16014 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

Fuel Tank Ignition Prevention

Measures

AGENCY: Federal Aviation

Administration, DOT.

NOTICE: Notice of request for comment

on National Transportation Safety Board

recommendations.

SUMMARY: This notice solicits public

comment on the feasibility of

implementing four recommendations

proposed by the National

Transportation Safety Board (NTSB) that

are intended to reduce the likelihood of

airplane fuel tank ignition. The NTSB

recommendations resulted from an

accident on a Boeing Model 747

operated by Trans World Airways

(TWA) that occurred after taking off

from Kennedy International Airport in

New York, on July 17, 1996. The cause

of the accident has not been determined.

However, evidence suggests that

explosion of fuel vapors within the

center wing fuel tank occurred due to a

yet to be determined ignition source.

The FAA is not currently considering or

proposing any regulatory action. The

purpose of this notice is to gather

technical information needed to

formally respond to the NTSB

recommendations.

DATES: Comments must be received on

or before August 1, 1997.

ADDRESSES: Comments on this notice

may be mailed to: Federal Aviation

Administration, Transport Airplane

Directorate, Aircraft Certification

Service, ANM–100 (Attn: Mike Dostert,

ANM–112), 1601 Lind Avenue SW.,

Renton, Washington 98055–4056.

FOR FURTHER INFORMATION CONTACT:

Mike Dostert, FAA, Airframe and

Propulsion Branch (ANM–112),

Transport Airplane Directorate, Aircraft

Certification Service, 1601 Lind Avenue

SW., Renton, Washington 98055–4056;

telephone (206) 227–2132.

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to

participate in evaluation of the NTSB

recommendations by submitting written

data, views, or arguments as they may

desire. Comments relating to the

environmental, energy, or economic

impact that might result from adopting

the recommendations contained in this

notice are invited. Substantive

comments should be accompanied by

cost estimates. All comments received

on or before the closing date for

comments will be considered by the

FAA before preparing a formal response

to the NTSB recommendations.

Background

On July 17, 1996, a Boeing Model 747

operated by Trans World Airways was

involved in an accident after taking off

from Kennedy International Airport in

New York. Although no specific cause

for the accident has been determined,

evidence suggests that the center wing

fuel tank exploded due to a yet to be

determined ignition source. The

accident investigation has focused on a

missile, bomb, or mechanical failure as

the possible source of ignition of fuel

vapors within the tank. On December

13, 1996, the NTSB issued four

recommendations to the FAA

requesting, in part, that the FAA require

the development and implementation of

design or operational changes that will

preclude the operation of transport

category airplanes with explosive fuelair mixtures in the fuel tanks. The

following is a summary of the four

recommendations that are published in

their entirety later within this notice.

The first recommendation would

require development of an airplane

design modification, such as nitrogeninerting systems, and the addition of

insulation between heat-generating

equipment and fuel tanks. (A–96–174)

The second recommendation would

require modifications in operational

procedures to reduce the potential for

explosive fuel-air mixtures in the fuel

tanks of transport category aircraft. In

the Model 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. (A–96–175)

The third recommendation would

require that the Model 747 Flight

Handbooks of TWA and other operators

of Model 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)

The fourth recommendation would

require modification of the CWT of

Model 747 airplanes and other airplanes

on which the fuel tanks are located near

heat sources, to incorporate temperature

probes and cockpit fuel tank

temperature displays to permit

determination of the fuel tank

temperatures. (A–96–177)

The flammability temperature range

of jet engine fuel vapors varies with the

type of jet fuel, the ambient pressure in

the tank, and the amount of dissolved

oxygen that may evolve from the fuel

due to vibration and sloshing that

occurs within the tank. At sea level

pressures and with no sloshing of

vibration present, Jet A fuel, the most

common commercial jet fuel in the

United States has flammability

characteristics that tend to make the

fuel-air mixture too ‘‘lean’’ to ignite at

temperatures below approximately

100°F and too ‘‘rich’’ to ignite at

temperatures above 175°F. This range of

flammability (100°F to 175°F) is reduced

to cooler temperatures as the airplane

gains altitude due to the corresponding

reduction of pressure. For example, at

an altitude of 30,000 ft. the flammability

temperature range is approximately

60°F to 120°F. The flammability region

of Jet B (JP–4), another fuel approved for

use on most commercial transport

category airplanes but primarily used

for military jets, is in the temperature

range of 15°F to 75°F at sea level, and

-20°F to 35°F at 30,000 ft. Therefore, Jet

B fuel characteristics result in

flammable fuel vapors being present

within airplane fuel tanks for a much

larger portion of the flight. Most

commercial transports are approved for

operation at altitudes in the range of

30,000 to 45,000 feet. The FAA has

always assumed that airplanes could be

operated for some portion of flights with

flammable fuel vapors in their fuel tank

ullage (the vapor space above the level

of the fuel in the tank). Commercial

transport operated in the United States,

and in most overseas locales, use Jet A

fuel, which minimizes exposure to

operation in the flammability region.

The FAA philosophy regarding

flammable fuel vapors is that the best

way to ensure airplane safety is to

preclude ignition sources within fuel

tanks. This philosophy includes

application of fail safe design

requirements to fuel tank components

(lightning design requirements, fuel

tank wiring, fuel tank temporary limits,

etc.), which would preclude ignition

sources from being present in fuel tanks

even when component failures occur.

Implementation of the NTSB

recommendations would require a

significant change in airplane design

and/or operational practices currently in

use. These changes could have major

effects on passengers and the aviation

community.

The effectiveness and feasibility of the

proposals need to be fully evaluated.

Past studies of nitrogen inerting have

shown that few benefits are provided by

nitrogen inerting of fuel tanks and that
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices 16015

the cost of these systems is prohibitive.

However, since these studies were

conducted, advances in technology for

separating nitrogen from air and

instances of tank ignition may now

make it possible to show that inerting of

fuel tanks is cost beneficial. The FAA

needs accurate information regarding

the NTSB proposals in order to prepare

a formal response to these

recommendations. This notice requests

information regarding the NTSB

proposals.

