Proposed Special Conditions: Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 Series Airplanes; Flammability Reduction Means (Fuel Tank Inerting), 34702-34714 [05-11762]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 70, Number 114 (Wednesday, June 15, 2005)]
[Proposed Rules]
[Pages 34702-34714]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 05-11762]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM309; Notice No. 25-05-06-SC]


Proposed Special Conditions: Boeing Model 737-200/200C/300/400/
500/600/700/700C/800/900 Series Airplanes; Flammability Reduction Means 
(Fuel Tank Inerting)

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: The Federal Aviation Administration (FAA) proposes special 
conditions for the Boeing Model 737-200/200C/300/400/500/600/700/700C/
800/900 series airplanes. These airplanes, as modified by Boeing 
Commercial Airplanes, include a new flammability reduction means that 
uses a nitrogen generation system to reduce the oxygen content in the 
center wing fuel tank so that exposure to a combustible mixture of fuel 
and air is substantially minimized. This system is intended to reduce 
the average flammability exposure of the fleet of airplanes with the 
system installed to a level equivalent to 3 percent of the airplane 
operating time. The applicable airworthiness regulations do not contain 
adequate or appropriate safety standards for the design and 
installation of this system. These proposed special conditions contain 
the additional safety standards the Administrator considers necessary 
to ensure an acceptable level of safety for the installation of the 
system and to define performance objectives the system must achieve to 
be considered an acceptable means for minimizing development of 
flammable vapors in the fuel tank installation.

DATES: Comments must be received on or before July 15, 2005.

ADDRESSES: Comments on this proposal may be mailed in duplicate to: 
Federal Aviation Administration, Transport Airplane Directorate, Attn: 
Rules Docket (ANM-113), Docket No. NM309, 1601 Lind Avenue SW., Renton, 
Washington, 98055-4056; or delivered in duplicate to the Transport 
Airplane Directorate at the above address. Comments must be marked: 
Docket No. NM309. Comments may be inspected in the Rules Docket 
weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.

FOR FURTHER INFORMATION CONTACT: Mike Dostert, Propulsion and 
Mechanical Systems Branch, FAA,

[[Page 34703]]

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. The most 
helpful comments reference a specific portion of the special 
conditions, 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 these special conditions. 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.
    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 
these proposed special conditions 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 back to you.

Background

    Boeing Commercial Airplanes intends to modify the Model 737 series 
airplanes to incorporate a new flammability reduction means (FRM) that 
will inert the center fuel tanks with nitrogen-enriched air (NEA). 
Though the provisions of Sec.  25.981, as amended by Amendment 25-102, 
will apply to this design change, these proposed special conditions 
address novel design features. This document proposes the same special 
conditions that were published in the Federal Register [Docket No. 
NM270; Special Conditions No. 25-285-SC] for incorporation of an FRM on 
Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/
400F series airplanes (70 FR 7800, January 24, 2005).
    Regulations used as the standard for certification of transport 
category airplanes prior to Amendment 25-102, effective June 6, 2001, 
were intended to prevent fuel tank explosions by eliminating possible 
ignition sources from inside the fuel tanks. Service experience of 
airplanes certificated to the earlier standards shows that ignition 
source prevention alone has not been totally effective at preventing 
accidents. Commercial transport airplane fuel tank safety requirements 
have remained relatively unchanged throughout the evolution of piston-
powered airplanes and later into the jet age. The fundamental premise 
for precluding fuel tank explosions has involved establishing that the 
design does not result in a condition that would cause an ignition 
source within the fuel tank ullage (the space in the tank occupied by 
fuel vapor and air). A basic assumption in this approach has been that 
the fuel tank could contain flammable vapors under a wide range of 
airplane operating conditions, even though there were periods of time 
in which the vapor space would not support combustion.

Fuel Properties

    Jet fuel vapors are flammable in certain temperature and pressure 
ranges. The flammability temperature range of jet engine fuel 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.
    Jet A fuel is the most commonly used commercial jet fuel in the 
United States. Jet A-1 fuel is commonly used in other parts of the 
world. At sea level and with no sloshing or vibration present, these 
fuels have flammability characteristics such that insufficient 
hydrocarbon molecules will be present in the fuel vapor-air mixture, to 
ignite when the temperature in the fuel tank is below approximately 100 
[deg]F. Too many hydrocarbon molecules will be present in the vapor to 
allow it to ignite when the fuel temperature is above approximately 175 
[deg]F. The temperature range where a flammable fuel vapor will form 
can vary with different batches of fuel, even for a specific fuel type. 
In between these temperatures the fuel vapor is flammable. This 
flammability temperature range decreases as the airplane gains altitude 
because of the corresponding decrease of internal tank air pressure. 
For example, at an altitude of 30,000 feet, the flammability 
temperature range is about 60 [deg]F to 120 [deg]F.
    Most transport category airplanes used in air carrier service are 
approved for operation at altitudes from sea level to 45,000 feet. 
Those airplanes operated in the United States and in most overseas 
locations use Jet A or Jet A-1 fuel, which typically limits exposure to 
operation in the flammability range to warmer days.
    We have always assumed that airplanes would sometimes be operated 
with flammable fuel vapors in their fuel tank ullage (the space in the 
tank occupied by fuel vapor and air).

Fire Triangle

    Three conditions must be present in a fuel tank to support 
combustion. These include the presence of a suitable amount of fuel 
vapor, the presence of sufficient oxygen, and the presence of an 
ignition source. This has been named the ``fire triangle.'' Each point 
of the triangle represents one of these conditions. Because of 
technological limitations in the past, the FAA philosophy regarding the 
prevention of fuel tank explosions to ensure airplane safety was to 
only preclude ignition sources within fuel tanks. This philosophy 
included application of fail-safe design requirements to fuel tank 
components (lightning design requirements, fuel tank wiring, fuel tank 
temperature limits, etc.) that are intended to preclude ignition 
sources from being present in fuel tanks even when component failures 
occur.

