The Reason Plane Tires Don't Explode During Landing

You know what they say: "The bigger they are, the harder they fall." And that's not just a metaphor, either — just ask the geneticist, physiologist, and biometrician J.B.S. Haldane, who once said:

"You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes."

So you can only imagine what a jumbo jet ought to do once it hits the ground. For context, a Boeing 747 weighs 403,000 pounds when empty, and 875,000 pounds at max weight. Planes usually land at around 170 miles an hour, too — and yet you've probably never seen a news story about an airplane tire bursting like a horse thrown down a mineshaft.

So why doesn't landing usually affect an aircraft's tires? Well, a commercial jet typically has 20 tires. Brandy Moorhead, the director of Off-Highway Product Development for Goodyear, explains that these tires are constructed using "a combination of proprietary synthetic rubber compounds" along with aluminum steel reinforcements and nylon fabrics. Moreover, they're filled with nitrogen, which is less sensitive to temperature and pressure changes than the air typically used in car tires. To ensure that the tires can withstand the weight of an aircraft, they're inflated six times as much as a car's tires.

A plane can usually land 500 times before the tires have to be retread. After seven retreads, however, the tires end up essentially useless. That means it takes roughly 3,500 landings for most tires to kick the bucket. Still, that's not to say it doesn't happen. In June 2019, a United Airlines flight skidded off a runway because its tires exploded mid-landing.

At least one person suffered a head injury while another injured their elbow. Then, in September 2019, a Qantas Airways flight had to reverse course because one of its tires exploded midair. Obviously, the landing process had nothing to do with that one, but it still goes to show that tires aren't necessarily safe just because you're putting no pressure on them. But why does this actually happen? Put short: human error. Goodyear Aviation's 2017 "Aircraft Tire and Maintenance" manual explains:

"Over-inflation can cause uneven treadwear, reduce traction, make the tread more susceptible to cutting and increase stress on aircraft wheels. Under-inflation produces uneven tire wear and greatly increases stress and flex-heating in the tire, which shortens tire life and can lead to tire incidents."

Furthermore, the manual warns that that tires exposed to higher-than average stress should be removed and scrapped. This can happen if a plane lands at too high a speed, or if the brakes are used an unusually high number of times.

Unfortunately, a damaged or improperly maintained plane can easily turn a tire explosion into a tragedy. In 1991, blown tires caused a fire on a Nigerian Airlines jet. The plane subsequently crashed, and all 261 people on board perished. The Associated Press have reported that it was the world's 10th-worst air disaster at the time. An investigation determined that the tires had insufficient pressure required for safe travel. Sadly, maintenance personnel recognized the problem but chose not to address it.

According to the New York Times, an exploded tire might also have caused a Mexicana Airlines jet to crash into a mountain in 1986, an event that killed 166 people. Experts believe that a broken wheel brake may have dragged across the ground, causing it to overheat, and in turn increase tire pressure to the point of bursting. Though planes have safety valves to prevent a dangerous buildup of pressure, sometimes fail-safes fail, and — in this case — the exploded tire could have caused a fire to erupt on the plane.

Although aircraft tires are usually heavily safeguarded against the stresses they're put through, the sad truth is that no form of engineering is perfect — and a combination of bad luck and human error can often lead to the very worst kind of tragedy.

#Airplanes #Aviation

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