National Transportation Safety hard 

Washington, D.C. 20594 

Safety Recommendation 

Date: December 13, 1996 

In reply refer to: A-96-174 through -177 

Honorable Linda Hall Daschle 

Acting-Administrato ---- 

Federal Aviation Administration 

Washington, D.C. 20591 

On July 17, 1996, about 2031 eastern daylight time, a Boeing 747-131, N93 119, operated 

as Trans World Airlines Flight 800 (TWASOO), crashed into the Atlantic Ocean, about 8 miles 

south of East Moriches, New York, after taking off from John F. Kennedy International Airport 

(JFK), Jamaica, New York. All 230 people aboard the airplane were killed The airplane, which 

was operated under Title 14 Code of Federal Regulations (CFR) Part 121, was bound for Charles 

De Gaulle International Airport (CDG), Paris, France. The flight data recorder (FDR) and 

cockpit voice recorder (CVR) ended simultaneously, about 13 minutes after takeoff Evidence 

indicates that as the airplane was climbing near 13,800 feet mean sea level (msl), an in-flight 

explosion occurred in the center wing fuel tank (CWT);' the CWT was nearly empty 

A substantial portion of the airplane wreckage has been recovered fiom the ocean floor. 

Amo_ng~e-ddebris-fo~nd~Q~g~h~~~~p~~f~he~ec~ge-pa~

beam Nos 2 and 3, the forward spar, and debris from beneath and forward of the center wing 

section (see Figure 1). The cockpit of the airplane and pieces of the forward fuselage were found 

in a second debris field that was more than 1 mile from the beginning of the wreckage path 

Fragmented wing and aft fuselage parts were recovered from a third debris field farther along the 

wreckage path. 

Portions of the airplane have been reconstructed, including the CWT, the passenger cabin 

above the CWT, and the air conditioning packs and associated ducting beneath the CWT. The 

reconstruction thus far shows outward deformation of the CWT walls and deformation of the 

internal components of the tank that are consistent with an explosion originating within the tank 

Airplane parts* from in and around the CWT recovered and identified to date contain no evidence 

The flight engineer from the previous flight remembend having left about 300 pounds, or about 50 gallons, of 

fuel in the approximately 13,000 gallon capacity taok. The recovered fuel gauge indicated slightly more than 600 

pounds (about 100 gallons) of fuel remaining in the CWT. 

Includes portions of the fuselage structure from above, air conditioning packs and ducting from below, wing 

StnrCrUre from both sides, all tires from behind, and numemus components that included the large fiberglass water 

and cargo fire extinguisher confainen from forward of the CWT. 

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of bomb or missile damage. The investigation into what might have provided the source of 

ignition of the fuel-air mixture (including a bomb or missile) in the CWT is continuing. 

Cabin Floor Level 

\ Re 

atrwise 

!ar 

Bi 

SPW 

eam 1 

Spanwise Beam 3 J 

Figure 1. Center Wing Fuel Tank 

Since 1985, the Safety Board has investigated or assisted in the investigation of two other 

fuel tank explosions involving commercial transport-category airplanes The most recent accident 

'nvolved a Philippine Airlines B-737-300 at Ninoy Aquino International Airport, Manila, 

'hilippines, on May 11, 1990. In that accident, the CWT ullage' fuel-air vapors exploded as the 

airplane was being pushed back from a terminal gate, resulting in 8 fatalities and 30 injuries. The 

ambient temperature at the time of the accident was about 95T, and the airplane had been parked 

in the sun. Although damage to wiring and a defective fuel quantity sensor were identified as 

possible sources of ignition, a definitive ignition source was never confirmed. 

The Safety Board also assisted in the investigation of the crash of Avianca flight 203, a B727, on November 27, 1989 The airplane had departed Bogota, Colombia, about 5 minutes 

before the crash. Examination of the wreckage revealed that a small bomb placed under a 

passenger seat, above the CWT, had exploded The bomb explosion did not compromise the 

structural integrity of the airplane; however, the explosion punctured the CWT and ignited the 

fuel-air vapors in the ullage, resulting in destruction of the airplane 

Earlier, the Safety Board conducted a special investigation of the May 9, 1976, explosion 

and in-flight separation of the left wing of an Iranian Air Force B-747-131, as it approached 

Madrid, Spain, following a flight from Iran Witnesses reported seeing a lightning strike to the 