History

Since the introduction of turbine

powered transport category airplanes,

the FAA and aviation industry have

evaluated numerous techniques and

systems for reducing the severity or

occurrence of airplane fires and

explosions. The evaluations have

focused primarily on post crash

situations because reviews of service

history showed existing design

standards provided adequate protection

from fuel tank ignition from causes

other than post crash fires. The

following methods have been evaluated

for reducing the post-crash fire/

explosion hazard: (1) Crash-Resistant

Fuel Tanks and Breakaway, (2) SelfClosing Fittings, (3) Engine Ignition

Suppression System, (4) Fuel Tank

Nitrogen Inerting System, (5) Fuel Tank

Foam Filler Explosion Suppression

System, (6) Fuel Tank Chemical Agent

Explosion Suppression System, (7) AntiMisting Kerosene (AMK), (8) Fuel Tank

Vent Flame Arrestor, (9) Surge Tank

Chemical Agent Explosion Suppression

System, (10) Design to Assure Fuel

Tank-to-Engine Shutoff Valve

Activation, (11) Fire-Resistant Fuel

Tank Access Panels, and (11) Revised

Location of Fuel Tank and Engines.

All of these techniques and systems,

with the exception of mandating the

location of fuel tanks and engines, have

been or are currently being considered

by the FAA. Initial consideration with

respect to crash-resistant fuel tanks, selfclosing breakaway fittings, and engine

ignition suppression was reflected to

Advance Notice of Proposed

Rulemaking (ANPRM) No. 64–12, which

was issued in 1964 to solicit the views

of all interested persons on the

practicability, and possible regulations

for these various techniques. The FAA

concluded, after consideration of

comments submitted in response to

Notice No. 64–12, the technical

information available at that time did

not provide a sufficient basis on which

to develop precise regulatory standards.

The FAA subsequently extended its

fuel system fire safety program to

include consideration of means to

prevent fires and explosion within the

fuel tank and the tank vapor and vent

spaces. Based on information developed

by FAA-sponsored government-industry

conferences on fuel system fire safety in

1967 and 1970, and an FAA-industry

advisory committee established in 1968,

the FAA concluded that there are three

systems capable of preventing fuel tank

and vent system fires and explosions

arising from ignition within the fuel

system. These are fuel tank nitrogen

inerting, foam filler, and chemical agent

explosion suppression systems.

In 1969, the FAA initiated research

into the feasibility of nitrogen inerting

of fuel tanks of transport category

airplanes based on systems under

development by the military. The

systems were intended to reduce the

likelihood of a fuel tank explosion due

to a fuel tank penetration by hostile

enemy fire. The FAA interest in these

systems focused on the potential for

reducing the likelihood of fuel tank

explosion due to post crash ground fire.

The FAA contracted with the Parker

Hannifin Company for designing and

manufacturing the inerting system, and

for installation in the DC–9 aircraft

under subcontract to Lockheed Aircraft

Services Company. The system

consisted of storage bottles, pressure

regulating hardware, and the

installation of valves to maintain a

constant positive pressure and the

desired concentration of nitrogen in the

fuel tanks. The combined system weight

was 643 pounds. Results of the testing

showed that the system provided

adequate inerting of the fuel tanks.

However, the penalty in airplane

performance due to increased weight

and maintenance costs was very high

and the costs of such a system were

shown to outweigh the benefits at that

time.

Since these studies were conducted,

new military nitrogen inerting designs

have been developed and are installed

in all Air Force C–5 and C–17 military

transport category airplanes, the F–22

fighter and the V–22 tiltrotor. Foam

filler explosion suppression systems are

installed in a variety of military

airplanes. Chemical agent explosion

suppression systems are installed in the

surge tanks of several civil transport

category airplanes. These systems are

intended to provide protection against

fuel tank ignition from external sources,

hostile enemy fire in the case of the

military aircraft, and lightning in the

case of the chemical agent explosion

suppression systems installed on civil

transports.

In 1971, NTSB Recommendation A–

71–59 requested action to require ‘‘fuel

system fire safety devices which will be

effective in prevention and control of

both inflight and post crash fuel system

fires and explosions.’’ This

recommendation resulted from an

accident in 1971 in New Haven,

Connecticut, where 27 of 28 passengers

survived the initial ground impact but

died due to post crash fire/explosion. In

1972, the Aviation Consumer Action

Project petitioned for rulemaking

requesting action to require nitrogen

fuel tank inerting systems on all

transport category airplanes. Based on

these requests, the FAA issued Notice of

Proposed Rulemaking (NPRM) No. 74–

16, which proposed fuel tank inerting in

transport category airplanes. The

majority of comments received opposed

this proposal because it was argued that

the explosion prevention systems would

have little or no effect in reducing the

fire and explosion hazards of impactsurvivable accidents when a fuel tank is

ruptured. Comments received and

subsequent cost benefit analysis showed

that fuel tank explosions had occurred

due to post crash fire ignition of fuel

tanks that remained intact and the

ignition of the fuel tank was caused by

propagation of fire through the fuel tank

vent system. However, no clear benefits

could be shown for the use of an

inerting system in the prevention of

ignition of fuel tanks. In addition, with

technology available at that time,

nitrogen inerting was not considered

feasible because: (1) inerting is not

effective in the majority of accidents

because fuel tank rupture occurs and

suppression of the fire would not occur

due to ignition from sources outside the

tank; and (2) in accidents where intact

fuel tank explosions occurred, it was

determined that installation of flame

arrestors in the vent lines would

eliminate the ignition source and offer

a lower cost means of reducing the

likelihood of post crash explosion. In

view of these comments, the FAA

concluded that a public hearing should

be held to obtain information needed to

determine whether a requirement

should be developed to reduce the fire

and explosion hazards to both inflight

and impact-survivable accidents.

In 1978, the FAA established a

Special Aviation Fire and Explosion

Reduction (SAFER) Advisory

Committee to recommend ways to

improve survivability in the post-crash

environment. The SAFER committee

reviewed service history at that time

and evaluated numerous potential

methods of reducing the incidents of

post crash fire and fuel tank explosions.

The committee concluded that nitrogen

inerting provided little or no benefit and

was very costly. The Aerospace
16016 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

Industries Association estimated that

total installation and operational costs

through 1996 would be 19 billion

dollars.

