Trans World Airlines Flight 800 (TWA 800) was a Boeing 747-100 that exploded and crashed into the Atlantic Ocean near East Moriches, New York, on July 17, 1996, at about 8:31p.m. EDT, 12 minutes after takeoff from John F. Kennedy International Airport on a scheduled international passenger flight to Rome, with a stopover in Paris.[1]:1 All 230 people on board died in the third-deadliest aviation accident in U.S. history.
Accident investigators from the National Transportation Safety Board (NTSB) traveled to the scene, arriving the following morning[1]:313 amid speculation that a terrorist attack was the cause of the crash.[2][3][4] Consequently, the Federal Bureau of Investigation and New York Police Department (FBI-NYPD) Joint Terrorism Task Force (JTTF) initiated a parallel criminal investigation.[5] Sixteen months later, the JTTF announced that no evidence had been found of a criminal act and closed its active investigation.[6]
The four-year NTSB investigation concluded with the approval of the Aircraft Accident Report on August 23, 2000, ending the most extensive, complex, and costly air disaster investigation in U.S. history.[7][8] The report's conclusion was that the probable cause of the accident was explosion of flammable fuel vapors in the center fuel tank. Although it could not be determined with certainty, the likely ignition source was a short circuit.[1]:xvi Problems with the aircraft's wiring were found, including evidence of arcing in the Fuel Quantity Indication System (FQIS) wiring which enters the tank. The FQIS on Flight 800 is known to have been malfunctioning; the captain remarked on what he called "crazy" readings from the system approximately two minutes and thirty seconds before the aircraft exploded. As a result of the investigation, new requirements were developed for aircraft to prevent future fuel tank explosions.[9]
Accident flight

The accident airplane, registration N93119, (Boeing 747-131) was manufactured by Boeing in July 1971; it had been ordered by Eastern Air Lines, but after Eastern canceled its 747 orders, the plane was purchased new by TWA. The aircraft had completed 16,869 flights with 93,303 hours of operation.[1]:6 On the day of the accident, the airplane departed from Ellinikon International Airport in Athens, Greece as TWA Flight 881 and arrived at John F. Kennedy International Airport (JFK) about 4:38pm. The aircraft was refueled, and there was a crew change; the new flight crew consisted of 58-year-old Captain Ralph G. Kevorkian, 57-year-old Captain/Check Airman Steven E. Snyder, and 63-year-old Flight Engineer/Check Airman Richard G. Campbell (all of whom were highly experienced veterans flying with TWA), as well as 25-year-old flight engineer trainee Oliver Krick, who was starting the sixth leg of his initial operating experience training.[1](pp4–5)[10][11][note 1]
The ground-maintenance crew locked out the thrust reverser for engine #3 (treated as a minimum equipment list item) because of technical problems with the thrust reverser sensors during the landing of TWA 881 at JFK, prior to Flight 800's departure. Additionally, severed cables for the engine #3 thrust reverser were replaced.[12] During refueling of the aircraft, the volumetric shutoff (VSO) control was believed to have been triggered before the tanks were full. To continue the pressure fueling, a TWA mechanic overrode the automatic VSO by pulling the volumetric fuse and an overflow circuit breaker. Maintenance records indicate that the airplane had numerous VSO-related maintenance writeups in the weeks before the accident.[1]:31
TWA 800 was scheduled to depart JFK for Paris around 7:00p.m., but the flight was delayed until 8:02p.m. by a disabled piece of ground equipment and a passenger/baggage mismatch.[1]:1 After the owner of the baggage in question was confirmed to be on board, the flight crew prepared for departure and the aircraft pushed back from Gate 27 at the TWA Flight Center. Taxi and takeoff proceeded uneventfully.
 TWA 800 then received a series of heading changes and generally increasing altitude assignments as it climbed to its intended cruising altitude.[1]:2 Weather in the area was light winds with scattered clouds,[1]:256 and there were dusk lighting conditions.[1]:4 The last radio transmission from the airplane occurred at 8:30p.m. when the flight crew received and then acknowledged instructions from Boston Center to climb to 15,000 feet (4,600m).[13]:4 The last recorded radar transponder return from the airplane was recorded by the Federal Aviation Administration (FAA) radar site at Trevose, Pennsylvania at 8:31:12p.m.[1]:3
Thirty-eight seconds later, the captain of an Eastwind Airlines Boeing 737 reported to Boston ARTCC that he "just saw an explosion out here", adding, "we just saw an explosion up ahead of us here... about 16,000 feet (4,900m) or something like that, it just went down into the water." Subsequently, many air traffic control facilities in the New York/Long Island area received reports of an explosion from other pilots operating in the area. Many witnesses in the vicinity of the crash stated that they saw or heard explosions, accompanied by a large fireball or fireballs over the ocean, and observed debris, some of which was burning while falling into the water.[1]:3
Various civilian, military, and police vessels reached the crash site and searched for survivors within minutes of the initial water impact, but found none,[1]:86 making TWA 800 the second-deadliest aircraft accident in United States history at that time.[14]
Initial investigation
The NTSB was notified about 8:50pm the day of the accident; a full go team was assembled in Washington, D.C. and arrived on scene early the next morning.[1]:313 Meanwhile, initial witness descriptions led many to believe the cause of the crash was a bomb or surface-to-air missile attack.[15][16][17] The NTSB does not investigate criminal activity. The Attorney General is empowered to declare an investigation to be linked to a criminal act, and require the NTSB to relinquish control of the investigation to the FBI.[18] In the case of TWA 800, the FBI initiated a parallel criminal investigation alongside the NTSB's accident investigation.[19]
Search and recovery operations
Search and recovery operations were conducted by federal, state, and local agencies, as well as government contractors.[1](pp363–365) An HH-60 helicopter of the New York Air National Guard saw the explosion from approximately eight miles away, and arrived on scene so quickly that debris was still raining down, and the aircraft had to pull away. They reported their sighting to the tower at Suffolk County Airport (now Gabreski ANG Base). Remote-operated vehicles (ROVs), side-scan sonar, and laser line-scanning equipment were used to search for and investigate underwater debris fields. Victims and wreckage were recovered by scuba divers and ROVs; later scallop trawlers were used to recover wreckage embedded in the ocean floor.[1]:63 In one of the largest diver-assisted salvage operations ever conducted, often working in very difficult and dangerous conditions, over 95% of the airplane wreckage was eventually recovered.[19][20]:1 The search and recovery effort identified three main areas of wreckage underwater.[1]:65 The yellow zone, red zone, and green zone contained wreckage from front, center, and rear sections of the airplane, respectively.[1](pp65–74) The green zone with the aft portion of the aircraft was located the furthest along the flight path.[1](pp71–74)

Wreckage recovery and debris fields
Wreckage recovered with tangled and damaged wires attached.[1](fig.24, p.80)
Map showing the locations of the red, yellow, and green zones.[1](fig.22a, p.66)
Wreckage found in each zone corresponded to specific areas of the aircraft.[1](fig.22b, p.67)
The pathways the wreckage took as it fell to the ocean.[1](fig.22c, p.68)

