Expanding Gases

Tire explosions present serious danger, so personnel should take proper precautions.

It’s a dangerous world out there. The ramps, shops and hangars all have their traps for the unwary. Partially, it is the environment: large pieces of machinery moving about, some driven by blasts of hot gases, others by whirling propellers, and still others by their wheels. The wheels are what these things have in common. Rubber sausages wrapped around metal hubs, holding their shape and supporting the load of the vehicle on some compressed gas. They can support enormous weights, as in a tug, or withstand the shock of a 100 tons of airplane contacting the concrete at 150 mph. They can also let go with enormous force and send out pieces of shrapnel like a proper bomb.

Dangerous Wheels
And therein lies the usually unrecognized danger of wheels, whether mounted on an aircraft, or supporting a push tractor or “merely” rolling along under a baggage cart. Of course, it is not the wheel itself; it is the compressed gas inside. I was scanning a Web site and found a short notice of another horrendous accident where a wheel let go and terribly injured a mechanic. Details were lacking in the story, but he had lost part of his foot and arm. In this case, it was apparently a nose gear tire on a smaller jet. In other words, not much gas volume needed to inflate it.

Sometimes the causes of accidents are self-created. I can remember one incident where a GSE mechanic was servicing a bag cart tire. It was more than 50 years ago now, but the impression it made on me has lasted to this very day. The tire on a cart bag had gone flat. The mechanic responded to the call and picked up a handy servicing cart to pump it up. The servicing rig was for aircraft, but the Schrader valves on the tires on automobiles, bag carts, bicycles, Hough tugs, Mack trucks, Constellation wheels and the high-pressure accumulators are all one size. Same servicing fitting fits them all.

Unfortunately, there is a great difference in the pressures required in each one. This cart the GSE mechanic chose had nitrogen bottles on it with up to 2000 psi in them.

There were two regulators to reduce or control the pressure. He chose the wrong one — the high-pressure one for servicing struts or accumulators on aircraft. Baggage cart tires are little bitty things, and the pressure requirements are not high. A snort of gas at over 1000 psi not only filled it but exploded the wheel almost instantaneously, and he suffered a gruesome facial injury that he has had to live with the rest of his life. I am glad I did not see him, but the blood at the scene was still there. It made a lifelong impression on me.

Proper Precaution
I have seen even dafter things. I have stopped people trying to fill tires directly off a high-pressure bottle without benefit of a regulator. The usual excuse is, “I only crack the valve on the bottle.” I hate to tell them, it doesn’t take much to over-pressurize a tire and just “cracking” the valve is not a good way to control it. Nothing should be serviced directly off a high-pressure source without a regulator to control it. That, by the way, is not limited to tire servicing with inert gasses. I stopped an even worse practice at another company when I found mechanics servicing oxygen bottles with nothing but a hose and fitting between the servicing bottle and the aircraft. What disturbed me most was the whining about how long it took to service a low bottle using a regulator. I told them my office was right upstairs from the hangar floor where they were working, and I didn’t want to have to leave it in a hurry because they had started an oxygen fire in order to save a few minutes.

There is also the danger of damaged wheels or in working on wheels in the shop and changing tires on them. A recurrent problem is people trying to disassemble wheels without letting the air out of the tire. Be sure to remove the valve core. It’s only a moment’s work, but it beats trying to take the through-bolts out of a split hub or the retainer ring out of an air-loaded wheel. The force is incredible and you are in the way. Of course, wheels should be deflated before removal from the axle. The axle nut or the wheel retaining nuts or bolts may be all that is holding it together if it is cracked or damaged. But don’t depend on someone else — it’s your face that’s in front of it if it comes apart.

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