Zones, halon, and the lesson of TWA 800

Fire protection on the 747 is divided into zones - each engine, the APU, the cargo holds and the toilets - each with its own detection and extinguishing hardware, all managed from a single fire-control module on the flight deck.

Key facts
Zones covered	All four engines, the APU, cargo holds, toilets
Extinguishing agent	Halon-type bottles discharged from the flight deck
Cargo compartments	Classed A/B/C; Class C holds must have detection plus automatic extinguishing
Fuel-tank safety	Fleet gained inerting after TWA Flight 800

The 747's integrated fire-control installation covers its four engines, the APU and the main cargo compartment from one module. Cargo compartments are classed by accessibility: a Class C hold, which cannot be reached in flight, must have both fire detection and fully automatic extinguishing.

TWA Flight 800, a 747-100 destroyed in 1996 when fuel vapour in a nearly empty centre-wing tank ignited, transformed the industry's approach. Subsequent 747s carry fuel-tank inerting hardware to keep oxygen out of tank ullage, and the 747-400's master minimum equipment list includes a dedicated inert-gas-system page (ATA 47).

Zoned by function

The 747's fire protection is divided into zones, each with its own detection sensors and extinguishing bottles, all supervised from a single fire-control module on the flight deck. The zones are the four engine nacelles, the APU compartment, the cargo holds, and the cabin including the lavatories.

Engine and APU zones use a combination of linear heat-detection and overheat elements, because those areas have characteristic, localised overheating signatures. Cargo holds, where fires can be smouldering and diffuse, use different detection. The lavatory zones are monitored and extinguished separately, because a lavatory fire is common, usually minor, and must not be allowed to escalate.

Cargo compartment classification

Cargo holds are classified by how accessible they are in flight, and the classification sets the required protection. A Class C compartment - one that cannot be reached in flight to fight a fire - must have both fire detection and fully automatic extinguishing. This is the strictest category, and it is the reason the loading manual's airflow requirement above stacked containers, discussed in weight and balance and loading, exists: smoke detection only works if air can circulate.

The TWA 800 lesson

TWA Flight 800, a 747-100 destroyed in 1996 after fuel vapour in a nearly empty centre-wing tank ignited, changed the industry approach to fuel-tank safety. The investigation found that the empty centre tank's ullage could accumulate vapour, and the ignition source was never conclusively established. The result was a new generation of requirements, implemented across the fleet, for centre-wing tank inerting systems that keep the oxygen content of the tank ullage below the level where fuel vapour can burn. Later 747s, including the 747-400, carry this hardware, and the type's minimum equipment list includes a dedicated inert-gas-system item.

Related

• Fuel system
• Hydraulic systems
• Electrical system
• 747-8 systems differences