Supplemental Oxygen Systems and Regulatory Requirements

Supplemental oxygen is both a physiological necessity and a regulatory requirement. The ATP must know the 14 CFR 91.211 crew/passenger thresholds, the differences between oxygen systems, and the special rules for pressurized aircraft.

• Above 12,500 ft up to and including 14,000 ft MSL: the required minimum flight crew must use oxygen for any portion of the flight exceeding 30 minutes at those altitudes.
• Above 14,000 ft MSL: the required minimum flight crew must use oxygen the entire time above 14,000.
• Above 15,000 ft MSL: each occupant of the aircraft must be provided with supplemental oxygen.

At high altitude additional 91.211 rules apply. Above FL250 at least a 10-minute supply of supplemental oxygen for each occupant must be available for emergency descent in case of pressurization loss. Above FL350, one pilot at the controls must wear and use an oxygen mask continuously unless the aircraft is below FL410, has quick-donning masks, and two pilots are at the controls — in which case the mask may be off. If one pilot leaves the controls above FL350, the remaining pilot must don the mask. (Part 121/135 carry parallel, sometimes stricter, requirements.)

• Continuous-flow systems deliver oxygen at a steady rate, often through a rebreather bag; common for passengers and lower altitudes.
• Diluter-demand regulators mix cabin air with oxygen and deliver gas only on inhalation, conserving supply; the regulator increases the oxygen fraction with altitude and offers a 100%/normal selector and an emergency (positive pressure) position.
• Pressure-demand systems force oxygen into the lungs under positive pressure, required above roughly FL400 where ambient pressure is too low for the lungs to draw in enough even at 100%.

Aviator's breathing oxygen must be dry to prevent valve icing — never substitute medical or industrial oxygen. Keep oil and grease away from oxygen fittings (fire/explosion hazard), and know the difference between gaseous bottled oxygen and chemical oxygen generators (passenger drop-down masks), which produce oxygen by exothermic reaction and cannot be shut off once initiated.