High pressure environment - the critical importance of aircraft tyre inflation and inspection

Depending on a range of factors, including concrete surface friction, ambient temperature, weather and even the actions of the pilots, aircraft tyres have a serviceable life of anywhere between 120 and 400 landing cycles. Unlike tyres on road transport vehicles, however, they aren’t inflated with air but instead, compressed nitrogen is used. 

 

Since nitrogen is an inert gas, it is especially suited to the aerospace industry, as it is chemically unreactive, which prevents the risk of combustion. As well as for tyres, nitrogen is also used in the fuel system and emergency evacuation slides, for example. 

Whilst aircraft tyres are designed and tested to ‘rated’ conditions, specified by international organisations, in practice most tyres will not operate at the rated limits. Operators must consult the aircraft’s Aircraft Maintenance Manual (AMM) or Pilot Operating Handbook (POH) to ensure the recommended operating pressure range is adhered to. 

 

 

Failure to keep aircraft tyres properly inflated can lead to very serious consequences, the most serious of which is structural collapse. If the tyre operates whilst under or over inflated, the nylon cords forming the structure go in and out of compression as the tyre rotates. This weakens the cords over time until they break and if enough cords break, the tyre structure will fail. 

 

Another serious consequence of under-inflation is over deflection, otherwise known as ‘thrown tread’. Over deflection increases shear between components in the tyre as it rolls in and out of contact and deforms. This results in a more rapid build‐up of heat within the tyre than would be evident if it were properly inflated. This excessive heat will eventually cause the rubber to reverse cure and decompose, resulting in the entire tread or pieces of it delaminating or being ‘thrown’ . 

 

Maintaining precise pressure (PSI) is therefore an absolute to aircraft safety but we must also consider the importance of correct inflation from a commercial perspective. Aircraft tyres on airliners are very expensive. Tyres for the Boeing 777-300ER, for example, can cost more than $5000 each and there are 14 per aircraft, so that’s around $70,000 of rubber. To put things into perspective, Emirates, the world’s largest Boeing 777-300ER operator with 123 of them in service, needs some 2,000 tyres and another 2,000 spares to keep its fleet operational. 

 

Tyres are under most load whilst the aircraft is taxiing and during take-off, so fuel efficiency at this point is also very important. If the tyres are accurately inflated and properly maintained, the aircraft will burn less fuel than if the tyre was under inflated, due to less drag. 

 

 

 

Because of the high pressures and extreme temperatures at which aircraft tyres operate, they are still prone to leakage. In fact, tyres can lose up to 5 percent of pressure in 24 hours and still be perfectly serviceable. 

 

It’s important to check pressures when tyres are cold, ideally pre-flight, using a calibrated pressure gauge. Whilst pilots and mechanics may think that they can identify under-inflated tyres by looking at them, it is impossible to tell visually if they are under-inflated because, on a two-wheel gear, a correctly inflated tyre assumes more of the load than the under-inflated one and both deflect the same amount. 

 

The target range is between 100 and 105 percent of the AMM-defined operating pressure. One of the world’s largest manufacturer of aircraft tyres, Michelin, recommends always servicing the tyres to the top of that range, i.e. 105 percent. If the pressure is found to be between 95 and 100 percent, it recommends servicing the tyre to 105 percent. If it is between 90 and 95 percent, the pressure loss is no longer normal. If the tyre has operated at less than 90 percent of the targeted operating pressure, it is considered no longer serviceable and must be removed. Tyres that have operated under these conditions have been over deflected to the point where their structure may be compromised. 

 

Finally, it is important to think about ambient temperature change. For every 5 degrees in Fahrenheit or 3 degrees Centigrade,, there is a corresponding 1 percent change in tyre pressure. If, for example, a tyre is serviced in Miami where it is 85F and the plane flies to Minneapolis where it is 25F, a tyre which was serviced to 100% of operating pressure, will be at 88 percent in Minneapolis (after cooling for three hours). If a mechanic checks the pressure in Minneapolis before the plane returns to Miami, the tyre will be unserviceable and need to be changed. 

This is another reason why Michelin recommends servicing tyres to the top of the acceptable range, 105 percent of operating pressure. If this same tyre had been serviced to 105 percent in Miami, it would have been at 93 percent in Minneapolis and therefore, still serviceable. 

 

Following these three actions is a good way to make sure operators ensure safety and get the most value out of their aircraft tyres: 

 

 

 

 

 

Supporting the global aviation industry for over fifty years, Newbow Aerospace is a leading manufacturer of ground support equipment, including nitrogen carts, calibrated gauges, hoses, regulators, wheel and brake change trailers and other tools and equipment. 

 

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