Need to Address Flammability

    Three accidents have occurred in the last 13 years as the result of 
unknown ignition sources within the fuel tank in spite of past efforts, 
highlighting the difficulty in continuously preventing ignition from 
occurring within fuel tanks. Between 1996 and 2000 the National 
Transportation Safety Board (NTSB) issued recommendations to improve 
fuel tank safety that included prevention of ignition sources and 
addressing fuel tank flammability (i.e., the other two points of the 
fire triangle).
    The FAA initiated safety reviews of all larger transport airplane 
type certificates to review the fail-safe features of previously 
approved designs and also initiated research into the feasibility of 
amending the regulations to address fuel tank flammability. Results 
from the safety reviews indicated a significant number of single and 
combinations of failures that can result in ignition sources within the 
fuel tanks. The FAA has adopted rulemaking to require design and/or 
maintenance actions to address these issues;

[[Page 34704]]

however, past experience indicates unforeseen design and maintenance 
errors can result in development of ignition sources. These findings 
show minimizing or preventing the formation of flammable vapors by 
addressing the flammability points of the fire triangle will enhance 
fuel tank safety.
    On April 3, 1997, the FAA published a notice in the Federal 
Register (62 FR 16014), Fuel Tank Ignition Prevention Measures, that 
requested comments concerning the 1996 NTSB recommendations regarding 
reduced flammability. That notice provided significant discussion of 
the service history, background, and issues related to reducing 
flammability in transport airplane fuel tanks. Comments submitted to 
that notice indicated additional information was needed before the FAA 
could initiate rulemaking action to address all of the recommendations.
    Past safety initiatives by the FAA and industry to reduce the 
likelihood of fuel tank explosions resulting from post crash ground 
fires have evaluated means to address other factors of the fire 
triangle. Previous attempts were made to develop commercially viable 
systems or features that would reduce or eliminate other aspects of the 
fire triangle (fuel or oxygen) such as fuel tank inerting or ullage 
space vapor ``scrubbing'' (ventilating the tank ullage with air to 
remove fuel vapor to prevent the accumulation of flammable 
concentrations of fuel vapor). Those initial attempts proved to be 
impractical for commercial transport airplanes due to the weight, 
complexity, and poor reliability of the systems, or undesirable 
secondary effects such as unacceptable atmospheric pollution.

Fuel Tank Harmonization Working Group

    On January 23, 1998, the FAA published a notice in the Federal 
Register that established an Aviation Rulemaking Advisory Committee 
(ARAC) working group, the Fuel Tank Harmonization Working Group 
(FTHWG). The FAA tasked the FTHWG with providing a report to the FAA 
recommending regulatory text to address limiting fuel tank flammability 
in both new type certificates and the fleet of in service airplanes. 
The ARAC consists of interested parties, including the public, and 
provides a public process to advise the FAA concerning development of 
new regulations. (Note: The FAA formally established ARAC in 1991 (56 
FR 2190, January 22, 1991), to provide advice and recommendations 
concerning the full range of the FAA's safety-related rulemaking 
activity.)
    The FTHWG evaluated numerous possible means of reducing or 
eliminating hazards associated with explosive vapors in fuel tanks. On 
July 23, 1998, the ARAC submitted its report to the FAA. The full 
report is in the docket created for this ARAC working group (Docket No. 
FAA-1998-4183). This docket can be reviewed on the U.S. Department of 
Transportation electronic Document Management System on the Internet at 
https://dms.dot.gov.
    The report provided a recommendation for the FAA to initiate 
rulemaking action to amend Sec.  25.981, applicable to new type design 
airplanes, to include a requirement to limit the time transport 
airplane fuel tanks could operate with flammable vapors in the vapor 
space of the tank. The recommended regulatory text proposed, ``Limiting 
the development of flammable conditions in the fuel tanks, based on the 
intended fuel types, to less than 7 percent of the expected fleet 
operational time (defined in this rule as flammability exposure 
evaluation time (FEET)), or providing means to mitigate the effects of 
an ignition of fuel vapors within the fuel tanks such that any damage 
caused by an ignition will not prevent continued safe flight and 
landing.'' The report included a discussion of various options for 
showing compliance with this proposal, including managing heat input to 
the fuel tanks, installation of inerting systems or polyurethane fire 
suppressing foam, and suppressing an explosion if one occurred.
    The level of flammability defined in the proposal was established 
based on a comparison of the safety record of center wing fuel tanks 
that, in certain airplanes, are heated by equipment located under the 
tank, and unheated fuel tanks located in the wing. The ARAC concluded 
that the safety record of fuel tanks located in the wings with a 
flammability exposure of 2 to 4 percent of the FEET was adequate and 
that if the same level could be achieved in center wing fuel tanks, the 
overall safety objective would be achieved. The thermal analyses 
documented in the report revealed that center wing fuel tanks that are 
heated by air conditioning equipment located beneath them contain 
flammable vapors, on a fleet average basis, in the range of 15 to 30 
percent of the fleet operating time.
    During the ARAC review, it was also determined that certain 
airplane types do not locate heat sources adjacent to the fuel tanks 
and have significant surface areas that allow cooling of the fuel tank 
by outside air. These airplanes provide significantly reduced 
flammability exposure, near the 2 to 4 percent value of the wing tanks. 
The group therefore determined that it would be feasible to design new 
airplanes such that airplane operation with fuel tanks that were 
flammable in the flammable range would be limited to nearly that of the 
wing fuel tanks. Findings from the ARAC report indicated that the 
primary method of compliance available at that time with the 
requirement proposed by the ARAC would likely be to control heat 
transfer into and out of fuel tanks. Design features such as locating 
the air conditioning equipment away from the fuel tanks, providing 
ventilation of the air conditioning bay to limit heating and to cool 
fuel tanks, and/or insulating the tanks from heat sources, would be 
practical means of complying with the regulation proposed by the ARAC.
    In addition to its recommendation to revise Sec.  25.981, the ARAC 
also recommended that the FAA continue to evaluate means for minimizing 
the development of flammable vapors within the fuel tanks to determine 
whether other alternatives, such as ground-based inerting of fuel 
tanks, could be shown to be cost effective.
    To address the ARAC recommendations, the FAA continued with 
research and development activity to determine the feasibility of 
requiring inerting for both new and existing designs.

