A continuous-flow oxygen system on commercial airliners is defined as which of the following?

• A. It vents cabin air to the atmosphere to maintain cabin pressure.
• B. It supplies oxygen only to the pilots' masks during emergencies.
• C. It uses a mixing system to blend cabin air with oxygen for crew.
• D. It supplies a constant flow of pure oxygen to a rebreather bag that dilutes the pure oxygen with exhaled gases and thus supplies a healthy mix of oxygen and ambient air to the mask, primarily used in passenger cabins of commercial airliners.

Answer: (D)

Continuous-flow oxygen systems push oxygen to the mask at a steady rate, not only when you inhale. In airliners, this is typically achieved by delivering oxygen to a reservoir (rebreather) bag in the mask circuit, so oxygen is available continuously and a breathable mix is supplied to the mask throughout the breathing cycle. This setup is characteristic of passenger cabins on commercial jets, making it the correct description. The other statements describe functions that don’t match how the continuous-flow system is designed to deliver oxygen: venting cabin air relates to pressurization, not the oxygen mask system; providing oxygen only to pilots excludes passengers; and blending cabin air with oxygen for crew isn’t how the standard passenger continuous-flow arrangement works.