Reduction of Fuel Tank Flammability in Transport Category Airplanes, 70922-70962 [05-23109]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 70, Number 225 (Wednesday, November 23, 2005)]
[Proposed Rules]
[Pages 70922-70962]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 05-23109]



[[Page 70921]]

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Part II





Department of Transportation





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Federal Aviation Administration



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14 CFR Parts 25, 91, et al.



Reduction of Fuel Tank Flammability in Transport Category Airplanes; 
Proposed Rule

Federal Register / Vol. 70, No. 225 / Wednesday, November 23, 2005 / 
Proposed Rules

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 25, 91, 121, 125, and 129

[Docket No. FAA-2005-22997; Notice No. 05-14]
RIN 2120-A123


Reduction of Fuel Tank Flammability in Transport Category 
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This NPRM proposes new rules that will require operators and 
manufacturers of transport-category airplanes to take steps that, in 
combination with other required actions, should greatly reduce the 
chances of a catastrophic fuel-tank explosion. The proposal follows 
seven years of intensive research by the FAA in collaboration with 
industry into promising technologies designed to make fuel tanks 
effectively inert, thus preventing electrical and other systems from 
igniting flammable vapors in the fuel tank ullage (vapor space). The 
result of that research is that fuel tank inerting, originally thought 
to be prohibitively expensive, can now be accomplished in a reasonably 
cost-effective fashion and protect the public from future calamities 
which, we have concluded, are otherwise virtually certain to occur. The 
new rules, if adopted, would not actually direct the adoption of 
specific inerting technology either by manufacturers or operators but 
would establish a performance-based set of requirements that do not 
specifically direct the use of fuel-inerting but rather set acceptable 
levels of flammability exposure in tanks most prone to explosion or 
require the installation of an ignition mitigation means in an affected 
fuel tank. Technology now provides a variety of commercially feasible 
methods to accomplish these vital safety objectives.

DATES: Send your comments on or before March 23, 2006.

ADDRESSES: You may send comments, identified by Docket No. FAA-2005-
22997, using any of the following methods:
    DOT Docket Web site: Go to https://dms.dot.gov and follow the 
instructions for sending your comments electronically.
    Government-wide rulemaking Web site: Go to https://
www.regulations.gov and follow the instructions for sending your 
comments electronically.
    Mail: Docket Management Facility; U.S. Department of 
Transportation, 400 Seventh Street, SW., Nassif Building, Room PL-401, 
Washington, DC 20590-001.
    Fax: 1-202-493-2251.
    Hand Delivery: Room PL-401 on the plaza level of the Nassif 
Building, 400 Seventh Street, SW., Washington, DC, between 9 a.m. and 5 
p.m., Monday through Friday, except Federal holidays.
    For more information on the rulemaking process, see the 
SUPPLEMENTARY INFORMATION section of this document.
    Privacy: We will post all comments we receive, without change, to 
https://dms.dot.gov, including any personal information you provide. For 
more information, see the Privacy Act discussion in the SUPPLEMENTARY 
INFORMATION section of this document.
    Docket: To read background documents or comments received, go to 
https://dms.dot.gov at any time or to Room PL-401 on the plaza level of 
the Nassif Building, 400 Seventh Street, SW., Washington, DC between 9 
a.m. and 5 p.m., Monday through Friday, except Federal holidays.

FOR FURTHER INFORMATION CONTACT: Michael E. Dostert, FAA, Propulsion/
Mechanical Systems Branch (ANM-112), Transport Airplane Directorate, 
Aircraft Certification Service, 1601 Lind Avenue, SW., Renton, 
Washington 98055-4056; telephone (425) 227-2132, facsimile (425) 227-
1320; e-mail: mike.dostert@faa.gov.

SUPPLEMENTARY INFORMATION:

Comments Invited

    The FAA invites interested persons to participate in this 
rulemaking by submitting written comments, data, or views. We also 
invite comments relating to the economic, environmental, energy, or 
federalism impacts that might result from adopting the proposals in 
this document. The most helpful comments reference a specific portion 
of the proposal, explain the reason for any recommended change, and 
include supporting data. We ask that you send us two copies of written 
comments.
    We will file in the docket all comments we receive, as well as a 
report summarizing each substantive public contact with FAA personnel 
concerning this proposed rulemaking. The docket is available for public 
inspection before and after the comment closing date. If you wish to 
review the docket in person, go to the address in the ADDRESSES section 
of this preamble between 9 a.m. and 5 p.m., Monday through Friday, 
except Federal holidays. You may also review the docket using the 
Internet at the web address in the ADDRESSES section. Comments that you 
may consider to be of a sensitive security nature should not be sent to 
the docket management system. Send those comments to the FAA, Office of 
Rulemaking, ARM-1, 800 Independence Avenue, SW., Washington, DC 20591.
    Privacy Act: Using the search function of our docket Web site, 
anyone can find and read the comments received into any of our dockets, 
including the name of the individual sending the comment (or signing 
the comment on behalf of an association, business, labor union, etc.). 
You may review DOT's complete Privacy Act Statement in the Federal 
Register published on April 11, 2000 (65 FR 19477-78) or you may visit 
https://dms.dot.gov. Before acting on this proposal, we will consider 
all comments we receive on or before the closing date for comments. We 
will consider comments filed late if it is possible to do so without 
incurring expense or delay. We may change this proposal in light of the 
comments we receive.
    If you want the FAA to acknowledge receipt of your comments on this 
proposal, include with your comments a pre-addressed, stamped postcard 
on which the docket number appears. We will stamp the date on the 
postcard and mail it to you.

Availability of Rulemaking Documents

    You can get an electronic copy using the Internet by:
    (1) Searching the Department of Transportation's electronic Docket 
Management System (DMS) web page (https://dms.dot.gov/search);
    (2) Visiting the Office of Rulemaking's web page at https://
www.faa.gov/avr/arm/index.cfm; or
    (3) Accessing the Government Printing Office's web page at https://
www.access.gpo.gov/su_docs/aces/aces140.html.
    You can also get a copy by submitting a request to the Federal 
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence 
Avenue, SW., Washington, DC 20591, or by calling (202) 267-9680. Make 
sure to identify the docket number, notice number, or amendment number 
of this rulemaking.

