Autunno 2000
NATIONAL TRANSPORTATION SAFETY
BOARD
Public Meeting of August 22-23, 2000
Abstract of Final Report
(Subject to Editing)
Aviation Accident Report
In-Flight Breakup Over the Atlantic
Ocean
Trans World Airlines (TWA) Flight
800
Boeing 747-131, N93119
near East Moriches, New York
July 17, 1996
NTSB AAR-00/03
This is an abstract from the Safety Board's report and does not include the Board's rationale for the conclusions, probable cause, and safety recommendations. Safety Board staff is currently making final revisions to the report from which the attached conclusions, probable cause, and safety recommendations have been extracted. The final report and pertinent safety recommendation letter will be distributed to recommendation recipients and interested parties as soon as possible. The attached information is subject to further review and editing.
EXECUTIVE SUMMARY
On July 17, 1996, about 8:31 p.m. eastern daylight time, TWA flight 800, a Boeing 747-131, N93229, broke up in flight and crashed in the Atlantic Ocean near East Moriches, New York. TWA flight 800 was operating under the provisions of 14 Code of Federal Regulations Part 121 as a scheduled international passenger flight from John F. Kennedy International Airport (JFK), New York, New York, to Charles DeGaulle International Airport, Paris, France. The flight departed JFK about 8:19 p.m., with 2 pilots, 2 flight engineers, 14 flight attendants, and 212 passengers on Board. All 230 people on board were killed and the airplane was destroyed. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules plan.
CONCLUSIONS
1. The flight crew was properly certificated and qualified and had received the training and off-duty time prescribed by Federal regulations. No evidence indicated any preexisting medical or behavioral conditions that might have adversely affected the flight crew's performance during the accident flight.
2. The airplane was certificated, equipped, and dispatched in accordance with Federal regulations and approved TWA procedures.
3. At the time of the accident, there were light winds and scattered clouds in the area, but there were no significant meteorological conditions that might have disrupted the flight.
4. The in-flight breakup of TWA flight 800 was not initiated by a preexisting condition resulting in a structural failure and decompression.
5. The in-flight breakup of TWA flight 800 was not initiated by a bomb or a missile strike.
6. The fuel/air vapor in the ullage of the TWA flight 800 center wing tank was flammable at the time of the accident.
7. A fuel/air explosion in the center wing tank of TWA flight 800 would have been capable of generating sufficient internal pressure to break apart the tank.
8. The witness observations of a streak of light were not related to a missile, and the streak of light reported by most of these witnesses was burning fuel from the accident airplane in crippled flight during some portion of the postexplosion preimpact breakup sequence. The witness observations of one or more fireballs were of the airplane's burning wreckage as it fell from the sky.
9. The TWA flight 800 in-flight breakup was initiated by a fuel/air explosion in the center wing tank.
10. Boeing's design practice of permitting parts less than 3 inches in any direction to be electrically unbonded may not provide adequate protection against potential ignition hazards as a result of static electricity generated by lightning and other high-energy discharges.
11. It is very unlikely that the flammable fuel/air vapor in the center wing tank on TWA flight 800 was ignited by a lightning or meteor strike; a missile fragment; a small explosive charge; auto ignition or hot surface ignition, resulting from elevated temperatures produced by sources external to the center wing tank; a fire migrating to the center wing tank from another fuel tank via the vent (stringer) system; an uncontained engine failure or a turbine burst in the air conditioning packs beneath the center wing tank; a malfunctioning center wing tank jettison/override pump; a malfunctioning center wing tank scavenge pump; or static electricity.
12. Electromagnetic interference from radio frequency sources external to TWA flight 800 did not produce enough energy to ignite the fuel/air vapor in the center wing tank.
13. Electromagnetic interference from personal electronic devices played no role in the ignition scenario for the TWA flight 800 center wing tank explosion.
14. It is unlikely that electromagnetic interference from aircraft system wiring played a role in the ignition scenario for the TWA flight 800 center wing tank explosion.
15. Existing standards for wire separation may not provide adequate protection against damage from short circuits.
16. A short circuit producing excess voltage that was transferred to the center wing tank fuel tank quantity indication system wiring is the most likely source of ignition energy for the TWA flight 800 center wing tank explosion.
17. Silver-sulfide deposits on fuel quantity indication system components inside fuel tanks pose a risk for ignition of flammable fuel/air vapor.
18. The ignition energy for the center wing tank explosion most likely entered the center wing tank through the fuel quantity indication system (FQIS) wiring and, although it is possible that the release of ignition energy inside the center wing tank was facilitated by the existence of silver-sulfide deposits on an FQIS component, neither the energy release mechanism nor the location of ignition inside the center wing tank could be determined from the available evidence.
19. Failure modes and effects analyses and fault tree analyses should not be relied upon as the sole means of demonstrating that an airplane's fuel tank system is not likely to experience a catastrophic failure.
