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I just watched the first episode of Inside Mighty Machine where civil engineer-turned-host, Chad Zdenek, discussed innovations of the 747. One innovation he discussed was the increased number of wheels and trucks, along with this tidbit:

When the 747 touches down, the wheels must accelerate from 0 to 150 mph in a heartbeat, making them skid before they start to spin. That's why they smoke on touchdown. That friction causes heat to build inside the wheel, creating the risk of an explosion, so the 747's tires are filled with nitrogen, an inert gas which, unlike oxygen, won't aid combustion. That helps protect the plane from blowouts and keep landings safe.

That seems like an oversimplification and backhandedly suggests that the alternative is to fill the tires with pure oxygen—a surefire recipe for combustion I would think.

So, (1) is nitrogen really "the thin line" between safe landings and blowouts on modern commercial airliners and (2) is nitrogen landing gear inflation a legitimate innovation of the 747?

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    $\begingroup$ Your title says "prevent", but the quote says "protect". Those are quite different. "Prevent" means "it doesn't happen", "protect" means "the frequency decreases". $\endgroup$ – Acccumulation May 24 at 14:40
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    $\begingroup$ Does "protect the plane from blowouts" mean reduce the frequency of blowouts, or minimise the damage from a blowout should one occur? $\endgroup$ – TripeHound May 24 at 15:38
  • $\begingroup$ @Acccumulation Thanks for the English lesson... armed with that I'm now going to march down to the local police station and demand they change their slogan to "To serve and prevent"! ;) $\endgroup$ – Michael May 24 at 23:45
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    $\begingroup$ @Michael No, that's the motto of Planned Parenthood. $\endgroup$ – Acccumulation May 24 at 23:59
  • $\begingroup$ @Acccumulation: Perhaps you’re referring to some technical/legal sense I’m not aware of, but un normal use, “prevent” can perfectly well mean “reduce the frequency of”. Most statements that “X prevents Y” mean “X prevents a significant amount of Y” not “X prevents 100% of Y”, just as most statements “X protects Z from Y” mean “X protects Z from most of the danger of Y” not “…from 100% of the danger of Y”. $\endgroup$ – Peter LeFanu Lumsdaine May 26 at 10:08
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Apart from the paragraph you quoted, here are a few more reasons.

  1. Dry nitrogen is specified for aircraft use. I believe the rules say that any gas used cannot exceed 5% oxygen content. Because its dry it reduces corrosion.

  2. Nitrogen moleculeas are slightly larger than oxygen and takes longer to escape.. hence tyres stay inflated longer.

  3. But more importantly, nitrogen has a lower rate of expansion/contraction than normal air. An aircraft wheel has to live in sub-zero (at 40,000ft) and blistering hot (IIRC about 500deg C will trigger hot brakes warning). A gas with a low expansion rate cf temp is desirable. This trait has been put to good use by unscrupulous 'performance auto shops' which claim better handling with nitrogen in the tyres...

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    $\begingroup$ How does 3) work? Doesn't nitrogen behave according to the ideal gas law? $\endgroup$ – DeltaLima May 24 at 8:54
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    $\begingroup$ @DeltaLima The ideal gas law is an approximation that assumes molecules have zero volume. The level of approximation you get will be different for different gases, so it's not impossible that something like this could be true. Wikipedia mentions the water content of compressed air vs nitrogen being a significant issue here, though surely one could dehydrate the air if that was the only issue. $\endgroup$ – David Richerby May 24 at 13:21
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    $\begingroup$ It should be noted that air contains almost 80% nitrogen. Nobody is filling tires with pure oxygen. The characteristics of air are in between those of its constituents. $\endgroup$ – bogl May 24 at 13:25
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    $\begingroup$ I was with you until #3. The ideal gas constant for air is 287 and for nitrogen it's 296. A difference of 3%. $\endgroup$ – Organic Marble May 24 at 13:30
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    $\begingroup$ @DavidRicherby I agree on the water being the issue. At lower temperature/ higher pressures, there will be condensation. But when comparing dry air to nitrogen, there is no difference in expansion vs temperature. $\endgroup$ – DeltaLima May 24 at 13:48
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There is some truth in the claim about inerting. Mostly, blowouts are as a result of high temperatures weakening the rim and tyre and increasing gas pressure within it, rather than being the result of chemical explosions. However, overheated tyres can decompose to produce gaseous products that can explode in air at high pressure and temperature and there have been airworthiness directives related to this (see, e.g., FAA Advisory Circular 20-97B).

Also, having oxygen in the tyres will oxidize the rubber to some extent, weakening it. Of course, the outside of the tyre is bathed in oxygen anyway, but at much lower pressure, so there's much less of it around.

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In addition to what the current answers mention, it should be noted that the danger is not merely theoretical, though blowouts on touchdown aren't necessarily the primary concern.

Mexicana flight 940 was the deadliest 727 accident as well as the deadliest accident on Mexican soil with 167 deaths. Accident investigators found that a tire had been serviced with regular air instead of pure Nitrogen. A brake malfunctioned and overheated during the takeoff run. This caused the tire to heat up to the point that the oxygen in the tire chemically reacted with the tire itself to the point that the tire exploded fifteen minutes after takeoff. This severed hydraulic, fuel, and electrical lines, which then resulted in an in-flight fire at cruising altitude. The pilots declared an emergency and tried to return to Mexico City, but ultimately lost control and crashed into a mountain before they could get back to MEX, killing everyone on board.

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  • $\begingroup$ This incidentally answers the second part of the question : if it was required on the 727, it probably pre-dates the 747. $\endgroup$ – Brian Drummond May 25 at 14:37

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