film of water so fast that the water can't be squeezed out from under the tyre footprint, and the tyre actually lifts up and rides on top of the water — like a water ski. The speed at which this happens is given a special symbol, VP. That's the dynamic hydroplaning speed. There's a simple formula to calculate it, and I want you to remember this because it's examinable. For rotating tyres, the dynamic hydroplaning speed in knots is equal to 9 times the square root of the tyre pressure in psi. So if you know the tyre pressure, you take its square root and multiply by 9, and that gives you the speed in knots. To give you a real-world feel for this, for a typical 737 the dynamic hydroplaning speed is between 90 and 120 knots. That's a very real operating speed — you can be hydroplaning on touchdown. But there's a second case. For non-rotating tyres — and think about a wheel that's locked, not spinning — the dynamic hydroplaning speed is lower. It's equal to 7.7 times the square root of the tyre pressure. So a locked wheel will start hydroplaning at a lower speed than a spinning wheel. Now, why is this so dangerous? The danger from hydroplaning is the virtually nil braking and steering effect. If your tyre is riding on water, it's not in contact with the runway, so your brakes have nothing to grip and your steering has nothing to grip. You're essentially a passenger. So what are the most positive methods of preventing dynamic hydroplaning? There are four, and I want you to note each one. First, groove the tyres — the tread grooves channel water away. Second, transversely groove the runway — that's grooves cut across the runway direction. Third, ensure the runway pavement is convex from the centre line — so the runway is slightly crowned, letting water run off to the sides. And fourth, ensure the runway has a macro-texture — that's a coarse surface texture at a larger scale. Now let's move to the second type, viscous hydroplaning. This one is different. It occurs because of the viscous properties of water acting like a lubricant. Here's the key detail: a thin film of fluid not more than 0.03 mm deep cannot be penetrated by the tyre in the footprint area. So even a film thinner than a human hair — 0.03 millimetres — acts like a lubricant, and the tyre rolls on top of the film. Viscous hydroplaning can occur at a much lower speed than dynamic hydroplaning, but it requires a smooth surface. So on a smooth, polished runway with just a thin film of water, you can lose grip at low speed. The most positive method of preventing viscous hydroplaning is to provide a micro-texture to the pavement surface. That's a fine-scale texture, and it works by breaking up the film of water, allowing it to collect into very small pockets. The result is that the tyre footprint sits on the peaks of the textured surface, not on the film of water. So you've got macro-texture for dynamic hydroplaning, and micro-texture for viscous hydroplaning — two different scales for two different problems. Now the third type, and this is the one that's been the subject of a variety of explanations over the years — it's a complex phenomenon. It's called reverted rubber hydroplaning. The key requirement here is a prolonged, locked wheel. And the excerpt cuts off right at that point, so I'll pause there — but I want you to hold onto that phrase: prolonged, locked wheel. That's the trigger condition, and we'll pick up right there with what happens next.