FAA Rulemaking Activity

    Based in part on the ARAC recommendations to limit fuel tank 
flammability exposure on new type designs, the FAA developed and 
published Amendment 25-102 in the Federal Register on May 7, 2001 (66 
FR 23085). The amendment included changes to Sec.  25.981 that require 
minimization of fuel tank flammability to address both reduction in the 
time fuel tanks contain flammable vapors, (Sec.  25.981(c)), and 
additional changes regarding prevention of ignition sources in fuel 
tanks. Section 25.981(c) was based on the FTHWG recommendation to 
achieve a safety level equivalent to that achieved by the fleet of 
transports with unheated aluminum wing tanks, between 2 to 4 percent 
flammability. The FAA stated in the preamble to Amendment 25-102 that 
the intent of the rule was to--

    * * * require that practical means, such as transferring heat 
from the fuel tank (e.g., use of ventilation or cooling air), be 
incorporated into the airplane design if heat sources were placed in 
or near the fuel tanks that significantly increased the formation of 
flammable fuel vapors in the tank, or if the tank is located in an 
area of the airplane where little or no cooling occurs. The intent

[[Page 34705]]

of the rule is to require that fuel tanks are not heated, and cool 
at a rate equivalent to that of a wing tank in the transport 
airplane being evaluated. 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.

    Advisory circulars associated with Amendment 25-102 include AC 
25.981-1B, ``Fuel Tank Ignition Source Prevention Guidelines,'' and AC 
25.981-2, ``Fuel Tank Flammability Minimization.'' Like all advisory 
material, these advisory circulars describe an acceptable means, but 
not the only means, for demonstrating compliance with the regulations.

FAA Research

    In addition to the notice published in the Federal Register on 
April 3, 1997, the FAA initiated research to provide a better 
understanding of the ignition process of commercial aviation fuel 
vapors and to explore new concepts for reducing or eliminating the 
presence of flammable fuel air mixtures within fuel tanks.

Fuel Tank Inerting

    In the public comments received in response to the 1997 notice, 
reference was made to hollow fiber membrane technology that had been 
developed and was in use in other applications, such as the medical 
community, to separate oxygen from nitrogen in air. Air is made up of 
about 78 percent nitrogen and 21 percent oxygen, and the hollow fiber 
membrane material uses the absorption difference between the nitrogen 
and oxygen molecules to separate the NEA from the oxygen. In airplane 
applications NEA is produced when pressurized air from an airplane 
source such as the engines is forced through the hollow fibers. The NEA 
is then directed, at appropriate nitrogen concentrations, into the 
ullage space of fuel tanks and displaces the normal fuel vapor/air 
mixture in the tank.
    Use of the hollow fiber technology allowed nitrogen to be separated 
from air, which eliminated the need to carry and store the nitrogen in 
the airplane. Researchers were aware of the earlier system's 
shortcomings in the areas of weight, reliability, cost, and 
performance. Recent advances in the technology have resolved those 
concerns and eliminated the need for storing nitrogen on board the 
airplane.

Criteria for Inerting

    Earlier fuel tank inerting designs produced for military 
applications were based on defining ``inert'' as a maximum oxygen 
concentration of 9 percent. This value was established by the military 
for protection of fuel tanks from battle damage. One major finding from 
the FAA's research and development efforts was the determination that 
the 9 percent maximum oxygen concentration level benchmark, established 
to protect military airplanes from high-energy ignition sources 
encountered in battle, was significantly lower than that needed to 
inert civilian transport airplane fuel tanks from ignition sources 
resulting from airplane system failures and malfunctions that have much 
lower energy. This FAA research established a maximum value of 12 
percent as being adequate at sea level. The test results are currently 
available on FAA Web site: https://www.fire.tc.faa.gov/pdf/tn02-79.pdf 
as FAA Technical Note ``Limiting Oxygen Concentrations Required to 
Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures,'' report 
number DOT/FAA/AR-TN02/79. As a result of this research, the quantity 
of NEA that is needed to inert commercial airplane fuel tanks was 
lessened so that an effective FRM can now be smaller and less complex 
than was originally assumed. The 12 percent value is based on the 
limited energy sources associated with an electrical arc that could be 
generated by airplane system failures on typical transport airplanes 
and does not include events such as explosives or hostile fire.
    As previously discussed, existing fuel tank system requirements 
(contained in earlier Civil Air Regulation (CAR) 4b and now in 14 Code 
of Federal Regulations (CFR) part 25) have focused solely on prevention 
of ignition sources. The FRM is intended to add an additional layer of 
safety by reducing the exposure to flammable vapors in the heated 
center wing tank, not necessarily eliminating them under all operating 
conditions. Consequently, ignition prevention measures will still be 
the principal layer of defense in fuel system safety, now augmented by 
substantially reducing the time that flammable vapors are present in 
higher flammability tanks. We expect that by combining these two 
approaches, particularly for tanks with high flammability exposure, 
such as the heated center wing tank or tanks with limited cooling, 
risks for future fuel tank explosions can be substantially reduced.

