What you should know about…aircraft wheels

A Boeing 747 coming from Hong Kong touches down on the Northwest Runway at the Frankfurt Airport while a blue-gray cloud hangs in the background. The 16 main landing gear tires safely bring the 260-metric-ton long-range aircraft to a stop. A fairly common scenario, but also one that could cause a curious onlooker to start wondering: Are the tires subjected to the greatest load during landing? How much does each tire cost? And why exactly are the tires filled with nitrogen?

 

 

 

Tires are changed every 120 to 400 landings depending on a number of factors

Aircraft tires need to withstand an extremely wide range of temperatures that go from minus 60 degrees Celsius at an altitude of 10,000 meters to extremely high temperatures when landing in the world’s hottest regions. Add to that the fact that a long-range airplane such as the B747 will complete a total of more than 15,000 takeoffs and landings throughout its life, and it should come as no surprise that the tires on that aircraft will be changed over one hundred times. However, it is also worth noting that aircraft tires are somewhat different from car tires: They are tubeless, have a pressure of approx. 12 to 18 bar, and are filled with nitrogen instead of just air. In fact, that last difference plays a key role in ensuring safety, since the oxygen in ambient air could otherwise quickly ignite when interacting with braking systems that reach temperatures of several hundred degrees. When all these variables are put together, the result is that aircraft tires need to be changed every 120 to 400 landings depending on the aircraft model and factors such as the runway, the weather, and the pilot.

 

Before every flight, an aircraft’s pilot or copilot needs to carry out a preflight check, which includes a visual inspection of all tires: Are any of the tires worn out? Are there any objects stuck in the rubber? This is absolutely crucial, as it is not uncommon for an inspection to reveal that tires need to be changed right away at the terminal. After all, safety comes first!

 

 

Retreading does not equal poor quality

Another difference between car tires and aircraft tires is something that is considered a sign of second-rate quality in the former but an indicator of first-rate engineering in the latter: Retreading. Aircraft tires are retreaded eight to ten times before being replaced, which is particularly important when one considers the fact that the cost of retreading is only one fourth of the cost of a new tire, which is about 1,200 euros for the landing gear on a jumbo jet. Interestingly enough, however, the tire manufacturers are actually the same for both aircraft and passenger cars. Bridgestone, Michelin, Goodyear, Continental, and Dunlop – they all are part of the market. Another couple of details: There is no difference between summer and winter tires when it comes to aircraft tires, which can have a width of up to 50 cm and a diameter of over 1.5 m. And the biggest tires are used on the Airbus A380 or A340 as of this writing.

 

 

Figure: A long-overdue change: Aircraft tires beyond the wear limit

 

 

 

So what is tougher on the tires: Landing? Or taxiing?

Before taking off with a large weight, a Boeing B747 will be exerting a load of up to 25 metric tons on its 16 tubeless main landing gear tires – in other words, a tire load that is one hundred times the load on a medium-sized passenger car. And yet, these tires can handle this at speeds of up to 380 kilometers per hour without a problem, even if only for a brief moment. Now, many would probably think that the touchdown is the actual moment when aircraft tires experience the most wear. They would also be wrong. While it is true that a main landing gear wheel on a jumbo jet will accelerate from zero to about 1,100 rpm upon touching down, the resulting wear will still pale in comparison to a takeoff, in which weight truly bears down, and to the taxiing between the landing and the next takeoff, which amounts to an average of 8 km of distance. In addition to this, the main wheels on airliners are mounted on fixed axles and need to absorb both radial and axial forces when turning, which results in heavy tire deformation that can even result in cracking on treads. A significant factor to take into account when considering the fact that an airplane will be moving up to 8,000 km on taxiways and runways each year!

 

 

Which ones are used: Circumferential grooves? Or lateral grooves?

A Boeing 747-400 tire weighs about 110 kg and has a diameter of 1.24 m. In contrast to car tires, however, an aircraft tire’s tread only has three to six circumferential grooves, part of which is due to the fact that tires on aircraft have a different function than tires on cars. More specifically, the lateral grooves on car tires help move the car and assist with acceleration, while the circumferential grooves on aircraft tires are primarily intended to prevent hydroplaning by diverting water when it is raining (aircraft engines take care of the acceleration). Moreover, there are two circumferential grooves that are simply reference grooves used to determine the degree of tire wear.

 

 

Figure: Six circumferential grooves at the B747 tires, to prevent hydroplaining

 

 

 

Who is responsible for changing tires?

At the Frankfurt Airport, for example, Lufthansa Technik takes care of tire changes. An average of 20,000 completely mounted wheels leave the wheel workshop every year, with the workload heavily depending on the season of the year. And while this can be partially traced back to increased air activity during the peak travel season, it is also due to the higher temperatures during the summer months, which result in heavier tread wear.

 

 

 

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