Table of Contents

I. Executive Summary
II. Background
    A. The Need for Safety Improvements in Fuel Tank Systems
    B. Fuel Properties
    C. National Transportation Safety Board (NTSB) Recommendations
    D. FAA Response
III. Proposed Requirements Relating to Fuel Tank Flammability

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    A. Overview of the Proposal
    B. Ongoing Responsibility of Type Certificate Holders for 
Continued Airworthiness
    C. Applicability
    1. Manufacturers and Holders of Type Certificates, Supplemental 
Type Certificates and Field Approvals
    2. Airplanes
    3. Fuel Tanks
    4. Airplane Operators
    D. Proposed Requirements for Manufacturers and Holders of Type 
Certificates, Supplemental Type Certificates and Field Approvals
    1. New Airplane Designs
    2. Existing Airplane Designs
    3. Auxiliary Fuel Tanks
    4. Methods of Mitigating the Likelihood of a Fuel Tank Explosion
    a. Flammability Analysis Using the Monte Carlo Method
    b. Ignition Mitigation Means
    c. Flammability Reduction Means
    i. Accounting for System Reliability and Performance Issues
    ii. Warm Day Fleet Flammability Exposure
    iii. Reliability Reporting
    iv. Reliability Indication and Maintenance Access
    d. Service Instructions and Service Bulletins
    e. Critical Design Configuration Control Limitations (CDCCL)
    f. Compliance Planning
    i. Compliance Plan for Flammability Exposure Analysis
    ii. Compliance Plan for Design Changes and Service Instructions
    iii. Compliance Plan for Auxiliary Fuel Tanks
    g. Compliance Schedule
    E. Proposed Requirements for Airplane Operators
    1. Requirement to Install and Operate FRM, IMM or FIMM
    2. Authority to Operate with an Inoperative FRM, IMM or FIMM
    3. Compliance Schedule
    F. Additional Provisions
    1. Relationship of this Proposal to Aging Airplane Regulatory 
Initiatives
    2. FAA Advisory Material
    3. FAA Oversight Office
    4. Workplace Safety Issues
IV. Rulemaking Analyses and Notices
V. The Proposed Amendment

I. Executive Summary

    Fuel tank explosions have been a constant threat with serious 
aviation safety implications for many years. Since 1960, some 17 
airplanes have been destroyed as the result of a fuel tank 
explosion.\1\ Four fatal airplane accidents have been caused by fuel 
tank explosions just since 1989. Two of the more recent accidents--one 
involving a Boeing Model 747 (TWA Flight 800) off Long Island, New York 
in 1996 and the other, a Boeing Model 727 accident (Avianca Flight 203) 
in Bogot[aacute], Columbia in 1989--occurred during flight and led to 
catastrophic losses, including the deaths of 337 individuals. The two 
other recent explosions occurred on the ground but led to nine 
fatalities.\2\ Although it was determined that a terrorist's bomb had 
caused the explosion of the center tank in the Bogot[aacute] accident, 
the NTSB determined the ``bomb explosion did not compromise the 
structural integrity of the airplane; however, the explosion punctured 
the [center wing tank] and ignited the fuel-air vapors in the ullage, 
resulting in destruction of the airplane.'' Investigations of the other 
three accidents failed to identify the ignition source that caused the 
explosion. But in each instance the weather was warm, with an outside 
air temperature over 80 [deg]F, the incident occurred during the 
initial (ground, takeoff or climb) phases of flight, and the explosion 
involved empty or nearly empty tanks that had been previously fueled. 
Additionally, investigators were able to conclude that the center wing 
fuel tank in all four airplanes contained flammable vapors in the 
ullage (that portion of the fuel tank not occupied by liquid fuel) when 
the fuel tanks exploded. While the proposed requirements are not 
intended to address terrorist initiated fuel tank explosions, a system 
designed to reduce the likelihood of a fuel tank fire, or mitigate the 
effects of a fire should one occur, would have prevented these four 
fuel tank explosions.
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    \1\ None of the 17 explosions occurred on an airplane 
manufactured by Airbus, who, along with Boeing, would be most 
affected by this rulemaking. Although Airbus currently delivers more 
airplanes worldwide than Boeing, their cumulative fleet hours are 
still relatively small, at approximately 65 million (approximately 
9% of total fleet hours for all transport category airplanes). Based 
on the FAA's projection of the likelihood of an explosion based on 
one accident every 60 million hours, there is a 40% chance that no 
Airbus accidents would have occurred to date.
    \2\ Philippine Airlines 737 accident in 1990 and the Thai 
Airlines accident in 2001.
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    A statistical evaluation of these accidents has led the FAA to 
project that nine more transport category airplanes will likely be 
destroyed by a fuel tank explosion in the next 50 years, unless 
remedial measures are taken. Although we cannot forecast precisely when 
these accidents would occur, computer modeling that has been an 
accurate predictor in the past indicates these events are virtually 
certain to occur. We believe at least eight of these explosions are 
preventable if we adopt a comprehensive safety regime to reduce both 
the incidence of ignition and the likelihood of an explosion following 
ignition. We have already taken steps through other regulatory actions 
to reduce the chances of ignition. Today's proposal attempts to address 
the risk of an explosion by reducing the likelihood that fuel tank 
vapors cause an explosion when an ignition source is introduced into 
the tank.
    Since the introduction of turbine powered airplanes, the FAA has 
premised its fuel tank rules on the assumption that fuel tanks will 
always contain flammable vapors and thus the best way to prevent 
explosions is to eliminate ignition sources. Since 2001, we have 
imposed airworthiness requirements (including airworthiness directives 
or ``ADs'') directed at the elimination of fuel tank ignition sources. 
Although these measures--particularly Special Federal Aviation 
Regulation 88 of 14 CFR part 21 (SFAR 88), which requires the detection 
and correction of potential system failures that can cause ignition--
should prevent some of the nine forecast explosions, review of the 
current designs of airplanes in the transport category of all major 
manufacturers has shown that unanticipated failures and maintenance 
errors will continue to generate unexpected ignition sources. We have 
concluded we are unlikely ever to identify and eradicate all possible 
sources of ignition.
    To ensure safety, therefore, we must also focus on the environment 
that permits combustion to occur in the first place. Technology now 
exists that can prevent ignition of flammable fuel vapors by reducing 
their oxygen concentration below the level that will support 
combustion. By thus making the vapors ``inert,'' we can significantly 
reduce the likelihood of an explosion when a fire source is introduced 
to the fuel tank. Prototype onboard fuel tank inerting systems have 
been successfully flight tested on Airbus A320, Boeing Model 747, and 
Model 737 airplanes. Boeing applied in 2002 for type certification of 
an inerting system for the Model 747 that it plans to install on all 
new production 747 aircraft.
    Because the chances of a fuel tank explosion naturally correlate 
with the exposure of the tank to flammable vapors, the proposed 
requirements would mitigate the effects of such exposure or limit such 
exposure to acceptable levels by mandating the installation of either a 
Flammability Reduction Means (FRM) or an Ignition Mitigation Means 
(IMM). In either case, the technology would have to adhere to 
performance and reliability standards that would be set by the FAA and 
contained in Appendices K and L to Title 14 Code of Federal Regulations 
(CFR) part 25.
    If adopted, this rulemaking would amend the existing airworthiness