Boeing Application for Certification of a Fuel Tank Inerting System

    On September 23, 2005 (737Classics) and December 2, 2005 (737NG), 
Boeing Commercial Airplanes applied for a change to Type Certificate 
A16WE to modify Model 737-200/200C/300/400/500/600/700/700C/800/900 
series airplanes to incorporate a new FRM that inerts the center fuel 
tanks with NEA. These airplanes, approved under Type Certificate No. 
A16WE, are two-engine transport airplanes with a passenger capacity up 
to 189, depending on the submodel. These airplanes have an approximate 
maximum gross weight of 174,700 lbs with an operating range up to 3,380 
miles.

Type Certification Basis

    Under the provisions of Sec.  21.101, Boeing Commercial Airplanes 
must show that the Model 737-200/200C/300/400/500/600/700/700C/800/900 
series airplanes, as changed, continue to meet the applicable 
provisions of the regulations incorporated by reference in Type 
Certificate No. A16WE, or the applicable regulations in effect on the 
date of application for the change. The regulations incorporated by 
reference in the type certificate are commonly referred to as the 
``original type certification basis.'' The regulations incorporated by 
reference in Type Certificate A16WE include 14 CFR part 25, dated 
February 1, 1965, as amended by Amendments 25-1 through 25-94, except 
for proposed special conditions and exceptions noted in Type 
Certificate Data Sheet A16WE.
    In addition, if the regulations incorporated by reference do not 
provide adequate standards with respect to the change, the applicant 
must comply with certain regulations in effect on the date of 
application for the change. The FAA has determined that the FRM 
installation on the Boeing Model 737-200/200C/300/400/500/600/700/700C/
800/900 series airplanes must also be shown to comply with Sec.  25.981 
at Amendment 25-102.
    If the Administrator finds that the applicable airworthiness 
regulations (14 CFR part 25) do not contain adequate or appropriate 
safety standards for the Boeing Model 737-200/200C/300/400/500/600/700/
700C/800/900 series airplanes because of a novel or unusual design 
feature, proposed special conditions are prescribed under the 
provisions of Sec.  21.16.
    In addition to the applicable airworthiness regulations and 
proposed special conditions, the Model 737-200/200C/300/400/500/600/
700/700C/800/900 series airplanes must comply with the fuel vent and 
exhaust emission requirements of 14 CFR part 34 and the acoustical 
change requirements of Sec.  21.93(b).
    Special conditions, as defined in Sec.  11.19, are issued in 
accordance with Sec.  11.38 and become part of the type certification 
basis in accordance with Sec.  21.101.

[[Page 34706]]

    Special conditions are initially applicable to the model for which 
they are issued. Should the type certificate for that model be amended 
later to include any other model that incorporates the same or similar 
novel or unusual design feature, or should any other model already 
included on the same type certificate be modified to incorporate the 
same or similar novel or unusual design feature, these proposed special 
conditions would also apply to the other model under the provisions of 
Sec.  21.101.

Novel or Unusual Design Features

    Boeing has applied for approval of an FRM to minimize the 
development of flammable vapors in the center fuel tanks of Model 737-
200/200C/300/400/500/600/700/700C/800/900 series airplanes. Boeing also 
plans to seek approval of this system on Boeing Model 747, 757, 767, 
and 777 airplanes.
    Boeing has proposed to voluntarily comply with Sec.  25.981(c), 
Amendment 25-102, which is normally only applicable to new type designs 
or type design changes affecting fuel tank flammability. The provisions 
of Sec.  21.101 require Boeing to also comply with Sec. Sec.  25.981(a) 
and (b), Amendment 25-102, for the changed aspects of the airplane by 
showing that the FRM does not introduce any additional potential 
sources of ignition into the fuel tanks.
    The FRM uses a nitrogen generation system (NGS) that comprises a 
bleed-air shutoff valve, ozone converter, heat exchanger, air 
conditioning pack air cooling flow shutoff valve, filter, air 
separation module, temperature regulating valve controller and sensor, 
high-flow descent control valve, float valve, and system ducting. The 
system is located in the air conditioning pack bay below the center 
wing fuel tank. Engine bleed air from the existing engine pneumatic 
bleed source flows through a control valve into an ozone converter and 
then through a heat exchanger, where it is cooled using outside cooling 
air. The cooled air flows through a filter into an air separation 
module (ASM) that generates NEA, which is supplied to the center fuel 
tank, and also discharges oxygen-enriched air (OEA). The OEA from the 
ASM is mixed with cooling air from the heat exchanger to dilute the 
oxygen concentration and then exhausted overboard. The FRM also 
includes modifications to the fuel vent system to minimize dilution of 
the nitrogen-enriched ullage in the center tank due to cross-venting 
characteristics of the existing center wing fuel tank vent design.
    Boeing has proposed that limited dispatch relief for operation with 
an inoperative NGS be allowed. Boeing has initially proposed a 10-day 
master minimum equipment list (MMEL) relief for the system. Boeing has 
stated that to meet operator needs and system reliability and 
availability objectives, built-in test functions would be included and 
system status indication of some kind would be provided. In addition, 
indications would be provided in the cockpit on certain airplane models 
that have engine indicating and crew alerting systems. The reliability 
of the system is expected to be designed to achieve a mean time between 
failure (MTBF) of 5000 hours or better.

Discussion

    The FAA policy for establishing the type design approval basis of 
the FRM design will result in application of Sec. Sec.  25.981(a) and 
(b), Amendment 25-102, for the changes to the airplane that might 
increase the risk of ignition of fuel vapors. Boeing will therefore be 
required to substantiate that changes introduced by the FRM will meet 
the ignition prevention requirements of Sec. Sec.  25.981(a) and (b), 
Amendment 25-102 and other applicable regulations.
    With respect to compliance with Sec.  25.981(c), AC 25.981-2 
provides guidance in addressing minimization of fuel tank flammability 
within a heated fuel tank, but there are no specific regulations that 
address the design and installation of an FRM that inerts the fuel 
tank. These proposed special conditions include additional requirements 
above that of Amendment 25-102 to Sec.  25.981(c) to minimize fuel tank 
flammability, such that the level of minimization in these proposed 
special conditions would prevent a fuel tank with an FRM from being 
flammable during specific warm day operating conditions, such as those 
present when recent accidents occurred.