The FAA research and development

testing showed that, during simulated

ground fire conditions, a fuel tank

explosion would not occur from an

under-wing fire as long as a small

volume of fuel remained within the fuel

tank. Therefore, only minimal benefits

could be shown. Two other methods for

reducing post crash fires; incorporation

of flame arrestors in fuel tank vents and

incorporation of a method for shutting

down fuel to the engines using both the

normal and emergency shutdown

means, were recommended by the

SAFER Committee. In addition, initial

testing of Anti Misting Kerosene showed

promising potential for reducing post

crash fires. Therefore, NPRM 74–16 was

withdrawn because other methods for

reducing post crash fires were

determined to be more practical and

effective.

Fuel Tank Ignition Experience

During the SAFER Committee’s

evaluation of the methods of reducing

post crash fires, the service history of

fuel tank explosions was prepared. A

list of civilian transport category

airplane accidents was compiled that

included fuel tank explosions resulting

from post crash ground fires. In

addition, during evaluation of the

benefits of nitrogen inerting systems as

proposed in NPRM 74–16, a list of fuel

tank explosions that occurred during

normal operations was prepared.

Experience on military aircraft was not

included in the SAFER committee

review. Evaluation of data available at

that time indicated that three accidents

resulted from fuel tank explosion

inflight where benefits of nitrogen

inerting could be claimed. In two of

these cases, design modifications were

made to eliminate the source of ignition.

The remaining case resulted from an

uncontrolled engine fire, and

improvement in engine fuel shutoff

features was incorporated to address

this issue. Therefore little or no benefit

could be shown for requiring nitrogen

inerting.

However, in the almost 20 years since

the SAFER Committee

recommendations were issued,

additional incidents of fuel tank ignition

have occurred. The FAA has compiled

an updated list of incidents of fuel tank

ignition that includes three inflight

incidents evaluated by the SAFER

Committee, other related events from

that time period, recent events, and also

military experience. A review of the

data shows that fuel tank ignition and

explosion events have occurred in all

portions of airplane operations and

maintenance. The majority of the events

have occurred in tanks loaded with JP–

4 fuel, a fuel type that produces

flammable vapors at lower temperatures

and a consequent increase in exposure

to ignition for typical airplane

operations. The cause of many of the

military accidents can be traced to a

combination of using JP–4 fuel and

maintenance or design practices that

differ from that of commercial airplanes.

It should be noted that the military has

phased out use of JP–4 fuel within the

United States and adopted JP–8, a fuel

similar to Jet A–1, as a replacement fuel.

However, the significant number of

military fuel tank explosion events in

relation to the number of total operating

hours indicates that use of more volatile

fuels increases the likelihood of fuel

tank ignition.

The following list includes incidents

where a specific cause was identified

and improved design standards have

prevented reoccurrence of incidents due

to these causes. The list should be

reviewed carefully when using the data

to derive benefits from implementing

the proposed NTSB safety

recommendations.

(a) COMMERCIAL FUEL TANK EXPLOSION/IGNITION EXPERIENCE

Model Operator/location Year Fatal Hull

loss

Fuel

type

Inerting

benefit

Phase of

operation Description/Cause

B707 .............. OSO ................. 1959 4 Yes UNK Yes Flight .................

B707 .............. Elkton ............... 1963 81 Yes JP–4 Yes Flight ................. Lightning, In flight explosion.

B707 .............. San Francisco .. 1965 0 Yes Jet A Possible Flight ................. #4 Engine fire heated wing upper surface above 900F—Partially full fuel

tank exploded resulting in loss of 21

ft. of wing. Landed safely.

B727 .............. Southern Air

Transport-Taiwan.

1964 1 No Jet A No Ground maintenance.While purging center tank for entry,

static discharge from CO2 Firex

Nozzle to center tank access door

caused wing tank explosion.

B727 .............. Minneapolis ...... 1968 0 No Jet A Yes Ground refueling Electrostatic Charge—Ground refueling system found as source of

charging—minor damage to wing

structure. Group equipment and airplane refueling system design

standards have eliminated reoccurrence.

B727 .............. Minneapolis ...... 1971 0 No Jet A Yes Ground refueling See Above.

DC–8 ............. Toronto Canada 1970

July

106 Yes JP–4 Yes Flight ................. Spolier deployed. Possible fuel tank

explosion during go-around following ground impact during attempted

landing.

DC–8 ............. Travis AFB ....... 1974 1 Yes JP–4 No Ground ............. World Airways DC–8 inboard main

tank, exploded and burned at Travis

AFB during maintenance. Open fuel

cell, mechanic forced circuit breaker

in.

DC–9 ............. Air Canada ....... 1982 0 Yes Jet A–1 Possible Ground maintenance.During maintenance center wing fuel

tank exploded. Dry running of

pumps suspected cause.
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices 16017

(a) COMMERCIAL FUEL TANK EXPLOSION/IGNITION EXPERIENCE—Continued

Model Operator/location Year Fatal Hull

loss

Fuel

type

Inerting

benefit

Phase of

operation Description/Cause

Beechjet 400 .. Jackson Miss .... 1989

June

0 No JP–4/

Jet A

Yes Ground Refueling.During refueling of auxiliary tank ignition occurred. Tank remained intact

but fuel leakage occurred. Electrostatic Charge discharge from polyurethane foam source of Ignition.

B727 .............. Avionca ............. 1989 107 Yes Jet A Possible Climb ................ Bomb located over center wing fuel

tank. Inerting benefit unknown.

B737 .............. Philippine Airlines.1990 8 Yes Jet A Yes Taxi ................... Not determined—Empty Center Wing

Fuel tank explosion.

B747 .............. TWA 800 .......... 1996

July

230 Yes Jet A Yes Climb ................ Bomb, Missile, Mechanical Failure?—

Empty center wing fuel tank explosion.

(B) MILITARY NON-COMBAT FUEL TANK EXPLOSION/IGNITION EXPERIENCE

Model Operator/location Year Fatal Hull

loss

Fuel

type

Inerting

benefit

Phase of operation Description/Cause

B52 ................ Loring AFB

Maine.

1970 July 0 Yes JP–4 Yes Maintenance ..... Most likely ignition source traced

to arcing or overheat of fuel

pump shaft or fuel quantity

probe.

B707 .............. USAF Spain ..... 1971 June Yes Yes JP4 Yes Decent 17K ...... Inflight explosion of #1 Main

Tank. USAF determined chafing of boost pump wires located in conduits as possible

ignition source.

B52H .............. Minot ND AFB .. 1975 Nov 0 Yes JP–4 Yes Maintenance

Prior to Refueling.

