Expanding Gases

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


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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