306 Aircraft Accident Report
3. Conclusions
3.1 Findings
I. 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 TWA flight 800's center wing fuel tank was
flammable at the time of the accident.
7. A fuel/air explosion in the center wing fuel 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 pre impact
breakup sequence. The witnesses' observations of one or more fireballs were of the
airplane's burning wreckage falling toward the ocean.
9. The TWA flight 800 in-flight breakup was initiated by a fuel/air explosion in the
center wing fuel tank.
10. Boeing's design practice that permits parts less than 3 inches long in any direction to
be electrically unbonded may not provide adequate protection against potential
ignition hazards created by static electricity generated by lightning or other
high-energy discharges.
Conclusions 307 Aircraft Accident Report
II. It is very unlikely that the flammable fuel/air vapor in the center wing fuel tank
(CWT) on TWA flight 800 was ignited by a lightning or meteor strike; a missile
fragment; a small explosive charge placed on the CWT; auto ignition or hot surface
ignition, resulting from elevated temperatures produced by sources external to the
CWT; a fire migrating to the CWT from another fucl tank via the vent (stringer)
system; an uncontained engine failure or a turbine burst in the air conditioning packs
beneath the CWT; a malfunctioning CWT jettison/override pump; a malfunctioning
CWT 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 fuel tank.
13. Elcctromagnetic interference from personal electronic devices played no role in
igniting the fuel/air vapor in TWA flight 800's center wing fuel tank.
14. It is unlikely that electromagnetic interference from aircraft system wiring played a
role in igniting the fuel/air vapor in TWA flight 800's center wing fuel tank.
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
(CWT) fuel quantity indication system wiring is the most likely source of ignition
energy for the TWA flight 800 CWT 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 fuel tank (CWT) explosion most likely
entered the CWT through the fuel quantity indication system (FQIS) wiring, and,
although it is possible that the release of ignition energy inside the CWT was
facilitated by the existence of silver-sulfide deposits on an FQIS component, neither
the energy release mechanism nor the location of the ignition inside the CWT 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 relics 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.
Conclusions 308 Aircraft Accident Report
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 (I) prevent the heat
from entering the center wing fuel tank (CWT) or (2) eliminate the flammable vapor
inside the CWT.
23. The condition of the wiring system in the accident airplane was not atypical for an
airplane of its age, and the airplane 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 must be fully addressed
through appropriate changes, including rulemaking.
3.2 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
CWT that allowed excessive voltage to enter it through electrical wiring associated with
the fuel quantity indication system.
Contributing factors to the accident were the design and certification concept that
fuel tank explosions could be prevented solely by precluding all ignition sources and the
design and certification of the Boeing 747 with heat sources located beneath the CWT
with no means to reduce the heat transferred into the CWT or to render the fuel vapor in
the tank nonflammable.