Pieces of wreckage were transported by boat to shore and then by truck to leased hangar space at the former Grumman Aircraft facility in Calverton, New York, for storage, examination, and reconstruction.[1]:63 This facility became the command center and headquarters for the investigation.[1](pp363–365) NTSB and FBI personnel were present to observe all transfers to preserve the evidentiary value of the wreckage.[1]:367 The cockpit voice recorder and flight data recorder were recovered by U.S. Navy divers 1 week after the accident; they were immediately shipped to the NTSB laboratory in Washington, D.C., for readout.[1]:58 The victims' remains were transported to the Suffolk County Medical Examiner's Office in Hauppauge, New York.[21]:2
Tensions in the investigation
Relatives of TWA 800 passengers and crew, as well as the media, gathered at the Ramada Plaza JFK Hotel.[22] Many waited until the remains of their family members had been recovered, identified, and released.[23]:1[24](pp3–4) This hotel became known as the "Heartbreak Hotel" for its role in handling families of victims of several airliner crashes.[25][26] The New York Daily News wrote that for the families the hotel became "a makeshift grief counseling center".[27]
Grief turned to anger at TWA's delay in confirming the passenger list,[22] conflicting information from agencies and officials,[28] and mistrust of the recovery operation's priorities.[29]:2 Although NTSB vice chairman Robert Francis stated that all bodies were being retrieved as soon as they were spotted, and that wreckage was being recovered only if divers believed that victims were hidden underneath,[29]:2 many families were suspicious that investigators were not being truthful, or withholding information.[29]:2[30]:7[31](pp1–2)
Much anger and political pressure was also directed at Suffolk County Medical Examiner Dr. Charles V. Wetli as recovered bodies backlogged at the morgue.[20]:3[30]:5[31](pp1–2) Under constant and considerable pressure to identify victims with minimal delay,[21]:3 pathologists worked non-stop.[30]:5 Since the primary objective was to identify all remains rather than performing a detailed forensic autopsy, the thoroughness of the examinations was highly variable.[21]:3 Ultimately, remains of all 230 victims were recovered and identified, the last over 10 months after the crash.[21]:2
With lines of authority unclear, differences in agendas and culture between the FBI and NTSB resulted in discord.[30]:1 The FBI, from the start assuming that a criminal act had occurred,[30]:3 saw the NTSB as indecisive. Expressing frustration at the NTSB's unwillingness to speculate on a cause, one FBI agent described the NTSB as "No opinions. No nothing".[30]:4 Meanwhile, the NTSB was required to refute or play down speculation about conclusions and evidence, frequently supplied to reporters by law enforcement officials and politicians.[20]:3[30]:4 The International Association of Machinists and Aerospace Workers (IAMAW), an invited party to the NTSB investigation, criticized the undocumented removal by FBI agents of wreckage from the hangar where it was stored.[32]
Witness interviews

Although there were considerable discrepancies between different accounts, most witnesses to the accident had seen a "streak of light" that was unanimously described as ascending,[1]:230 moving to a point where a large fireball appeared, with several witnesses reporting that the fireball split in two as it descended toward the water.[1]:3 There was intense public interest in these witness reports and much speculation that the reported streak of light was a missile that had struck TWA 800, causing the airplane to explode.[1]:262 These witness accounts were a major reason for the initiation and duration of the FBI's criminal investigation.[34]:5
Approximately 80 FBI agents conducted interviews with potential witnesses daily.[34]:7 No verbatim records of the witness interviews were produced; instead, the agents who conducted the interviews wrote summaries that they then submitted.[34]:5 Witnesses were not asked to review or correct the summaries.[34]:5 Included in some of the witness summaries were drawings or diagrams of what the witness observed. Witnesses were not allowed to testify at the court hearings.[33]:165[35]:184
Within days of the crash the NTSB announced its intent to form its own witness group and to interview witnesses to the crash.[34]:6 After the FBI raised concerns about non-governmental parties in the NTSB's investigation having access to this information and possible prosecutorial difficulties resulting from multiple interviews of the same witness,[34]:6 the NTSB deferred and did not interview witnesses to the crash. A Safety Board investigator later reviewed FBI interview notes and briefed other Board investigators on their contents. In November 1996, the FBI agreed to allow the NTSB access to summaries of witness accounts in which personally identifying information had been redacted and to conduct a limited number of witness interviews. In April 1998, the FBI provided the NTSB with the identities of the witnesses but due to the time elapsed a decision was made to rely on the original FBI documents rather than reinterview witnesses.[1]:229
Further investigation and analysis
Examination of the cockpit voice recorder (CVR) and flight data recorder data showed a normal takeoff and climb,[13]:4 with the aircraft in normal flight[36]:2 before both abruptly stopped at 8:31:12pm.[1]:3 At 8:29:15pm the captain was heard to say, "Look at that crazy fuel flow indicator there on number four... see that?"[1]:2 A noise recorded on the last few tenths of a second of the CVR was similar to the last noises recorded from other airplanes that had experienced in-flight breakups.[1]:256 This, together with the distribution of wreckage and witness reports, all indicated a sudden catastrophic in-flight breakup of TWA 800.[1]:256
Possible causes of the in-flight breakup
Investigators considered several possible causes for the structural breakup: structural failure and decompression, detonation of a high-energy explosive device, such as a missile warhead exploding either upon impact with the airplane, or just before impact, a bomb exploding inside the airplane, or a fuel-air explosion in the center wing fuel tank.[1](pp256–257)
Structural failure and decompression
Close examination of the wreckage revealed no evidence of structural faults such as fatigue, corrosion or mechanical damage that could have caused the in-flight breakup.[1]:257 It was also suggested that the breakup could have been initiated by an in-flight separation of the forward cargo door like the disasters on board Turkish Airlines Flight 981 or United Airlines Flight 811, but all evidence indicated that the door was closed and locked at impact.[1]:257 The NTSB concluded that "the in-flight breakup of TWA flight 800 was not initiated by a preexisting condition resulting in a structural failure and decompression."[1]:257
Live missile or bomb detonation
A review of recorded data from long-range and airport surveillance radars revealed multiple contacts of airplanes or objects in TWA800's vicinity at the time of the accident.[1](pp87–89) None of these contacts intersected TWA 800's position at any time.[1]:89 Attention was drawn to data from the Islip, New York, ARTCC facility that showed three tracks in the vicinity of TWA 800 that did not appear in any of the other radar data.[1]:93 None of these sequences intersected TWA 800's position at any time either.[1]:93 All the reviewed radar data showed no radar returns consistent with a missile or other projectile traveling toward TWA 800.[1]:89
The NTSB addressed allegations that the Islip radar data showed groups of military surface targets converging in a suspicious manner in an area around the accident, and that a 30-knot radar track, never identified and 3 nautical miles (5.6km; 3.5mi) from the crash site, was involved in foul play, as evidenced by its failure to divert from its course and assist with the search and rescue operations.[1]:93 Military records examined by the NTSB showed no military surface vessels within 15 nautical miles (28km; 17mi) of TWA800 at the time of the accident.[1]:93 In addition, the records indicated that the closest area scheduled for military use, warning area W-387A/B, was 160 nautical miles (296km; 184mi) south.[1]:93
The NTSB reviewed the 30-knot target track to try to determine why it did not divert from its course and proceed to the area where the TWA800 wreckage had fallen. TWA800 was behind the target, and with the likely forward-looking perspective of the target's occupant(s), the occupants would not have been in a position to observe the aircraft's breakup or subsequent explosions or fireball(s).[1]:94 Additionally, it was unlikely that the occupants of the target track would have been able to hear the explosions over the sound of its engines and the noise of the hull traveling through water, even more so if the occupants were in an enclosed bridge or cabin.[1]:94 Further, review of the Islip radar data for other similar summer days and nights in 1999 indicated that the 30-knot track was consistent with normal commercial fishing, recreational, and cargo vessel traffic.[1]:94

Recorded radar data
Radar data showing vehicle and/or object tracks within 10 nautical miles (19km; 12mi) of TWA flight 800 just before the accident.[1](fig. 25, p. 90)
Three sequences of primary returns near TWA 800 that were only recorded by the Islip radar.[1](fig. 26, p. 91)
Primary radar returns that appeared near the TWA 800 after 8:31:12pm. The 30-knot track is at the bottom center of the image.[1](fig. 27, p. 92)

