# National Transportation Safety hard Washington, 

### D.C. 20594 

## Safety Recommendation

Date: December 13, 1996



### Inreply referto: A-96-174 through -177

HonorableLindaHall 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 ofEast Moriches, New York, after taking off from John F. Kennedy International Airport 

### (JFK),Jamaica, New York. 

### All 230 people aboard the airplanewere killed The airplane, which
was operated under Title 14Code ofFederal 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 occurredin the centerwing fueltank (CWT);' the CWTwas 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~~~-C~-p~s~msp~~se beam Nos 2 and 3, the forward spar, and debris from beneath and forward of the center wing section (see Figure 1). The cockpit ofthe airplane and pieces ofthe forward fuselage were found 

### in a second debris field that was more 

### than 1 mile from the beginning of the wreckage path Fragmentedwing and 

### aft fuselage parts were recovered from 

### a third debris field farther along the wreckage path.

Portions ofthe 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

### ofthe tank that are consistent with an explosion originatingwithin the tank Airplane parts*from in and around the CWT recovered and identified to date contain no evidence

The flight engineerfrom the previous flight remembend having left about 300 pounds, or about 50 gallons, of fuel in the approximately 13,000gallon capacity taok. The recoveredfuel gauge indicated slightlymore than600 pounds (about 100gallons)offuel remainingin the CWT.

Includes portions of the fuselage structure from above, air conditioning packs and ducting from below, wing StnrCrUrefromboth sides, alltiresfrombehind, and numemuscomponents that included the large fiberglasswater and cargofireextinguisher confainen from forwardofthe CWT. 1

67882 of bomb or missile damage. The investigation into what might have provided the source of ignition ofthefuel-airmixture (including a bomb or missile)in the CWT is continuing.



### CabinFloor

### Level



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atrwise

### !ar


### Bi SPW

eam

### 1

SpanwiseBeam 

### 3

## J
Figure 1. CenterWing Fuel Tank
Since 1985, the SafetyBoard has investigatedor assisted in the investigationoftwo other fuel tank explosionsinvolvingcommercialtransport-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 airplanewasbeing pushed back from a terminal gate, resulting in 8 fatalities and 30 injuries. The ambienttemperature at the time of the accidentwas about 95T, and the airplanehad beenparked in the sun. Although damage to wiring and a defective fuel quantity sensor were identified as possible sourcesofignition, a definitive ignition sourcewas never confirmed.

The Safety Board also assisted in the investigation ofthe crash ofAviancaflight 203, aB727, 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 compromisethe structural integrity 

### of the airplane; however, the explosion punctured the 

### CWT and ignited the fuel-air vaporsin theullage, resulting 

### in destruction of the airplane
Earlier, the SafetyBoard conducted a special investigation ofthe 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 oftiiefuel in the tank.3 left wing, followed by fire, explosion, and separation of the wing The wreckage revealed evidenceof an explosion that originatednear a fuel valve installationin the left outboard main fuel tank The Safety Board’s report4noted 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 ignitethe fuel-airmixture,but that 

### energy levels required to produce

a spark

### will not necessarily damage metal or leave marks at the point ofignition

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~ecomesourcesofFuel 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 strikesto areas having a high probabilityof stroke attachment, (b) swept lightning strikes to areas where swept strokes are highly probable; and (c) corona and streameringat fuel vent outlets 

### (Part25 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 conditionsof any component whose operation, failure, or malfunction could increasethetemperature insidethe tank (Part 25 981)

However, a 1990, Society of Automotive Engineers technical paper comments, “ if the ignition sourceis sufficiently strong (such as in combat threats), it can raise the fluid temperature locally and thus ignitea fiiel that is below itsflashpoint 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 ofthe IranianB-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 thereforethemixtures areflammable.” Marks’ states further, “The autoignition temperature ofanair-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 ConferenceandExposition, Long Beach CAifornia,on October 14,1990. 44 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 conditioningpacks were operated on the ground for about 2 hours togenerateheat beneath the 

### CW. The airplanewasthen climbed to an altitude of 18,000feet msl. Thetemperature

### ofthe fuel in the center tank 

### ofthetest airplanewas measured at one location, and the 

### airtemperaturewithin the tank was measured at

### four locations. 

### Inthis test, the fuel-air mixture in the 

### CW ullage was stabkedat 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 explosivefuel-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 firesand 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 systemsin 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 SystemDesignand Analysis,paragraph5.al states, “Inany systemor subsystem, the failure of any single element, component, or connection during any one flight @rake release through ground decelerationto stop) should be assumed, regardless of‘ its improbability. Such single failures should not prevent continuedsafe flight and landing, or signiiicantlyreduce the capability ofthe airplane or the abilityofthe crewto cope with the resulting failureconditions.” 

### (5 systems are expensive and may be impractical because of system weight and maintenance requirements in some airplanes.
Therefore, the SafetyBoard has considered other modifications ofthe 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

TheBoard recognizes that such design modifications take time to implement and believes that in the interim, operationalchanges are needed to reducethe likelihood ofthe 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 

### tanktemperatures 

### 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 modificationsin 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

### inthe 

### CWT.

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

### CWTby an additional 10to 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 WAS00would 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 otherAirbus Industriecommercial transport airplanes are similar Iothose &om Boeing in that the airconditioningpack and ducts are beneaththe CWT. 86
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 requirethat theB-747 Flight Handbooks ofTWA and other operators ofB-747s and other aircraft in which fuel tank temperature cannot be determined by flightcrews be immediately revised to reflect the increasesin CWT temperatures found by flight tests, including operational procedurestoreducethe potential for exceedingCWT 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 (1IOF),” the only fueltank temperatureindication displayedforflightcrewsis that 

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

no means to measurethe temperature ofthe fuel or ullage offuel tanks that are located near heat

sources The Safety Board believesthat flightcrewsneed to monitorthe temperature offuel 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 ofother airplanesthat are located near heat sourcesto incorporate temperature probes and cockpit fuel tank temperature displaysto permit determination of the fuel tank temperatures.
Therefore, the National Transportation Safety Board recommends that the Federal Aviation Administration:
Require the developmentof and implementation of design or operational changes that wiU precludethe operation oftransport-category airplanes with explosivefuelair mixtures in thefueltanks:
(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 airplanesand, wherefeasible,toexisting 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, considerationshouldbe given to refuelingthe center wing fuel tank 

### (CWT) before flight whenever possible from cooler ground 

### fuel tanks, proper monitoringand management ofthe CWT fuel temperature, and maintaining

an appropriate minimum fuel quantityin the CWT (Urgent) (A-96-175)
Require that the B-747 Flight Handbooks of TWA and other operators ofB-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 potentialfor exceedingCWT temperature limitations (A-96-176)Require modification 

### of the CWT of B-747 airplanes and the fbel tanks of other airplanesthat are located 

### near heat sourcesto 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 andBLACK concurredin these recommendations.