If you are flying at a cabin altitude of 13,000 feet MSL and plan a 45-minute leg, how many minutes of supplemental oxygen would be required?

• A. 0
• B. 15
• C. 30
• D. 45

Answer: (B)

Oxygen requirements hinge on how long you stay at cabin altitude above 12,000 feet. In this scenario you’re at 13,000 feet for a 45-minute leg. The rule used here implies a 30-minute period that can pass without supplemental oxygen, and the remaining time must have oxygen available. So out of 45 minutes, 30 minutes can be flown without oxygen, leaving 15 minutes for which supplemental oxygen is required. That’s why the minimum oxygen you’d need for this leg is 15 minutes. If you were at higher altitudes (above 14,000 feet) for extended periods, the requirements are more stringent, but for this case the 15-minute figure applies.