Swissair Probe Hinges On Systems Analyses |
![]() |
James T. McKenna/Washington
Having
recovered more than 90% of Swissair Flight 111's wreckage from 200-ft.-deep Atlantic
waters, investigators face what could be the most daunting task yet of their probe into
the crash.They must now determine why, after 6 min. of silent and apparent controlled
flight, the MD-11 plunged into the ocean.``It's a very complicated and complex
investigation,'' said Benoit Bouchard, chairman of the Transportation Safety Board of
Canada (TSB), which is running the probe. ``We have to go about this in a systematic, thorough, almost painstaking method,'' Vic Gerden, the TSB's investigator-in-charge, said. Public attention since the crash Sept. 2 southwest of Halifax, Nova Scotia, has focused on the problems known to have hampered Flight 111. Smoke in the cockpit spurred the pilots to divert the New York-Geneva flight to Halifax. Electrical glitches cropped up as the captain tried to isolate the source of the smoke. In a sequence that appears unrelated to steps in the checklist, the autopilot disconnected, the first officer apparently lost his primary flight displays and then the flight data (FDR) and cockpit voice recorders (CVR) stopped. The transponder stopped transmitting and voice communications were lost. MUCH HAS BEEN MADE outside the investigation of the potential problems on the aircraft--Kapton-insulated wiring or a hot-running inflight entertainment system that might have started a fire and insulation blankets that might have fed one. But none of the problems--real or theorized--can yet answer the investigation's most basic question: Why did an MD-11 crewed by skilled and professional pilots fly a descending right turn away from Halifax International Airport, then nose over and plunge several hundred feet into the ocean? According to officials involved in the investigation, the answer likely lies in months of minute analysis of clues from the aircraft's systems and in the forensic reconstruction of Flight 111's cockpit. On Nov. 10, engineers and systems analysts from Boeing's accident-investigation unit in Seattle and its Long Beach Div., where the MD-11 was built, traveled to Halifax. There they briefed Gerden and his TSB team on their analysis of the string of system fault indications that Flight 111's FDR captured before it stopped. Safety board engineers and technicians in Halifax and the TSB's laboratory in Ottawa have been working since mid-September to determine if the indications reflected actual problems in the systems or were the result of power-supply or FDR-sensor-input disruptions. They also have been assessing whether the fault indications collectively point toward a specific problem at a specific location of the aircraft. The Boeing team laid out its analysis of the combination and sequence of the fault indications and the most likely failure scenarios they would represent. Investigators are particularly interested in how the progression of individual failure scenarios would have affected the crew's ability to control and navigate the MD-11. Investigators are debating the plausibility of the scenarios, given the complex interaction of the MD-11's electrical, flight control and flight display systems. Meanwhile, teams began reconstructing the forward 30 ft. of Flight 111. Their objective is to assemble the physical evidence that will identify the source of the smoke and the extent of its damage. The reconstruction is being done in a hangar at the Canadian Forces Base Shearwater outside Halifax. Reassembling the cockpit, the cabin area immediately behind it and the avionics bay below should allow investigators to better analyze the patterns of heat, smoke or fire damage to components in the forward section. They will compare the extent of that damage to the melting point, design-versus-residual hardness and other characteristics of the affected materials. Through such comparisons, accident investigators can deduce the peak temperatures to which fabrics, plastics and metals in the MD-11 were exposed as well as the minimum duration of their exposure. The location and extent of damage to specific components could help investigators narrow down when in the accident sequence the damage occurred and the attitude of the aircraft when it occurred. Take, for instance, heat and fire damage to the fore and aft faces of a wall like that separating the cockpit and cabin. Damage to similar heights and degrees on both faces could indicate the part was exposed to heat while the aircraft was in a level attitude. Disparities in the location and degree of damage on such mating faces could be the result of a sustained pitch attitude during the heat exposure. Investigators will look for other clues in the reconstruction, such as whether heat damage is widespread or localized near a wire bundle. The degree of damage to plastic interior panels, and the layers of wiring, plumbing, tubing and insulation underneath it, and the metal skin and structure underneath them, should give investigators indications of the heat conditions in those locations. Combining detailed technical damage analyses of materials in and around the cockpit with data from Flight 111's FDR, CVR and various components' non-volatile memory computer chips, investigators should be able to piece together a detailed time line of what happened to the MD-11 (AW&ST Oct. 26, p. 38). Investigators also should be able to derive plausible answers to the questions of whether conditions in the cockpit were survivable before the impact with the ocean and whether the crew was able to control the aircraft throughout the accident sequence. INVESTIGATORS MUST OFFER a hypothesis of what happened to the pilots and aircraft in the 6 min. between the last recordings on the FDR and CVR and impact. Theories abound: -- The crew may have lost all but their standby flight instruments. With their aircraft headed out toward the dark, with no moonlight evident below the 13,000-ft. ceiling, or outside visual references from lights on land, the crew may have become disoriented and flown the MD-11 into the ocean. Some MD-11 pilots have said the instrument scan with the standby units is a difficult one. -- The crew somehow became fixated on solving the problem of the smoke and stopped flying the aircraft. -- The pilots were incapacitated by smoke and fumes. -- Some people unconnected to the investigation have speculated that conditions in the cockpit became intolerable and the crew fled. Most of the theories have little basis in fact yet and none fully explains the crash sequence. Officials familiar with the CVR recording said Capt. Urs Zimmerman and First Officer Stephan Loew appeared calm and professional throughout. They said they had donned their oxygen masks, a safeguard against incapacitation. They were working through a checklist to isolate the smoke source. The MD-11 checklists are designed to ensure that one set of primary flight displays is available to the crew throughout such an emergency. But at one point, according to individuals familiar with the investigation, Loew makes mention of going ``back to the bad ones.'' Some investigators believe the reference is to other flight displays that were disabled earlier. It is unclear whether Halifax's approach lights would have been visible to the crew during the latter part of their descending right turn, as the nose came back around toward the airport. Investigators have duplicated the accident flight path with a Canadian Forces Aurora. The MD-11 is designed to be flyable even if two of the aircraft's three main electrical systems are lost. Investigators are looking closely at the electrical systems' design and the performance of the emergency batteries and other power sources. At the very end of the CVR recording, something prompted both pilots simultaneously to try to transmit an emergency call to the Halifax controller. It is unclear what that event was. The final moments of Flight 111 remain a mystery. Gerden's challenge lies in developing an explanation that can stand up to technical criticism from Boeing, the pilots' union, Swissair and other parties who will fight to avoid blame for the crash. |
|||
|