In a fuel tank, the ullage is the vapor-laden space above the level oftiie fuel in the tank. 
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left wing, followed by fire, explosion, and separation of the wing The wreckage revealed 

evidence of an explosion that originated near a fuel valve installation in the left outboard main fuel 

tank The Safety Board’s report4 noted that almost all of the electrical current of a lightning 

strike would have been conducted through the aluminum structure around the ullage While the 

report did not identifL a specific point of ignition, it noted that static discharges could produce 

sufficient electrical energy to ignite the fuel-air mixture, but that energy levels required to produce 

a spark will not necessarily damage metal or leave marks at the point of ignition 

Fuel tank explosions require an energy source sufEicient for ignition and temperatures 

between the lower explosive (flammability) limit and upper explosive limit (UEL), which 

will result in a combustible mixture of fuel and air Current Federal Aviation Administration 

(FAA) regulations require protection against the ignition of fuel vapor by lightning, components 

ignition Specifically 

~otenougbto-create_a~oigni~o~~~p~or-system~~r~s-~~Qul~ecsourcesofFuel system lightning protection. The &el system must be designed and 

arranged to prevent the ignition of fuel vapor within the system by (a) direct 

lightning strikes to areas having a high probability of stroke attachment, (b) swept 

lightning strikes to areas where swept strokes are highly probable; and (c) corona 

and streamering at fuel vent outlets (Part 25 954) 

Fuel Tank Temperature. (a) The highest temperature allowing a safe margin 

below the lowest expected auto ignition temperature of the &el in the fuel tanks 

must be determined (b) No temperature at any place inside any &el tank where 

fuel ignition is possible may exceed the temperature determined under paragraph 

(a) of this section This must be shown under all probable operating, failure, and 

malfunction conditions of any component whose operation, failure, or malfunction 

could increase the temperature inside the tank (Part 25 981) 

However, a 1990, Society of Automotive Engineers technical paper comments, “ if the 

ignition source is sufficiently strong (such as in combat threats), it can raise the fluid temperature 

locally and thus ignite a fiiel that is below its flash point temperature This is particularly true with 

a kel mist where small droplets require little energy to heat up ’‘6 Elevated, possibly extremely 

high local temperatures would have been associated with the lightning strike of the Iranian B-747 

in 1976 

NTSB-AAR-78-12“ The Safety Board did not determine the probable cause of this foreign accident because it 

had no statutory authority to do so. Several hypotheses addressing the sequence of even& and possible causes of 

the accident were presented in the Board’s report., 

Marks’ Standard Handbook for Mechanical Engineers, Eighth Edition, slates, “The lower and upper limits of 

flammability indicate the percentage of combustible gas in air below which and above which flame will not 

propagate.. When a flame is initiated in mixtures having compositions within these limits, it will propagate and 

therefore the mixtures are flammable.” Marks’ states further, “The autoignition temperature of an air-fuel mixture 

is the lowest temperam at which chemical reaction proceeds at a rate suBicient to result eventually (long time 

lag) in innammation.” (In the TWA800 CWT, the LEL was about 1 WF, and the autoignition temperature was 

about 440’F.) 

Society of Automotive Engineers (SAEJ Technical Paper Series 901949, Flammability of Aircraft Fuels, by N. 

Albert Mow4 BlazeTech Cop , Winchester, Massachusetts, as presented at the Aerospace Technology 

Conference and Exposition, Long Beach CAifornia, on October 14, 1990. 

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Despite the current aircraft certification regulations, airlines, at times, operate transportcategory turbojet airplanes under environmental conditions and operational circumstances that /II 

allow the temperature in a fuel tank ullage to exceed the EL, thereby creating a potentially 

explosive fuel-air mixture. For example, on August 26, 1996, Boeing conducted flight tests with 

an instrumented B-747 airplane that carried about the same small amount of fuel in the center 

wing tank as that carried aboard TWASOO. AU three air conditioning packs were operated on the 

ground for about 2 hours to generate heat beneath the CW. The airplane was then climbed to an 

altitude of 18,000 feet msl. The temperature of the fuel in the center tank of the test airplane was 

measured at one location, and the air temperature within the tank was measured at four locations. 

In this test, the fuel-air mixture in the CW ullage was stabked at a temperature below the LEL 

on the ground. However, as the airplane climbed, the atmospheric pressure decreased (the LEL 

decreases with decreasing atmospheric pressure) reducing the LEL temperature and allowing an 

explosive fuel-air mixture to exist in the tank ullage. 