Trace amounts of explosive residue were detected on three samples of material from three separate locations of the recovered airplane wreckage (described by the FBI as a piece of canvas-like material and two pieces of a floor panel).[1]:118 These samples were submitted to the FBI's laboratory in Washington, D.C., which determined that one sample contained traces of cyclotrimethylenetrinitramine (RDX), another nitroglycerin, and the third a combination of RDX and pentaerythritol tetranitrate (PETN);[1]:118 these findings received much media attention at the time.[37][38] In addition, the backs of several damaged passenger seats were observed to have an unknown red/brown-shaded substance on them.[1]:118 According to the seat manufacturer, the locations and appearance of this substance were consistent with adhesive used in the construction of the seats, and additional laboratory testing by NASA identified the substance as being consistent with adhesives.[1]:118
Further examination of the airplane structure, seats, and other interior components found no damage typically associated with a high-energy explosion of a bomb or missile warhead ("severe pitting, cratering, petalling, or hot gas washing").[1]:258 This included the pieces on which trace amounts of explosives were found.[1]:258 Of the 5 percent of the fuselage that was not recovered, none of the missing areas were large enough to have covered all the damage that would have been caused by the detonation of a bomb or missile.[1]:258 None of the victims' remains showed any evidence of injuries that could have been caused by high-energy explosives.[1]:258
The NTSB considered the possibility that the explosive residue was due to contamination from the aircraft's use in 1991 transporting troops during the Gulf War or its use in a dog-training explosive detection exercise about one month before the accident.[1](pp258–259) Testing conducted by the FAA's Technical Center indicated that residues of the type of explosives found on the wreckage would dissipate completely after two days of immersion in sea water (almost all recovered wreckage was immersed longer than two days).[1]:259 The NTSB concluded that it was "quite possible" that the explosive residue detected was transferred from military ships or ground vehicles, or the clothing and boots of military personnel, onto the wreckage during or after the recovery operation and was not present when the aircraft crashed into the water.[1]:259
Although it was unable to determine the exact source of the trace amounts of explosive residue found on the wreckage, the lack of any other corroborating evidence associated with a high-energy explosion led the NTSB to conclude that "the in-flight breakup of TWA flight 800 was not initiated by a bomb or missile strike."[1]:259
Fuel-air explosion in the center wing fuel tank

In order to evaluate the sequence of structural breakup of the airplane, the NTSB formed the Sequencing Group,[1]: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 airplane.[1]: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.[1]: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).[39]:29 An overpressure event was defined as a rapid increase in pressure resulting in failure of the structure of the CWT.[1]:85
Because there was no evidence that an explosive device detonated in this (or any other) area of the airplane, this overpressure event could only have been caused by a fuel/air explosion in the CWT.[1]:261 There were 50USgal (190L) of fuel in the CWT of TWA 800;[40] tests recreating the conditions of the flight showed the combination of liquid fuel and fuel/air vapor to be flammable.[1]: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;[1]:298 with the CWT temperature raised to a sufficient level, a single ignition source could cause an explosion.[1]:298
Computer modeling[1](pp122–123) and scale-model testing[1]: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, where the explosion would extinguish itself as it passed through the complex structure of the CWT.[1]:123 Because the research data regarding quenching was limited, a complete understanding of quenching behavior was not possible, and the issue of quenching remained unresolved.[1]:137
In order 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 Airfield, England. These tests simulated a fuel/air explosion in the CWT by igniting a propane/air mixture; this resulted in the failure of the tank structure due to overpressure.[1]:261 While the NTSB acknowledged that the test conditions at Bruntingthorpe were not fully comparable to the conditions that existed on TWA 800 at the time of the accident,[1]:261 previous fuel explosions in the CWTs of commercial airliners such as 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.[1]: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,"[1]:271 the NTSB concluded that "the TWA flight 800 in-flight breakup was initiated by a fuel/air explosion in the CWT."[1]:63
In-flight breakup sequence and crippled flight

Recovery locations of the wreckage from the ocean (the red, yellow, and green zones) clearly indicated that: (1) the red area pieces (from the forward portion of the wing center section and a ring of fuselage directly in front) were the earliest pieces to separate from the airplane; (2) the forward fuselage section departed simultaneously with or shortly after the red area pieces, landing relatively intact in the yellow zone; (3) the green area pieces (wings and the aft portion of the fuselage) remained intact for a period after the separation of the forward fuselage, and impacted the water in the green zone.[41]

Fire damage and soot deposits on the recovered wreckage indicated that some areas of fire existed on the airplane as it continued in crippled flight after the loss of the forward fuselage.[1]:109 After about 34 seconds (based on information from witness documents), the outer portions of both the right and left wings failed.[1](pp109, 263) Shortly after, the left wing separated from what remained of the main fuselage, which resulted in further development of the fuel-fed fireballs as the pieces of wreckage fell to the ocean.[1]:263
Only the FAA radar facility in North Truro, Massachusetts, using specialized processing software from the United States Air Force 84th Radar Evaluation Squadron, was capable of estimating the altitude of TWA 800 after it lost power due to the CWT explosion.[1]:87 Because of accuracy limitations, this radar data could not be used to determine whether the aircraft climbed after the nose separated.[1]:87 Instead, the NTSB conducted a series of computer simulations to examine the flightpath of the main portion of the fuselage.[1](pp95–96) Hundreds of simulations were run using various combinations of possible times the nose of TWA 800 separated (the exact time was unknown), different models of the behavior of the crippled aircraft (the aerodynamic properties of the aircraft without its nose could only be estimated), and longitudinal radar data (the recorded radar tracks of the east/west position of TWA 800 from various sites differed).[1](pp96–97) These simulations indicated that after the loss of the forward fuselage the remainder of the aircraft continued in crippled flight, then pitched up while rolling to the left (north),[1]:263 climbing to a maximum altitude between 15,537 and 16,678 feet (4,736 and 5,083m)[1]:97 from its last recorded altitude, 13,760 feet (4,190m).[1]:256
Analysis of reported witness observations

At the start of FBI's investigation, because of the possibility that international terrorists might have been involved, assistance was requested from the Central Intelligence Agency (CIA).[42]:2 CIA analysts, relying on sound-propagation analysis, concluded that the witnesses could not be describing a missile approaching an intact aircraft, but were seeing a trail of burning fuel coming from the aircraft after the initial explosion.[42](pp5–6) This conclusion was reached after calculating how long it took for the sound of the initial explosion to reach the witnesses, and using that to correlate the witness observations with the accident sequence.[42]:5 In all cases the witnesses could not be describing a missile approaching an intact aircraft, as the plane had already exploded before their observations began.[42]:6
As the investigation progressed, the NTSB decided to form a witness group to more fully address the accounts of witnesses.[34]:7 From November 1996 through April 1997 this group reviewed summaries of witness accounts on loan from the FBI (with personal information redacted), and conducted interviews with crewmembers from a New York Air National Guard HH-60 helicopter and C-130 airplane, as well as a U.S. Navy P-3 airplane that was flying in the vicinity of TWA 800 at the time of the accident.[34](pp7–8)
In February 1998, the FBI, having closed its active investigation, agreed to fully release the witness summaries to the NTSB.[34]:10 With access to these documents no longer controlled by the FBI, the NTSB formed a second witness group to review the documents.[34]:10 Because of the amount of time that had elapsed (about 21 months) before the NTSB received information about the identity of the witnesses, the witness group chose not to re-interview the witnesses, but instead to rely on the original summaries of witness statements written by FBI agents as the best available evidence of the observations initially reported by the witnesses.[1]:230 Despite the two and a half years that had elapsed since the accident, the witness group did interview the captain of Eastwind Airlines Flight 507, who was the first to report the explosion of TWA 800, because of his vantage point and experience as an airline pilot.[34]:12

