Supplemental oxygen is used to prevent hypoxia from occurring in pilots and passengers. Since the chance of hypoxia increases with altitude, part 91.211 lists key altitudes where supplemental oxygen must be used for both unpressurized and pressurized aircraft. The human body can generally function at atmospheric altitudes ranging from sea level to 12,000ft MSL. From sea level to 12,000ft, the oxygen saturation in an average individual can stay within an allowable range, with the optimal oxygen saturation level at 96%. At 12,000ft MSL, oxygen saturation decreases to approximately 87% and starts to approach a level that could affect performance. While regulations allow pilots to fly at cabin pressures below 12,500ft, it is recommended to use supplemental oxygen when flying above 10,000ft during the day. At night the recommended altitude decreases due to the eyes being more sensitive to oxygen deprivation in low light environments, down to 5,000ft. This part of the regulations mainly applies to non-pressurized aircraft. However, the regulation applies to all aircraft when they are operating at cabin altitudes:
This is to prevent pressurized aircraft from operating at high cabin altitudes without using supplemental oxygen—the obvious danger being the onset of hypoxia. Therefore, whenever an aircraft experiences a cabin altitude above 12,500ft up to and including 14,000ft, the minimum flight crew must be provided with and use supplemental oxygen for the parts of the flight greater than 30 minutes in duration. If the cabin altitude climbs above 14,000ft, the minimum flight crew must be provided with and use supplemental oxygen for the entire time at those altitudes. Finally, each occupant, i.e., passengers and remaining crew members, must be provided with supplemental oxygen when above 15,000ft. While these regulations allow pilots to fly higher than recommended altitudes without supplemental oxygen legally, hypoxia can occur much lower. The exact altitude and strength that hypoxia occurs is different for each person and should be kept in mind while operating between 5,000 and 10,000ft. Pressurized aircraft can maintain cabin altitudes lower than the outside atmospheric pressure. This gives a greater sense of comfort and removes supplemental oxygen requirements while operating at higher altitudes. However, as previously stated, if a pressurized aircraft intends to fly at cabin altitudes previously listed, the respective supplemental oxygen requirement must be followed. If a pressurized aircraft operates at altitudes above FL250, then a 10 minute supply of oxygen must be available for every occupant. This oxygen supply is intended to give the flight crew enough time to initiate an emergency descent below 10,000ft. When a pressurized aircraft operates above FL350, one pilot must be wearing and using an oxygen mask while at the controls. The oxygen mask must either provide oxygen at all times or automatically supply oxygen when cabin altitudes climb above 14,000ft. However, this rule does not apply if two pilots are at the controls, and both possess and have a quick don mask available. If both pilots have a quick don mask, then neither pilot needs to wear a mask while at or below FL410. This exception to the rule does not apply if one pilot leaves the controls. If a pilot must leave the controls for any reason, the remaining pilot must wear and use an oxygen mask until the other pilot returns or the aircraft descends to FL350 and below.  The zero-sideslip is necessary for a pilot to maintain control of an aircraft and have the greatest climb performance during an engine failure in a twin. Aerodynamics | February 10, 2022 The critical engine on a multi-engine aircraft is the engine whose failure will result in the most adverse effects on aircraft performance and controllability. Aerodynamics | January 27, 2022 Pilots use medical certificates to show that they meet the health requirements to perform the duties and privileges of their pilot certificate. Physiology | January 4, 2022 Runway markings vary depending on the type of operations conducted at the airport Systems | December 29, 2021 An Emergency Locator Transmitter (ELT) is a battery-operated transmitter developed to locate a downed aircraft and is a component of the various emergency services available to pilots. Systems | November 14, 2021
The FARs for Part 91 pilots only require that supplemental oxygen be used after 30 minutes of exposure to altitudes between 12,500 and 14,000 feet. But consider that pilots for hire (Part 135 and 121) are required to use oxygen above 10,000 feet. I would favor the conservative approach that professional pilots are required to adhere to and use supplemental oxygen anytime above 10,000 feet. If you regularly fly at altitudes above 8-10,000 feet, you should invest in a pulse oximeter. These relatively inexpensive devices clip onto a fingertip and read out blood oxygen levels. The minimum blood oxygen level you should see is 90%, anything less than that means you need to either use oxygen or descend to a lower altitude.” Here’s a quick quiz on hypoxia from the Air Safety Institute.
What is the lowest altitude you've experienced symptoms of hypoxia? Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. Symptoms include headache, dizziness, confusion, shortness of breath, restlessness, changes in skin color.
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At what altitude must a pilot use supplemental oxygen for the duration of the flight that is longer than 30 minutes?
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