Body tank exploded after midnight while on ramp. No specific evidence but suspected

fuel pump locket rotor ignition

source.

B747 .............. Iranian Fuel

Tanker.

1976 7 Yes JP–4/

Jet A

Yes Decent 8K ft ..... Lightning—wing tank.

KC135Q ......... Plattsburg AFB

NY.

1980 Feb .......... Yes JP–4 Yes Refueling .......... Aft body tank, faulty fuel probe

found as problem.

B52G ............. Robins AFB

Georgia.

1980 Aug Yes Yes JP–4 Yes Maintenance on

ramp.

While transferring fuel from body

tanks to wing tanks the empty

mid body tank exploded. Investigation showed electrical

arcing occurred in the mid

body boost pump due to mis

positioned phase lead wire inside the pump.

KC135A ......... Near Chicago ... 1982 March Yes Yes JP–4 Yes 12K descent ..... Forward body tank exploded, initial cause listed as VHF antenna.

B52G ............. Grand Forks

AFB ND.

1983 Jan .......... Yes JP–4 Yes Maintenance on

ramp.

While troubleshooting a fuel

transfer malfunction center

wing tank exploded due to an

electrical fault associated with

the EMI filter on a valve.

KC135A ......... Altus AFB Okl ... 1987 Feb Yes Yes JP–4 Yes Landing roll out During landing roll out an explosion and fire occurred following copilot transmission on

UHF radio. The UHF wire run

near the right aft wing root in

the fuselage was melted due

to an electrical fault. Fuel vapors in the area of the aft

body tank were ignited.

B52H .............. Swayer AFB

Mich.

1988 Dec Yes Yes JP–4 Yes During touch

and go landing.

At 20 feet AGL the empty aft

body tank exploded. Pump

num operating in the aft body

tank was cause. Evidence of

arcing a overheat was found.
16018 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

(B) MILITARY NON-COMBAT FUEL TANK EXPLOSION/IGNITION EXPERIENCE—Continued

Model Operator/location Year Fatal Hull

loss

Fuel

type

Inerting

benefit

Phase of operation Description/Cause

KC135A ......... Loring AFB

Maine.

1989 Sept Yes Yes JP–4 Yes Parked following

flight.

During system shutdown explosion in the aft fuselage tank

occurred. Source of ignition

was believed to be a hydraulically driven fuel pump mounted inside the aft body fuel

tank.

KC135A ......... Loring AFB

Maine.

1989 Oct Yes Yes JP–4 Yes In flight local

pattern.

Explosion in the aft body fuel

tank caused hull loss. Aft

body f hydraulically driven

pump implicated as source of

ignition.

KC135R ......... Mitchell Field

Milwaukee.

1993 Dec Yes Yes JP–4 Yes Ground maintenance.During maintenance center wing

tank exploded. Center wing

fuel tank fuel pump implicated

as source of ignition.

National Transportation Safety Board

Recommendations: The following text is

from NTSB letter to the FAA dated

December 13, 1996, that transmitted

Recommendations A–96–174 through

–177.

On July 17, 1996, about 20:31 eastern

daylight time, a Boeing 747–131,

N93119, operated as Trans World

Airlines Flight 800 (TWA800), 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 flight engineer

from the previous flight remembered

having left about 300 pounds, or about

50 gallons, of fuel in the approximately

13,000 gallon capacity tank. The

recovered fuel gauge indicated slightly

more than 600 pounds (about 100

gallons) of fuel remaining in the CWT.)

The CWT was nearly empty.

A substantial portion of the airplane

wreckage has been recovered from the

ocean floor. Among the debris found

along the first part of the wreckage path

were CWT parts from spanwise section.

The cockpit of the airplane and pieces

of the forward fuselage were found in a

second debris field that was more than

a 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 (includes portions of the

fuselage structure from above, air

conditioning packs and ducting from

below, wing structure from both sides,

all tires from behind, and numerous

components that included the large

fiberglass water and cargo fire

extinguisher containers from forward of

the CWT) from in and around the CWT

recovered and identified to date contain

no evidence 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.

Since 1985, the Board has

investigated or assisted in the

investigation of two other fuel tank

explosions involving commercial

transport category airplanes. The most

recent accident involved a Philippine

Airlines Model 737–300 at Nimoy

Aquino International Airport, Manila,

Philippines, on May 11, 1990. In the

accident, the CWT ullage (In a fuel tank,

the ullage is the vapor-laden space

above the level of the fuel in the tank.)

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 95°F, 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 Board also assisted in the

investigation of the crash of Avianca

Flight 203, a Model 727, 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, about 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 Board conducted a special

investigation of the May 9, 1976,

explosion and in-flight separation of the

left wing of an Iranian Air Force Model

747–131, as it approached Madrid,

Spain, following a flight from Iran.

Witnesses reported seeing a lightning

strike to the 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 Board’s report

(NTSB–AAR–78–12. The 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

events and possible causes of the

accident were presented in the Board’s

report.) 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 identify 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
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices 16019

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 sufficient for ignition and

temperatures between the lower

explosive (flammability) limit (LEL)

(Marks’ Standard Handbook for

Mechanical Engineers, Eighth Edition,

states, ‘‘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 temperature at which chemical

reaction proceeds at a rate sufficient to

result eventually (long time lag) in

inflammation.’’ In the TWA800 CWT,

the LEL was about 115°F, and the

autoignition temperature was about

440°F.) and upper explosive limit (UEL),

which will result in a combustible

mixture of fuel and air. Current FAA

regulations require protection against

the ignition of fuel vapor by lightning,

components hot enough to create an

autoignition, and parts or systems

failures that could become sources of

ignition. Specifically: (1) Fuel system

lightning protection. The fuel 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.

(§ 25.954), and (2) Fuel Tank

Temperature. (a) The highest

temperature allowing a safe margin

below the lowest expected autoignition

temperature of the fuel in the fuel tanks

must be determined. (b) Not at any place

inside any fuel 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. (§ 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

fuel that is below its flash point

temperature. This is particularly true

with a fuel mist where small droplets

require little energy to heat up.’’

(Society of Automotive Engineers (SAE)

Technical Paper Series 901949,

Flammability of Aircraft Fuels, by N.