Definition of ``Inert''

    For the purpose of these proposed special conditions, the tank is 
considered inert when the bulk average oxygen concentration within each 
compartment of the tank is 12 percent or less at sea level up to 10,000 
feet, then linearly increasing from 12 percent at 10,000 feet to 14.5 
percent at 40,000 feet and extrapolated linearly above that altitude. 
The reference to each section of the tank is necessary because fuel 
tanks that are compartmentalized may encounter localized oxygen 
concentrations in one or more compartments that exceed the 12 percent 
value. Currently there is not adequate data available to establish 
whether exceeding the 12 percent limit in one compartment of a fuel 
tank could create a hazard. For example, ignition of vapors in one 
compartment could result in a flame front within the compartment that 
travels to adjacent compartments and results in an ignition source that 
exceeds the ignition energy (the minimum amount of energy required to 
ignite fuel vapors) values used to establish the 12 percent limit. 
Therefore, ignition in other compartments of the tank may be possible. 
Technical discussions with the applicant indicate the pressure rise in 
a fuel tank that was at or near the 12 percent oxygen concentration 
level would likely be well below the value that would rupture a typical 
transport airplane fuel tank. While this may be possible to show, it is 
not within the scope of these proposed special conditions. Therefore, 
the effect of the definition of ``inert'' within these proposed special 
conditions is that the bulk average of each individual compartment or 
bay of the tank must be evaluated and shown to meet the oxygen 
concentration limits specified in the definitions section of these 
proposed special conditions (12 percent or less at sea level) to be 
considered inert.

Determining Flammability

    The methodology for determining fuel tank flammability defined for 
use in these proposed special conditions is based on that used by ARAC 
to compare the flammability of unheated aluminum wing fuel tanks to 
that of tanks that are heated by adjacent equipment. The ARAC evaluated 
the relative flammability of airplane fuel tanks using a statistical 
analysis commonly referred to as a ``Monte Carlo'' analysis that 
considered a number of factors affecting formation of flammable vapors 
in the fuel tanks. The Monte Carlo analysis calculates values for the 
parameter of interest by randomly selecting values for each of the 
uncertain variables from distribution tables. This calculation is 
conducted over and over to simulate a process where the variables are 
randomly selected from defined distributions for each of the variables. 
The results of changing these variables for a large number of flights 
can then be used to approximate the results of the real world exposure 
of a large fleet of airplanes.
    Factors that are considered in the Monte Carlo analysis required by 
these proposed special conditions include those affecting all airplane 
models in the transport airplane fleet such as: a statistical 
distribution of ground, overnight, and cruise air temperatures likely 
to be experienced worldwide, a

[[Page 34707]]

statistical distribution of likely fuel types, and properties of those 
fuels, and a definition of the conditions when the tank in question 
will be considered flammable. The analysis also includes factors 
affecting specific airplane models such as climb and descent profiles, 
fuel management, heat transfer characteristics of the fuel tanks, 
statistical distribution of flight lengths (mission durations) expected 
for the airplane model worldwide, etc. To quantify the fleet exposure, 
the Monte Carlo analysis approach is applied to a statistically 
significant number (1,000,000) of flights where each of the factors 
described above is randomly selected. The flights are then selected to 
be representative of the fleet using the defined distributions of the 
factors described previously. For example, flight one may be a short 
mission on a cold day with an average flash point fuel, and flight two 
may be a long mission on an average day with a low flash point fuel, 
and on and on until 1,000,000 flights have been defined in this manner. 
For every one of the 1,000,000 flights, the time that the fuel 
temperature is above the flash point of the fuel, and the tank is not 
inert, is calculated and used to establish if the fuel tank is 
flammable. Averaging the results for all 1,000,000 flights provides an 
average percentage of the flight time that any particular flight is 
considered to be flammable. While these proposed special conditions do 
not require that the analysis be conducted for 1,000,000 flights, the 
accuracy of the Monte Carlo analysis improves as the number of flights 
increases. Therefore, to account for this improved accuracy, appendix 2 
of these proposed special conditions defines lower flammability limits 
if the applicant chooses to use fewer than 1,000,000 flights.
    The determination of whether the fuel tank is flammable is based on 
the temperature of the fuel in the tank determined from the tank 
thermal model, the atmospheric pressure in the fuel tank, and 
properties of the fuel quantity loaded for a given flight, which is 
randomly selected from a database consisting of worldwide data. The 
criteria in the model are based on the assumption that as these 
variables change, the concentration of vapors in the tank 
instantaneously stabilizes and that the fuel tank is at a uniform 
temperature. This model does not include consideration of the time lag 
for the vapor concentration to reach equilibrium, the condensation of 
fuel vapors from differences in temperature that occur in the fuel 
tanks, or the effect of mass loading (times when the fuel tank is at 
the unusable fuel level and there is insufficient fuel at a given 
temperature to form flammable vapors). However, fresh air drawn into an 
otherwise inert tank during descent does not immediately saturate with 
fuel vapors so localized concentrations above the inert level during 
descent do not represent a hazardous condition. These proposed special 
conditions allow the time during descent, where a localized amount of 
fresh air may enter a fuel tank, to be excluded from the determination 
of fuel tank flammability exposure.

