The National Transportation Safety Board today determined that the probable cause of the crash of TWA flight 800 was "an explosion of the center wing fuel tank resulting from ignition of the flammable fuel/air mixture in the tank."
"The source of ignition energy for the explosion could not be determined with certainty," the Board said, "but, of the sources evaluated by the investigation, the most likely was a short circuit outside the center wing tank that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system."
TWA 800, a Boeing 747-131 on a scheduled flight from John F. Kennedy International Airport, New York, NY, to Charles DeGaulle International Airport, Paris, France, broke up and crashed into the Atlantic Ocean near East Moriches, NY, on July 17, 1996. All 230 people on board were killed and the airplane was destroyed.
Cited as contributing factors to the accident were: (1) the design and certification concept that fuel tank explosions could be prevented solely by precluding all ignition sources; and (2) the design and certification of the Boeing 747 with heat sources located beneath the center wing tank with no means to reduce the heat transferred into the center wing tank or to render the fuel vapors in the tank nonflammable.
The five-member Safety Board met in Washington, August 22-23, to review with the NTSB staff the results of a four-year investigation into the flight 800 tragedy - which, as noted by NTSB Chairman Jim Hall, had become "the most extensive, complex and expensive investigation in the Safety Board's 33-year history."
From the beginning, the scope and dimensions of the investigation have been extraordinary. The salvage effort organized by the Navy extended over four months and was followed by months of work by contracted fishing trawlers that scoured hundreds of miles of the ocean floor. In the end, remains of all 230 victims and more than 95% of the aircraft were recovered.
The reconstruction of a 93-foot segment of the aircraft fuselage, including the center wing fuel tank, was unique in size and scope, and involved more than 30 people working meticulously for many months on the project to help the Board better understand what had happened to flight 800. The amount of research and testing undertaken by Safety Board staff and by various government laboratories and private research organizations under contract to the NTSB also were unprecedented.
NTSB investigators examined every piece of wreckage for any physical evidence that the crash of flight 800 could have been caused by a bomb or missile. No such evidence was found.
In the early months of the investigation, it became clear that an explosion of flammable vapors in the aircraft's center wing tank initiated the breakup and subsequent crash of flight 800. In December 1996, based on the Board's conclusion that heated, flammable vapors in an aircraft fuel tank poses a serious risk to safe flight, the Board recommended that the FAA study design changes to deal with this problem and that, in the interim, they require operational changes to enhance safety. In April 1998, the Board followed with another set of recommendations focused on wiring and the aircraft's fuel quantity indication system.
More broadly, the flight 800 investigation uncovered and focused the attention of the aviation community on some very important safety issues -- fuel tank flammability, numerous potential ignition sources, and a complex of issues related to maintenance and aging of aircraft electrical wiring.
Consequently, the Board issued four new recommendations to the FAA:
1. Examine manufacturers' design practices with regard to bonding of components inside fuel tanks and require changes in those practices, as necessary, to eliminate potential ignition hazards.
2. Review the design specifications for aircraft wiring systems of all U.S.-certified aircraft to (a) identify which systems are critical to safety and (b) require revisions, as necessary, to ensure that adequate separation is provided for the wiring related to those critical systems.
3. Require the development and implementation of corrective actions to eliminate the ignition risk posed by silver-sulfide deposits on fuel quantity indication system components inside fuel tanks.
4. Regardless of the scope of the Aging Transport Systems Rulemaking Advisory Committee's eventual recommendations, address (through rulemaking or other means) all of the issues identified in the Aging Transport Non-Structural Systems Plan, including:
- The need for improved training to maintenance personnel to ensure adequate recognition and repair of potentially unsafe wiring conditions;
- The need for improved documentation and reporting of potentially unsafe electrical wiring conditions; and
- The need to incorporate the use of new technology, such as arc-fault circuit breakers and automated wire test equipment.
To determine whether adequate progress is being made in these areas, the Safety Board asked that, within 90 days, the FAA provide a briefing on the status of its efforts to address all of the issues identified in the Aging Transport Non-Structural Systems Plan.
Further information on the TWA flight 800 investigation is available on the Safety Board's web site (http://www.ntsb.gov). Printed copies of the report, when available, can be purchased from the National Technical Information Service by calling (800) 533-NTIS.