Definition
An oxygen system built permanently into the aircraft, consisting of one or more storage cylinders mounted in the airframe, plumbed through fixed lines and regulators to outlets at each crew and passenger station, where masks or cannulas are connected for use at altitude.
Plain English
An oxygen system that is part of the airplane itself. The bottles, hoses, and outlets are installed in the aircraft, so the pilot just plugs in a mask and turns it on rather than carrying separate equipment on board.
Context Anchor
Seen in aircraft equipment descriptions, oxygen system discussions, and preflight checks for aircraft that provide built-in oxygen for high-altitude flight.
Derivation
Fixed comes from a Latin word meaning “fastened.” In this term, it points to oxygen equipment that is fastened into the aircraft as installed equipment.
Why Pilots Care
Supplies reliable supplemental oxygen above 10,000 feet or during pressurization problems, directly reducing hypoxia risk on longer flights.
Intuition Check
Fixed does not mean the oxygen flow is unchangeable or that the system never needs attention. Here, fixed means installed as part of the aircraft rather than carried as a portable unit.
Example Sentence 1
During preflight, the pilot checked the gauge for the fixed oxygen installation and confirmed the cylinder showed full pressure before the high-altitude leg.
Example Sentence 2
Because the aircraft had a fixed oxygen installation, the crew could remain above 12,500 feet without portable bottles.