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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?
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.
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.
Here is a video that shows these plugs blowing out:
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.
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.