Fuel tank temperatures may also become elevated, allowing explosive fuel-air mixtures to 

exist in the ullage, when airplanes are on the ground between flights at many airports worldwide 

during warm weather months. When the temperature of a combustible fuel-air mixture exceeds 

the LEL, a single ignition source exposed to the ullage could cause an explosion and loss of the 

airplane.. This situation is inconsistent with the basic tenet of transport aircraft design-that no 

single-point failure should prevent continued safe flight? 

Without oxygen in the fuel-air mixture, the fuel tank ullage could not ignite, regardless of 

temperature or ignition considerations. The military has prevented fuel tank ignition in some 

aircraft through the creation of a nitrogen-enriched atmosphere (nitrogen-inerting) in fuel tank 

ullage, thereby creating an oxygen-deficient fuel-air mixture that will not ignite. Although this 

technology could be applied to civil aircraft, there are no transport-category airplanes of which 

the Safety Board is aware that currently incorporate nitrogen-inerting systems to reduce the 

potential for fuel tank fires and explosions. 

Nitrogen-inerting has been accomplished several ways: by adding nitrogen to fuel tank($ 

from a ground source before flight; by discharging onboard supplies of compressed or liquified 

nitrogen in flight; or by the use of on-board inert gas generation systems that separate air into 

nitrogen and oxygen,. Such systems in current-generation military aircraft incorporate lightweight, 

permeable plastic membrane systems that produce high nitrogen flow rates and require only “oncondition” maintenance. Nitrogen-inerting using a ground source of nitrogen might prevent 

explosions such as those that occurTed to the TWASOO and Avianca airplanes, but may not 

prevent an explosion after the fuel tanks have been emptied during flight through fuel 

consumption, or when ullage is exposed to warmer air as an airplane descends-situations that 

existed in the Iranian Air Force B-747 accident. Nitrogen-inerting fuel tank ullage has been used 

for more than 25 years in military airplanes and could be used to protect commercid air 

transportation. However, the Safety Board recognizes that development and installation of such 

’ FAA Advisory Circular (AC) 25.1309-1A System Design and Analysis, paragraph 5.al states, “In any system or 

subsystem, the failure of any single element, component, or connection during any one flight @rake release 

through ground deceleration to stop) should be assumed, regardless of‘ its improbability. Such single failures 

should not prevent continued safe flight and landing, or signiiicantly reduce the capability of the airplane or the 

ability of the crew to cope with the resulting failure conditions.” 

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systems are expensive and may be impractical because of system weight and maintenance 

requirements in some airplanes. 

Therefore, the Safety Board has considered other modifications of the airplane that would 

reduce the potential for aircraft fuel tank explosions. A reduction in the potential for fuel tank 

explosions could be attained by reducing the heat transfer to fuel tanks from sources such as hot 

air ducts and air conditioning packs* that are now located under or near fuel tanks in some 

transport-category airplanes. This may be achieved by installing additional insulation between 

such heat sources and fuel tanks that must be collocated with heat-generating equipment such as 

hot air ducting and air conditioning packs. 

Because the Safety Board believes that the FAA should require the development and 

~implementation-of-design-or-operational-change~that-~-preclud~th~opera~ion-of-tr~sportcategory airplanes with explosive fuel-air mixtures in the fuel tanks, significant consideration 

should be given to the development of airplane design modifications, such as nitrogen-inerting 

systems and the addition of insulation between heat-generating equipment and the fuel tanks 

Appropriate moditications should apply to newly certificated airplanes, and where feasible, to 

existing airplanes 

The Board recognizes that such design modifications take time to implement and believes 

that in the interim, operational changes are needed to reduce the likelihood of the development of 

explosive mixtures in fuel tanks. Two ways to reduce the potential of an explosive fuel-air 

mixture could be by refueling the CWT to a minimum level from cooler ground fuel tanks or by 

canying additional fuel. Therefore, by monitoring fuel quantities and temperatures (when soequipped), by controlling the use of air conditioning packs and other heat-generating devices or 

systems on the ground, and by managing fuel distribution among various tanks to keep all fuel 

tank temperatures in safe operating ranges and a to-be-determined minimum fuel quantity in the 

CWT, flightcrews could reduce the potential for fuel tank explosions in the B-747. The Safety 

Board believes that pending implementation of design modifications, the FAA should require 

modifications in operational procedures to reduce the potential for explosive fuel-air mixtures in 

the fuel tanks of transport-category aircraft. In the B-747, consideration should be given to 

refueling the CWT before flight whenever possible from cooler ground fuel tanks, proper 

monitoring and management of the CWT temperature, and maintaining an appropriate minimum 

fuel quantity in the CWT. 

