California Institute of Technology

Explosion Dynamics Laboratory

Explosion Research at Caltech and TWA Flight 800

Background | Facts | Documents | 1/4-Scale Experiments | Misconceptions |
Why was the Explosion Dynamics Laboratory of the California Institute of Technology involved with aviation kerosene explosion research for the TWA Flight 800 Investigation? We became involved in the TWA Flight 800 crash investigation in the fall of 1996 at the request of the National Transportation Safety Board. The Explosion Dynamics Laboratory has carried out a number of studies on explosions and safety hazards with explosive gases over the last decade and has a unique set of laboratory facilities available for studying explosions. From the fall of 1996 to the fall of 2000, we worked with the NTSB investigators and other groups in performing experiments and analyses in order to improve our understanding of flammability and combustion under the conditions of TWA flight 800.

Where is the CIT research documented? An introduction (containing material through mid 1997) can be obtained from an article that appeared in Engineering and Applied Science. The detailed technical results of the Caltech studies can be found on this website in the documents section, or in the and also discussed in the NTSB final report.

Why study aviation kerosene explosions?Aviation crash investigators and safety researchers have long known that fuels such as JP-8 and Jet A can pose an explosion hazard under common flight conditions. The nominal explosion limits were determined in the 1960s and the explosive range is approximately known as a function of pressure (conventionally expressed in terms of altitude) and temperature within the tank. Click on the graph to see numerical values of lean and rich flammability limits.

Jet A flammability diagram Aviation fuel combustion and explosion hazards are extensively discussed in the standard handbook for military crash investigators:

Kuchta, J. M. and R. G. Clodfelter (1985). Aircraft mishap fire pattern investigations. Final Report APWAL-TR-85-2057, Aero Propulsion Laboratory.

However, despite over 30 years of using Jet A in commercial aviation and JP-8 in military aviation, the amount of publicly available data on flammability and explosion is rather meager. At the start of the TWA 800 investigation, information available came from three separate studies carried out in 1967, 1970 and 1971. These studies are reported in:

Nestor, L. (1967). Investigation of turbine fuel flammability within aircraft fuel tanks. Final Report DS-67-7, Naval Air Propulsion Test Center, Naval Base, Philadelphia.

Ott, E. (1970). Effects of fuel slosh and vibration on the flammability hazards of hydrocarbon turbine fuels within aircraft fuel tanks. Technical Report AFAPL-TR-70-65, Fire Protection Branch of the Fuels and Lubrication Division, Wright-Patterson Air Force Base, Ohio.

Kosvic, T., L. Zung, and M. Gerstein (1971). Analysis of aircraft fuel tank fire and explosion hazards. Technical Report AFAPL-TR-71-7, Air Force Aero Propulsion Laboratory, Wright-Patterson Air Force Base, Ohio.

None of these previous studies examined the issues at the heart of the TWA 800 investigation; specifically, the effect of a thin layer of Jet A, the ignition energy for Jet A vapor at the appropriate temperatures and pressures, and the propagation of the flame inside the CWT structure. For these reasons, we carried out new studies on the physical chemistry of Jet A, the conditions within the center wing tank at the time of the explosion, the effects of airplane operation on the flammability of the fuel and finally, the initiation and development of an explosion in the center wing tank itself. These studies were considered by the NTSB in reaching their findings of probable cause in the TWA Flight 800 Accident.

What was the outcome of the TWA 800 investigation? The investigation has been completed and the final report was discussed at a two-day meeting on August 22 and 23, 2000. The presentation materials, transcripts of the oral presentations, downloadable portions of the public docket, and a CD of other docket materials (including our reports) available at that time on the NTSB website but has now been archived. The final report provides the following statement of 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."

Brief Timeline on Flight 800 and Fuel Tank Inerting

July 17, 1996 At about 2031 EDT, TWA flight 800, a Boeing 747-13, broke up in flight with a loss of life of all 230 passengers and crew. The crash debris fell into the Atlantic Ocean south of East Moriches, Long Island, NY. The accident investigation was one of the longest and most expensive in the NTSB's history. A substantial fraction of the aircraft was recovered and reconstructed, and numerous studies were carried in the effort to determine the probable cause. The Explosion Dynamics Laboratory at Caltech was asked by the NTSB to participate in the investigation and lead a group of researchers to examine the issues of fuel flammability, ignition, and flame propagation. EDL staff were involved from the fall of 1996 until the final hearing in August 2000.

December 13, 1996 Safety Recommendation Letter A-96-174 published.

TO THE FEDERAL AVIATION ADMINISTRATION: Require the development of and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel-air mixtures in the fuel tank: (a) significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems & the addition of insulation between heat-generating equipment & fuel tanks. Appropriate modifications should apply to newly certificated airplanes &, where feasible to existing airplanes.

May 20, 1997 Added fuel tank flammability reduction to the Ten-Mosted Wanted List of Transportation Safety Improvements:

"Reduce the potential for explosive fuel-air mixtures in fuel tanks of transport category aircraft. The NTSB has urged the FAA to make operational changes. They include refueling the center wing tank from cooler ground fuel tanks before flight, monitoring temperatures and maintaining a proper minimum amount of fuel in the tanks."

December 8-9, 1997 NTSB Investigative hearing.

August 22 and 23, 2000 Final hearing by NTSB and announcement of probable cause.

2002 Fuel-tank inerting added to Ten-Most Wanted List (removed in 2008)

Feb 17, 2004 The FAA announced that it is considering issuing a Notice of Proposed Rulemaking (NPR) requiring a fuel tank inerting system to be installed on existing aircraft with center wing tank flammability hazards.

Feb 15, 2005 The FAA issued the special conditions for the certification of the flammability reduction means (FRM) or fuel tank inerting system proposed by Boeing for the 747 family of aircraft. This system will use hollow fiber membranes to generate "nitrogen enhanced air" to fill the vapor space of the center fuel tank in order to reduce the O2 concentration below 12% for a sufficient duration of the flight that the center fuel is not flammable for greater than 3% of the fleet operational time.

Nov 15, 2005 The FAA has finally put on public display the Notice of Proposed Rulemaking on fuel tank inerting.

November 23, 2005 The (NPRM) was published in the Federal register.

March 21, 2006 The FAA has extended the deadline for comment on the NPRM to May 8, 2006.

July 12, 2006 From the NTSB website: "The investigation into a wing fuel tank explosion on a Transmile Airlines B-727 airplane in Bangalore, India, on May 4, 2006, is ongoing. The evidence indicates that an explosion in the left wing fuel tank destroyed the structural integrity of the wing."

July 21, 2008 The FAA has issued the the final rule: "Reduction of Fuel Tank Flammability in Transport Aircraft." The rule requires retrofitting of certain aircraft with heated center wing tanks and use of flammability reduction means (inerting systems) or ignition mitigation means (foam) on future aircraft to meet a target flammability exposure of 3% fleet average flammability and specific risk of 3% during ground operation and climb out on warm day, above 80 F. The present value of the total compliance cost is estimated by the FAA to be 1 billion USD. Boeing has developed and placed into production inerting systems based on hollow fiber membrane technology for the 747 and 737 type

October 16, 2008 Safety Recommendation A-96-174 closed as an acceptable action.

The FAA has summarized the issues and regulatory actions derived from TWA 800 and on the Lessons Learned from Transport Airplane Accidents web page.

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