Rare occurrence of an inflight fire leads to setting a higher materials flammability standard

FRANCES FIORINO/NEW YORK

Swissair Flight 111 leaves a valuable safety legacy: Investigators were able to review aircraft flammability standards and improve testing and certification of materials. It also leaves a painful legacy: The lead investigator says there wouldn't have been an accident if flammable materials hadn't been positioned next to arcing wires.

Flight 111 investigators found regions of wire copper melt in the fire initiation area. The fire likely started with electrical arcing "involving one or more wires."

According to the Transportation Safety Board (TSB) of Canada's final accident report, the inflight fire that led to the Sept. 2, 1998, crash of the MD-11 had "most likely started from an electrical arcing event that occurred above the ceiling on the right side of the cockpit near the cockpit rear wall" (see photo above).

The arcing of one or more wires in turn ignited the flammable cover material--metallized polyethylene terephthalate (MPET)--on nearby thermal acoustic insulation blankets (see photo, p. 63). A segment of arced electrical cable from the inflight entertainment network (IFEN) is believed to be associated with one or more of the arcing events.

However, the TSB was unable to conclude whether arcing of an IFEN wire was involved in the initial fire event. Other flammable materials in the area, including silicone elastomeric end caps and metallized polyvinyl fluoride insulation blanket materials, helped to sustain and propagate the fire.

Deteriorating conditions in the cockpit resulted in the pilots losing control of the aircraft, which plunged at a speed of 300 kt. into the waters 5 naut. mi. southwest of Peggy's Cove, Nova Scotia. All 215 passengers and 14 crewmembers died in the crash.

The FAA's June 2000 airworthiness directive gave operators five years to remove flammable MPET materials from MD-11s.

It took the TSB 4.5 years, C$57 million ($38.8 million) and up to 4,000 people at one stage to complete the investigation. Recovery, sorting and cataloging of the wreckage took 18 months. Divers, a heavy ship barge and remotely operated vehicles (ROVs) retrieved about 2 million pieces of wreckage, or 98% of the aircraft (measured by weight) from 200-ft.-deep waters.

When recovery was complete, investigators set out to piece together the events that unfolded during the flight's final minutes:

Flight 111 departed New York JFK International Airport at 8:18 p.m.local time on Sept. 2, 1998, en route to Geneva. About 53 min. later, when the MD-11 was at Flight Level 330 (33,000 ft.), the first officer reported an unusual odor in the cockpit, and there was a small amount of smoke visible on the flight deck.

Both pilots concentrated on trying to determine its cause, instead of expediting plans to land immediately, according to the report. At 9:14 p.m., when the aircraft was about 66 naut. mi. southwest of Halifax, the crew informed Moncton Control Center of smoke in the cockpit and issued a "pan, pan, pan" message. Controllers suggested Halifax International as a diversion airport instead of Boston.

Meanwhile, failed end caps on air-conditioning ducts fed a steady supply of air to the fire that raged in an inaccessible area, while smoke and fumes seeped into the cockpit.

The aircraft was cleared to proceed directly to Halifax and descend to FL290 from FL328 when it was 56 naut. mi. from Halifax Runway 06.

The crew, who had donned oxygen masks, was focused on dealing with a diversion to an unfamiliar airport at night, and the approach charts to Halifax were not within easy reach, according to the report. The aircraft's descent rate increased to 4,000 fpm.

Aircraft checklists did not deal adequately with smoke conditions, according to the TSB. The report notes that during the lead arcing event, the associated circuit breakers did not trip. No fire suppression equipment was near the area, and there was no integrated inflight firefighting plan in place--nor was there any regulatory requirement for either.

The accident report points out that the crew was "essentially powerless to aggressively locate and eliminate the source of fire or to expedite plans for emergency landing."

The crew declared an emergency at 9:24 p.m. This was followed by a series of electrical and navigation equipment system failures. The cabin crew indicated electrical power was lost in the cabin, and flashlights were being used to prepare for emergency landing.

When the primary flight displays failed, the crew had to adjust to small standby instruments, which added to the workload (AW&ST Jan. 7, 2002, p. 43). Gradually overcome with heat and fumes, the pilots lost situational awareness, and the MD-11 crashed into the sea at 10:31 p.m. The report notes that although the crew recognized the necessity for a diversion, they did not believe the threat to the aircraft was sufficient to declare an emergency or initiate an emergency descent profile. The report notes that from the time the peculiar odor was detected, "the time required to complete an approach and landing to Halifax . . . would have exceeded time available before the fire-related conditions in the cockpit would have precluded a safe landing."

ACCORDING TO Investigator-in-Charge Vic Gerden, "One of the most important aspects of this investigation was our examination of the flammability standards and the flammability of various materials. It is rare to have a fire on board a large commercial aircraft, and we were able to glean a lot of information about the materials and look to the tests used to certify those materials. Without the readily flammable material in this airplane, this accident wouldn't have happened."

Among the 11 causes and contributing factors, the TSB found materials flammability as the "most significant deficiency" uncovered in the most complex aviation safety investigation it had ever undertaken.

The Flight 111 final report also said the certification testing procedures, mandated by flammability standards in effect at the time, were "not sufficiently stringent or comprehensive to adequately represent the full range of potential ignition sources." Nor did the procedures mirror the behavior of materials installed in combination, at various aircraft locations and in "realistic" operating environments. "The lack of adequate standards allowed materials to be approved for use in aircraft, even though they could be ignited and propagate flame."

The report said two factors shaped those standards. In the mid-1970s, the FAA concentrated its fire prevention efforts on improving cabin interior materials and setting higher standards for materials in designated fire zones--with lower priority given to fire threats in other areas. The non-fire-zone hidden areas were viewed as not having potential ignition sources and flammable materials, two elements required for a fire, according to the report.

Canada's TSB alone issued 23 safety recommendations, along with myriad safety advisories and information letters. The board's latest recommendations, which were issued with the final report, address the testing and flammability standards of in-service thermal acoustic insulation materials. They also call for taking extra measures in the certification of add-on electrical systems and setting industry standards for circuit breaker testing. The TSB also has proposed improvements to capture and store mandatory and non-mandatory flight data, as well as the installation of cockpit image recording systems.

Previous recommendations included a call for wire inspections, the removal of MPET from aircraft, and development of new flammability testing criteria. They also urged that crews be provided with additional guidance material to deal with smoke situations, and that checklists be modified.

A number of safety actions have already been taken. The FAA's June 2000 Airworthiness Directive ordered operators to remove MPET from aircraft. The AD, which affects Douglas heritage aircraft along with the MD-11, gave operators five years to comply--and this means the flammable materials will remain installed on in-service aircraft until 2005.

The FAA also conducted a review of problems in the MD-11 service life and developed a plan to correct wiring deficiencies, which a Boeing official said essentially calls for a re-rigging of wire systems. The FAA plan includes 61 final-rule ADs and 59 NPRMs (notices of proposed rulemaking). The agency also started the Enhanced Airworthiness Program for Airplane Systems for increased awareness of wire system degradation and improvements in wiring maintenance.

There's still work to do, said Gerden. "Now it's our job to ensure that follow-ups on safety actions are taken."

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©April 14, 2003, The McGraw-Hill Companies Inc.