Albert Moussa, Blaze Tech Corp.,

Winchester, Massachusetts, as presented

at the Aerospace Technology

Conference and Exposition, Long Beach,

California, on October 1–4, 1990.)

Elevated, possibly extremely high local

temperatures would have been

associated with the lightning strike of

the Iranian Model 747 in 1976.

Despite the current aircraft

certification regulations, airlines, at

times, operate transport category

turbojet airplanes under environmental

conditions and operational

circumstances that allow the

temperature in a fuel tank ullage to

exceed the LEL, thereby creating a

potentially explosive fuel-air mixture.

For example, on August 26, 1996,

Boeing conducted flight tests with an

instrumented Model 747 airplane that

carried about the same small amount of

fuel in the center wing tank as that

carried aboard TWA800. All three air

conditioning packs were operated on

the ground for about 2 hours to generate

heat beneath the CWT. 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 CWT ullage

was stabilized at a temperature below

the LEL on the ground. However, as the

airplane climbed, the 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. (FAA Advisory

Circular (AC) 25.1309–1A, System

Design and Analysis, paragraph 5.a.1

states, ‘‘In any system or subsystem, the

failure of any single element,

component, or connection during any

one flight (brake 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

significantly reduce the capability of the

airplane or the ability of the crew to

cope with the resulting failure

conditions.’’)

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 (nitrogeninerting) in fuel tank ullage, there by

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 Board is aware

that currently incorporate nitrogeninerting systems to reduce the potential

for fuel tank fires and explosions.

Nitrogen-inerting has been

accomplished several ways: (1) By

adding nitrogen to fuel tank(s) from a

ground source before flight; (2) By

charging onboard supplies of

compressed or liquefied nitrogen in

flight; or (3) 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 ‘‘on-condition’’ maintenance.

Nitrogen-inerting using a ground source

of nitrogen might prevent explosions

such as those that occurred to the

TWA800 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 Model

747 accident. Nitrogen-inerting fuel tank

ullage has been used for more than 25

years in military airplanes and could be

used to protect commercial air

transportation. However, the Board

recognizes that development and

installation of such systems are

expensive and may be impractical

because of system weight and

maintenance requirements in some

airplanes.

Therefore, the 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

(Airplanes other than the Model 747

also have heat-producing equipment in

the vicinity of fuel tanks. For example,

the A–320 and other Airbus Industries

commercial transport category airplanes

are similar to those from Boeing in that

the air conditioning packs and ducts are

beneath the CWT.) that are now located
16020 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

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 Board believes that the

FAA should require the development

and implementation of design or

operational changes that will preclude

the operation of transport category

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

modifications 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 carrying additional fuel.

Therefore, by monitoring fuel quantities

and temperatures (when so-equipped),

by controlling the use of air

conditioning packs and other heatgenerating 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 operations in the Model

747. The 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 Model 747, consideration

should be given to refueling the CWT

before flight whenever possible from

cooler ground fuel tanks, proper

monitoring and managing of the CWT

temperature, and maintaining an

appropriate minimum fuel quantity in

the CWT.

The Board has also found that the

Trans World Airlines 747 Flight

Handbook used by crewmembers

understates the extent to which the air

conditioning packs can elevate the

temperature of the Model 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, Model 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 TWA800 would

have raised the temperature of the

ullage above the LEL of its fuel-air

mixture. The handbook also states,

‘‘warm fuel . . . may cause pump

cavitation and low pressure warning

lights may come on steady or flashing.’’

The Board is concerned that the flight

handbooks of other operators of the

Model 747 may have similar

deficiencies, Therefore, the Board

believes that the FAA should require

that the Model 747 Flight Handbooks of

TWA and other operators of Model 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 Model 747 Flight

handbook (and the Boeing Airplane

Flight Manual) instruct flightcrews not

to exceed fuel temperatures of ‘‘54.5C

(130F), except JP–4 which is 43C

(110F),’’ the only fuel tank temperature

indication displayed for flightcrews is

that of the outboard main tank in the left

wing. The designs of the Model 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 Board

believes that flightcrews need to

monitor the temperature of fuel tanks

that are located near heat sources,

including the CWT in Model 747s.

Therefore, the Board believes that the

FAA should require modification of the

CWT of Model 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 Board recommends that

the FAA:

(1) Require the development of and

implementation of design or operational

changes that will preclude the operation

of transport category airplanes with

explosive fuel-air mixtures in the fuel

tanks:

(a) Significant consideration should

be given to the development of airplane

design modification, such as nitrogeninserting 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 in

operational procedures to reduce the

potential for explosive fuel-air mixtures

in the fuel tanks of transport category

aircraft. In the Model 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 fuel temperature, and maintaining

an appropriate minimum fuel quantity

in the CWT. (Urgent) (A–96–175)

(2) Require that the Model 747 Flight

Handbooks of TWA and other operators

of Model 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)

(3) Require modification of the CWT

of Model 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. (A–96–177)

Chairman Hall, Vice Chairman

Francis, and Members Hammerschmidt,

Goglia, and Black concurred in these

recommendations.

FAA Discussion of NTSB

Recommendations: The discussion that

follows provides additional information

and clarification of the NTSB

recommendations.

As part of the discussion providing

the background for the

recommendations, the NTSB letter cites

§ 25.954, Fuel system lightning

protection, and § 25.981, Fuel tank

temperature, of 14 CFR part 25. The

letter then states, ‘‘Despite the current

aircraft certification regulations,

airlines, at times, operate under

environmental conditions and

operational circumstances that allow the

temperature in a fuel tank ullage to

exceed the LEL (lower explosive limit),

thereby creating a potentially explosive

fuel-air mixture. 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.’’ A footnote is

then made referring to FAA Advisory

Circular (AC) 25.1309–1A.
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices 16021

These statements in the NTSB letter

appear to indicate a belief that the

airworthiness standards of part 25 do

not allow operation of airplanes with

flammable vapors in the fuel tank

ullage. In fact, the FAA has never

attempted to preclude the operation of

transport category airplanes with

flammable fuel-air mixtures in the fuel

tanks. Section 25.981 requires that the

temperature of fuel in a tank on

transport category airplanes be below

the lowest expected auto ignition

temperature of the fuel; not below the

lower explosive limit. The auto ignition

temperature is the temperature at which

spontaneous ignition of the fuel will

take place, which, for aviation turbine

fuels, is in the range of 440°F to 490°F.

