Expanding Gases

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.

Reinflating wheels for the first time after buildup can be exciting if there is a flaw in the hub. This should be only be done with the wheel inside a cage built for the purpose.

Split hubs with through-bolts can fail and so can split-rim wheels where the locking ring comes loose and, typically, there will be folks inflating the wheel. That ring can take your head off. I noticed a little story on the use of tire cages on the same Web site I mentioned. A young apprentice was told to use the safety cage for the first inflation of a “tyre” (hey, he’s a Brit) after the wheel was assembled. Two minutes later, the apprentice was inside the cage with the wheel on the outside. Spannerhead, who wrote the blog, said, “No it wasn’t me.” Well, at least the apprentice was trying to be safe. Hint for next time: The wheel goes inside, you stay outside.

Tire explosions are ugly things — I inspected the wheel well of a B727-200 after it was partially repaired, having suffered a tire explosion in Havana. We finished the repairs, which were extensive. It looked like a bomb had gone off there and had the aircraft been in flight, it would have gone down. Had personnel been in the area, they would have been killed. The same airline had another wheel explode a bit later. Unfortunately, it was in-flight and caused a fatal crash. That accident led to the Airworthiness Directive against using compressed air in braked wheels on aircraft. Only dry nitrogen is allowed in aircraft wheels with brakes. A hot wheel filled with “air” has lots of oxygen in it. The hot rubber of the wheel gives off gases, and it can actually combust inside. You can only imagine how high the pressure will go before the now-weakened tire/wheel explodes.

Brainless occurrences can be dangerous, too. A mechanic was charging the brake accumulator on a Constellation. It is quite large and required a lot of air. He was using the tool of the day (mid-1950s), which was a bootstrap compressor. It took in compressed air off the hangar system and used it to compress a small bit of it to a much higher pressure. It was nicknamed the “knicker-knocker” due to the sound it made and also some other things due to the slow rate of charging. The mechanic went to lunch leaving the bootstrap compressor running. He figured he had time to eat, play a hand or two of cards and get back in time to see the accumulator topped off. He was a bit slow getting back, the knicker-knocker exceeded expectations and the accumulator exploded. Considerable damage ensued to the nose compartment of the Connie but no one injured, thankfully. We can be our own worst enemies.

One other item is always interesting. Brakes sometimes drag or an aircraft may make a rejected takeoff, which means lots of heat in the brake and the wheel. Hot means hot enough to melt the rubber in the tire or actually set it alight. Fusible plugs may, or may not release the air in the tire. Never approach or work on an overheated wheel on the sides where the hub is. Approach from the tread side only. But how do you let the air out? One airline developed a spiked chock for deflating tires that were dangerously hot or actually afire, or had cracked or pieces of rim broken out. You placed it against the tire like a chock (approaching from the tread side of the tire) with the spike against the tire. You used a tug on the nose to shove the aircraft and the wheel back against the hollow spike. It would pierce the tire and deflate it. Quite effective and keeps you away from the side of the wheel where the danger is. It would be worthwhile having one available and probably something the fire service people should have, too.

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