New Technologies Demonstrate Ability to Defeat Aircraft Fires

Air Safety Week, June 26, 2000

"Very promising results."-- Richard Healing, Director, U.S. Navy Safety &

Survivability; chairman, Aircraft Wiring and Inert Gas Generator (AWIGG) working group

INDIANAPOLIS, Ind. - Tests of new water mist technologies show significant promise to provide added protection against in-flight fire at a negligible increase in aircraft weight or system complexity.

Another approach, based on fast-acting chemical discharge units already in use in marine transportation, also appears to present the potential for protecting inaccessible areas in the aircraft, such as the electronics and equipment (E&E) bay.

Both the water mist and the chemical reaction technologies were tested last May at Greenwood, Mississippi, utilizing the fuselage of a recently retired B737-200 as the test bed. The tests were conducted under the auspices of the U.S. Navy's directorate for safety and survivability. The results of the tests were presented at a meeting here of the Aircraft Wiring and Inert Gas Generator (AWIGG) working group. The meeting was hosted by United Airlines [UAL] at its massive heavy maintenance facility. During the course of the AWIGG meetings, United officials illustrated some of the difficulties they are experiencing in coping with wiring-related failures, particularly in high-time aircraft where the effects of vibration, moisture, chafing, and other assaults take their toll on wiring.

An Initial Effort

The Greenwood tests were intended to establish "proof of concept" of three approaches to combating in-aircraft fires: (1) a high pressure water mist system, (2) a twin fluid low pressure mist of air and water, and (3) a commercially available aerosol generator. The water mist systems were tested primarily in occupied areas of the aircraft, although the effectiveness of the low pressure system was tested against an electrical fire in the space between the cabin ceiling panels and the outer fuselage skins. The gas-generated aerosol system was tested for its potential to quench fires in the E & E bay.

The high pressure water mist system, which operates at a pressure of about 1,000 pounds per square inch, was developed by the Naval Research Laboratory (NRL) as a potential substitute for Halon to fight shipboard fires. A system of steel pipes and nozzles was installed in the retired B737 to demonstrate the potential of this approach to defeat fires in commercial aircraft.

"Our goal was not 100 percent extinguishment, but to control the event so the pilot could make a decision," said Fred Williams, an NRL scientist who headed the high-pressure system tests. The high pressure system was tested against four types of fires: newspapers and other flammable materials in the lavatory, a fire in a passenger seat, a fire in a trash can in the galley, and a fire in the overhead bin. In each case, six ounces of nail polish remover was added to the trash, newspaper, clothing, etc., depending upon scenario, to act as an "accelerant." The fires were lit and allowed to burn for 15 seconds to "take hold" before the water mist system was triggered.

The lavatory fire, in which flames were leaping well above the sink, was suppressed within 8 seconds with about one-tenth of a gallon of water. It took about a minute and 20 seconds to shut down the fire in the galley trash can. The melting Styrofoam cups, Williams believes, acted as a form of "Greek fire," dripping down into the trash receptacle, making it less accessible to the quenching effects of the water mist.

Even though nozzles were routed into the overhead bins, the overhead bin fire took the longest to douse. The bin was loaded with typical materials: duffel bags stuffed with sweatshirts and other clothing items, a gift box containing tissue paper and, per the other tests, the 6 oz. "accelerant" of nail polish remover.

Burning Bins

Videotapes of the bin tests showed that the fire burned through the top of the overhead bin with astonishing speed, vividly demonstrating the capability of the fire to spread rapidly into the void above the cabin ceiling panels. Even so, the high pressure water mist system extinguished the inferno in less than a minute and a half, using less than a gallon and a half of water.

Lawson believes that an intumescent paint, which would expand under heat, could provide protection against burn-through for overhead bins. The paint would expand, anywhere from 20-100 times its thickness, helping to seal the gap between the bin and adjacent overhead structure.

"We feel that we have demonstrated the capability of water mist to control a fire in the cabin," Lawson said. He believes the logical next step would be a series of tests where the cabin is pressurized, and the ventilation system is functioning, as it would be in flight. In addition, Lawson believes various aerosol cans, like those containing hair spray and other cosmetics products found in passenger baggage, should be included to provide a more demanding test scenario.

Although the high pressure water mist system would not require more water than contained in the potable water supply on the aircraft, the system of steel piping would add weight, and the "proof of concept" installation would require considerable additional design to be outfitted in the cabin of an airliner. The system, although impressive in the immediacy of its effects against raging flames, might not assure protection in a post-crash fire. Severe impact forces could sever and/or rupture the steel pipes, compromising coverage or rendering the system altogether ineffective.