Wednesday, July 30, 2008

Quantas Jet and the “Oxygen Cylinder”

A recent Quantas flight from Melbourne to Hong Kong had to make an emergency landing after a hole was blown out of its fuselage at 30,000 feet. Miraculously, the pilot was able to land the jet liner, and no one was hurt. From a system safety point of view, it’s amazing since the failure of a system is built upon the serial dependence of all the system’s part’s failure rates*…Redundancy is thus required for these complex systems. I’m talking about an airplane in this context, but the same goes for all systems.

Preliminary investigations apparently are pointing to an oxygen cylinder that was housed in the cargo area of the plane. These cylinders store the oxygen for the masks that deploy in the event of a rapid decompression. The theory is that the cylinder, which holds pressurized oxygen, ruptured, and this rupturing created a force that blew the tank out of the plane. The plane’s fuselage, now compromised, proceeded to widen.

I hope they are right. This sounds very ominous to me, and Quantas has recently had a series of issues with its flights (who hasn't really?). So either you can draw the conclusion that their service maintenance has been an issue, but either way, it’s a potentially scary development. The best evidence to support the oxygen tank theory comes from one of the passenger stories. Apparently when the oxygen masks deployed, one passenger complained that their mask did not provide oxygen. Let’s hope that’s true.

* System Failure...The reliability of a system is the geometric series of each independent part's reliability...So if s system has 3 parts, and each has a reliability of 99% ( i.e. each fails 1% of the time), then the system's reliability is 97.0% (97% = 0.99 x 0.99 x 0.99). You can see how system engineers need to be build redundancy in their systems because as systems become more complex and have more "moving parts", you cannot escape this serial dependence. Safety is truly about your risk tolerance. Since you cannot escape this fundamental law, the game becomes, how many failures can I tolerate, which ultimately leads you to the economic benefits vs. costs of building the system you are designing. Which is again why, if you read my post on antibiotic resistance, you realize that nature is truly efficient in its design. Everytime you look at a plane, it is truly amazing we can put a piece of metal in the sky. I'm quite certain that when a bird sees a big plane, they probably say "that's amazing...the wings aren't moving..."

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