Editing TWA Flight 800 (section)

====Fuel-air explosion in the center wing fuel tank====
[[File:Center Wing Fuel Tank.png|upright=0.9|thumbnail|right|The wing center section of a Boeing 747-100, including the CWT<ref name="Final Report"/>{{rp|at=fig. 4a, p. 13}}]]
[[File:Ntsb cwt scale test.PNG|thumbnail|right|Scale-model test of a CWT fuel/air vapor explosion]]
To evaluate the sequence of structural breakup of the airplane, the NTSB formed the Sequencing Group,<ref name="Final Report"/>{{rp|100}} which examined individual pieces of the recovered structure, two-dimensional reconstructions or layouts of sections of the airplane and various-sized three-dimensional reconstructions of portions of the aircraft.<ref name="Final Report"/>{{rp|100}} In addition, the locations of pieces of wreckage at the time of recovery and differences in fire effects on pieces that are normally adjacent to each other were evaluated.<ref name="Final Report"/>{{rp|100}} The Sequencing Group concluded that the first event in the breakup sequence was a fracture in the wing center section of the aircraft caused by an "overpressure event" in the center-wing fuel tank (CWT).<ref name="NTSB Seq p.29">{{cite journal |title=Metallurgy/Structures Group Chairman Factual Report Sequencing Study |url=https://data.ntsb.gov/Docket/Document/docBLOB?ID=40058820&FileExtension=.PDF&FileName=Sequencing%20Group%2018%20-%20Exhibit%20No.%2018A%20-%20Metallurgy%2FStructures%20Sequencing%20Group%20Chairman%27s%20Report-Master.PDF |journal=Docket No. 5A-516, Exhibit No. 18A |publisher=National Transportation Safety Board |access-date=January 31, 2010 |archive-date=July 23, 2021 |archive-url=https://web.archive.org/web/20210723174918/https://data.ntsb.gov/Docket/Document/docBLOB?ID=40058820&FileExtension=.PDF&FileName=Sequencing%20Group%2018%20-%20Exhibit%20No.%2018A%20-%20Metallurgy%2FStructures%20Sequencing%20Group%20Chairman%27s%20Report-Master.PDF |url-status=live}}</ref>{{rp|29}} An overpressure event was defined as a rapid increase in pressure resulting in failure of the structure of the CWT.<ref name="Final Report"/>{{rp|85}}

Because no evidence was found that an explosive device detonated in this (or any other) area of the airplane, the overpressure event could only have been caused by a fuel-air explosion in the CWT.<ref name="Final Report"/>{{rp|261}} There were {{convert|50|gal|L|abbr=on}} of fuel in the CWT of TWA 800;<ref>{{cite journal |title=Fire in the sky |url=http://nsc.nasa.gov/SFCS/SystemFailureCaseStudyFile/Download/172 |format=PDF |journal=System Failure Case Studies |publisher=National Aeronautics and Space Administration |access-date=June 30, 2013 |archive-date=July 1, 2013 |archive-url=https://web.archive.org/web/20130701061505/http://nsc.nasa.gov/SFCS/SystemFailureCaseStudyFile/Download/172 |url-status=live}}</ref> tests recreating the flight conditions showed the combination of liquid fuel and fuel-air vapor to be flammable.<ref name="Final Report"/>{{rp|261}} A major reason for the flammability of the fuel-air vapor in the CWT of the 747 was the large amount of heat generated and transferred to the CWT by air-conditioning packs located directly below the tank;<ref name="Final Report"/>{{rp|298}} with the CWT temperature raised to a sufficient level, a single ignition source could cause an explosion.<ref name="Final Report"/>{{rp|298}}

Computer modeling<ref name="Final Report"/>{{rp|122–123}} and scale-model testing<ref name="Final Report"/>{{rp|123}} were used to predict and demonstrate how an explosion would progress in a 747 CWT. During this time, quenching was identified as an issue, a phenomenon in which the explosion would extinguish itself as it passed through the complex structure of the CWT.<ref name="Final Report"/>{{rp|123}} Because the research data regarding quenching were limited, a complete understanding of quenching behavior was not possible, and the issue of quenching remained unresolved.<ref name="Final Report"/>{{rp|137}}

To better determine whether a fuel-air vapor explosion in the CWT would generate sufficient pressure to break apart the fuel tank and lead to the destruction of the airplane, tests were conducted in July and August 1997 using a retired [[Air France]] 747 at [[Bruntingthorpe Aerodrome|Bruntingthorpe Airfield]], England. These tests simulated a fuel-air explosion in the CWT by igniting a propane-air mixture, which failed the tank structure from overpressure.<ref name="Final Report"/>{{rp|261}} While the NTSB acknowledged that the test conditions at Bruntingthorpe were not fully comparable to the conditions that existed on TWA 800,<ref name="Final Report"/>{{rp|261}} previous fuel explosions in the CWTs of commercial airliners such as those of [[Avianca Flight 203]] and [[Philippine Airlines Flight 143]] confirmed that a CWT explosion could break apart the fuel tank and lead to the destruction of an airplane.<ref name="Final Report"/>{{rp|261}}

Ultimately, based on "the accident airplane's breakup sequence; wreckage damage characteristics; scientific tests and research on fuels, fuel tank explosions, and the conditions in the CWT at the time of the accident; and analysis of witness information,"<ref name="Final Report"/>{{rp|271}} the NTSB concluded that "the TWA flight 800 in-flight breakup was initiated by a fuel/air explosion in the CWT."<ref name="Final Report"/>{{rp|63}}