Definition
A supplemental oxygen system that delivers oxygen to the user only when the user inhales. A regulator senses the drop in pressure caused by the pilot's inhalation and releases a metered flow of oxygen through the mask; during exhalation and between breaths, no oxygen flows. This conserves the oxygen supply compared with a continuous-flow system.
Plain English
An oxygen system that gives you oxygen only when you breathe in, instead of letting it flow nonstop. It saves oxygen by not wasting any between breaths.
Context Anchor
Seen in aircraft oxygen equipment descriptions, high-altitude operations, and oxygen system checks.
Derivation
The word 'demand' comes from the Latin 'demandare', meaning 'to entrust' or 'to ask for'. Here it means the system supplies oxygen only on demand — that is, only when the pilot's breath asks for it.
Why Pilots Care
It conserves the aircraft's limited oxygen supply while still protecting against hypoxia during flight above 10,000 feet.
Intuition Check
Demand does not mean the pilot manually asks for oxygen each time. Here, demand means the system automatically responds when the user inhales.
Example Sentence 1
Because the aircraft was equipped with a demand oxygen system, the crew's oxygen bottles lasted well beyond the planned cruise time.
Example Sentence 2
Because the demand oxygen system supplies gas only on inhalation, the bottle lasted the entire cross-country flight.