The pressurisation module built a bubble of breathable air; this closing lesson is about the day the bubble bursts. Oxygen on board comes in three families: the crew's regulated system, the portable equipment, and the passengers' drop-out masks. The exam wants the modes, the actuation, and the sharp differences between a chemical candle and a gaseous cylinder.
The cockpit system and its regulator
The cockpit oxygen system is gaseous: high-pressure cylinders, a shutoff and pressure-reducing stage, and lines to each pilot's quick-donning mask, whose diluter-demand regulator gives the system its three modes:
Normal (diluter demand) — the regulator mixes cabin air into the oxygen, richer in oxygen as the cabin altitude climbs, and delivers only on the demand of an inhalation, stretching the supply.
100 per cent — cuts the dilution and gives pure oxygen on demand, for when the cabin air itself is suspect.
Emergency — adds continuous positive pressure, holding the mask's interior above cabin pressure so that any leak at the face seal blows outward: smoke and fumes cannot seep in.
Portable oxygen
Smoke hood (protective breathing equipment, PBE) — a sealed enclosure for the whole head with its own oxygen supply for roughly a quarter of an hour: the equipment a cabin crew member wears to walk toward a fire and fight it inside the smoke.
Portable bottle — a cylinder with a mask and simple flow settings, carried to wherever oxygen is needed: first aid for an unwell passenger, and mobility for cabin crew who must work while the cabin is still high after a decompression.