[[Page 70924]]

standards contained in 14 CFR 25.981 so as to require all type 
certificate (TC) holders and their licensees to develop FRM or IMM for 
many large turbine powered transport category airplanes with high risk 
fuel tanks. We would also amend 14 CFR parts 91, 121, 125 and 129 so as 
to require operators of these airplanes to incorporate the approved FRM 
or IMM and to keep them operational. We estimate that approximately 
3,800 Airbus and Boeing airplanes operated in the United States would 
be affected. Fuel tank system designs in several pending type-
certification applications, including the Airbus A380 and the Boeing 
Model 7E7, would also have to meet the proposed requirements.
    We acknowledge that the proposed requirements are costly and 
propose these steps only after spending several years, in cooperation 
with scientists and other experts from the affected industry, 
researching the most cost-effective ways to prevent fuel tank 
explosions. Those efforts have resulted in the development of fuel-
inerting technology that is vastly cheaper than originally thought.
    The loss of a single, fully loaded large passenger aircraft in 
flight, such as a Boeing Model 747 or Airbus A380, moreover, would 
result in death and destruction causing societal loss of at least $1.2 
billion based on prior calamities, and we project that the new rule 
would prevent four accidents of some type (for analytical purposes we 
assume the accidents would involve ``average'' aircraft with 
``average'' passenger loads) over 50 years. Such estimates of harm do 
not account for the intangible costs of a series of in-flight 
explosions (such as a loss of confidence in aviation) or the indirect 
costs (such as trip cancellations following these incidents).
    Our philosophy is to address aviation safety threats whenever 
practicable solutions are found, especially when dealing with 
intractable and catastrophic risks like fuel tank explosions that are 
virtually certain to occur. Thus, now that solutions are reasonably 
cost-effective, the Administrator has tentatively determined that it is 
necessary for safety and in the public's best interest to adopt the 
requirements proposed today. This action is in response to an NTSB 
recommendation.

II. Background

A. The Need for Safety Improvements in Fuel Tank Systems

    Fuel tank explosions continue to occur despite many safety 
improvements over the last 40 years aimed at removing ignition sources 
from fuel tanks. Experience tells us that even with the latest and most 
comprehensive initiative, SFAR 88, we cannot adequately protect the 
public from fuel tank explosions absent measures designed to lessen the 
exposure of vulnerable tanks to highly flammable jet fuel vapors. 
Fortunately, by taking such steps now to complement ignition-source 
reduction measures already taken, we are confident that fuel tank 
explosions in affected aircraft will be nearly eliminated.
    For a variety of reasons, SFAR 88, though a significant advancement 
in safety, will never provide a complete safeguard against fuel tank 
explosions; thus our analysis has assumed that SFAR 88 will not reduce 
the possibility of a fuel explosion occurring by more than 50 percent. 
To be sure, SFAR 88 has resulted in several significant changes in fuel 
tank system design and maintenance, including (1) new features to 
prevent dry running of fuel pumps within the fuel tanks; (2) use of 
ground fault protection of fuel pump power supplies for pumps or wires 
exposed to the fuel tank ullage; (3) addition of electrical bonds on 
some components; (4) use of electrical energy limiters on wiring 
entering fuel tanks that are ``normally emptied'' \3\ and located 
within the fuselage contour; (5) electrical bond integrity checks; and 
(6) improved maintenance programs. These design improvements, however, 
do not and cannot address all sources of ignition (such as external 
ignition sources resulting from fire).
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    \3\ The phrase ``normally emptied'' refers to fuel tanks that 
contain a substantial vapor space during a significant portion of 
the airplane operating time. Tanks that are designed to be normally 
emptied have been installed in various locations including the 
center wing structure, horizontal stabilizers, wings and cargo 
compartments. Fuel loading and usage management practices on certain 
airplane models use the auxiliary fuel tanks for controlling the 
center of gravity.
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    Past experience, moreover, shows that it is not possible to 
pinpoint and remove every ignition source from a large, complex 
transport aircraft. For example, the FAA is aware of one case where a 
manufacturer had conducted an exhaustive design review to identify 
possible sources of arcing within the fuel tank after a fuel tank 
exploded due to lightning. The manufacturer identified several possible 
sources of the arcing, and the FAA issued ADs to correct these 
deficiencies. The same airplane design was then evaluated as a result 
of SFAR 88, and additional sources of lightning-induced ignition were 
identified. In another instance, a TC holder submitted a safety 
analysis to the FAA claiming that certain airplane models met existing 
system safety requirements of Sec.  25.1309 and thus that the 
likelihood of an ignition source developing was extremely improbable 
(one in a billion flight hours). When the requirements of the SFAR 88 
safety review and unsafe condition criteria were applied, however, 
approximately 80 new unsafe conditions were found. These conditions 
will now be addressed by AD for those airplane models but, in 
retrospect, it was clear that the manufacturer's claims were erroneous.
    The safety reviews have also identified the potential for system 
failures (or ``failure modes'') that cannot be eliminated as possible 
ignition sources at reasonable cost. For example, use of ground fault 
protection for fuel pump power supplies will protect the fuel pumps 
from shorts to ground (such as one might find from lightning), but will 
not protect the fuel pumps from shorts between the three power wires to 
the pump, commonly referred to as ``phase-to-phase shorts.'' Currently 
there is no proven component available to address this failure mode. 
Combinations of failure modes are even more problematic. We could 
require installation of redundant bond paths to prevent the latent 
failure of a critical electrical bond, but doing so would be cost-
prohibitive.
    Finally, human error creates continuing risk. Each attempt to fix 
an electrical system presents the possibility of an inadvertent 
introduction of a new ignition source. Maintenance oversights, such as 
the failure to properly install electrical bonds or improper 
installation or overhaul of components, compound the possibility of an 
ignition source developing.
    Carrier fuel carrying practices could impact the possibility of an 
explosion as well. If a carrier decides to carry only that fuel 
necessary to meet the FAA's fuel reserve requirements, the likelihood 
of an explosion is greater than if it carries excess fuel. This 
potential exists because more ignition sources within the fuel tank are 
exposed to the ullage and because the fuel has insulating properties 
which keeps the fuel tank cooler. Thus, ``tankering'', or carrying 
excess fuel, could theoretically lower the risk of an explosion. 
Current fuel management practices, where excess fuel is carried only 
when cost beneficial to the carrier, are largely market driven because 
airlines try to minimize their fuel costs to the maximum extent 
possible. Both the FAA and industry explored mandatory refueling of 
center wing tanks after the NTSB suggested the FAA adopt an interim 
flammability reduction measure in 1996. We determined that the 
reduction in

