Fine Print: Bad Air

A human factors case study: Qantas’ nitrogen cart


Internet news item, Dec. 16, 2007: Probe after Qantas pumps wrong gas into jets.

Potentially fatal gas being pumped into passenger jet emergency oxygen tanks in Australia sparked a worldwide safety investigation. The Australian Safety Transport Bureau confirmed yesterday the Qantas engineers (mechanics) accidentally put nitrogen into the emergency oxygen tanks of a Boeing 747-300 passenger jet at Melbourne Airport. Before the mistake was discovered, 51 aircraft were serviced with nitrogen instead of oxygen. Qantas sent out advisories to all operators of the affected aircraft and took the necessary measures to prevent this mistake from happening again.

Information gleaned from later news releases reported that instead of the original 51 aircraft that were serviced incorrectly, Qantas advised an additional 124 operators of aircraft that came into Melbourne to check out their aircraft’s oxygen systems for nitrogen contamination.

Before I am accused of playing the blame game, I want you to know that Qantas airlines has the best airline safety record in the world. It has never had a fatal accident in its long history of providing air transportation. Based on its safety record, study of this incident to find out how this mix-up could have happened is made even more important.

How did the oxygen bottles get serviced with nitrogen when the fittings are different sizes? The problem started in March of 2007 when Qantas purchased a brand new nitrogen cart that looked identical to the oxygen carts it was using on the ramp. Using the reasoning that if it looks like a duck, quacks like a duck, and walks like a duck, then it must be a duck, Qantas engineers (mechanics) quickly found out that the nitrogen fittings on the new cart did not fit the oxygen ports on the airplanes. They must have figured that the cart’s wrong fittings must have been the manufacturer’s mistake. They removed the new cart nitrogen fittings and installed the oxygen fittings from an old oxygen cart; their new duck was back in business. The cart was used off and on for 10 months until another mechanic saw the nitrogen cart being used to service pilot’s oxygen in the B 747-300. He stopped them from servicing the aircraft and reported the incident to his superior.

Cockpit oxygen bottles
Now just how serious was this maintenance error? Doing a little research on the incident reveals that the emergency oxygen tanks they described in the news release were actually the cockpit oxygen bottles. To the best of my knowledge, the affected aircraft were not U.S. registered and not subject to our FAR rules. However, I am sure many foreign operators have similar rules on the use of oxygen by pilots that are comparable to FAR 121.333 (c) (3). This rule requires a pilot to don his or her mask, if the aircraft is above 25,000 feet and the other pilot leaves the cockpit to take a bathroom break. Oxygen masks are also donned in emergency situations like an explosive cabin depressurization or smoke in the cockpit. Since the cockpit oxygen is used more frequently than the passenger oxygen bottles, they are usually topped-off every day or serviced at least a couple of times per week. That is why such a large number of aircraft were serviced with bad air in such a short time.

Depending on the ratio of nitrogen to oxygen in the affected bottles, it might only take a few minutes after donning the mask for the pilot to feel the effects of bad air. After that happens, I see only two possible outcomes.

(1) Upon discovering that the pilot passed out, it would be reasonable for those giving first aid to figure that the pilot had suffered a stroke or a heart attack. It would be logical to keep the mask on the pilot to ensure that he or she was getting 100 percent oxygen, while the pilot would still be getting bad air and would slowly suffocate. Result: one fatality, or:

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