Aircraft Tyres — Page 129, Lesson 167

I want to walk you through the subject of aircraft tyres, and we're going to start with the things that can damage them and the pressures that keep them safe. This is a topic where the details really matter, because a tyre failure on the ground can be just as serious as one in the air. First, let's talk about tyre contamination. Tyres are made of rubber, and rubber has enemies. The book is very specific here: tyres must be protected from excessive heat, dampness, bright sunlight, and contact with oil, fuel, glycol, and hydraulic fluid. All of these have a harmful effect on rubber. So when an aircraft is parked for any length of time, or when oil, fuel, cooling, or hydraulic systems are being drained or replenished, you place oilskin covers over the tyres. And here's the key operational rule: any fluid that is inadvertently spilt or allowed to drip onto a tyre must be wiped off immediately. Not later, not when you get a chance — immediately. Now let's move to a phenomenon called creep, which is also called slippage. When tyres are first fitted to a wheel, they tend to move slightly around the rim. That movement is called creep, and at this stage it's considered normal. After the tyres settle down, that movement should cease. But in service, the tyre may tend to continue to creep around the wheel. And this is where it gets dangerous. If creep is excessive on a tyre fitted with an inner tube, it will tear out the inflation valve and cause the tyre to burst. So that's the failure mode with tube-type tyres. Creep is less of a problem with tubeless tyres, as long as the tyre bead is undamaged and any pressure drop is within limits. Now, what makes creep less likely to occur? Correctly maintained tyre air pressure. To assist in this, tyre manufacturers specify a rated inflation pressure for each tyre. Let me be precise about what that figure means. The rated inflation pressure applies to a cold tyre not under load — that is, a tyre not fitted to an aircraft. Here's the practical consequence: when the weight of the aircraft is on the tyre, the distortion of the tyre cover causes the pressure to rise by 4%. So when you're checking the pressure of a cold tyre fitted to an aircraft, you should mentally add 4% to the rated tyre pressure. And during use — that is, during taxiing, take-off, or landing — the tyres become heated, and that can cause up to a further 10% rise in pressure. So let me give you the full picture of pressure behaviour. The rated inflation pressure is the cold, unloaded figure. Add 4% for the aircraft's weight on the tyre. Add up to another 10% for heat during taxi, take-off, and landing. That's why you check pressures on a cold tyre and you know the reading will be higher once the aircraft is moving. Finally, let's look at correct tyre pressures. Tyres in use must be kept inflated to the correct pressures using nitrogen or other inert gas, with a maximum 5% oxygen content. Why nitrogen or inert gas? Because oxygen is reactive and promotes degradation of the rubber and corrosion of the wheel. Under-inflated tyres may move — that's the creep we talked about — round the wheel. Over-inflated tyres will cause other types of failure. And here's a striking statistic: it is estimated that 90% of all tyre failures can be attributed to incorrect gas pressure. That's nine out of ten failures coming down to pressure. Modern aircraft can even display tyre pressures on the electronic systems monitoring screen, so the crew can see the state of each tyre without a manual check. So the whole lesson here is about protecting the rubber and managing pressure. Protect from heat, dampness, sunlight, and fluids. Understand creep and what it does to valves. Know your rated inflation pressure, add 4% for load, and up to 10% more for heat. And keep the tyres inflated with nitrogen or inert gas at no more than 5% oxygen, because 90% of failures trace back to incorrect pressure.