[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  
                                                                                                                                        CO<INF>2 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. 
--------------------------------------------------------------------------------------------------------------------------------------------------------


                                             (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