October 2008 Issue

Causes of Hypoxia and Flying Non-Pressurized Aircraft at Lower Altitudes

The FARs say one thing about when to use oxygen but the reality is quite different. You probably need it sooner and lower.

Twelve thousand five hundred feet. Fourteen thousand. Fifteen thousand feet. If you’re a pilot, you immediately recognize the significance of these altitudes. Each triggers different requirements for supplemental oxygen use. Most of us learn the FARs associated with these requirements early in our primary training so we can spout them back on written exams and in the oral portion of the Practical Tests. After that, we may never think much more about them. But like most FARs, the oxygen rules are a minimum standard of safety. Of what real-world relevance are the oxygen requirements of FAR 91.211? From the standpoint of safety, when should you be using supplemental oxygen? Supplemental oxygen, for those not familiar with the term, is additional oxygen added to ambient air. The goal is to provide enough "added air" to bring the O2 user’s oxygen intake up to the same level it would be at a target altitude (usually sea level). The need for additional oxygen increases with altitude, since (obviously) the higher you go, the more O2 you have to add to give the breather sea-level air. For example, one aircraft manufacturer’s automatically regulated oxygen system meters supplemental air at the rate of 0.5 liters/minute/person at 5000 feet, scaling up to 2.8 liters/minute/person at Flight Level 250.

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