Section 25.961 requires that the fuel

system (e.g. pumps, valves etc.,) operate

satisfactorily in hot weather. No

regulation or policy currently in place is

intended to prevent the operation of

transport category airplanes with a

flammable fuel-air mixture in the fuel

tanks.

Based on the flammability

characteristics of the various fuels

approved for use on transport category

airplanes, it has always been assumed

by the FAA that airplanes may operate

during some significant portion of the

flight with flammable mixtures in their

fuel tank ullage. The FAA has

considered that design features which

are intended to preclude the presence of

an ignition source within the fuel tanks

would provide an acceptable level of

safety.

The NTSB statements also appear to

indicate that the FAA has knowingly

approved transport airplane fuel

systems which have the potential for

single failures to create an ignition

source in the fuel tanks. In fact, the FAA

has not knowingly approved any such

fuel systems. At the time of its

certification, the Model 747 fuel system

design was found to comply with 14

CFR 25.901(b)(2), which stated, ‘‘The

components of the installation must be

constructed, arranged, and installed so

as to ensure their continued safe

operation between normal inspections

and overhauls.’’ It was also found to

comply with § 25.1309(b), which stated,

‘‘The equipment, systems, and

installations whose functioning is

required by this subpart (F) must be

designed to prevent hazards to the

airplane if they malfunction or fail.’’

While the current versions of

§§ 25.901(c) and 25.1309(b) (and AC

25.1309–1A) did not exist at the time of

application for the Model 747 type

certificate and were therefore not part of

the Model 747 certification basis, the

FAA did apply §§ 25.901(b) and

25.1309(b), as they existed at that time,

in a manner that was intended to

require a fuel system which was fail-safe

(i.e., single failures cannot be

catastrophic) with respect to the

creation of ignition sources inside the

fuel tanks. On the Model 747, the

approval of the installation of

mechanical and electrical components

inside of the fuel tanks was based on a

system safety analysis and component

testing that showed: (1) mechanical

components were fail safe, and (2)

electrical devices would not create arcs

of sufficient energy to ignite a fuel-air

mixture in the event of a single failure

or a probable combination of failures.

The FAA approved the Model 747

fuel system, as well as many other

transport airplane models, on this basis.

The operational situation and the fuel

tank temperature and loading

conditions that existed in the center

wing tank of the TWA airplane in the

hours leading up to the accident were in

no way unique. During warm and hot

weather, most commercial transport

category airplanes operate with

flammable vapor within center wing,

auxiliary, and main fuel tanks. Model

747 airplanes operating on many routes

are regularly operated without mission

fuel in the center wing tank. One to

three air conditioning packs are

normally operated on the airplane once

the flightcrew is on board, depending on

outside air temperature and passenger

load, and extended delays in warm or

hot weather have occurred many times

since the Model 747 was certificated in

1970. The obvious difference on the day

of the accident was that an ignition

source of some sort made contact with

the flammable mixture in the center

wing tank.

The FAA has examined the service

history of the Model 747 and other

transport category airplane models and

has performed a preliminary analysis of

the history of fuel tank explosions on

civil transport category airplanes and on

military transport category airplanes

which are based on a civil airplane type.

While there were a significant number

of fuel tank fires and explosions that

occurred during the 1960’s and 1970’s

on several airplane types, in most cases

the fire or explosion was found to be

related to maintenance errors or

improper modification of fuel pumps

which provided an ignition source.

Some of the events were apparently

caused by lightning strikes, including

the 1976 Imperial Iranian Air Force 747

accident in Spain. In almost every case,

the ignition source was identified and

actions were taken to prevent similar

occurrences. Because of the lessons

learned from these events, the transport

airplane industry has significantly

improved its capability to provide

airplanes that are fail-safe with respect

to ignition sources in fuel tanks and

which are able to maintain those failsafe characteristics over the life of

individual airplanes.

The FAA recognizes, however, that

the Philippine Airlines 737 accident in

1990 and the TWA Flight 800 accident

are inconsistent with this perceived

trend toward a very low rate of tank

explosions. While no probable cause has

yet been identified in either of these

accidents, the presence of an ignition

source originating with the accident

airplanes has not been ruled out. In

addition, it is clear that fuel tanks of all

current designs are also vulnerable to

ignition from bombs or missiles.

Therefore the FAA has initiated

evaluation of possible methods of

reducing or eliminating the potential of

fuel tank ignition. However, such

evaluation requires analyses of the

potential benefits of such design

changes in terms of accident prevention,

analyses of the additional costs to the

industry and risks to an airplane caused

by any additional systems.

Request for Information

Before initiating any action regarding

these recommendations the FAA must

determine the feasibility and the

effectiveness of any proposed methods

of reducing the potential of an explosive

fuel-air mixture within airplane fuel

tanks. The FAA therefore requests

comments in that regard from the

public, including the aviation industry,

airplane manufacturers (both domestic

and foreign), and any other interested

persons. This information may include

technical and economic data and

information, arguments pro or con

concerning technical feasibility, and any

other information deemed pertinent.

The modern commercial transport

category airplane requires maximum

safety; however, new protective features

must be justified by an increased level

of safety with minimum added

complexity, weight, and operational

constraints. Estimates of probable costs

and benefits derived from implementing

the NTSB recommendations are

important.

The following questions are intended

to solicit comments regarding the NTSB

recommendations.

Specific Questions

NTSB Recommendations 96–174 and

–175 focus on controlling fuel

temperatures within fuel tanks as a

short term method of reducing the

potential of an explosive fuel-air

mixture within fuel tanks. Nitrogen
16022 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

inerting is proposed as a longer term

methodology of reducing the potential

of an explosive fuel-air mixture. These

proposals are applicable to transport

category airplanes. Recommendations

number A–96–176 and –177 propose

revisions to airplane flight manuals to

include limitations on fuel temperatures

and incorporation of fuel temperature

indication systems to determine fuel

tank temperatures, respectively. These

two proposals are applicable to all

airplanes. Therefore, comments to the

questions below relating to

Recommendations A–96–176 and –177

should include consideration of the

appropriateness to transport category

airplanes (which would include

airplanes designed for business travel as

well as airline service) and nontransport category airplanes. The latter

would include airplanes intended for

general aviation use as well as

commuter airline service. Questions

regarding each of these proposals are

provided below. The FAA is

particularly interested in comments to

the specific questions in the following

areas:

Controlling Fuel Temperatures

Initial evaluation indicates that if the

NTSB proposal to modify airplane

operational procedures to limit fuel

temperatures was implemented, the use

of more volatile fuels such as Jet B

would likely be unacceptable. The use

of fuels produced in countries outside

the United States that are more volatile

would also likely be unacceptable under

certain conditions. In addition, the

flammability characteristics of Jet A fuel

vapors are such that fuel temperatures

would be limited throughout the flight.

