WHAT YOU SHOULD KNOW ABOUT…AIRCRAFT WHEELS

WHAT YOU SHOULD KNOW ABOUT…AIRCRAFT WHEELS


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!

How do an Airplane's Tires Help It Land Safely?

Have you ever flown in an airplane? If so, you have felt that funny bump when you touch down at the end of the journey. But have you ever wondered how airplane tires handle all that stress and pressure? Why don’t they burst into contact with the hard runway? Well, airplane tires need to be specially designed. Thanks to their design, they are strong enough to stay intact during takeoff and landing. Airplane tires are very different from car, truck, or bicycle tires. In fact, airplane tires have about as much in common with these tires as they do with running shoes! They are all made of rubber. They all contain air. They all provide support and cushioning for what’s on top of them. But that's where the similarities end.

When a plane lands, airplane tires have to support the weight of the airplane and all the people in it. The passengers and cargo in an airplane are called the payload. Airplane tires have to deal with enormous forces when the plane lands. They especially have to deal with friction. Friction occurs when two surfaces move against each other, like when you rub your hands together. Airplane tires create friction when they hit the runway. This friction generates heat. It also and wears down the outside layer of the tire. Because of this, airplane tires are reinforced with strong, flexible materials. One of these materials is a super-strong plastic called Kevlar. Kevlar is strong, flexible, heat-resistant, and lightweight.

It’s important for airplane tires to be flexible. Flexibility lets airplane tires absorb more of the shock of landing. Flexibility also slows the wearing down of tires.

Airplane tires also have conducting strips built into the tire grooves. These strips discharge any electric charges that may have built up. This is important. If static electricity were to build up during takeoff or landing, it could cause a spark. A spark could ignite the fuel in the plane! Airplane tires are also protected by what’s inside. Usually, they are filled with nitrogen. Nitrogen is a non-flammable gas. It doesn’t corrode the metal parts of a plane. Nitrogen also doesn’t oxidize (break down) the rubber in the tires. The tires themselves are made from at least three layers of rubber. Each layer is laid down in a different direction. This makes the tire stronger and gives it better traction when it lands.

Airplane tires also have six times more pressure than car tires do.

But airplane tires are more than just rubber (and Kevlar). They can have 14 different parts. Each of these parts serves a specific purpose in making takeoffs and landings safer and easier.

The typical airplane tire can go through about 500 landings before it needs to be repaired. Usually, the top layer of tread is simply peeled off and replaced with new tread. That way, the other parts don’t need to be replaced. That’s a good thing because the other parts are very expensive.

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.

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 referenced grooves used to determine the degree of tire wear. 

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.

So the next time you see an airplane in the sky, think about its tires. Remember, there is a lot of engineering that goes into them. This helps the plane take off and land safely!

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