The NTSB's review of the released witness documents determined that they contained 736 witness accounts, of which 258 were characterized as "streak of light" witnesses ("an object moving in the sky... variously described [as] a point of light, fireworks, a flare, a shooting star, or something similar.")[1]:230 The NTSB Witness Group concluded that the streak of light reported by witnesses might have been the actual airplane during some stage of its flight before the fireball developed, noting that most of the 258 streak of light accounts were generally consistent with the calculated flightpath of the accident airplane after the CWT explosion.[1]:262
Thirty-eight witnesses described a streak of light that ascended vertically, or nearly so, and these accounts "seem[ed] to be inconsistent with the accident airplane's flightpath."[1]:265 In addition, 18 witnesses reported seeing a streak of light that originated at the surface, or the horizon, which did not "appear to be consistent with the airplane's calculated flightpath and other known aspects of the accident sequence."[1]:265 Regarding these differing accounts, the NTSB noted that based on their experience in previous investigations "witness reports are often inconsistent with the known facts or with other witnesses' reports of the same events."[1]:237 The interviews conducted by the FBI focused on the possibility of a missile attack; suggested interview questions given to FBI agents such as "Where was the sun in relation to the aircraft and the missile launch point?" and "How long did the missile fly?" could have biased interviewees' responses in some cases.[1]:266 The NTSB concluded that given the large number of witnesses in this case, they "did not expect all of the documented witness observations to be consistent with one another"[1]:269 and "did not view these apparently anomalous witness reports as persuasive evidence that some witnesses might have observed a missile."[1]:270
After missile visibility tests were conducted in April 2000, at Eglin Air Force Base, Fort Walton Beach, Florida,[1]:254 the NTSB determined that if witnesses had observed a missile attack they would have seen:
a light from the burning missile motor ascending very rapidly and steeply for about 8 seconds;
the light disappearing for up to 7 seconds;
upon the missile striking the aircraft and igniting the CWT, another light, moving considerably more slowly and more laterally than the first, for about 30 seconds;
this light descending while simultaneously developing into a fireball falling toward the ocean.[1]:270 None of the witness documents described such a scenario.[1]:270

Because of their unique vantage points or the level of precision and detail provided in their accounts, five witness accounts generated special interest:[1](pp242–243) the pilot of Eastwind Airlines Flight 507, the crew members in the HH-60 helicopter, a streak-of-light witness aboard US Airways Flight 217, a land witness on the Beach Lane Bridge in Westhampton Beach, New York, and a witness on a boat near Great Gun Beach.[1](pp243–247) Advocates of a missile-attack scenario asserted that some of these witnesses observed a missile;[1]:264 analysis demonstrated that the observations were not consistent with a missile attack on TWA800, but instead were consistent with these witnesses having observed part of the in-flight fire and breakup sequence after the CWT explosion.[1]:264
The NTSB concluded that "the witness observations of a streak of light were not related to a missile and that 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 post-explosion, preimpact breakup sequence".[1]:270 The NTSB further concluded that "the witnesses' observations of one or more fireballs were of the airplane's burning wreckage falling toward the ocean".[1]:270
Possible ignition sources of the center wing fuel tank
In an attempt to determine what ignited the flammable fuel-air vapor in the CWT and caused the explosion, the NTSB evaluated numerous potential ignition sources. All but one were considered very unlikely to have been the source of ignition.[1]:279
Missile fragment or small explosive charge
Although the NTSB had already reached the conclusion that a missile strike did not cause the structural failure of the airplane, the possibility that a missile could have exploded close enough to TWA800 for a missile fragment to have entered the CWT and ignited the fuel/air vapor, yet far enough away not to have left any damage characteristic of a missile strike, was considered.[1]:272 Computer simulations using missile performance data simulated a missile detonating in a location such that a fragment from the warhead could penetrate the CWT.[1]:273 Based on these simulations, the NTSB concluded that it was "very unlikely" that a warhead detonated in such a location where a fragment could penetrate the CWT, but no other fragments impact the surrounding airplane structure leaving distinctive impact marks.[1]:273
Similarly, the investigation considered the possibility that a small explosive charge placed on the CWT could have been the ignition source.[1]:273 Testing by the NTSB and the British Defence Evaluation and Research Agency demonstrated that when metal of the same type and thickness of the CWT was penetrated by a small charge, there was petalling of the surface where the charge was placed, pitting on the adjacent surfaces, and visible hot gas washing damage in the surrounding area.[1](pp273–274) Since none of the recovered CWT wreckage exhibited these damage characteristics, and none of the areas of missing wreckage were large enough to encompass all the expected damage, the investigation concluded that this scenario was "very unlikely."[1]:274
Other potential sources
The NTSB also investigated whether the fuel/air mixture in the CWT could have been ignited by lightning strike, meteor strike, auto-ignition or hot surface ignition, a fire migrating to the CWT from another fuel tank via the vent system, an uncontained engine failure, a turbine burst in the air conditioning packs beneath the CWT, a malfunctioning CWT jettison/override pump, a malfunctioning CWT scavenger pump, or static electricity.[1](pp272–279) After analysis the investigation determined that these potential sources were "very unlikely" to have been the source of ignition.[1]:279
Fuel quantity indication system
Because a combustible fuel/air mixture will always exist in fuel tanks, Boeing designers had attempted to eliminate all possible sources of ignition in the 747's tanks. To do so, all devices are protected from vapor intrusion, and voltages and currents used by the Fuel Quantity Indication System (FQIS) are kept very low. In the case of the 747-100 series, the only wiring located inside the CWT is that which is associated with the FQIS.
In order for the FQIS to have been Flight 800's ignition source, a transfer of higher-than-normal voltage to the FQIS would have needed to occur, as well as some mechanism whereby the excess energy was released by the FQIS wiring into the CWT. While the NTSB determined that factors suggesting the likelihood of a short circuit event existed, they added that "neither the release mechanism nor the location of the ignition inside the CWT could be determined from the available evidence." Nonetheless, the NTSB concluded that "the ignition energy for the CWT explosion most likely entered the CWT through the FQIS wiring".
Though the FQIS itself was designed to prevent danger by minimizing voltages and currents, the innermost tube of Flight 800's FQIS compensator showed damage similar to that of the compensator tube identified as the ignition source for the surge tank fire that destroyed a 747 near Madrid in 1976.[1](pp293–294) This was not considered proof of a source of ignition. Evidence of arcing was found in a wire bundle that included FQIS wiring connecting to the center wing tank.[1]:288 Arcing signs were also seen on two wires sharing a cable raceway with FQIS wiring at station 955.[1]:288
The captain's cockpit voice recorder channel showed two "dropouts" of background power harmonics in the second before the recording ended (with the separation of the nose).[1]:289 This might well be the signature of an arc on cockpit wiring adjacent to the FQIS wiring. The captain commented on the "crazy" readings of the number 4 engine fuel flow gauge about 21/2 minutes before the CVR recording ended.[1]:290 Finally, the Center Wing Tank fuel quantity gauge was recovered and indicated 640pounds instead of the 300pounds that had been loaded into that tank.[1]:290 Experiments showed that applying power to a wire leading to the fuel quantity gauge can cause the digital display to change by several hundred pounds before the circuit breaker trips. Thus the gauge anomaly could have been caused by a short to the FQIS wiring.[1]:290 The NTSB concluded that the most likely source of sufficient voltage to cause ignition was a short from damaged wiring, or within electrical components of the FQIS. As not all components and wiring were recovered, it was not possible to pinpoint the source of the necessary voltage.
Conclusions
The NTSB investigation ended with the adoption of its final report on August 23, 2000. In it the Board determined that the probable cause of the TWA 800 accident was:[1]:308
[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.

