Definition
An oxygen delivery system that supplies oxygen to the user only when the user inhales (on demand), and at altitudes above roughly 40,000 feet delivers that oxygen under positive pressure so that it is forced into the lungs rather than simply being drawn in by normal breathing. Below the pressure-breathing threshold the system functions as a standard demand system, releasing oxygen through a tight-sealing mask each time the user inhales.
Plain English
An oxygen system that gives the pilot oxygen only when they breathe in, and at very high altitudes actually pushes the oxygen into their lungs so they can keep getting enough at altitudes where regular breathing isn't strong enough.
Context Anchor
Seen in high-altitude oxygen system discussions, especially when comparing continuous-flow, demand, and pressure-demand equipment.
Derivation
Pressure refers to the positive pressure used to push oxygen into the lungs at high altitude. Demand means the system only releases oxygen when the user inhales, rather than flowing all the time. Together: an on-demand system that adds pressure when needed.
Why Pilots Care
Prevents hypoxia during flight above 35,000 feet where normal breathing becomes impossible without pressurized delivery.
Grounding Statement
At very high altitude, the air is so thin that the system may need to help push oxygen through the mask instead of simply letting the pilot breathe it in normally.
Intuition Check
“Demand” does not mean the pilot manually asks for oxygen. Here it means the system responds to the user’s inhaling. “Pressure” does not mean a forceful blast. It means controlled gas pressure that helps oxygen enter the lungs.
Example Sentence 1
The aircraft was equipped with a pressure-demand oxygen system because its service ceiling was well above 40,000 feet.
Example Sentence 2
The aircraft checklist required switching to pressure-demand mode before climbing through the transition altitude.