[[Page 70925]]

flammability exposure would not be significant and would not address 
the warm day flammability risk. Thus, while either reducing or 
increasing the amount of fuel carried in the center wing tank could 
theoretically have some impact on the risk of an explosion, the FAA 
does not believe that current fuel carrying practices are likely either 
to change significantly or to have a measurable impact on the overall 
risk of an explosion. We seek comment on this position.

B. Fuel Properties

    Three conditions must be present in a fuel tank to support 
combustion and a fuel-tank explosion: Fuel vapor in the right amount, 
enough oxygen, and an ignition source. As discussed earlier, our 
regulatory efforts since piston-powered aircraft evolved into the jet 
age have been focused almost exclusively on the last item, ignition 
sources. A basic assumption in this approach has been that the fuel 
tank would contain flammable vapors under a wide range of airplane 
operating conditions. The question is, what level of exposure is safe?
    Jet fuel vapors are flammable only in certain temperature and 
pressure ranges. The flammability temperature range of such vapors 
varies with the type and properties of the fuel, the ambient pressure 
in the tank, and the amount of dissolved oxygen released from the fuel 
into the tank. The amount of dissolved oxygen in a tank will also vary 
depending on the amount of vibration and sloshing of the fuel that 
occurs within the tank. The temperature range in which a flammable fuel 
vapor will form can vary with different batches of fuel even for a 
specific fuel type, but the threshold temperature for flammability 
decreases as the airplane gains altitude because of the corresponding 
decrease of internal tank air pressure. Thus, the higher the airplane 
is flying, the lower the ambient temperature required for a fuel tank 
to explode when an ignition source introduced.
    Jet A fuel is the most commonly used commercial jet fuel in the 
United States and is widely used in other parts of the world. At sea 
level and with no sloshing or vibration present, these fuels have 
flammability characteristics that make it unlikely that the fuel 
molecules present in the fuel vapor-air mixture will ignite when the 
temperature in the fuel tank is below approximately 100 [deg]F. The 
vapor will ignite, however, once the fuel temperature reaches 
approximately 175 [deg]F, because of the increased concentration of 
fuel molecules at higher temperatures. At an altitude of 30,000 feet, 
the flammability temperature range drops to approximately 60 to 120 
[deg]F.\4\ Use of Jet A or Jet A-1 fuel thus tends to limit the risk of 
high flammability to warmer days.
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    \4\ Most transport category airplanes used in air carrier 
service are approved for operation at altitudes from sea level to 
45,000 feet.
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    Jet B (JP-4) is another fuel approved for use on most commercial 
transport category airplanes, although it is no longer used as a 
primary fuel for commercial transports. The flammability range of Jet B 
(JP-4) is about 15 to 75 [deg]F at sea level and 20 to 35 [deg]F at 
30,000 feet. Because the flammable temperature range of Jet B fuel is 
more within the range of typical air temperatures at those altitudes 
where the airplane is likely to be operated, airplane fuel tanks with 
Jet B fuel are flammable for a much larger portion of the flight.

C. National Transportation Safety Board (NTSB) Recommendations

    The NTSB determined that the probable cause of the in-flight 
explosion on TWA Flight 800 was the ignition of the flammable fuel/air 
mixture in the center wing fuel tank. However, the source of ignition 
energy for the explosion could not be determined with certainty. The 
Board also faulted, as contributing to the accident, the FAA's design 
and certification approach to transport-category airplanes, as it (1) 
concentrated solely on precluding all ignition sources, and (2) allowed 
heat sources to be located beneath the center wing fuel tank.
    In 1996, the NTSB issued recommendations to improve fuel tank 
safety. The NTSB recommended both eradicating ignition sources and 
reducing fuel tank flammability.\5\ In their accident report, the Board 
concluded that ``a fuel tank design and certification philosophy that 
relies solely on the elimination of all ignition sources, while 
accepting the existence of fuel tank flammability, is fundamentally 
flawed because experience has demonstrated that all possible ignition 
sources cannot be determined and reliably eliminated.''
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    \5\ NTSB recommendations provided on page 309 of NTSB Accident 
Report, ``In-flight Breakup Over the Atlantic Ocean, TransWorld 
Airlines Flight 800 Boeing 747-131, N93119 Near East Moriches, New 
York, July 17, 1996, Report number NTSB/AAR-00/03, DCA96MA070, 
Adopted August 23, 2000.
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D. FAA Response

    The FAA conducted ignition-prevention safety reviews following the 
1996 accident, which revealed many new single-component failure modes 
that could ignite fuel tanks. We continue to issue ADs that require 
design or maintenance actions to address these deficiencies. These 
safety reviews also identified combinations of failures that could 
result in an ignition source, but as these combinations were less 
likely to occur than single failures, we determined that it was not 
practical to address them in existing airplanes. The safety reviews 
also confirmed that unforeseen design and maintenance errors could 
create ignition sources.
    Recognizing the need to focus on flammability rather than just 
ignition, on April 3, 1997, the FAA published a notice in the Federal 
Register seeking comments on the 1996 NTSB recommendations on 
flammability exposure (62 FR 16014). That notice reviewed the service 
history of transport category airplane fuel tanks and the challenges 
underlying fuel-tank flammability reduction. Public comment indicated 
that more information was needed before we could begin a rulemaking on 
this safety issue.
    Given that control of flammable vapors was a new concept, we 
assigned two Aviation Rulemaking Advisory Committee (ARAC) working 
groups to study the issues and provide recommendations. (The ARAC 
consists of interested parties, including the public, and provides a 
process to advise us on the development of new regulations.) The first 
working group reviewed the practicality of requiring flammability 
reduction, evaluating many different flammability reduction methods. 
Upon the recommendation of the first working group, the second working 
group then focused exclusively on fuel tank inerting.
    On January 23, 1998, we published a notice in the Federal Register 
that established the Fuel Tank Harmonization Working Group as part of 
ARAC (63 FR 3614). This group was asked to recommend regulations on 
fuel tank flammability for both newly certificated and existing 
airplanes. The working group looked at fuel tank explosions that 
occurred after Jet A fuel had replaced Jet B fuel as the predominant 
type used on transport airplanes. The group examined the performance of 
two types of fuel tanks: the center wing fuel tanks located within the 
fuselage contour, and wing fuel tanks. Fuel tanks located in an 
aluminum wing are typically unheated and cool quickly when the wing 
surfaces are exposed to colder air during flight. Conversely, the 
center wing fuel tanks in certain airplanes have equipment underneath 
the tank radiating heat; in addition, with no surfaces exposed to 
outside air, the tank

