Let's pick this up right where the tyre's sidewall markings leave off — Page 122, Lesson 166

Let's pick this up right where the tyre's sidewall markings leave off. We've got the speed rating embossed on the sidewall, and that rating is what lets the engineers calculate the maximum take-off mass — the MTOM — that the tyres can sustain. That rating isn't just a number pulled from thin air; it takes account of three specific things: pressure altitude, ambient temperature, and wind component. So when you're doing your performance calculations, those three variables feed into that tyre rating to tell you the heaviest the aircraft can be for take-off. Now, look for the green or grey dots painted on the sidewall. Those dots mark the position of what are called the "awl" vents. Let me explain what those do, because they're easy to overlook. Awl vents prevent pressure from being trapped between the plies. Remember, a tyre is built up of layers — the plies — and if air gets trapped between them, and then the tyre is exposed to the low pressures of high altitude flight, that trapped pressure would cause disruption of the tyre carcase. So the awl vents are essentially tiny escape routes that let that trapped pressure bleed out, protecting the structural integrity of the carcase. Next, a red dot or a red triangle on the sidewall indicates the lightest part of the tyre. This is a fitting aid. When you're mounting the tyre on the wheel, you place that red dot or triangle opposite the valve. Why? Because it assists in balancing the wheel assembly. You're putting the heavy point of the wheel — the valve — against the light point of the tyre, and that helps the whole assembly run balanced. Finally, there are two markings that tell you about the tread construction. You'll see the letters DRR printed in the code panel, and the words "REINFORCED TREAD" printed on the sidewall. Both of these indicate that the tyre has a layer of fabric woven into the tread. That fabric layer may become visible during normal wear as the tread rubber wears down. And here's the critical caution: that woven fabric layer must not be confused with the casing cords. The casing cords are the structural reinforcement deeper in the tyre body — they're what hold the tyre together under load. The DRR reinforced tread layer is a separate, shallower layer, and mistaking one for the other could lead you to condemn a serviceable tyre or, worse, keep an unserviceable one in use. So when you see that fabric showing, you need to know exactly which layer you're looking at. That figure shows the make-up of a tyre, and it's worth studying because it ties all these markings to the physical layers — the plies, the tread, the casing cords — that the sidewall markings are telling you about.