In addition to the probable cause, the NTSB found the following contributing factors to the accident:[1]:308
The design and certification concept that fuel tank explosions could be prevented solely by precluding all ignition sources.
The 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 tank vapor non-combustible.
During the course of its investigation, and in its final report, the NTSB issued fifteen safety recommendations, mostly covering fuel tank and wiring-related issues.[1](pp309–312) Among the recommendations was that significant consideration should be given to the development of modifications such as nitrogen-inerting systems for new airplane designs and, where feasible, for existing airplanes.[43]:6
Controversy

The NTSB's conclusions about the cause of the TWA 800 disaster took four years and one month to be published. The FBI's earliest investigations and interviews, later used by the NTSB, were performed under the assumption of a missile attack, a fact noted in the NTSB's final report. Six months into the investigation, the NTSB's chairman, Jim Hall, was quoted as saying, "All three theories—a bomb, a missile, or mechanical failure—remain."[44] Speculation was fueled in part by early descriptions, visuals and eyewitness accounts of the disaster that indicated a sudden explosion and trails of fire moving in an upward direction.
On July 17, 2013, the 17th anniversary of the tragedy, the Epix premium TV channel aired the documentary TWA Flight 800, which alleges that the crash investigation was a cover-up. The film highlights extensive eyewitness interviews, with many interviewees directly objecting to publicly described versions of their own descriptions of events. It also highlights interviews with investigators who had been involved in the original inquest, six of whom had filed a petition to reopen the probe. Their petition was based on eyewitness accounts, radar evidence indicating a possible missile and claims of evidence tampering. They dubbed it "The TWA 800 Project". Former NTSB investigator Henry Hughes has been quoted that he believes a bomb or a missile caused the crash.[45][46][47][48][49]
To consider the petition, the NTSB assembled a team of investigators not previously involved with the original investigation. On July 2, 2014, the NTSB said it would not reconsider its finding that the crash was caused by a fuel tank explosion. In a press release they stated: "After a thorough review of all the information provided by the petitioners, the NTSB denied the petition in its entirety because the evidence and analysis presented did not show the original findings were incorrect."[49]
Aftermath
Many internet users responded to the incident; the resulting web traffic set records for internet activity at the time. CNN's traffic quadrupled to 3.9 million hits per day. After the tragedy, the website of The New York Times saw its traffic increase to 1.5 million hits per day, 50% higher than its previous rate. In 1996, few U.S. government websites were updated daily, but the United States Navy's crash website was constantly updated and had detailed information about the salvage of the crash site.[50]
The wreckage is permanently stored in an NTSB facility in Ashburn, Loudoun County, Virginia, that was custom-built for the purpose. The reconstructed aircraft is used to train accident investigators.[51][52]
On July 18, 2008, the Secretary of Transportation visited the facility and announced a final rule designed to prevent accidents caused by fuel-tank explosions. The rule would require airlines to pump inert gas into the tanks, and will cover the centre-wing tank on all new passenger and cargo airliners, and passenger planes built in most of the 1990s, but not old cargo planes.[53] The NTSB had first recommended such a rule just five months after the incident and 33 years after a similar recommendation issued by the Civil Aeronautics Board Bureau of Safety on December 17, 1963, nine days after the crash of Pan Am Flight 214.[54]
The crash of TWA Flight 800, and that of ValuJet Flight 592 earlier in 1996, prompted Congress to pass the Aviation Disaster Family Assistance Act of 1996 as part of the federal aviation appropriations bill. Among other things, the Act gives NTSB, instead of the particular airline involved, responsibility for coordinating services to the families of victims of fatal aircraft accidents in the United States. In addition, it restricts lawyers and other parties from contacting family members within 30 days of the accident.[55]

During the investigation, the NTSB and the FBI clashed with each other many times. The agencies needed a detailed protocol describing which agency takes the lead when it is initially unclear whether an event is an accident or a criminal act. 49 Code of Federal Regulations 831.5 specified in 1996 (as it does now) that the NTSB's aviation accident investigations have priority over all other federal investigations. After the TWA flight 800 investigation, the NTSB recognized the need for better clarity. The NTSB sought and secured language to clarify the issue in 49 USC 1131(a)(2)(B), which was amended in 2000 to read:
If the Attorney General, in consultation with the Chairman of the [NTSB], determines and notifies the [NTSB] that circumstances reasonably indicate that the accident may have been caused by an intentional criminal act, the [NTSB] shall relinquish investigative priority to the [FBI]. The relinquishment of investigative priority by the [NTSB] shall not otherwise affect the authority of the [NTSB] to continue its investigation under this section

In 2005, the NTSB and the FBI entered into a Memorandum of Understanding (MOU) that states that, “[i]n the immediate aftermath of a transportation accident, the NTSB is the presumptive lead investigative agency and will assume control of the accident scene.” The FBI may still conduct a criminal investigation, but the NTSB investigation has priority. When investigative priority remains with the NTSB, the FBI must coordinate its investigative activities with the NTSB investigator-in-charge. This authority includes interviewing witnesses. The MOU states that: “[t]his procedure is intended…to ensure that neither NTSB nor FBI investigative activity unnecessarily complicates or compromises the other agency’s investigation. The new statutory language and the MOU have improved coordination between the NTSB and FBI since the TWA flight 800 accident. Today, FBI and NTSB personnel conduct joint exercises. They can call upon one another’s laboratories and other assets. The NTSB and the FBI have designated liaisons to ensure that information flows between agencies, and to coordinate on-scene operations.[56]
Heidi Snow, the fiancée of TWA800 victim Michel Breistroff, established the AirCraft Casualty Emotional Support Services (ACCESS) nonprofit group together with families of victims of Pan Am Flight 103.[57]
International Memorial

The TWA Flight 800 International Memorial was dedicated in a 2-acre (8,100m2) parcel immediately adjoining the main pavilion at Smith Point County Park in Shirley, New York, on July 14, 2004. The memorial is located at . Funds for the memorial were raised by the Families of TWA Flight 800 Association. David Busch of Busch Associates in Bay Shore, New York designed the memorial. The memorial includes landscaped grounds, flags from the 13 countries of the victims, and a curved Cambrian Black granite memorial with the names engraved on one side and an illustration on the other of a wave releasing 230 seagulls. In July 2006, an abstract design of a 10-foot-high (3.0m) lighthouse in black granite designed by Harry Edward Seaman, who had lost his cousin in the crash, was added. The lighthouse sits above a tomb holding many of the victims' personal belongings.[58] The 567 Air Force Band of the Northeast played the dedication, performing an arrangement of Brahms "Blessed are They" while all victims names were read aloud.
Passengers and crew

There were 230 people on board TWA 800 including 18 crew and 20 off-duty employees,[59] most of whom were crew meant to cover the Paris-Rome leg of the flight.
Of the 230 people on board there were passengers from the United States, France, Israel, the Ivory Coast, Italy, Norway, Spain, Sweden and Germany.