[[Page 70926]]

cools much more slowly than a wing fuel tank.
    The working group concluded that the safety records of fuel tanks 
located in aluminum wings of airplanes fueled with Jet A type fuel were 
satisfactory. These tanks had an average flammability exposure (as 
calculated under a methodology contained in proposed Part 25, Appendix 
L) of approximately 2 to 6 percent. However, the group found that on 
some airplane fleets the center wing fuel tanks had an average 
flammability exposure ranging from 7 percent to a high of 30 percent, a 
dangerous level.
    The working group then evaluated many possible means of reducing or 
removing the hazards associated with explosive vapors in fuel tanks, 
such as fuel tank inerting, fuel tank cooling, fuel property 
alteration, fire suppression systems and polyurethane foam treatments. 
The ARAC sent the working group's report to the FAA on July 23, 1998 
(Docket No. FAA-1998-4183, viewable on the U.S. Department of 
Transportation electronic Document Management System at https://
dms.dot.gov).
    The working group report concluded that flammability reduction was 
practical for new airplane designs, but impractical for current 
production designs or retrofit in the current fleet of transport 
category airplanes. The report recommended that the FAA begin 
rulemaking to add a requirement to Sec.  25.981, so that fuel tanks in 
new airplane designs would have an average flammability exposure of 
less than 7 percent. The report also recommended requiring by 
regulation that each newly designed airplane incorporate means to 
mitigate the effects of an ignition of fuel vapors, such that any 
damage caused would not prevent continued safe flight and landing. The 
report reviewed various technical solutions, including control of heat 
transmission into fuel tanks, use of inerting systems, or ignition 
mitigation means like polyurethane foam. The report concluded that the 
best solution was likely to be control of heat transmission and 
suggested that the most practical means of control were (1) relocation 
of the air-conditioning equipment away from the fuel tanks; (2) 
ventilation of the air-conditioning bay to limit heating and cool fuel 
tanks; or (3) insulation of the tanks from heat. Nevertheless, the ARAC 
also recommended that we continue to evaluate the cost-effectiveness of 
other means for reducing flammable vapors in the fuel tanks, such as 
ground-based inerting of fuel tanks.
    Based in part on the ARAC recommendations, we issued a rule 
entitled ``Transport Airplane Fuel Tank System Design, and Maintenance 
and Inspection Requirements'' in the Federal Register on May 7, 2001 
(66 FR 23085). The rule added current Sec.  25.981(c) which requires 
minimization of fuel tank flammability exposure in new type designs 
without setting a specific safety standard. Section 25.981(c) thus 
states:

    (c) The fuel tank installation must include either--
    (1) Means to minimize the development of flammable vapors in the 
fuel tanks (in the context of this rule, ``minimize'' means to 
incorporate practicable design methods to reduce the likelihood of 
flammable vapors); or
    (2) Means to mitigate the effects of an ignition of fuel vapors 
within fuel tanks such that no damage caused by an ignition will 
prevent continued safe flight and landing.

    Higher flammability tanks are typically located in the center wing 
box, in the horizontal stabilizer where little surface area is exposed 
to outside air, or in the cargo compartment. Our intent, as discussed 
in that rule's preamble was to ``require that [such] fuel tanks are not 
heated, and cool at a rate equivalent to that of a wing tank in the 
transport airplane being evaluated.'' We noted that, ``This may require 
incorporating design features to reduce flammability, for example 
cooling and ventilation means, or inerting for fuel tanks located in 
the center wing box, horizontal stabilizer, or auxiliary fuel tanks 
located in the cargo compartment.'' (Our reference to a wing tank was 
to a conventional subsonic airplane with aluminum wing tanks.) We also 
stated, ``At such time as the FAA has completed the necessary research 
and identified an appropriate definitive standard to address this 
issue, new rulemaking would be considered to revise the standard 
proposed in this rulemaking.''
    We then issued two Advisory Circulars, AC 25.981-1B, ``Fuel Tank 
Ignition Source Prevention Guidelines,'' and AC 25.981-2, ``Fuel Tank 
Flammability Minimization.'' These ACs described acceptable means of 
showing compliance with Sec.  25.981(c). AC 25.981-2 specifically 
discussed the use of fuel tank inerting as a method of compliance with 
the flammability exposure requirements. To ``inert'' a fuel tank, as 
defined in AC 25.981-2, the percentage of oxygen in a fuel tank's air 
should not exceed 10 percent. (Later research, discussed below, showed 
that containing oxygen concentrations to 12 percent or less would inert 
a fuel tank.)
    After revising Sec.  25.981, we began scientific research, hoping 
to gain a better understanding of the ignition properties of commercial 
aviation jet fuel vapors. We also explored new ideas for removing 
flammable fuel air mixtures from fuel tanks, as well as other methods 
for improving fuel tank safety. Initially, efforts to develop 
commercially viable ways to remove flammable fuel vapors from tanks 
failed. For example, to lower the danger of fuel tank explosions after 
post-crash ground fires, systems were considered that would ``scrub'' 
the vapor in the ullage--ventilating the tank with air so as to prevent 
the build-up of flammable concentrations of fuel vapor. At the time, we 
found these systems to be impractical because of their weight, 
complexity, unreliability, and undesirable secondary effects on the 
environment.
    On the recommendation of the ARAC, we refocused our efforts on 
reducing fuel tank flammability through nitrogen inerting. Public 
comment on the 1997 notice had suggested inerting was possible through 
adoption of a hollow fiber membrane technology, which separates oxygen 
from nitrogen in the atmosphere. (Air is made up of about 78 percent 
nitrogen and 21 percent oxygen.) The hollow fiber membrane material 
uses the absorption difference between the nitrogen and oxygen 
molecules to separate nitrogen-enriched air from oxygen. The technology 
had been used for many years in non-aerospace applications, such as 
obtaining oxygen-enriched air for medical purposes and generating 
nitrogen-enriched air to preserve produce in transport. In airplane 
applications, nitrogen-enriched air could be produced when pressurized 
air is forced through a canister that contains the hollow fibers. The 
created nitrogen-enriched air is then directed, at appropriate 
concentrations, into the ullage of fuel tanks and displaces the normal 
fuel vapor/air mixture in the tank. Use of this technology allows 
nitrogen to be separated from the available pressurized air onboard the 
airplane, which eliminates the need to carry and store nitrogen in the 
airplane.
    Initially, we found that airplanes in the current transport 
category fleet were not designed with optimized air sources for 
creating nitrogen-enriched air. As a result, early designs required 
installation of an air compressor, adding significant weight and cost. 
Aware of the earlier system's disadvantages, our researchers worked to 
address those issues. Earlier fuel tank inerting designs, primarily 
produced for military applications to prevent fuel tank

