Aircraft Tyres — Page 129, Lesson 169

Let’s get into aircraft tyres. I want to start with a phenomenon that can be a real hazard on a wet runway: aquaplaning. Aquaplaning is what happens when a wedge of water builds up under the tread of the tyre and breaks its contact with the ground. So instead of the rubber touching the runway, you have a layer of water in between, and the tyre essentially rides on top of it. That means you lose braking and steering effectiveness. There’s a formula to find the aquaplaning speed — that’s the speed, in nautical miles per hour, at which the tyre loses contact with the ground. The formula is: aquaplaning speed equals 9 times the square root of P, where P is the tyre pressure in pounds per square inch, psi. Alternatively, you can use aquaplaning speed equals 34 times the square root of P, where P is the tyre pressure in kilograms per square centimetre, which is the same as bar. So the higher the tyre pressure, the higher the speed at which aquaplaning can occur. Now, the possibility of aquaplaning increases as the depth of the tread is reduced. That’s why it’s so important that the amount of tread remaining is accurately assessed. When aquaplaning occurs, the coefficient of dynamic friction drops to very low values — typically zero. So you have essentially no grip at all. Next, let’s talk about MAT limits. MAT stands for mass, altitude, and temperature. When you’re calculating take-off distance or obstacle clearance with increased V2 speeds — V2 being the take-off safety speed — it’s important not to exceed the speed rating of the tyres fitted to the aircraft. For example, it may be necessary to reduce mass in order to satisfy the mass, altitude, and temperature limits. So the tyre’s speed rating can actually constrain your take-off performance. Now, tyre damage. During servicing, tyre covers must be examined for cuts, bulges, embedded stones, metal or glass, signs of wear, creep, and local sponginess. These defects can make the cover unserviceable, and each gets specific treatment. Let’s go through them one by one. First, cuts. A cut in the tyre cover that penetrates to the cords renders the tyre unserviceable and must be repaired. The cords are the reinforcing fabric inside the tyre, so if a cut reaches them, the structural integrity is compromised. Second, bulges. A bulge may indicate partial failure of the casing. If the casing has failed — meaning the fabric is fractured — you must renew the cover. So a bulge is a warning sign of internal damage. Third, foreign bodies. Embedded stones, metal, glass, and so on. These must NOT be removed by you. Instead, they must be reported to maintenance staff, and the cuts probed with a blunt tool to ascertain their depth. Repair or renewal of the cover is governed by the extent of the damage — referring back to the first point about cuts penetrating to the cords. Fourth, wear. For pattern tread covers — those with tread grooves — if they’re worn to the base of the marker grooves or marker tie bars for 25% of the tyre circumference, they must not be used. For plain tread covers, if they’re worn to the casing fabric, they must not be used. So there’s a specific wear limit: 25% of the circumference for pattern treads, and exposure of the casing fabric for plain treads. Take a look at Figure 5.3, which shows the wear markers. Fifth, creep. Creep is movement of the tyre round the wheel. The limit is: creep must not exceed 1 inch for tyres of up to 24 inches outside diameter, and 1½ inches for tyres over 24 inches outside diameter. If these limits are exceeded, the tyre must be removed from the wheel, and the tube examined for signs of tearing at the valve. Also examine the valve stem for deformation. If the tube is serviceable, the tyre may be refitted and creep marks re-applied. So to summarise the key points: aquaplaning speed depends on tyre pressure, tread depth affects aquaplaning risk, MAT limits tie into tyre speed ratings, and during servicing you check for cuts, bulges, foreign bodies, wear, and creep — each with its own specific limit or treatment.