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According to the CAAIP (Civil Aircraft Airworthiness Information and Procedures):/

10.4 Tests have shown that the safest extinguishant to use is a dry chemical agent and this must be used whenever possible. It should be applied by an operator standing in line with the tyre’s rolling path and at a safe distance; an overheated wheel should never be approached in line with the axle.

But it doesn't state why it should be so. Can you explain this?

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    $\begingroup$ Highly related $\endgroup$
    – FreeMan
    Commented Aug 25, 2022 at 16:06
  • $\begingroup$ I'll link to this related Question/Answer. The diagram it quotes for "wheel/brake overheat hazard areas" makes a slightly different suggestion: you're apparently supposed to approach from from an oblique angle to the direct front/back, as directly front/back is the path for tire debris. $\endgroup$
    – mbrig
    Commented Aug 26, 2022 at 0:59

4 Answers 4

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Besides staying clear of the fuse plug in the rim, it's partly because the sidewall of the tire is the weakest part of the carcass, so that a rupture from overpressure from overheating means most of the force goes sideways when the tire is stationary. The key word is "stationary".

When the tire is spinning, centrifugal forces mean the tread lets go from the sidewall all the way around, and the bits all go perpendicular to the axle, but when sitting still, it mostly goes sideways.

A secondary factor is the rims themselves. Aircraft wheels are built in halves and bolted together. They have to be made that way to ensure that there is enough overlap of the edge of the rim over the bead to prevent the tire from being forced off the rim by side loads.

You don't mount the tire the way a car tire is, by stretching the bead over the rim using a bar or in a machine. You place each half of the rim into the tire from each side and bolt it together.

If you disassemble an aircraft tire by unbolting the wheel halves and forget to release the pressure in the tire, you get a nice surprise when you remove the last fastener (this happens with forgetful mechanics removing light aircraft tires from time to time).

Anyway, this means that if the wheel itself fails and breaks into pieces, the large chunks of metal will go sideways, mostly.

Bottom line is with a stationary tire, the heavy structure of the belted tread acts like a containment structure to protect you, so you take advantage of that.

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    $\begingroup$ Light truck rims (in the 3/4 to 1 1/2 ton range, maybe heavier but I was never around those) had split rims long ago -- and as late as the 1970s. I knew someone who worked in a tire shop, and said split rims were the most dangerous to work on. $\endgroup$
    – Zeiss Ikon
    Commented Aug 25, 2022 at 15:41
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    $\begingroup$ Yeah on those I believe the flange was bolted on instead of having the entire wheel in two halves, but dangerous nonetheless. I mount my own car tires (because I'm nuts) using one of those harbor freight tire stands that you bolt to the floor. The trick is to use LOTS of bead lubricant, and they go on easy peasy. When I change a tire on my plane, I check the pressure's out prolly 3 times before I start unbolting it. $\endgroup$
    – John K
    Commented Aug 25, 2022 at 16:28
  • $\begingroup$ @JohnK multipart rims are still a thing in the automotive aftermarket today; for street cars I think it's mostly about simplifying logistics by being able to mix and match parts to offer more configurations with less inventory needed. $\endgroup$ Commented Aug 26, 2022 at 21:47
  • $\begingroup$ In fact go to any truck stop and you’re likely to see a tire cage—a cage enclosure that is placed around a semi truck’s tires when a rim set is first inflated. Split-rim failures can easily be fatal and cause a lot of injuries every year. A typical semi or bus rim failure can generate 60-80,000 pound-feet of kinetic energy. I’m guessing an over-heated high pressure nitrogen 747 tire heated by a brake overheat could generate pressures more near a million pound feet were it not for fuse plugs. There’s zero chance of survival and in fact zero chance you’d stay in one piece. $\endgroup$
    – Max R
    Commented Aug 27, 2022 at 3:46
  • $\begingroup$ Those aircraft mechanics who forget to release the pressure ... do they live to tell the tale? Like, how bad are the results? $\endgroup$
    – Sixtyfive
    Commented Aug 28, 2022 at 13:38
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Aircraft wheels have fusible plugs that blow out when high temperatures are reached. I am just guessing, but it stands to reason that it’s safer to approach an overheated tire from the front or rear in case the wheel plugs blow out.

The International Fire Service Training Association says to approach an overheated tire at a 45 degree angle. enter image description here enter image description here

Here is a video that shows these plugs blowing out:

B777 brake test enter image description here

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The weakest part of tire's structure is the sidewall, unless the wheel has been locked and the tire tread is severely worn out. If a tire is blown simply due to overheating, it most likely will eject shrapnel to its sides.

It is also possible that shock cooling of the wheel by extinguishing agent will cause it to shatter, again sending shrapnel mostly in the direction of the wheel axle. The cooling effect will largely depend on the type of extinguishing agent.

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To add more detail to another, too-short answer, the primary hazard when using an extinguisher to cool an overheated wheel is blowout of the tire. If this should occur, the relatively heavy tread and belts will tend to limit the velocity of tire fragments thrown in the rolling direction, as well as largely directing the pressure wave parallel to the axle.

For reference, I've seen a car tire, at a mere 2.3 bar during installation, blow out and break shirt buttons on someone standing a couple meters from the tire. Many aircraft tires, especially those on relatively heavy airplanes, run at significantly higher pressures than a couple bar -- some heavy military tires well above 15 bar. One of these tires can produce a very injurious pressure wave if it bursts, independent of any fragments.

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    $\begingroup$ The tire on a B52's were pressurized to 265 PSI, over 18 BAR. I did not work on F106's so I am not sure what their tire pressure was, but we had two airmen killed at Griffiss AFB back in 1986 when a F106 tire exploded during maintenance. You were suppose to depressurize the tire and pull the valve core when the tire was removed from the aircraft. Check the core was out when the tire was brought to the shop, and check again it was removed before working on the tire. All three times this was not done and men died because of it. $\endgroup$
    – Jim
    Commented Aug 25, 2022 at 15:05
  • $\begingroup$ And there was an earlier case, also at Griffiss. 2 guys inflating a B-52 tire. Did not have all the bolts in and tight. Eventually, it let go. One of the bullets...er...bolts went through the shop door, across the adjoining office, and embedded itself into the far wall. No one died, luckily. $\endgroup$
    – WPNSGuy
    Commented Aug 25, 2022 at 16:53

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