Passengers who died included:[60]

Michel Breistroff, French hockey player
Marcel Dadi, French guitarist
David Hogan, American composer
Jed Johnson, Andy Warhol's partner of twelve years, interior designer, and director of the 1977 movie Andy Warhol's Bad
Pam Lychner, American crime victims' rights advocate and former TWA flight attendant
Rico Puhlmann, German fashion photographer[61]
In addition, 16 students and 5 adult chaperones from the French Club of the Montoursville Area High School in Pennsylvania were on board.[62][63]
See also
Philippine Airlines Flight 143 - A similar incident in 1990 involving a 737, experienced a fuel tank explosion on the ground, possibly caused by faulty wiring
Pan Am Flight 214 - An aircraft that suffered a fuel tank explosion in mid-air in 1963, after being hit by lightning
Night Fall (novel) - 2004 New York Times Best Seller that revolves around a couple who witnessed and videotaped the crash of TWA Flight 800
Louis Freeh - Director of the FBI from 1993 - 2001 who assisted in the crash investigation
Notes

National Transportation Safety Board

 (Archive)
 (), posted on NTSB website

: United States Navy (Archive)
: Federal Bureau of Investigation New York Field Office (Archive)
 - Boeing

"". The Washington Post.

 - Houston Press
(courtesy Ryan Bomar)

 on IMDb (the film mentioned in the above link ^ )
Coordinates:

What happens when a airplane explodes in the air?


Racehorse injuries and fatalities are a side effect of training and competition. The problem with equine injuries is that they so often result in death.[1][2]  A 2005 study by the United States Department of Agriculture found that injuries are the second leading cause of death in horses, second only to old age.[3] Nureyev's recovery from a broken leg while retired at stud in 1987 typifies the struggle horses have after being injured.[4]
Two years after Secretariat's recordbreaking US Triple Crown took the sport in the United States to a new level of popularity, the breakdown and death of Ruffian brought on a new era of safety concerns. Racehorses had been breaking down for centuries, but never before in an event so widely seen as the great match race between Ruffian and Foolish Pleasure at Belmont Park with 18 million viewers.[5]  The horse racing industry has been trying to adapt to the increased safety concerns ever since. "I personally think that in a very real way, we're almost held to a higher standard because of the horses—they don't make the decision to do this. And I think that's fair," said Nick Nicholson, the president and chief executive of Keeneland.[6]
Classification
The racing industry will always be faced with the unavoidable and inevitable accident when horses get bumped or tangled in traffic, trip, or take a bad step. 1885 Kentucky Derby winner Joe Cotton had to be euthanized after breaking his leg while tripping over two other horses who had fallen in front of him in an 1888 race.[7]  Accidents and disputes were so bad in 1732 England, soon after nobility first started racing Thoroughbreds, that participants were ending up in court.[8]  The "Sport of Kings" has been described as an exercise in controlled chaos. The jockeys can be daredevils, and the racehorses can be unpredictable.[9]  As a response to the chaos, stewardship of racing began to evolve in 1751 when the first set of recorded racing rules were printed and in 1752 when Britain's Jockey Club started to become racing's first regulatory authority.[8][10]
Injuries, unlike accidents, do not happen by chance. The science of injury prevention has demonstrated that injuries and the events leading up to injuries are not random. Like disease, they follow a distinct pattern. Studying these patterns has made it possible to learn to predict and prevent injuries from occurring. Injuries, whether unintentional or intentional, can be considered any physical damage or harm caused to the body resulting in impairment or destruction of health. Injuries lead to death, disability, and financial loss.[11]  In 1990, apprentice jockey Benny Narvaez was paralyzed from the chest down after his horse threw him while jumping over another horse who had broken down directly in front of him during a race at Tampa Bay Downs. A jury found that Tampa Bay Downs was responsible for Narvaez's injury because the track veterinarian failed to perform an adequate pre-race examination on the horse that broke down.[12]
International Thoroughbred fatality rates
National television coverage of the Breeders' Cup by NBC helped generate millions in revenue and publicity for Thoroughbred racing in the mid to late 1980s. Since the inaugural running of the event in 1984, though, there has been an injury in virtually every Cup renewal. That national publicity backfired when horse racing increasingly came under attack from animal rights groups in the early 1990s. In the 1990 Breeders' Cup Distaff, champion two-year-old and three-year-old filly Go for Wand got back up after breaking her ankle and crossed the finish line on three legs.[5]  Then, in the 1992 Breeders' Cup Sprint, the five-year-old European horse Mr Brooks fractured his cannon bone and fell on top of his jockey, Lester Piggott. Piggott had told fellow European jockey Walter Swinburn before the race that the horse did not warm up well. Mr Brooks had also been reluctant to go in the starting gate before the race.[13]  There was so much controversy over the handling of Mr Brooks that the Breeders' Cup implemented rigid pre-race inspections in 1993.[14]  The resulting media pressure from the deaths of Go for Wand and Mr Brooks prompted racing industries around the world to make a concerted effort at determining the extent of the problem and the causes.[15]

* Rate statistics are based on occurrences per 1,000 starts.
As evidenced by the death of 2006 Kentucky Derby winner Barbaro, not all racehorse fatalities are the direct result of injury. It wasn't the break in Barbaro's leg that necessitated his destruction, but the hoof disease laminitis.[20]  Barbaro's team of doctors, led by Dr. Dean Richardson, were able to repair his broken leg, but not the excruciatingly painful laminitis that followed.  Support limb laminitis, which is the specific type Barbaro had, is caused by over-stressing the good leg during recovery from an injury in the opposite limb.[21][22]  A 1986 survey done by the Morris Animal Foundation found that laminitis was the fourth leading cause of death among horses.[23]  AAEP members ranked laminitis as the most important disease needing further research in 2009.[24]
After the death of Barbaro in 2006, the North American Thoroughbred industry realized that individual studies weren't adequate tools for evaluating ongoing efforts in injury prevention.[25]  The idea for an equine injury database came from the first Welfare and Safety of the Racehorse Summit in 2006. The database has three objectives: 1) to identify the frequency, type, and outcome of horse racing injuries using a standardized format that will generate valid composite statistics; 2) identify markers for horses at increased risk of injury; and 3) to serve as a data source for research directed at improving safety and preventing injuries.[26]  The Equine Injury Database (EID) was launched in 2008 following a one-year pilot project where injury reports were submitted by racetracks in a hard copy form. InCompass Solutions, Inc. and The Jockey Club Technology Services, Inc., subsidiaries of The Jockey Club, underwrote the costs of the Database.[27]

At a rate of 2.00 fatalities per 1,000 starts, two Thoroughbred racehorses die (on average) every day in North America. That incidence rate is also higher than it was in 1992, when it was 1.60 fatalities per 1,000 starts. Neither of those rates include training fatalities. In states that track training fatalities, training accounts for nearly a third of the total fatalities.[29][30] That would mean three Thoroughbred racehorses die every day from training and racing combined.
Biological systems and associated injuries
Injuries are a consequence of demand exceeding the capacity of the tissues and structures of the biological systems.[31]  The musculoskeletal system is by far the most commonly injured system.