Definition of Transport Effects

    The effects of low fuel conditions (mass loading) and the effects 
of fuel vaporization and condensation with time and temperature 
changes, referred to as ``transport effects'' in these proposed special 
conditions, are excluded from consideration in the Monte Carlo model 
used for demonstrating compliance with these proposed special 
conditions. These effects have been excluded because they were not 
considered in the original ARAC analysis, which was based on a relative 
measure of flammability. For example, the 3 percent flammability value 
established by the ARAC as the benchmark for fuel tank safety for wing 
fuel tanks did not include the effects of cooling of the wing tank 
surfaces and the associated condensation of vapors from the tank 
ullage. If this effect had been included in the wing tank flammability 
calculation, it would have resulted in a significantly lower wing tank 
flammability benchmark value. The ARAC analysis also did not consider 
the effects of mass loading which would significantly lower the 
calculated flammability value for fuel tanks that are routinely emptied 
(e.g., center wing tanks). The FAA and European Aviation Safety Agency 
(EASA) have determined that using the ARAC methodology provides a 
suitable basis for determining the adequacy of an FRM system.
    The effect of condensation and vaporization in reducing the 
flammability exposure of wing tanks is comparable to the effect of the 
low fuel condition in reducing the flammability exposure of center 
tanks. We therefore consider these effects to be offsetting, so that by 
eliminating their consideration, the analysis will produce results for 
both types of tanks that are comparable. Using this approach, it is 
possible to follow the ARAC recommendation of using the unheated 
aluminum wing tank as the standard for evaluating the flammability 
exposure of all other tanks. For this reason, both factors have been 
excluded when establishing the flammability exposure limits. During 
development of these harmonized proposed special conditions, the FAA 
and EASA agreed that using the ARAC methodology provides a suitable 
basis for determining the flammability of a fuel tank and consideration 
of transport effects should not be permitted.

Flammability Limit

    The FAA, in conjunction with EASA and Transport Canada, has 
developed criteria within these proposed special conditions that 
require overall fuel tank flammability to be limited to 3 percent of 
the fleet average operating time. This overall average flammability 
limit consists of times when the system performance cannot maintain an 
inert tank ullage, primarily during descent when the change in ambient 
pressures draws air into the fuel tanks and those times when the FRM is 
inoperative due to failures of the system and the airplane is 
dispatched with the system inoperative.

Specific Risk Flammability Limit

    These proposed special conditions also include a requirement to 
limit fuel tank flammability to 3 percent during ground operations, and 
climb phases of flight to address the specific risk associated with 
operation during warmer day conditions when accidents have occurred. 
The specific risk requirement is intended to establish minimum system 
performance levels and therefore the 3 percent flammability limit 
excludes reliability related contributions, which are addressed in the 
average flammability assessment. The specific risk requirement may be 
met by conducting a separate Monte Carlo analysis for each of the 
specific phases of flight during warmer day conditions defined in the 
proposed special conditions, without including the times when the FRM 
is not available because of failures of the system or dispatch with the 
FRM inoperative.

Inerting System Indications

    Fleet average flammability exposure involves several elements, 
including--
     The time the FRM is working properly and inerts the tank 
or when the tank is not flammable;
     The time when the FRM is working properly but fails to 
inert the tank or part of the tank, because of mission variation or 
other effects;
     The time the FRM is not functioning properly and the 
operator is unaware of the failure; and
     The time the FRM is not functioning properly and the 
operator is aware of the failure and is operating the

[[Page 34708]]

airplane for a limited time under MEL relief.
    The applicant may propose that MMEL relief is provided for aircraft 
operation with the FRM unavailable; however, since the intent of Sec.  
25.981(c)(1) is to minimize flammability, the FRM system should be 
operational to the maximum extent practical. Therefore, these proposed 
special conditions include reliability and reporting requirements to 
enhance system reliability so that dispatch of airplanes with the FRM 
inoperative would be very infrequent. Cockpit indication of the system 
function that is accessible to the flightcrew is not an explicit 
requirement, but may be required if the results of the Monte Carlo 
analysis show the system cannot otherwise meet the flammability and 
reliability requirements defined in these proposed special conditions. 
Flight test demonstration and analysis will be required to demonstrate 
that the performance of the inerting system is effective in inerting 
the tank during those portions of ground and the flight operations 
where inerting is needed to meet the flammability requirements of these 
proposed special conditions.
    Various means may be used to ensure system reliability and 
performance. These may include: system integrity monitoring and 
indication, redundancy of components, and maintenance actions. A 
combination of maintenance indication and/or maintenance check 
procedures will be required to limit exposure to latent failures within 
the system, or high inherent reliability is needed to assure the system 
will meet the fuel tank flammability requirements. The applicant's 
inerting system does not incorporate redundant features and includes a 
number of components essential for proper system operation. Past 
experience has shown inherent reliability of this type of system would 
be difficult to achieve. Therefore, if system maintenance indication is 
not provided for features of the system essential for proper system 
operation, system functional checks at appropriate intervals determined 
by the reliability analysis will be required for these features. 
Validation of proper function of essential features of the system would 
likely be required once per day by maintenance review of indications, 
reading of stored maintenance messages or functional checks (possibly 
prior to the first flight of the day) to meet the reliability levels 
defined in these special conditions. The determination of a proper 
interval and procedure will follow completion of the certification 
testing and demonstration of the system's reliability and performance 
prior to certification.
    Any features or maintenance actions needed to achieve the minimum 
reliability of the FRM will result in fuel system airworthiness 
limitations similar to those defined in Sec.  25.981(b). Boeing will be 
required to include in the instructions for continued airworthiness 
(ICA) the replacement times, inspection intervals, inspection 
procedures, and the fuel system limitations required by Sec.  
25.981(b). Overall system performance and reliability must achieve a 
fleet average flammability that meets the requirements of these 
proposed special conditions. If the system reliability falls to a point 
where the fleet average flammability exposure exceeds these 
requirements, Boeing will be required to define appropriate corrective 
actions, to be approved by the FAA, that will bring the exposure back 
down to the acceptable level.
    Boeing proposed that the FRM be eligible for a 10-day MMEL dispatch 
interval. The Flight Operations Evaluation Board (FOEB) will establish 
the approved interval based on data the applicant submits to the FAA. 
The MMEL dispatch interval is one of the factors affecting system 
reliability analyses that must be considered early in the design of the 
FRM, prior to FAA approval of the MMEL. Boeing requested that the 
authorities agree to use of an MMEL inoperative dispatch interval for 
design of the system. Boeing data indicates that certain systems on the 
airplane are routinely repaired prior to the maximum allowable 
interval. These proposed special conditions require that Boeing use an 
MMEL inoperative dispatch interval of 60 hours in the analysis as 
representative of the mean time for which an inoperative condition may 
occur for the 10-day MMEL maximum interval requested. Boeing must also 
include actual dispatch inoperative interval data in the quarterly 
reports required by Special Condition III(c)(2). Boeing may request to 
use an alternative interval in the reliability analysis. Use of a value 
less than 60 hours would be a factor considered by the FOEB in 
establishing the maximum MMEL dispatch limit. The reporting requirement 
will provide data necessary to validate that the reliability of the FRM 
achieved in service meets the levels used in the analysis.
    Appropriate maintenance and operational limitations with the FRM 
inoperative may also be required and noted in the MMEL. The MMEL 
limitations and any operational procedures should be established based 
on results of the Monte Carlo assessment, including the results 
associated with operations in warmer climates where the fuel tanks are 
flammable a significant portion of the FEET when not inert. While the 
system reliability analysis may show that it is possible to achieve an 
overall average fleet exposure equal to or less than that of a typical 
unheated aluminum wing tank, even with an MMEL allowing very long 
inoperative intervals, the intent of the rule is to minimize 
flammability. Therefore, the shortest practical MMEL relief interval 
should be proposed. To ensure limited airplane operation with the 
system inoperative and to meet the reliability requirements of these 
proposed special conditions, appropriate level messages that are needed 
to comply with any dispatch limitations of the MMEL must be provided.

