Aircraft Wheels — Page 122, Lesson 158

Let’s talk about the fusible plug, because it’s a small component with a very serious job. Under extra hard braking conditions — think of a rejected take-off or a very heavy landing — the heat generated in the wheel, tyre, and brake assembly can get high enough to cause a tyre blowout. And a blowout at that moment can have a catastrophic effect on the aircraft. So the design goal is to prevent a sudden blowout. That’s exactly what the fusible plug does. It’s fitted in some tubeless wheels. The plug is held in position in the wheel hub by a fusible alloy — a metal that melts at a set temperature. When excessive heat conditions are reached, that alloy melts, and the plug is then blown out by the tyre air pressure. So instead of the tyre bursting violently, you get a controlled deflation. The plug blows out, the air escapes in a controlled way, and that prevents excessive pressure build-up inside the tyre. Now, these plugs are made for three different temperatures, and they’re colour coded so you can identify them at a glance. Red is 155°C, green is 177°C, and amber is 199°C. So the colour tells you the melting point of the alloy holding that plug in place. That’s the fusible plug — a simple, reliable safety device that trades a controlled deflation for a potentially catastrophic blowout. Now, that brings us to the next chapter, which is all about the aircraft tyre itself. And I want to give you the map of what we’re going to cover, because this is a big subject. We’ll start with an introduction to tyres, then look at the tyre covers — the actual structure of the tyre. Then we’ll go through the regions of the tyre, the inner tubes, the inflation valve, and tubeless tyres. After that, tyre pressures, tyre markings, and tyre contamination. Then creep — that’s slippage — and correct tyre pressures. Then aquaplaning, MAT limits, tyre damage, tread separation and tyre burst, and finally reduction of tyre wear. So let’s begin with the tyre covers. This is the structure of the tyre itself, and I want you to picture it as a layered construction. shows you the make-up of a tyre — the tyre covers. The key idea here is that a tyre is not a single piece of rubber; it’s built up from several layers, each with a specific job. The covers are the main structural plies that give the tyre its strength and shape. Now, let me walk you through the regions of the tyre, because each region has a distinct name and function. illustrates these points. The tyre has a bead — that’s the edge that seats against the wheel rim. Then there’s the sidewall, which is the flexible part that flexes as the tyre rolls. And then there’s the tread, which is the part that contacts the ground. Each region is designed for a different job: the bead holds the tyre on the wheel, the sidewall flexes and carries the load, and the tread provides grip and wears over time. So when we talk about the regions of the tyre, we’re really talking about the anatomy — the bead, the sidewall, the tread — and each one has to work with the others for the tyre to perform correctly. Now, let’s move on to inner tubes. Some tyres use an inner tube — a separate inflatable bladder inside the tyre that holds the air. The tyre itself is just the cover; the tube does the sealing. But there’s a trend toward tubeless tyres, which we’ll get to shortly. Next, the inflation valve. This is the fitting through which you inflate the tyre — the valve that lets air in and keeps it in. It’s a simple but critical component, because if the valve fails, you lose pressure. Then we come to tubeless tyres. These don’t use an inner tube; instead, the tyre itself seals against the wheel rim. That’s why the fusible plug we talked about is fitted in tubeless wheels — because the tyre and wheel form a sealed unit, and the plug is part of that system. Now, tyre pressures. This is about the correct inflation pressure for the tyre, and it’s critical for performance and safety. We’ll cover how pressure affects the tyre’s footprint, its wear, and its ability to carry load. Then tyre markings. Every tyre has markings on it — size, ply rating, speed rating, and so on. These markings tell you what the tyre is rated for, and you need to read them correctly. Tyre contamination is next. This is about what happens when the tyre gets contaminated — oil, grease, or other substances — and how that affects grip and safety. Then creep, or slippage. This is the phenomenon where the tyre doesn’t rotate perfectly with the wheel — it creeps or slips slightly. We’ll look at why that happens and what it means. Correct tyre pressures — this is about maintaining the right pressure, and we’ll look at the consequences of getting it wrong. Aquaplaning is a big one. This is when the tyre rides on a film of water and loses contact with the runway — a serious safety issue during landing and take-off in wet conditions. MAT limits — that’s Maximum Allowable Temperature, I believe, and it relates to the temperature limits for the tyre and brake assembly. Then tyre damage — the types of damage a tyre can sustain, and how to identify them. Tread separation and tyre burst — this is when the tread peels away from the tyre body, or when the tyre bursts. These are failure modes we need to understand and prevent. And finally, reduction of tyre wear — the techniques and practices that extend tyre life. So that’s the full map of the tyre chapter. We’ll work through each of these in turn, starting with the tyre covers and the regions of the tyre. Let’s begin there.