If the aircraft is traveling at ______ MSL then everyone on board must wear oxygen if the cabin is not pressurized.

• A. 10,000 ft MSL
• B. 8,000 ft MSL
• C. 20,000 ft MSL
• D. 15,000 ft MSL

Answer: (D)

Oxygen becomes essential for everyone once the cabin/flight altitude reaches 15,000 feet MSL in an unpressurized aircraft. At that point the air is too thin for safe function, and the risk of hypoxia rises quickly, so all occupants must use supplemental oxygen. Lower altitudes like 8,000 or 10,000 feet don’t trigger universal oxygen use for the whole cabin (crew oxygen is required above about 12,000 feet, but not all occupants). While higher altitudes such as 20,000 feet also require oxygen, 15,000 feet is the stated threshold for everyone on board to don oxygen in unpressurized conditions.