For example, at an altitude of 30,000 ft.

the maximum fuel temperature would

be limited to approximately 60°F and at

an altitude of 40,000 ft. it would be

limited to approximately 50°F. When

the effects of fuel shoshing and

vibration are considered the allowable

temperature would be reduced by

approximately 10°F to 50 and 40°F

respectively. The need to limit

maximum fuel temperatures to this

value is due to the change in the

flammability temperature range with

ambient pressure as discussed earlier in

this notice. The fuel temperature limit

established for each airplane type would

vary due to differing cruise altitudes

and fuel heating differences between

airplane types. Therefore, for the

purposes of cost estimates requested in

this notice, a maximum fuel

temperature limit in the range of 50–

50°F is proposed. Within some fuel

tanks, such as the center wing tank on

many airplane types, fuel cools very

slowly because very little of the fuel

tank surface is exposed to ambient air,

and the lower tank surfaces are heated

by the air conditioning packs.

Installation of insulation to reduce

heating of the fuel, carrying reserve fuel

within the center tank and/or

transferring cooler fuel during flight, are

proposed by the NTSB as possible

means to maintain fuel temperatures

below the proposed limit value.

Refueling Fuel Tanks From Cooler

Ground Sources

While ‘‘cool’’ fuel may be available at

some airports, a survey conducted in the

1970’s of fuel temperatures from ground

sources at major worldwide airports

indicated that average fuel temperatures

were in the range of 60–65°F. Fuel

temperatures will increase in tanks

adjacent to heat sources and on warmer

days following refueling; therefore,

cooling of fuel at many airports would

likely be required to maintain fuel

temperatures below the proposed

maximum limit, which would vary with

approved maximum altitude limits of

each airplane model. The FAA is

requesting additional information/

opinions on the following:

(1) What is the maximum fuel

temperature within a fuel tank that prior

to flight would preclude a flammable

mixture of fuel within the fuel tank

during the subsequent flight?

(2) In consideration of the fuel

properties noted above, is control of fuel

temperatures a practical and effective

way to reduce the likelihood of fuel tank

explosions?

(3) Is more recent fuel temperature

data available for fuel from ground

sources at major airports worldwide?

(4) Is it technically feasible and

operationally practical to cool fuel prior

to loading into fuel tanks?

(5) Is equipment currently available

for cooling of fuel prior to or during the

airplane loading process.

Limiting Environmental Control System

(ECS) Pack Operation

The NTSB also suggests controlling

the use of ECS packs to reduce fuel

heating within the center wing tank.

The recommendation would likely

require an alternate source of cool air for

passenger comfort during ground

operations.

(1) Would it be practical to limit ECS

pack operation while on ground and

inflight to reduce heat input to the

center wing fuel tank?

(2) Is it practical to assume that

external air conditioning is available at

all international airports?

(3) If other sources of air conditioning

were required, what would be the added

recurring (including labor to monitor

fuel temperatures and cabin

temperatures) and non-recurring costs?

Carrying Additional Fuel

(1) Assuming that an airplane was

dispatched with cooler fuel and fuel

tanks were insulated from heat sources,

what would be the minimum fuel level

that would be required to maintain fuel

temperatures below that where an

explosive fuel-air mixture forms in the

tank?

(2) Would fuel transfer from other fuel

tanks with cooler fuel be a practical

means of reducing the amount of fuel

carried within the tank to maintain

temperatures below that where an

explosive fuel-air mixture forms in the

tank?

Request for Cost Information for

Limiting Fuel Temperatures

The NTSB recommendations focus on

limiting fuel temperatures primarily on

Model 747 airplanes. Many other

airplane types, such as the Boeing

Model 737, 757, 767, 777, and Airbus

A320, A330, A340, have features such

as hydraulic heat exchangers within

wing fuel tanks or ECS packs located

below the center wing fuel tank that

may result in fuel tank heating.

(1) Regarding airplane type, what

should be the applicability of the

proposed recommendations?

(2) What would be the costs

associated with:

(a) Eliminating the use of more

volatile fuels such as Jet B, and JP–4?

(b) Tankering fuel within otherwise

empty fuel tanks for the purpose of

maintaining fuel temperatures below the

flammability limits?

(c) Installing a fuel temperature

indication system within each airplane

fuel tank to monitor fuel temperatures?

(d) Cooling fuel during the fueling of

airplanes when fuel temperatures from

the airport fueling hydrant are above the

limit of 40–50°F?

(e) Insulating fuel tanks from heat

sources?

(f) Transferring from other fuel tanks

with cooler fuel, while on ground and

inflight?

(3) What are the operational

considerations of such procedures?

(4) Are there additional near term

possibilities to reduce the potential of

an explosive fuel-air mixture within fuel

tanks? For any possible methods, the

above questions should be answered.

Nitrogen Inerting

Information available from military

airplanes indicates that with currently

available technology, On Board Inert

Gas Generating Systems (OBIGGS),
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices 16023

possibly supplemented for ground

conditions with ground based nitrogen

sources, would be an effective means of

inerting fuel tanks.

Results of the FAA test and other

military tests would indicate that an

effective inerting system would require

a constant supply of nitrogen to the fuel

tank. In 1993, McDonnell Douglas

installed an inerting system on the C–

17 military cargo airplane to reduce fuel

tank ignition from penetration by

unfriendly weapons fire. The system

utilizes an on-board inerting system that

separates nitrogen enriched air (NEA)

from compressed air supplied by the

engines. Each fuel tank is continuously

supplied with NEA. The NEA is

compressed to 3,000 psi and stored in

4 tanks to provide protection for onground use. Although a more modest

system may be possible for transport

category airplanes, the feasibility of

using the C–17 system is questionable

for commercial transport category

airplanes. Total system weight is 2,146

pounds (including 328 lbs. of stored

NEA). Additionally, the system design

and hardware costs, increased fuel burn

to provide compressed air to the system,

and increased maintenance costs would

have to be factored into an assessment

of the feasibility of installing such a

system on transport category airplanes.