Confined Space Hazard Markings

    Introduction of the FRM will result in NEA within the center wing 
fuel tank and the possibility of NEA in compartments adjacent to the 
fuel tank if leakage from the tank or NEA supply lines were to occur. 
Lack of oxygen in these areas could be hazardous to maintenance 
personnel, the passengers, or flightcrew. Existing certification 
requirements do not address all aspects of these hazards. Paragraph 
II(f) of the proposed special conditions requires the applicant to 
provide markings to emphasize the potential hazards associated with 
confined spaces and areas where a hazardous atmosphere could be present 
due to the addition of an FRM.
    For the purposes of these proposed special conditions, a confined 
space is an enclosed or partially enclosed area that is big enough for 
a worker to enter and perform assigned work and has limited or 
restricted means for entry or exit. It is not designed for someone to 
work in regularly, but workers may need to enter the confined space for 
tasks such as inspection, cleaning, maintenance, and repair. (Reference 
U.S. Department of Labor Occupational Safety & Health Administration 
(OSHA), 29 CFR 1910.146(b).) The requirement in the proposed special 
conditions does not significantly change the procedures maintenance 
personnel use to enter fuel tanks and are not intended to conflict with 
existing government agency requirements (e.g., OSHA). Fuel tanks are 
classified as confined spaces and contain high concentrations of fuel 
vapors that must be exhausted from the fuel tank before entry. Other 
precautions such as measurement of the oxygen concentrations before 
entering a fuel tank are already required. Addition of

[[Page 34709]]

the FRM that utilizes inerting may result in reduced oxygen 
concentrations due to leakage of the system in locations in the 
airplane where service personnel would not expect it. A worker is 
considered to have entered a confined space just by putting his or her 
head across the plane of the opening. If the confined space contains 
high concentrations of inert gases, workers who are simply working near 
the opening may be at risk. Any hazards associated with working in 
adjacent spaces near the opening should be identified in the marking of 
the opening to the confined space. A large percentage of the work 
involved in properly inspecting and modifying airplane fuel tanks and 
their associated systems must be done in the interior of the tanks. 
Performing the necessary tasks requires inspection and maintenance 
personnel to physically enter the tank, where many environmental 
hazards exist. These potential hazards that exist in any fuel tank, 
regardless of whether nitrogen inerting has been installed, include 
fire and explosion, toxic and irritating chemicals, oxygen deficiency, 
and the confined nature of the fuel tank itself. In order to prevent 
related injuries, operator and repair station maintenance organizations 
have developed specific procedures for identifying, controlling, or 
eliminating the hazards associated with fuel-tank entry. In addition 
government agencies have adopted safety requirements for use when 
entering fuel tanks and other confined spaces. These same procedures 
would be applied to the reduced oxygen environment likely to be present 
in an inerted fuel tank.
    The designs currently under consideration locate the FRM in the 
fairing below the center wing fuel tank. Access to these areas is 
obtained by opening doors or removing panels which could allow some 
ventilation of the spaces adjacent to the FRM. But this may not be 
enough to avoid creating a hazard. Therefore, we intend that marking be 
provided to warn service personnel of possible hazards associated with 
the reduced oxygen concentrations in the areas adjacent to the FRM.
    Appropriate markings would be required for all inerted fuel tanks, 
tanks adjacent to inerted fuel tanks and all fuel tanks communicating 
with the inerted tanks via plumbing. The plumbing includes, but is not 
limited to, plumbing for the vent system, fuel feed system, refuel 
system, transfer system and cross-feed system. NEA could enter adjacent 
fuel tanks via structural leaks. It could also enter other fuel tanks 
through plumbing if valves are operated or fail in the open position. 
The markings should also be stenciled on the external upper and lower 
surfaces of the inerted tank adjacent to any openings to ensure 
maintenance personnel understand the possible contents of the fuel 
tank. Advisory Circular 25.981-2 will provide additional guidance 
regarding markings and placards.

