The gas seal on a tubeless tyre depends on a wedge fit — Page 122, Lesson 164

Let's pick up right where we left off — we were talking about tubeless tyres, and I want to finish that thought before we move on. The gas seal on a tubeless tyre depends on a wedge fit. That means the underside of the tyre bead — the stiffened edge of the tyre that sits against the wheel — is forced into a taper on the wheel rim. The two surfaces wedge together, and that mechanical fit is what holds the gas in. There's no inner tube doing the sealing for you. The inflation valve is the usual type, but it's fitted with a rubber gasket and it's situated in the wheel rim itself. That gasket is what seals the valve against the rim. Now, the advantages of tubeless over conventional tyres — there are five, and I want you to hold each one clearly. First, the gas pressure is maintained over longer periods because the lining is unstretched. In a tubeless tyre, the inner lining isn't stretched by an inner tube, so it doesn't work loose and let gas seep out over time. Second, penetration by a nail or similar sharp object will not cause rapid loss of pressure. Because the lining is unstretched, it clings to the object that penetrates it, and that clinging action prevents loss of nitrogen. So a nail can go in, and the tyre won't deflate instantly. Third, the tyre is more resistant to impact blows and rough treatment. That's because of the increased thickness of the casing, and the lining distributes the stresses and prevents them from causing local damage. So the forces from a hard landing or a rough surface get spread out rather than concentrating at one weak point. Fourth, the lack of an inner tube means an overall saving of approximately 7.5% in weight. That's a direct weight saving on every wheel. Fifth, inflation valve damage by creep — that's slippage — is eliminated. With no inner tube, the valve isn't being pulled and shifted by tube movement, so it doesn't get damaged that way. Now let's move on to tyre pressures. The difference in landing speeds, loading, landing surfaces, and landing gear construction across different aircraft makes it necessary to provide a wide range of tyre sizes, types of construction, and inflation pressures. There are four main categories. Low Pressure operates at 25 to 35 psi, which is 1.73 to 2.42 bar, and it's used on grass surfaces. Medium Pressure operates at 35 to 70 psi, 2.42 to 4.83 bar, and it's used on grass surfaces or on medium firm surfaces without a consolidated base. High Pressure operates at 70 to 90 psi, 4.83 to 6.21 bar, and it's suitable for concrete runways. Extra High Pressure operates at pressures over 90 psi — some tyres of this type are inflated to 350 psi, which is 6.21 to 24.2 bar — and this tyre is suitable for concrete runways. Now, tyre markings. The letters ECTA or "Conducting" are used to indicate a tyre that has extra carbon added to the rubber compound to make it electrically conducting. That provides earthing — grounding — between the aircraft and the ground. So static electricity can bleed off safely. The size of a tyre is marked on its sidewall and includes the outside diameter in inches or millimetres, the nominal width in inches or millimetres, and the inside diameter in inches. The ply rating — the index of the tyre's strength — is also marked on the sidewall. Normally it's shown as an abbreviation, like 16PR, but occasionally it's shown in full as "16 PLY RATING." And finally, the speed rating. That denotes the maximum rated ground speed in mph to which the tyre has been tested and approved. It's embossed on the sidewall. The rating takes account of pressure altitude, ambient temperature, and wind component — and that's where we'll pick up next, because those factors are what let you interpret the rating in real operating conditions.