Although the added weight and cost

of the C–17 system may be prohibitive

for commercial transport airplane

operations, it may be possible to achieve

the desired level of safety with a more

modest inerting system. Based on

review of transport airplane operations,

the need for on-board storage of nitrogen

can be eliminated if the system is

designed for typical altitude changes

and dissolved oxygen in the fuel is

removed during the refueling process.

Therefore, for the purposes of this

notice, the FAA is assuming the

portions of the airplane operating

envelope to include only normal climb

and decent rates and that scrubbing of

oxygen from the fuel be completed

during the refueling process while the

airplane is on the ground. Possible

sources of nitrogen for the scrubbing

process may be on ground storage

systems or from the OBIGGS installed

on the airplane.

(1) What design and safety criteria

should be developed and used to define

a nitrogen inerting system providing

protection for the scenario described by

the NTSB recommendations?

(a) Would a system optimized for

normal airplane climb and decent rates

provide a desired level of safety

enhancement?

(b) Is it appropriate to allow dispatch

of an airplane with the inerting system

inoperative under minimum equipment

list requirements?

(c) Would the OBIGGS or ground

based sources be the most cost effective

source of nitrogen for scrubbing of the

fuel? What would be the costs

associated with two sources of nitrogen

for fuel scrubbing?

(2) Incorporation of nitrogen inerting

systems could result in negative impacts

on other airplane systems, and could

introduce additional safety concerns.

(a) What, if any, are the potential

safety concerns regarding

implementation of nitrogen inerting

systems (e.g., overpressurization of

airplane fuel tanks, and maintenance of

personnel entering previously inerted

tanks without appropriate breathing

apparatus)?

(b) What, if any, negative impact

could introduction of nitrogen inerting

have on airplane systems?

(3) What would be the cost of

incorporating a nitrogen inerting system

utilizing OBIGGS sized to inert the

tanks while on the ground and during

normal climb and decent conditions:

(a) Cost of the hardware?

(b) Weight of the system?

(c) Cost of maintenance of the system?

(d) Added fuel consumption to supply

bleed air to the inert gas separation

system?

(e) Cost of modifications to airplane

fuel/vent system?

(f) Cost of lost revenue due to

increased weight of airplane with

inerting system?

(g) Cost of reduced dispatch

reliability?

(h) Cost of developing inerting

systems consistent with commercial

standards of reliability?

(4) If nitrogen inerting were

implemented to reduce the potential for

fuel tank ignition, additional benefits

may result. Possible benefits include

reduction of water within fuel tanks, the

allowance of the use of more volatile

fuels, and any oxygen generated by the

OBIGGS system might be used to

replace or supplement passenger oxygen

systems.

(a) Would the reduction in water

within fuel tanks result in less corrosion

and any quantifiable reduction in

airplane maintenance?

(b) Would the reduction in water

within fuel tanks allow reduced

intervals for sumping of fuel tanks and

an associated reduction in labor costs?

(c) Would the continued use of more

volatile fuels provide a benefit,

particularly for engine starting in colder

climates?

(d) Could oxygen generated by the

OBIGGS system be used to replace or

supplement passenger oxygen systems

and provide a quantifiable benefit in

weight and costs?

(e) Several accidents have been

associated with oxygen bottles used for

the passenger oxygen system. If onboard storage of oxygen could be

reduced or eliminated by the OBIGGS,

what, if any, safety benefits would result

due to reduced potential for oxygen fed

fires?

(5) What other methods, other than

nitrogen inerting, will provide the

desired level of safety enhancement and

what costs are associated with these

methods.

Applicability

The recommendations by the NTSB

refer to transport category airplanes,

aircraft, or airplanes, and appear to use

the terms with intent. Thus, the desired

applicability of each of the NTSB

recommendations is different. These

terms have specific definitions that are

recognized throughout the aviation

industry and the FAA regulations. The

more generic term is aircraft. Part 1 of

Title 14 of the Code of Federal

Regulations defines aircraft as ‘‘a device

that is used or intended to be used for

flight in the air.’’ Airplane is a subset of

aircraft and means ‘‘an engine-driven

fixed wing aircraft heavier than air, that

is supported in flight by the dynamic

reaction of air against its wings.’’ A

transport category airplane is an

airplane that is certificated in

accordance with the airworthiness

standards of Part 25. The term

‘‘airplane’’ also includes non-transport

category airplanes such as those

intended for general aviation on

commuter airline service.

When commenting on the technical

feasibility and economic implications of

the NTSB recommendations, the FAA is

requesting that specific attention be

given to the intended scope of those

recommendations.

(1) What might be technically feasible

for a transport category airplane may not

be feasible for all aircraft. What is

technically feasible for the range of

products identified, and is there a range

where the recommendations seem

inappropriate?

(2) Transport category airplanes

include those designed for business

travel as well as those used for airline

service. The FAA is interested in

specific comments as to the feasibility of

applying some of the concepts

envisioned by the NTSB to that class of

airplanes.

(3) It is also recognized that some

airplanes and other aircraft have

reciprocating engines that use a

different and more volatile fuel than

that used by turbine engines. What
16024 Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 / Notices

unique situations does this present

relative to the NTSB recommendations?

(4) The NTSB recommendations also

distinguish in some cases between what

might be done for new designs and what

might be done for existing airplanes.

The FAA is interested in specific

comments as to the technical feasibility

and economic impacts of applying the

concepts in the NTSB recommendations

separately to newly certificated aircraft,

new production aircraft at some time in

the future, or existing aircraft in service.

Conclusion

This notice seeks information from

interested persons, including

manufacturers and users of transport

category airplanes and components, the

general public, and foreign

airworthiness authorities in determining

the feasibility of NTSB

recommendations to limit airplane

operation with explosive fuel vapors

within fuel tanks.

Issued in Renton, Washington, on March

28, 1997.

Darrell M. Pederson,

Acting Manager, Manager, Transport

Airplane Directorate, Aircraft Certification

Service, ANM–100.

[FR Doc. 97–8495 Filed 3–31–97; 12:57 am]

BILLING CODE 4910–13–M