General Principles - Landing — Page 289, Lesson 359

Right, let's pick this up with the landing phase. We've been talking about how the runway surface affects your stopping performance, and now we're getting into the real enemy of a good landing roll: hydroplaning. We have three distinct types to master, and they behave very differently. Let's start with the first one, which is the one you've probably heard the most about: dynamic hydroplaning. Think of it this way. When you land on a wet runway, your tyre has to push the water out of the way to make contact with the pavement. At low speed, it does that fine. But as your speed increases, the water doesn't have time to get out of the way. It builds up in front of the tyre, and eventually, the tyre is literally riding on a wedge of water, completely separated from the runway surface. That's dynamic hydroplaning. The speed at which this happens is a specific, calculable value, and we call it VP. There's a simple formula for it, and I want you to remember this one because it's a classic exam question. For rotating tyres — that's your main wheels spinning up as you touch down — the dynamic hydroplaning speed in knots is equal to 9 times the square root of the tyre pressure in psi. So, VP = 9 × √(tyre pressure in psi). To give you a sense of scale, for a typical 737, that dynamic hydroplaning speed works out to be between 90 and 120 knots. That's a significant speed, well within your landing roll. Now, here's the twist. For non-rotating tyres — and think about a wheel that's locked up, or one that hasn't spun up yet — the formula changes. The dynamic hydroplaning speed is equal to 7.7 times the square root of the tyre pressure. So it's a lower speed, which means a locked wheel will start hydroplaning sooner than a spinning one. That's a critical distinction. Why is this so dangerous? The danger from hydroplaning is the virtually nil braking and steering effect. You have no grip, so you can't slow down with the brakes, and you can't steer. You're a passenger on a water ski. So how do we prevent it? The most positive methods are to groove the tyres, to transversely groove the runway, to ensure the runway pavement is convex from the centre line — so water runs off to the sides — and to ensure the runway has a macro-texture. That macro-texture is the coarse, gritty surface that gives the water somewhere to go and gives the tyre something to bite into. Now, let's move to the second type: viscous hydroplaning. This one is different. It's not about a deep wedge of water. It occurs because of the viscous properties of water — the water's thickness and stickiness — acting like a lubricant. Here's the key: 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 microscopic film of water, thinner than a human hair, can act as a lubricant, and the tyre just rolls on top of it. The crucial point is that viscous hydroplaning can occur at a much lower speed than dynamic hydroplaning. But it requires a smooth surface. On a smooth runway, that thin film has nothing to break it up. So how do we defeat this one? The most positive method is to provide a micro-texture to the pavement surface. This is a much finer texture than the macro-texture we talked about for dynamic. The micro-texture breaks up the film of water, allowing it to collect into very small pockets. This means the tyre footprint will sit on the peaks of the textured surface and not on the film of water. So the tyre makes contact with the pavement, not the lubricant. Now, the third type, and this is the nasty one: reverted rubber hydroplaning. I want to be honest with you here. This is a complex phenomenon which over the years has been the subject of a variety of explanations. So we're going to treat it as such. The key trigger is that reverted rubber hydroplaning requires a prolonged, locked wheel. And that's where the excerpt cuts off, so we'll pick up right there with what happens during that prolonged lock-up.