4.3 Oxygen, Water, Waste & Convenience Systems

Three oxygen systems, three different designs

Nig. CARs 7.9.1.13 requires that all aircraft intended to be operated at altitudes requiring supplemental oxygen be equipped with adequate oxygen storage and dispensing apparatus, with the apparatus, the minimum rate of flow and the supply prescribed by the Authority. Part 8 adds the operational rule that passengers use oxygen continuously at cabin pressure altitudes above 4,000 m (13,000 ft).

How passenger chemical oxygen works, and why the briefing wording matters

A chemical oxygen generator contains a solid oxidiser core. When the firing mechanism is triggered, the core burns in a self-sustaining exothermic reaction that liberates oxygen. Three consequences follow, and all three are examinable:

1. It must be started by pulling the mask. The masks drop when the system deploys, but no oxygen flows until the lanyard attached to the mask is pulled sharply, firing the percussion cap. This is precisely why safety briefings say to pull the mask firmly towards you — a passenger who simply places the dropped mask on their face receives nothing.
2. It cannot be switched off. Once started, the reaction runs to completion. The unit will produce oxygen for a fixed period — typically of the order of 12 to 22 minutes depending on the generator fitted — which is designed to cover the time needed for an emergency descent to a safe altitude.
3. It gets very hot. The canister reaches high temperatures and may produce a burning smell or slight haze in the first moments. Cabin crew must expect this and reassure passengers, and must not touch the canister body.

The reservoir bag on a continuous-flow mask does not need to inflate for the mask to be working. Passengers frequently report "my mask isn't working" because the bag is not visibly full. The correct response is to confirm the lanyard was pulled and that the mask is over the nose and mouth with the elastic tightened.

Order of donning is fixed: fit your own mask first, then help others, including children and passengers requiring assistance. A crew member who reverses that order is likely to be the second casualty.

Potable water

The potable water system supplies galleys and lavatories from tanks filled through a service panel, pressurised either by air from the pneumatic system or by an electric pump.

Cabin crew responsibilities are largely public health responsibilities:

• Do not use water from an unknown or suspect source. Water quality is managed by the operator's approved water sampling and disinfection programme.
• Report a low quantity indication before departure; on a long sector, running out affects lavatories as well as service.
• Report discoloured, cloudy or odorous water immediately and stop using it.
• Report leaks. Water leaking into the cabin floor or into a cargo compartment can reach electrical equipment and, on a long high-altitude sector, freeze.
• Do not pour hot liquids, coffee grounds, tea leaves or food debris into lavatory basins — the drains are narrow and blockages disable the lavatory for the rest of the flight.

Waste system

Lavatory waste is collected into sealed tanks and serviced on the ground; the toilet is normally a vacuum system that uses the pressure differential in flight and a vacuum generator on the ground.

Rules that recur in examinations:

• The waste tank drain is serviced only by ground staff through an external panel. Cabin crew never open waste system panels.
• Blue disinfectant fluid leaking from a lavatory or visible externally must be reported. Externally, leaking fluid can freeze and detach.
• A strong smell of waste in the cabin may indicate a tank or seal problem and is reported.
• Nothing but the intended waste enters the toilet. Nappies, sanitary items, bottles and cloths cause blockages; blockages take the lavatory out of service and can back up the system.
• The waste bin in the lavatory is a fire risk area. Its flap must close fully, and its built-in extinguisher — required by Nig. CARs 7.9.1.7 — must be checked as part of pre-flight lavatory checks.

Entertainment and convenience systems

In-flight entertainment (IFE), seat power outlets, USB charging, cabin Wi-Fi and premium seat actuation are listed in the syllabus as aircraft systems because they carry safety consequences:

• IFE boxes are frequently mounted under passenger seats and are a recognised source of electrical heat and smoke; a hot or smoking under-seat unit is treated as an electrical fire event.
• Seat power outlets must be isolated if a device connected to one overheats.
• Powered seats must not be operated if a personal electronic device has fallen into the mechanism, because crushing a lithium battery can start a thermal runaway.
• IFE must be capable of being switched off and the PA must override it, so that emergency announcements are heard.
• A failed IFE system is a service issue, but a failed PA or a failed emergency announcement path is a safety issue and is reported as such.