[32]
Musculoskeletal system

The musculoskeletal system consists of the bones, cartilage, muscles, ligaments, and tendons.[33]
Skeletal fractures account for 87% of fatal injuries. When a horse's leg hits the ground at racing speed on a straightaway, it bears a load that is three times its weight (with the exception of harness racing). When negotiating a turn, centrifugal force increases the load to between 5 and 10 times body weight. When a horse hits the ground, the repetitive impact produces microscopic cracks and crevices inside bone so tiny they are undetectable by standard X-rays. If the horse isn't given enough time for healthy bone tissue to repair the damage with a process called remodeling, the cumulative stress can progress silently to the point where overload causes bones to break.[34]  The American Association of Equine Practitioners (AAEP) recommends a period of rest of at least 10 days between races for all horses to provide an opportunity to refresh and diminish the volume of persistent cyclic loading.[35]
Bucked shins is an inflammatory condition of the cannon bones. Bucked shins is a result of strain and excessive concussion to the cannon bone. The concussion comes from the rigorous training regimen that 2-year-olds often face and inability of the bone to adapt fast enough. 70% of young Thoroughbred racehorses in training develop the problem, usually in the first six months. As horses become older, the cannon bone becomes stiffer and thus bucked shins rarely occur again. Approximately 12% of horses that develop bucked shins go on to have stress or saucer fractures later.[36]  Bucked shins force 7% of racehorses to retire.[37]
Splints are new bone formation (exostoses) along the involved splint bone.[38] In the young horse the interosseous ligament which attaches the splint bones to the cannon can become damaged or torn from the concussive and rotational forces of exercise. This will cause heat, pain and swelling in the area between the splint and the cannon. In an attempt to stabilise this damaged attachment, new bone will be laid down around the ligament resulting in the formation of a bony lump known as a splint.[39] The size and position of this bony lump  determine whether a splint is likely to cause long-term lameness. The lump can interfere with the knee joint or the suspensory ligament, which runs down the back of the cannon bone.[40] Splints force 7% of racehorses to retire.[37]
Luxations are joint dislocations and account for 8% of fatal injuries. Fetlock luxations account for 91% of all fatal luxation injuries. Due to the construction of the fetlock joint, luxation will result in either a complete rupture of the flexor tendons and suspensory ligament or a lateral disarticulation. The joint capsule may also be completely ruptured and the articular portion of the bones exposed to view.[41]
An osselet is inflammation (arthritis) of the metacarpophalangeal joint (fetlock) of the equine front leg.[42] Osselets are a result of trauma, such as hard, heavy, or fast use, a slip or fall, or a direct blow to the joint.[43] When the fetlock suffers trauma, enzymes and other agents from the joint lining are released that destroy tissue inside the joint.[44] As the condition worsens, so does the horse's lameness, and the interior structures of the joints become more and more irritated.[42] Osselets force 16% of racehorses to retire.[37]
Carpitis is inflammation (arthritis) of the carpal joint (knee). Carpitis is caused by overextension of the carpus.[45] Knee injuries, the second most common non-fatal career-ending injury, force 16% of racehorses to retire.[37]
A ruptured tendon usually refers to the complete separation of a tendon. Tendon separation results in a complete loss of the tendon fibers, a marked increase in tendon cross-sectional area, and loss of support in the limb.[46]  A totally ruptured superficial digital flexor tendon (SDFT) will cause a visible drop in the angle of the fetlock.[47]  Catastrophic ruptured tendons account for as much as 3% of all tendon injuries.[48]
Tendinitis is inflammation of a tendon. Tendons connect muscles to bone and normally have an elastic property so that they can stretch. Tendinitis occurs when the tendon is overstretched or overloaded, which causes straining (tearing) of individual or multiple fibers and the formation of a lesion.[49] A lesion is strained tendon fibers with associated hemorrhage (bleeding) and edema (fluid retention).[50] Often strained tendons go undetected or may be subclinical before the clinical signs of tendonitis are observed.[51]  Tendinitis is characterized by heat, swelling, and pain. Racehorses often experience tendinitis when transitioning from sedentary activity to conditioning work. Thus young horses and those returning to exercise from lay-off are those most expected to suffer from tendinitis.[52] The incidence of tendon injuries is approximately 30% among Thoroughbred racehorses in training, mostly in the SDFT of the forelimbs.[53] Horsemen frequently refer to tendonitis as bowed tendon due to the bowed appearance of the SDFT.[54]  Bowed tendons force 25% of racehorses to retire and are the most common non-fatal career-ending injury.[37]  Racehorses are so often retired after tendinitis because as many as 70% will not be able to return to their previous level of performance and more than 66% of them will have a recurrence of injury.[55][56]
Tendinosis is chronic degeneration of a tendon without inflammation. It is caused by repetitive microtrauma and aging.[57]  Tendinosis is characterized by painful thickening and structural changes of the tendon.[58] Tendinosis predisposes horses for Tendinitis and catastrophic ruptured tendons.[55][59]
Desmitis is inflammation of a ligament. Ligaments connect bone to bone and, depending on individual function, are either compliant or noncompliant.[60] Ligaments have less elasticity than tendons and can therefore be injured easily.[61] Desmitis occurs most often when a horse overstrides, which causes a sprain (tear).[62][63] Some occurrences of desmitis may be difficult to detect because the ligament is deep within tissue. An injury to the collateral ligament of the fetlock predisposes the horse to fetlock luxation, the second most common fatal injury.[64][65]
Respiratory system
The equine respiratory system consists of the nostrils, pharynx (throat), larynx (voice box), trachea (windpipe), diaphragm, and lungs.
Exercise induced pulmonary hemorrhage (EIPH) is bleeding in the lungs. There are many theories regarding the causes of EIPH. The most widely accepted theory is that the high blood pressure from heavy exercise coupled with vacuum-like effects that occur during a deep inhalation causes the capillaries to rupture.[66]  The prevalence of EIPH varies with the method used to detect it and the frequency with which horses are examined. Almost all Thoroughbred racehorses in active training have some degree of EIPH, as many as 93% according to The Merck Veterinary Manual. When examined after each of three races, 87% of Standardbred racehorses had evidence of EIPH on at least one occasion. That suggests that EIPH is as common in Standardbred racehorses as it is in Thoroughbred racehorses. Higher degrees of EIPH undoubtedly result in poor performance and, on rare occasions, death.[67]
Epistaxis is EIPH characterized by blood appearing at the nostrils. Epistaxis is observed in approximately 5% of horses with EIPH.[68]  There are slight differences in the definition of a "bleeder" in various racing jurisdictions throughout the world. Some jurisdictions define bleeding as the appearance of blood at both nostrils, while other jurisdictions only require the appearance of blood at one nostril.[69]  There are also various regulations for each incident of bleeding throughout the world.[70]

[71]
Integumentary system
The integumentary system consists of the skin, hooves, hair, and glands.[72]
Hoof cracks are separations or breaks in the wall of a hoof. The most common type of hoof cracks are quarter cracks, which occur at the quarter, the thinnest and most delicate part of the hoof wall. Injury to the site may be the result of common things such as hard racetracks and uneven surfaces.[73] Quarter cracks originate from the coronet and run down the hoof wall, rather than a sand crack that begins at the ground surface and runs up the hoof wall.[74] Hoof cracks force 2% of racehorses to retire.[37]
Risk factors
The international community has identified important risk factors for injury or fatality in horse racing. These include: horse-related risk factors such as age, gender, and breeding; management-related risk factors such as training, medication, stewardship, economics, and jockeys; race-related factors such as type (flat or jump), distance, and field size; and track-related factors such as weather, surface, maintenance, and design.[75][76][77][78] Injury, in particular catastrophic injury, is a multi‐factorial event that involves the complex interaction of a number of risk factors. The effect of each individual factor and its importance can be measured by evaluating its relative risk.

