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Compressed Gas Is Not for Wimps

Story and Photos by: H. Dean Chamberlain
Reprinted with permission from FAA Aviation News

Compressed gases are not for wimps. While working at the Reno "air races" this year, I saw proof why compressed gases are not for wimps. In fact, compressed gases are downright dangerous. A P-51 Mustang was severely damaged when its oxygen tanks exploded while being refilled. Because of any potential liability issues that may result from the explosion, I don't profess to know the facts of the accident, nor do I portray myself as an expert when dealing with compressed gases. I do have a working knowledge of compressed air (3,000 PSI tanks) used for Scuba diving and compressed gases used for welding. My knowledge can be summarized as knowing not to apply more pressure than the tanks and connecting hoses or lines are rated for and always handling combustible gases and tanks very carefully.

As I understand the incident, the aircraft's pilot wanted to top off the Mustang's onboard oxygen tanks. In talking to him, he said he had enough oxygen for the proposed flight, but he decided to have the tanks filled. The contract oxygen service provider's oxygen cart, a collection of oxygen tanks plumbed to a common distribution manifold controlled by pressure shut off valves and pressure regulators mounted on a wheeled cart, arrived and began filling the aircraft's oxygen filler port. At this point, the details become fuzzy.

What is known is as the Mustang sat in its pit, its oxygen tanks, there was more than one in the system, exploded while being filled. Fortunately, there were no serious injuries. I was told one aircraft ground crewman suffered a ringing in his ears.

The aircraft suffered major damage. The oxygen tanks are located in the aft fuselage. When they exploded, they ripped open both sides of the Mustang's fuselage just forward of the tail section. In addition, the exploding tanks and metal punctured the aircraft's hydraulic system and damaged some control cables. Hydraulic fluid then leaked out under the aircraft. I was told, because the fuselage skins blew upward, they directed the force of the explosion up away from anyone near the aircraft. Eventually, the Mustang's wings, engine, and other parts had to be removed so that the aircraft could be loaded on a flatbed semi-tractor trailer for the trip home.

Although I don't know if an "official" investigation has determined the cause of the incident, various people were discussing the following points at the time of the incident. The Mustang's oxygen system was a low-pressure system. Its maximum rated pressure was placarded at 450 pounds per square inch (PSI). I was told this oxygen system might have been the only low-pressure system at the races. I was told the other aircraft oxygen systems on the ramp were rated up to 1,800 PSI. These systems are considered "high pressure" systems.

At issue is what caused the Mustang's oxygen tanks to explode? Were they filled with "high pressure" oxygen as some people speculated? Or, were they filled to some other lower pressure as per the aircraft's crew's instructions? In cases like this, I know the FAA aviation safety inspector who responded to the aircraft incident asked for the isolation of the oxygen cart so that its pressure regulator could be checked to determine its accuracy. This is normal procedure in such cases to try to eliminate or identify potential causes.

As a result of this incident, changes were made concerning how other aircrafts' oxygen systems could be refilled. The revised procedures require that oxygen systems be filled out on the ramp area away from the pits and people.

I think this incident brought up an important safety point. Because many aircraft operate with installed onboard oxygen systems and many pilots of non-pressurized aircraft may carry portable oxygen systems, I think this incident highlights the need for everyone involved with servicing aircraft oxygen systems to know and following the recommended safety requirements for handing pressurized gases. This means ensuring that the system and its tanks meet the appropriate Department of Transportation's or other recognized safety and inspection standards. That everyone involved in refilling installed and portable tanks knows how to refill the system or tanks. This includes knowing permitted pressures and quantities involved. It also means that portable systems be handled in a safe manner as recommended by their manufacturers to include how to carry, handle and safely store the tanks and related hoses and masks.

Although the incident involved oxygen, the same rules apply to other pressurized systems in aviation. From welding gases to high pressure struts to pressurized tires to even high pressurized hydraulic lines, high pressure systems can be potentially deadly if not treated with respect and system knowledge.