Cabin Fire Suppression System A Life Saver

SAVE commenced work on a system which would neither require costly and complex modifications (such as installing more exit doors or all new interior fittings) nor make it harder for passengers to evacuate a burning aircraft. It developed a spray system, designed to fill the cabin with a fine water mist which would suppress flames and maintain a survivable atmosphere (conditions in which people are not incapacitated by fumes, smoke or heat) for three minutes, without hindering the emergency evacuation.

Following successful proof of concept testing in 1988, both SAVE and the rights to the spray system were acquired by Darchem, which has since continued development and constructed a production prototype. T he Darchem spray system uses water from underfloor tanks fed under pressure to longitudinal titanium pipes running the length of the cabin behind the ceiling and fascia panels. Nozzles at intervals along these tubes provide a fine spray of water droplets when the system is pressurised. It is made even more effective by external fittings which allow fire services to supply water from outside the aircraft to extend the evacuation time.

The water mist has several functions: it cools the air inside the cabin; cools the passengers; suppresses the ignition of cabin furnishings, thus reducing the generation of smoke and fumes; absorbs toxic and irritant combustion gases in the confined cabin atmosphere; washes smoke particles out of the atmosphere; and inhibits flashover (when the heat radiated by a burning object causes an adjacent object not in contact with it to ignite). It must operate for at least three minutes from the time of activation The spray system is entirely self contained so as to ensure operation in the event of other systems, such as electric power, failing. Pressure is provided by compressed nitrogen, while dedicated batteries provide the system’s electric power. The cabin is divided into forward, centre and aft areas in case the fuselage breaks up on impact. Each has its own independent sub-system with water supply and pressurisation, connected to the overhead longitudinal pipes. If the fuselage does break up, easily frangible self sealing joints (designed to break easily and cleanly if twisted of bent) are installed at intervals in the longitudinal pipes. They immediately seal when broken, minimising water loss and maintaining spray pressure within the separated sections of the fuselage.

To operate the system, a control panel on the flight deck incorporates system status indication (including water level and nitrogen pressure) plus the master arming switch which completes the circuit that allows the indication switch to operate, an activation switch (which pressurises the water system), and an emergency stop button. In addition, each sub-system has its own activation switch and stop button for local use by cabin crew. Should it be necessary, the system can be operated in flight, in which event the water coverage is less than three millimetres, which would be easily absorbed by furnishings and clothing.

Airlines and air safety organisations are examining the system with much interest, for there are several operational prerequisites which they believe must be met for the system to be practical. Not the least of these is that a commercial water spray system must impose minimum weight and cost penalties. It must also be simple to both operate and maintain; it must not be able to activate inadvertently so as to present an inflight hazard; and it must be simple to install within existing airframes.

Development of the Darchem SAVE spray system has therefore focused on the primary concerns of weight and installation. Installation itself is quite simple, costing just $A225,000 for a Boeing 757 and substantially less for business jets and smaller regional airliners. As for weight, designers have reduced the volume of water needed to protect passengers for three minutes, and all components are of lightweight construction. Typical system weights (including water) for the production SAVE system range from 50-60kg for an executive aircraft, such as a Jetstream, to 700-B00kg for a widebody Airbus A330 type aircraft. With a number of rival spray systems now also under development, water weight is not being publicly disclosed.

With demonstrations showing the fire suppressor to be both practical and cost efficient, interest in the product is strong. Darchem is certain that it will soon be approved for installation onboard aircraft, making it the first such commercially available fire suppression system, one which should at last improve airliner crash survivability.