Race-related
Type
Jumps racing has long been steeped in controversy due to its high mortality rate. This brand of racing requires Thoroughbred horses to leap over a succession of fences and are generally run over longer distances. There are both hurdles (generally shorter with lower obstacles), and steeplechases (generally longer with higher obstacles). Animal welfare groups have been campaigning for around 30 years to abolish hurdle and steeplechase events in Australia. Many aspects to the sport pose serious risk to horses, and it harms horses at a rate far above that of flat racing. In 1991, an Australian Senate Select Committee address on animal welfare concluded they had serious concerns about the welfare of horses in jumps races and recommended that state governments across the country phase out jumps racing over a three-year period. New South Wales and Tasmania abandoned the sport in 1997 and 2008 respectively, but Victoria and South Australia continue.[79] Jumps racing has never been the massive industry in Australia that it is in Britain and Ireland, but even there some tracks have discontinued National Hunt racing.[80] Nottingham was the first to discontinue jumps racing in 1996, followed by Windsor in 1998 and Wolverhampton in 2002.[81] Lingfield and Kempton have considered discontinuing jumps racing as well.[82]

[78]
Management-related
Training
Catastrophic injuries are often the acute manifestation of a more chronic process.[83] Tissues with the greatest blood supply respond most quickly to conditioning. The first tissues to respond are the horse's heart and lungs, then his muscles, and last are tendons and bones.[84]  The heart, lungs and muscles possess an excellent blood supply. Ligaments, tendons and joint capsules are relatively poorly supplied with blood and strengthening them can take two to four times as long.  Bones and hooves require the longest period of time to develop their full density and strength.[85] A British study showed that 78% of fractures occur during training. The substantial proportion of fractures that occurred during training emphasised the importance of studying these injuries away from the racecourse and the large number of stress injuries suggested that training regimes for young Thoroughbreds could be improved to create a more robust skeleton, able to withstand injury and fatality.[86]
The Thoroughbred Owners and Breeders Association (TOBA) statistics on racing related career ending injuries show that high grade races have a much lower rate of injury than low grade races.[87] While this could be attributed to many factors, including the quality of the horse, economics, or medications, the trainer still must make the decision to run a horse or not to run a horse. Because leading trainers consistently get the best horses, injury rates by race grade should correspond to injury rates by trainer. John Gaines even said that trainers are so important, transcendent trainers like Ben Jones, Hirsch Jacobs, Woody Stephens, Bob Baffert, and Wayne Lukas are worth as much as a great stallion or a great broodmare.[88] High grade races are considered stakes, allowance, and maiden special weight races. Low grade races are considered claiming and maiden claiming races.

Medication
Dan Hartman, Chair of Racing Commissioners International (RCI), said in March 2011 that a five-year phase out of drugs and medication is reasonable to bring North American racing policies in line with what is going on in other parts of the world like Europe and Hong Kong. Hartman said that a phased approach would give horsemen and owners sufficient time to adjust to the change. Hartman went on to say that he thinks that the regulators are the only voice in North American racing for the horse.[89]  RCI was originally called National Association of State Racing Commissioners and consists of mainly North American regulators whose goal is uniform rules and practices and reciprocity in enforcing each member's official rulings.[90] Feeling no pain, an injured horse on drugs may continue to charge down the track, endangering every horse and jockey in the race. Drugs may account for the fact that the North American fatality rate on turf is higher than it is in Australia on turf, or even Great Britain (0.38), where race day medications are not allowed.[91]

[78][92]
Track-related
Michael Peterson of the University of Maine states there are three components that affect a racetrack's performance as it relates to injuries: the surface of the track, weather and maintenance. [93]
Surface
In Thoroughbred racing there are three main track surfaces: dirt, turf and synthetic. Dirt tracks are generally a mixture of sand, silt and clay. Turf tracks are usually Bermuda grass in warm climates and a mixture of Fescue, Bluegrass and Rye in more temperate climates. Synthetic tracks are generally a mixture of sand, synthetic fibers, rubber and wax. Synthetics are differentiated by brand names such as Polytrack, Tapeta Footings and Cushion Track.
North American racing is run on all three types of tracks and provides the best basis for a comparison on how track composition affects injuries and fatalities. The EID was finally able to provide statistics on racing related fatalities in March 2010. [26] The TOBA leaked statistics on racing related career ending injuries three months later in June. [87] The EID was able to supplement its March data with a release of a second year of statistics in December. Dr. Tim Parkin, a veterinarian and epidemiologist from the University of Glasgow, who has been involved in the EID for more than a year, said that the addition of a second year of information enabled them to statistically validate the data.[92] Not included in EID or TOBA statistics are training or non-race injuries.[94]
The following tables show that the prevalence of Thoroughbred fatalities or career-ending injuries is greater on dirt tracks than it is on either turf or synthetic tracks.
2009-2010 EID Stats

† Approximations based on standard distribution of races per surface.
2008-2009 TOBA Stats

Weather
For both dirt and turf surfaces there is a need to understand the relationship between the water content and the surface performance. For these surfaces moisture is the single most important variable in the maintenance of the surface. As synthetic surfaces wear it is also likely that the sensitivity to moisture will increase as the wax coating is lost from the surface.[95]
The TOBA statistics on racing related career ending injuries show that moisture has the opposite effect on dirt and turf tracks. Moisture causes more injuries on dirt tracks, while it reduces injuries on turf tracks.[87] This table does not include data from tracks rated "Good" because that term applies to both dirt and turf.
2008-2009 TOBA Stats

Moisture can cause even bigger problems when combined with freezing temperatures. At Philadelphia Park Racetrack in Bensalem, a rash of breakdowns in the first two months of 2004 was blamed on the winter weather and track problems caused by thawing frost. In that short stretch of time, 12 horses were euthanized following races, and another one was destroyed after sustaining a training injury.[96] The TOBA statistics on racing related career ending injuries show that cold weather racing causes almost twice as many injuries as warm weather racing (February's rate of 3.16 is 1.67 times larger than July's rate of 1.90).[87] This graph is a plot of injury rates for different tracks based on when their racing seasons begin and end during the calendar year. To eliminate as many conflicting variables as possible, only North American tracks in temperate climates that race on dirt/turf and that run graded stakes races were used. The tracks were Aqueduct, Belmont, Churchill Downs, Colonial Downs, Hawthorne, Laurel Park, Monmouth, and Saratoga.

Without better data, it is difficult to determine the effect moisture and freezing temperatures have on synthetic tracks. Golden Gate Fields has a relatively high career-ending injury rate for a synthetic track and their racing season takes place during winter months, but the coldest temperatures average about 50 degrees Fahrenheit (18 degrees above freezing) in that part of California during that time of year. In Great Britain the fatal injury rate on turf is less than it is on synthetics, which is opposite of North American racing.[97] Freezing temperatures could be the reason because winter racing on the flat only takes place on synthetic tracks in Great Britain. That could result in an increase of the fatality rate on synthetics, but not turf.[98] This anecdotal evidence seems to indicate that synthetic tracks are negatively affected by moisture and freezing temperatures just like dirt and turf.
Maintenance
Current evidence indicates that consistency of each surface and limited variability among surfaces seen by each horse are more important than the exact values of each property. Consistency allows for the horse to adapt through training.[95] Weather and the amount of traffic on a track make consistent surfaces difficult. One of the biggest challenges with a dirt track is that you can design a dirt track for a particular moisture content, but you can’t do it for all moisture contents. In the case of synthetic tracks, wax and polymers used in them will change over time, as will the length of the fibers as they break down from use. The sand in various areas of the track also wears differently.[99] The oils in synthetic tracks also tend to seep into the base of the surface over time, which may explain why some synthetic tracks have not performed as well in their third or fourth years as they did in their first years.[100] The Racing Surfaces Testing Laboratory is working to collect data from racetracks and correlate it with injury rates. Such information will allow track maintenance personnel to make the best decisions for any variable.
Horse-related
Breeding
There has been a lot of debate since 2006 about how sound the Thoroughbred is today compared to the 1960s and earlier.[77][101]  The only real data even remotely pertaining to the subject involves the average number of starts per horse, which The Jockey Club started tracking in 1950.[102]

[103]
Those who argue the Thoroughbred is unsound also point to the fact that horse races are run over shorter distances today. Monmouth Park Racetrack general manager Bob Kulina said long distance races disappeared across America because not enough horses entered to race.[104]  A prime example of a race that reduced its distance to keep drawing horses is the Jockey Club Gold Cup.

Horse conformation, which is a visible outcome of breeding practices, clearly plays a significant role in injury.

Why do horses get put down if they break a leg?