The Safety Board has also found that the Trans World Airlines B-747 Flight Handbook 

used by crewmembers understates the extent to which the air conditioning packs can elevate the 

temperature of the B-747 CWT. The Handbook notes that pack operation may elevate the 

temperature of the CWT by an additional 10 to 20°F. However, in the August 26, 1996, B-747 

flight tests with three air conditioning packs in operation, the temperature of the center tank fuel 

increased by approximately 40°F A 40°F temperature increase in the CWT of WAS00 would 

have raised the temperature of the ullage above the LEL of its fuel-air mixture The Handbook 

also states, ‘‘warm fuel I may cause pump cavitation and low pressure warning lights may come 

Airplanes other than the B-747 also have heat-producing equipment in the vicinity of fuel tanks. For example, 

the A-320 and other Airbus Industrie commercial transport airplanes are similar Io those &om Boeing in that the 

air conditioning pack and ducts are beneath the CWT. 

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on steady or flashing ” The Board is concerned that the flight handbooks of other operators of 

the B-747 may have similar deficiencies. Therefore, the Safety Board believes that the FAA 

should require that the B-747 Flight Handbooks of TWA and other operators of B-747s and other 

aircraft in which fuel tank temperature cannot be determined by flightcrews be immediately 

revised to reflect the increases in CWT temperatures found by flight tests, including operational 

procedures to reduce the potential for exceeding CWT temperature limitations 

Although the TWA B-747 Flight Handbook (and the Boeing Auplane Flight Manual) 

instruct flightcrews not to exceed fuel temperatures of “54 5C (130F), except JP-4 which is 43C 

(1 IOF),” the only fuel tank temperature indication displayed for flightcrews is that of the outboard 

main tank in the left wing. The designs of the B-747 and some other airplanes currently provide 

no means to measure the temperature of the fuel or ullage of fuel tanks that are located near heat 

sources The Safety Board believes that flightcrews need to monitor the temperature of fuel tanks 

that are located near heat sources, including the CWT in B-747s. Therefore, the Safety Board 

believes that the FAA should require modification of the CWT of B-747 airplanes and the fuel 

tanks of other airplanes that are located near heat sources to incorporate temperature probes and 

cockpit fuel tank temperature displays to permit determination of the fuel tank temperatures. 

Therefore, the National Transportation Safety Board recommends that the Federal 

Aviation Administration: 

Require the development of and implementation of design or operational changes 

that wiU preclude the operation of transport-category airplanes with explosive fuelair mixtures in the fuel tanks: 

(a) Significant consideration should be given to the development of 

airplane design modifications, such as nitrogen-inerting systems and the 

addition of insulation between heat-generating equipment and fuel tanks 

Appropriate modifications should apply to newly certificated airplanes and, 

where feasible, to existing airplanes (A-96-174) 

(b) Pending implementation of design modifications, require modifications f in operational procedures to reduce the potential for explosive fuel-air 

mixtures in the fuel tanks of transport-category aircraft In the B-747, 

consideration should be given to refueling the center wing fuel tank (CWT) 

before flight whenever possible from cooler ground fuel tanks, proper 

monitoring and management of the CWT fuel temperature, and maintaining 

an appropriate minimum fuel quantity in the CWT (Urgent) (A-96-175) 

Require that the B-747 Flight Handbooks of TWA and other operators of B-747s 

and other aircraft in which fuel tank temperature cannot be determined by 

flightcrews be immediately revised to reflect the increases in CWT fuel 

temperatures found by flight tests, including operational procedures to reduce the 

potential for exceeding CWT temperature limitations (A-96-176) 
Require modification of the CWT of B-747 airplanes and the fbel tanks of other 

airplanes that are located near heat sources to incorporate temperature probes and 

cockpit fbel tank temperature displays to permit determination of the fbel tank 

temperatures. (A-96-177) 

Chairman HALL, Vice Chairman FRANCIS, and Members HAMMEWCHMIDT, 

CmGLZq and BLACK concurred in these recommendations.