[Federal Register Volume 62, Number 64 (Thursday, April 3, 1997)]
[Notices]
[Pages 16014-16024]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-8495]
[[Page 16013]]
_______________________________________________________________________
Part IV
Department of Transportation
_______________________________________________________________________
Federal Aviation Administration
_______________________________________________________________________
Fuel Tank Ignition Prevention Measures; Notice
Federal Register / Vol. 62, No. 64 / Thursday, April 3, 1997 /
Notices
[[Page 16014]]
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 fuel-air
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 nitrogen-inerting 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 deg.F and too ``rich''
to ignite at temperatures above 175 deg.F. This range of flammability
(100 deg.F to 175 deg.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 deg.F to 120 deg.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 deg.F to 75 deg.F at sea level, and -
20 deg.F to 35 deg.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
[[Page 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) Self-Closing
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) Anti-Misting 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, self-closing 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 impact-survivable 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
[[Page 16016]]
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
--------------------------------------------------------------------------------------------------------------------------------------------------------
Inerting
Model Operator/location Year Fatal Hull loss Fuel type 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- 1964 1 No Jet A No Ground maintenance.... While purging
Taiwan. 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.
[[Page 16017]]
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.
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(b) Military Non-Combat Fuel Tank Explosion/Ignition Experience
--------------------------------------------------------------------------------------------------------------------------------------------------------
Inerting Description/
Model Operator/location Year Fatal Hull loss Fuel type benefit Phase of operation 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 Body tank
Refueling. 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 At 20 feet AGL
landing. the empty aft
body tank
exploded. Pump
num operating
in the aft body
tank was cause.
Evidence of
arcing a
overheat was
found.
[[Page 16018]]
KC135A.............. Loring AFB Maine...... 1989 Sept Yes Yes JP-4 Yes Parked following During system
flight. 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 Explosion in the
pattern. aft body fuel
tank caused
hull loss. Aft
body f
hydraulically
driven pump
implicated as
source of
ignition.
KC135R.............. Mitchell Field 1993 Dec Yes Yes JP-4 Yes Ground maintenance... During
Milwaukee. 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 deg.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
[[Page 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 deg.F, and the autoignition temperature was about 440 deg.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. (Sec. 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. (Sec. 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 (nitrogen-inerting) 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 nitrogen-inerting 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
[[Page 16020]]
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
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
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 deg.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 deg.F. A 40 deg.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 nitrogen-inserting 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 Sec. 25.954, Fuel system
lightning protection, and Sec. 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.
[[Page 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 deg.F to 490 deg.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
Sec. 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 Secs. 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 Secs. 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
fail-safe 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
[[Page 16022]]
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 non-transport
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 deg.F and
at an altitude of 40,000 ft. it would be limited to approximately
50 deg.F. When the effects of fuel shoshing and vibration are
considered the allowable temperature would be reduced by approximately
10 deg.F to 50 and 40 deg.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 deg.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 deg.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 deg.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),
[[Page 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 on-ground 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 on-board 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
[[Page 16024]]
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