[[Page 70927]]

explosions from battle damage, assumed a fuel tank was ``inert'' with a 
maximum of 9 percent oxygen content in the ullage. Achieving this level 
of concentration was not needed for transport category airplanes, as 
our research determined that a maximum oxygen content of 12 percent 
would be sufficient to protect airplanes from less powerful ignition 
sources typical of airplane system failures and malfunctions at sea 
level. Thus, our testing excluded turbulent flow flame propagation, or 
external fuel tank events, such as explosives and hostile fire. (The 
FAA test results are available in an FAA Technical Note: ``Limiting 
Oxygen Concentrations Required to Inert Jet Fuel Vapors Existing at 
Reduced Fuel Tank Pressures'' (DOT/FAA/AR-TN02/79). See: https://
www.fire.tc.faa.gov/pdf/TN02-79.pdf.)
    Terrorist initiated accidents were also excluded from consideration 
in the earlier ARAC reports and the possible benefits in the regulatory 
evaluation within this notice. While the proposed FRM requirements are 
not intended to address terrorist initiated explosions, such as the 
Bogata 727 accident discussed earlier, inerting fuel tanks may provide 
other significant secondary safety benefits by addressing flammability 
exposure. Testing conducted by China Lake Naval Weapons Center \6\ 
showed that inerting a fuel tank to 12 percent oxygen offers a high 
degree of protection from a fuel tank explosion when 30-millimeter high 
explosive incendiary projectiles shot into fuel tanks. The FAA invites 
comments related to the potential additional security benefits that may 
be achieved by imposing FRM.
---------------------------------------------------------------------------

    \6\ The Effectiveness of Ullage Nitrogen-Inerting Systems 
Against 30-mm High-Explosive Incendiary Projectiles, China Lake 
Naval Weapons Center, J. Hardy Tyson and John F Barnes, May 1991.
---------------------------------------------------------------------------

    Based on our research, we identified a simplified inerting system 
that, using existing airplane pressurized air sources, could limit a 
fuel tank to the 12 percent oxygen content level. This concept 
eliminated the need for an air compressor, thus reducing the size and 
complexity of the system. Our research determined that the method of 
distributing the nitrogen-enriched air to the fuel tank could also be 
simplified, which further reduced the system's weight and installation 
cost. We now estimate that a simplified inerting system adequate to 
protect the center wing tank on airplanes in the existing fleet should 
weigh from 100 to 250 pounds and cost from $140,000 to $225,000 to 
procure and install in existing airplanes, depending on fuel tank 
capacity. (More information on the costs of these systems is provided 
in the Preliminary Regulatory Evaluation.)
    The FAA has openly shared with industry information on the 
simplified inerting system design ever since it was first developed in 
May 2002. This design concept was adopted by Boeing when applying for a 
series of type certification and production approvals to incorporate a 
fuel inerting system using nitrogen air enrichment in all currently 
produced Boeing model airplanes. Thus, on November 15, 2002, Boeing 
applied for a change to TC No. A20WE to modify Boeing Model 747 series 
airplanes to incorporate the system into its center wing fuel tanks. It 
has since applied for similar approvals for the Boeing Model 737 
series, Boeing Model 757 series, Boeing Model 767 series, and Boeing 
Model 777 series airplanes. We published a request for and received 
public comments on a Notice of Proposed Special Conditions for 
flammability reduction on the Boeing Model 747 on December 9, 2003 (68 
FR 68563). Final Special Conditions No. 25-285-SC was issued on January 
24, 2005 (70 FR 7800; February 15, 2005).

III. Proposed Requirements Relating to Fuel Tank Flammability

    We are proposing today a performance-based set of requirements that 
do not specifically direct the use of fuel inerting, but rather set 
acceptable levels of flammability exposure in tanks most prone to 
explosion or require the installation of an ignition mitigation means 
in an affected fuel tank. We also by separate notice propose to revise 
Advisory Circular 25.981-2 so as to describe several means of 
compliance with these requirements, including both flammability-
reduction means, such as cooling, inerting using nitrogen or carbon 
dioxide, and ignition-mitigation means, such as use of polyurethane 
foam or explosion suppression systems. The revised AC sets out detailed 
parameters for such systems if used as a means of achieving the 
targeted safety standards.
    The rule, if adopted, would require a retrofit of much of the 
existing fleet of large airplanes but would not necessarily affect all 
transport aircraft. We will require retrofit based on safety needs, 
using a fleet average flammability exposure limit of seven (7) percent, 
the level recommended by ARAC. We know that this level is routinely 
exceeded in tanks that are incidentally heated by nearby air 
conditioning equipment and in unpressurized auxiliary fuel tanks that 
are located in the cargo compartment and that do not significantly 
cool. The vast majority of large transport category airplanes operating 
in the U.S., including all Airbus models and most Boeing models, have 
center wing tanks that are above this level. We estimate that 3,800 
airplanes with flammability exposure level above 7 percent would be 
retrofitted if this rule is adopted.
    As is the case for new production airplanes, all airplanes 
currently equipped with a normally emptied or auxiliary fuel tanks that 
have a flammability level above 7 percent could not have center wing 
tanks that are flammable more than 3 percent on average and 3 percent 
on hot days. Lowering the flammability levels of these fuel tanks in 
the existing fleet and limiting the permissible level of flammability 
on new production airplanes would result in an overall reduction in the 
flammability potential of these airplanes of approximately 95 percent.
    Some airplane models have center tanks with a fleet average 
flammability exposure level that does not exceed 7 percent, including 
to the best of our information the Lockheed L-1011, and Boeing MD-11, 
DC10, MD80, and Boeing Model 727, and Fokker F28 MK100. At this time we 
do not believe that these airplanes would need FRM or IMM for their 
center tanks, unless the certificate holder has also installed an 
auxiliary fuel tank that is found to be affected.\7\
---------------------------------------------------------------------------