20. A fuel tank design and certification philosophy that relies solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated.
21. Operating transport-category airplanes with flammable fuel/air mixtures in fuel tanks presents an avoidable risk of an explosion.
22. The placement of heat-generating equipment under a fuel tank containing Jet A fuel can unnecessarily increase the amount of time that the airplane is operating with a flammable fuel/air mixture unless measures are in place to either (1) prevent the heat from entering the center wing tank or (2) eliminate the flammable vapors inside the center wing tank.
23. The condition of the wiring system in the accident airplane was not atypical for an airplane of its age and it was maintained in accordance with prevailing accepted industry practices.
24. Until recently, insufficient attention has been paid to the condition of aircraft electrical wiring, resulting in potential safety hazards.
25. The issues defined in the Federal Aviation Administration's Aging Transport Non-Structural Systems Plan are important safety issues that need to be addressed through appropriate changes, including rulemaking.
PROBABLE CAUSE
The National Transportation Safety Board determines that the probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT) 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 but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the center wing tank that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.
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.
NEW SAFETY RECOMMENDATIONS
As a result of the investigation of the TWA flight 800 accident, the National Transportation Safety Board makes the following recommendations to the Federal Aviation Administration (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 and (1) identify which systems are critical to safety and (2) 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 believes that, within 90 days, the FAA should brief the Safety Board on the status of its efforts to address all of the issues identified in the Aging Transport Non-Structural Systems Plan.
PREVIOUSLY ISSUED SAFETY RECOMMENDATIONS RESULTING FROM THIS ACCIDENT INVESTIGATION
As a result of the TWA flight 800 accident investigation, the Safety Board issued the following safety recommendations to the Federal Aviation Administration on December 13, 1996:
A-96-174 and -175 -- Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks:
(a) Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes.
(b) Pending implementation of design modifications, require modifications in operational procedures to reduce the potential for explosive fuel/air mixtures in the fuel tanks of transport-category aircraft. In the 747, consideration should be given to refueling the center wing fuel tank (CWT) before flight whenever possible from cooler ground fuel tanks, proper monitoring and management of the CWT fuel temperature, and maintaining an appropriate minimum fuel quantity in the CWT.
A-96-176 -- Require that the 747 Flight Handbooks of TWA and other operators of 747s and other aircraft in which fuel tank temperature cannot be determined by flight crews be immediately revised to reflect the increases in CWT fuel temperatures found by flight tests, including operational procedures to reduce the potential for exceeding CWT temperature limits.
A-96-177 -- Require modification of the CWT of 747 airplanes and the fuel tanks of other airplanes that are located near heat sources to incorporate temperature probes and cockpit fuel tank temperature displays to permit determination of fuel tank temperatures.
As a result of information learned during this investigation, the Safety Board also issued the following recommendation to the FAA on February 18, 1997:
A-97-11 -- Develop and implement procedures, including a checklist of safety-related items, for the handling and placement of explosive training aids by K-9 explosives detection teams to prevent contamination of aircraft and airport facilities and to ensure an effective K-9 explosives detection program.
As a result of this accident investigation, the Safety Board also issued the following recommendations to the FAA on April 7, 1998:
A-98-34 -- Issue, as soon as possible, an airworthiness directive (AD) to require a detailed inspection of fuel quantity indication system (FQIS) wiring in Boeing 747-100, -200, and -300 series airplane fuel tanks for damage, and the replacement or the repair of any wires found to be damaged. Wires on Honeywell Series 1-3 probes and compensators should be removed for examination
A-98-35 -- Issue an AD to require the earliest possible replacement of the Honeywell Corporation Series 1-3 terminal blocks used on Boeing 747 fuel probes with terminal blocks that do not have knurled surfaces or sharp edges that may damage FQIS wiring.
A-98-36 -- Conduct a survey of FQIS probes and wires in Boeing 747s equipped with systems other than Honeywell Series 1-3 probes and compensators and in other model airplanes that are used in Title 14 Code of Federal Regulations Part 121 service to determine whether potential fuel tank ignition sources exist that are similar to those found in the 747. The survey should include removing wires from fuel probes and examining the wires for damage. Repair or replacement procedures for any damaged wires that are found should be developed
A-98-37 -- Require research into copper-sulfide deposits on FQIS parts in fuel tanks to determine the levels of deposits that may be hazardous, how to inspect and clean the deposits, and when to replace the components
A-98-38 -- Require in Boeing 747 airplanes, and in other airplanes with FQIS wire installations that are corouted with wires that may be powered, the physical separation and electrical shielding of FQIS wires to the maximum extent possible.
A-98-39 -- Require, in all applicable transport airplane fuel tanks, surge protection systems to prevent electrical power surges from entering fuel tanks through FQIS wires.
