As a flyer, you will know the airline that you are flying, what the in-flight service is going to be like and perhaps also the variety of aircraft you are flying. But would you know about the tyres and how important they are for your flight safety?
Perhaps not. But tyres are as important as the engine and the other hi-tech equipment that help fly a plane. According to Captain PP Singh, Senior Vice-President, JetLite, and currently examiner Airbus 330, Nepal Airlines, how important tyres are can be gauged from the Concorde crash in Paris in 2000. Some debris from a plane that took off before the Concorde had peeled off and was on the runway. When the Concorde took off it sucked in this debris which ruptured its fuel tank leading to its crashing, killing all those on board. The crash was also responsible for the Concorde never flying again.
Little wonder then that airlines treat aircraft tyres with all the respect and caution that they can. An Air India engineer based in Delhi says that aircraft tyres are inspected first by the engineering team and then by a member of the cockpit crew before a flight takes off. The tyre pressure is checked during a night halt. “The tyres have to cool off before the pressure can be checked, hence this is done at night,” he explains.
300 cycles on average
A Mumbai based AI engineer adds that the approximate life of a tyre is about 300 cycles, which is counted as one take-off and one landing per cycle. He adds that there is no hard and fast rule about how many cycles an aircraft tyre will be able to undertake before being taken off. “Three hundred cycles is an average, sometimes it may happen at 250 cycles as well,” he adds.
The placement of the tyres on the aircraft depends on the load that it is carrying. So, in a narrowbody like the Airbus A-320. there are two tyres on the nose and four on the main body as that portion carries the weight of the passengers, cargo and fuel.
The aircraft tyres are filled with nitrogen as this does not support a fire. “Suppose there is a tyre blowout on the aircraft, the brake units are next to the tyres which would typically heat up to 400 to 500 degrees Celsius during normal operations. If there was oxygen in the tyres it would be a recipe for a fire,” Captain Singh points out.
Maximum pressure during landing? No!
An aircraft tyre, when inflated for the first time, is left for at least 12 hours, because the volume of a new tyre increases after the first inflation, and as a result its pressure tends to drop a little. “It is only after that, that the tyre is checked for any leaks,” Captain Singh says, pointing out that this is the reason why tyres are kept in a ready shape to be mounted on aircraft.
Of course, like all things flying, even tyres are surrounded by myths — for instance, that tyres come under maximum pressure during landing. This is not true as the maximum pressure on the tyres is during take-off.
“There are several reasons for this — the aircraft is at maximum weight due to all the fuel, passengers, bags and cargo it is carrying. The temperature of the tyres keeps rising with every rotation of the wheel at the time of pushback, taxi and eventually when you roll for take-off when the speed is also much higher than that at the time of landing,” explains Captain Singh.
Further, from a design perspective, the tyres need to have the strength required to absorb the stress of an aborted take-off from high speed making the pressure on the tyres more at the time of take-off.