    \7\ Auxiliary fuel tanks are installed subject to amended 
supplemental type certificates or field approvals. As such they are 
``aftermarket'' installations not contemplated by the original 
manufacturer of the airplane. Auxiliary fuel tanks are installed to 
permit airplanes to fly for longer periods of time by increasing the 
amount of available fuel. While all auxiliary fuel tanks are 
normally emptied, some ``normally emptied'' tanks are included in 
the original type design, such as the center wing tank on the Boeing 
747.
---------------------------------------------------------------------------

A. Overview of the Proposal

    Our proposal would require manufacturers and operators of most 
large transport category airplanes to reduce the average flammability 
exposure in affected fleets to tolerable levels of risk. Fleet average 
flammability exposure represents the percent of flight time that fuel 
vapors in the ullage are flammable, calculated across a fleet of an 
airplane type operating over the range of actual or expected flights 
and based on a wide range of environmental conditions and fuel 
properties.\8\ This

[[Page 70928]]

rulemaking is premised on our finding that fuel tanks whose fleet-wide 
average flammability exposure is more than 7 percent have a ``high 
flammability exposure,'' which we consider unduly dangerous. This 
finding, in turn, is based on the reports and findings of the ARAC and 
our own risk assessment of the current transport category airplane 
fleet.
---------------------------------------------------------------------------

    \8\ The airplane flammability exposure evaluation time begins 
when the airplane is prepared for flight (which commences upon the 
start of preparing the airplane for flight by turning on the 
auxiliary power unit/ground power, starting the environmental 
control systems, or taking other steps that begin the initial 
preparation of the airplane), continues through the actual flight 
and landing, and ends when all payload has been unloaded and all 
passengers and crew have disembarked.
---------------------------------------------------------------------------

    Our proposal would modify current regulations in several important 
respects, affecting both manufacturers (TC holders and STC holders) and 
operators (air carriers). We would significantly expand the coverage of 
part 25 by making manufacturers generally responsible for the 
development of service information and safety improvements (including 
design changes) where needed to ensure the continued airworthiness of 
previously certificated airplanes. This proposal would apply to holders 
of existing TCs, holders of STCs, applicants for changes to existing 
TCs, and certain other airplane manufacturers. We are proposing to 
specify the new requirements for these entities in a new subpart I to 
part 25, although we may decide to relocate these requirements at the 
time the final rule is issued to simplify harmonization efforts.
    As to fuel tank flammability specifically, manufacturers, including 
holders of listed airplane TCs and of auxiliary fuel tank STCs, would 
be required to conduct a flammability exposure analysis of their fuel 
tanks, unless they have already notified the FAA that they will utilize 
an ignition mitigation means instead. A new Appendix L to part 25 will 
regulate the conduct of these analyses.\9\ As discussed later in this 
document, the Appendix contains the method for calculating overall and 
warm day fuel tank flammability exposure values needed to show that the 
affected aircraft tanks comply with proposed limitations on 
flammability exposure levels, described below.
---------------------------------------------------------------------------

    \9\ Rather than relying on the analysis already conducted 
pursuant to SFAR 88 and then simply regulating those airplanes with 
a demonstrated exposure level of 7 percent or greater, today's 
proposal contemplates requiring a new exposure analysis. The 
existing analyses, while helpful in positing which airplanes are 
likely to be affected by a final rule, were derived from incomplete, 
and sometimes differing, assumptions. Appendix L would correct such 
inconsistencies by establishing a single methodology for calculating 
average flammability exposure.
---------------------------------------------------------------------------

    Where the required analyses indicate that the fuel tank has an 
average flammability exposure level below 7 percent, no changes would 
be required. However, for the other fuel tanks, manufacturers would be 
required to develop design modifications to support a retrofit of the 
airplane. Under today's proposal, the average flammability exposure 
level of any affected wing tank would have to be reduced to no more 
than 7 percent. In addition, for any normally emptied fuel tank 
(including auxiliary fuel tanks) located in whole or in part in the 
fuselage, flammability exposure would have to be reduced to 3 percent, 
both for the overall fleet average and for operations on warm days.
    For long-pending certification projects that have not received a 
type certificate from the FAA prior to the date of the final rule 
(where application was received by the FAA before June 6, 2001, the 
effective date of 14 CFR 25.981(c), applicants would be required to 
limit the flammability exposure of any wing tank to no more than 7 
percent. Any of those applicants whose proposals include any normally 
emptied or auxiliary fuel tank with a flammability exposure level that 
exceeds 7 percent would also have to meet the same flammability 
exposure requirements proposed for retrofit (i.e., 3 percent), if any 
portion of the tank is located within the fuselage contour. Applicants 
for more recent certification projects (where application was received 
after June 6, 2001), and all applicants for a TC or STC submitted after 
the effective date of the final rule would need to meet the new 
requirements of that section set forth in today's proposal.
    We would set more stringent safety levels for certain critically 
located fuel tanks in most new type designs, while maintaining the 
current, general standard under Sec.  25.981 for all other fuel tanks. 
We expect that as a result of this rule the design of most normally 
emptied and auxiliary tanks located, in whole or in part, in the 
fuselage of transport-category airplanes would need to incorporate some 
form of FRM or IMM. Regulations in a new proposed Appendix K to Part 25 
contain detailed specifications for all FRM, if they are used to meet 
the flammability exposure limitations. These additional requirements 
are designed to ensure the reliability of flammability-reduction means, 
reporting of performance metrics and warnings of possible hazards in 
and around fuel tanks. Specifications for IMM are detailed in the 
current AC-25.981-2 and are not generally discussed in this document.
    Type certificate holders for specific airplane models with high 
flammability exposure fuel tanks would be required to develop design 
changes and service instructions to facilitate the adoption of IMM or 
FRM. Manufacturers of these airplanes would have to incorporate these 
design changes in airplanes produced in the future. In addition, these 
sections would require design approval holders (TC and STC holders) and 
applicants to develop airworthiness limitations to ensure that 
maintenance actions and future modifications do not increase 
flammability exposure above the limits in this proposal. These design 
approval holders would have to submit binding certification plans by a 
specified date, and these plans would be closely monitored by the 
holders' FAA oversight offices to ensure timely progress.
    Lastly, the proposal requires affected operators to incorporate FRM 
or IMM where required for high-risk fuel tanks in their existing fleet 
of affected airplane models. Air carriers would also have to revise 
their maintenance and inspection programs to incorporate the 
airworthiness limitations developed under the other proposals. We also 
intend to establish strict retrofit deadlines, which are premised on 
prompt compliance by manufacturers with their certification plans.
    Table 1 summarizes the proposed regulatory changes that relate to 
fuel tank flammability safety. This table does not summarize the 
proposed regulatory changes that are common between this proposal and 
other aging airplane initiatives. Those changes are discussed in detail 
later.

[[Page 70929]]



                   Table 1.--Summary of Proposed Rules
------------------------------------------------------------------------
                                 Description of
           14 CFR                   proposal             Applies to
------------------------------------------------------------------------
25.1, 25.2..................  Expand applicability  Applicants for TCs,
                               to current holders    and changes to
                               of TCs, STCs, and     those TCs for
                               certain               transport category
                               manufacturers.        airplanes.
                              Amend Sec.   25.2 to   Manufacturers of
                               make reference to     certain airplane
                               the proposed          models.
                               subpart I..
25.981......................  Revise paragraph (b)  Applicants for
                               to specify limits     future TCs and
                               on fuel tank          design changes to
                               flammability.         those certificates.
                              Add paragraph (c) to
                               restate current
                               option of providing
                               ignition mitigation
                               means (IMM)..
                              Add paragraph (d) to
                               include
                               airworthiness
                               limitation items
                               (ALI) for IMM or
                               Flammability
                               Reduction Means
                               (FRM), and move the
                               existing ignition
                               prevention ALI
                               requirements into
                               this paragraph.
Subpart I 25.1801...........  Defines the intent    TCs, and design
                               of the subpart.       changes to those
                                                     TCs for transport
                                                     category airplanes.
                                                     Manufacturers of
                                                     certain airplane
                                                     models.
25.1815.....................  Require flammability  TC holders.
                               exposure analysis
                               of all fuel tanks
                               within 150 days
                               after effective
                               date. If below 7
                               percent no
                               flammability
                               reduction required.
                               Compliance with
                               Sec.   25.981(d) to
                               define ALI required.
                              If above 7 percent    Large transport
                               and in fuselage and   category passenger
                               normally emptied,     airplanes, with
                               must develop          passenger capacity
                               service               of 30 or more or a
                               instructions to       payload of 7500 lbs
                               meet Sec.             or more (original
                               25.981(b), (c) and    TC or later
                               (d).                  increase).
                              If above 7 percent
                               and other tank
                               type, must develop
                               service
                               instructions to
                               incorporate IMM
                               (meet Sec.
                               25.981(c), or
                               reduce flammability
                               to 7 percent)..
                              Specific compliance
                               dates for each
                               Boeing and Airbus
                               airplane model.
                               Other models within
                               24 months.
25.1817.....................  Require flammability  Auxiliary tank STC
                               exposure analysis     holders for large
                               of all fuel tanks     transport category
                               installed under STC   passenger
                               within 12 months      airplanes, with
                               after effective       passenger capacity
                               date.                 of 30 or more or a
                              Require impact         payload of 7500
                               assessment of fuel    lbs. or more
                               tanks installed by    (original TC or
                               STCs, and (for        later increase).
                               pending and future
                               applicants) other
                               STCs affecting fuel
                               tank flammability,
                               on IMM or FRM
                               developed by TC
                               holder under Sec.
                               25.1815 to
                               determine if any
                               ALI has been
                               violated 6 months
                               after FAA approval
                               of ALI submitted by
                               TC holders under
                               Sec.   25.1815 or
                               before
                               certification,
                               whichever is later..
                              Require development   Applicants for
                               of service            future STCs or
                               instructions to       amendments to TCs
                               correct designs       that affect fuel
                               that compromise ALI   tank system or IMM/
                               defined by TC         FRM.
                               holder under Sec.
                               25.1815 within 24
                               months. Require
                               within 24 months
                               after TC holder
                               compliance with
                               25.1815 development
                               of service
                               instructions for a
                               IMM or FRM for any
                               tank with
                               flammability above
                               7 percent, if
                               located within the
                               fuselage and
                               normally emptied.
25.1819.....................  Requires IMM or FRM   Pending
                               for any fuel tank     certification
                               on a passenger        projects.
                               airplane with a      Pre Amendment 102.
                               flammability level
                               that exceeds 7
                               percent. Fuel tanks
                               located in the
                               fuselage and
                               normally emptied
                               must meet Sec.
                               25.981(b) level.
                               Other fuel tanks
                               must not exceed 7
                               percent.
                              Requires compliance   Post Amendment 102.
                               with Sec.
                               25.981(c).
25.1821.....................  Requires any          Manufacturers of
                               affected airplanes    certain airplane
                               produced after a      models.
                               certain date to
                               incorporate IMM or
                               FRM.
Appendix 25 K...............  Establishes           Applicants for
                               performance,          approval of
                               reliability and       flammability
                               reporting             reduction means.
                               requirements for
                               flammability
                               reduction means.
Appendix 25 L...............  Defines flammability  Any person required
                               analysis method and   to perform
                               input parameters      flammability
                               that must be used     analysis.
                               in the analysis.
91.1509, 121.917, 125.509,    Require retrofit of   U.S. certificate
 129.117.                      IMM or FRM into       holders and foreign
                               large airplanes       persons operating
                               with high             U.S.-registered
                               flammability fuel     large transport
                               tanks. Require        category passenger
                               large transport       airplanes.
                               category airplanes
                               manufactured after
                               specific dates to
                               have IMM or FRM in
                               high flammability
                               fuel tanks. Require
                               incorporation of
                               ALI into the
                               maintenance program.
------------------------------------------------------------------------

B. Ongoing Responsibility of Type Certificate Holders for Continued 
Airworthiness

    Several recent safety regulations necessitated action by air 
carriers and other operators but did not require design approval 
holders to develop and provide the necessary data and documents to 
facilitate the operators' compliance. Operators are often dependent on 
action by a d