Affect of FRM on Auxiliary Fuel Tank System Supplemental Type 
Certificates

    Boeing plans to offer a service bulletin that will describe 
installation of the FRM on existing in-service airplanes. Some in-
service airplanes have auxiliary fuel tank systems installed that 
interface with the center wing tank. The Boeing FRM design is intended 
to provide inerting of the center wing fuel tank volume of the 737 and 
does not include consideration of the auxiliary tank installations. 
Installation of the FRM on existing airplanes with auxiliary fuel tank 
systems may therefore require additional modifications to the auxiliary 
fuel tank system to prevent development of a condition that may cause 
the tank to exceed the 12 percent oxygen limit. The FAA will address 
these issues during development and approval of the service bulletin 
for the FRM.

Disposal of Oxygen-Enriched Air (OEA)

    The FRM produces both NEA and OEA. The OEA generated by the FRM 
could result in an increased fire hazard if not disposed of properly. 
The OEA produced in the proposed design is diluted with air from a heat 
exchanger, which is intended to reduce the OEA concentration to non-
hazardous levels. Special requirements are included in these proposed 
special conditions to address potential leakage of OEA due to failures 
and safe disposal of the OEA during normal operation.
    To ensure that an acceptable level of safety is achieved for the 
modified airplanes using a system that inerts heated fuel tanks with 
NEA, proposed special conditions (per Sec.  21.16) are needed to 
address the unusual design features of an FRM. These proposed special 
conditions contain the additional safety standards that the 
Administrator considers necessary to establish a level of safety 
equivalent to that established by the existing airworthiness standards.

Applicability

    As discussed above, these proposed special conditions are 
applicable to the Boeing Model 737-200/200C/300/400/500/600/700/700C/
800/900 series airplanes. Should the type certificate be amended later 
to include any other model that incorporates the same or similar novel 
or unusual design feature, or should any other model already included 
on the same type certificate be modified to incorporate the same or 
similar novel or unusual design feature, the proposed special 
conditions would also apply to the other model under the provisions of 
Sec.  21.101.

Conclusion

    This action affects only certain novel or unusual design features 
on Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 series 
airplanes. It is not a rule of general applicability and affects only 
the applicant who applied to the FAA for approval of these features on 
the airplane.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

    The authority citation for these proposed special conditions is as 
follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions as part of the type certification basis 
for the Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 
series airplanes, modified by Boeing Commercial Airplanes to include a 
flammability reduction means (FRM) that uses a nitrogen generation 
system to inert the center wing tank with nitrogen-enriched air (NEA).
    Compliance with these proposed special conditions does not relieve 
the applicant from compliance with the existing certification 
requirements.
    I. Definitions.
    (a) Bulk Average Fuel Temperature. The average fuel temperature 
within the fuel tank, or different sections of the tank if the tank is 
subdivided by baffles or compartments.
    (b) Flammability Exposure Evaluation Time (FEET). For the purpose 
of these proposed special conditions, the time from the start of 
preparing the airplane for flight, through the flight and landing, 
until all payload is unloaded and all passengers and crew have 
disembarked. In the Monte Carlo program, the flight time is randomly 
selected from the Mission Range Distribution (Table 3), the pre-flight 
times are provided as a function of the flight time, and the post-
flight time is a constant 30 minutes.
    (c) Flammable. With respect to a fluid or gas, flammable means 
susceptible to igniting readily or to exploding (14 CFR

[[Page 34710]]

part 1, Definitions). A non-flammable ullage is one where the gas 
mixture is too lean or too rich to burn and/or is inert per the 
definition below.
    (d) Flash Point. The flash point of a flammable fluid is the lowest 
temperature at which the application of a flame to a heated sample 
causes the vapor to ignite momentarily, or ``flash.'' The test for jet 
fuel is defined in ASTM Specification D56, ``Standard Test Method for 
Flash Point by Tag Close Cup Tester.''
    (e) Hazardous Atmosphere. An atmosphere that may expose any 
person(s) to the risk of death, incapacitation, impairment of ability 
to self-rescue (escape unaided from a space), injury, or acute illness.
    (f) Inert. For the purpose of these proposed special conditions, 
the tank is considered inert when the bulk average oxygen concentration 
within each compartment of the tank is 12 percent or less at sea level 
up to 10,000 feet, then linearly increasing from 12 percent at 10,000 
feet to 14.5 percent at 40,000 feet and extrapolated linearly above 
that altitude.
    (g) Inerting. A process where a noncombustible gas is introduced 
into the ullage of a fuel tank to displace sufficient oxygen so that 
the ullage becomes inert.
    (h) Monte Carlo Analysis. An analytical tool that provides a means 
to assess the degree of fleet average and warm day flammability 
exposure time for a fuel tank. See appendices 1 and 2 of these proposed 
special conditions for specific requirements for conducting the Monte 
Carlo analysis.
    (i) Transport Effects. Transport effects are the effects on fuel 
vapor concentration caused by low fuel conditions (mass loading), fuel 
condensation, and vaporization.
    (j) Ullage, or Ullage Space. The volume within the fuel tank not 
occupied by liquid fuel at the time interval under evaluation.
    II. System Performance and Reliability. The FRM, for the airplane 
model under evaluation, must comply with the following performance and 
reliability requirements:
    (a) The applicant must submit a Monte Carlo analysis, as defined in 
appendices 1 and 2 of these proposed special conditions, that--
    (1) demonstrates that the overall fleet average flammability 
exposure of each fuel tank with an FRM installed is equal to or less 
than 3 percent of the FEET; and
    (2) demonstrates that neither the performance (when the FRM is 
operational) nor reliability (including all periods when the FRM is 
inoperative) contributions to the overall fleet average flammability 
exposure of a tank with an FRM installed is more than 1.8 percent (this 
will establish appropriate maintenance inspection procedures and 
intervals as required in paragraph III(a) of these proposed special 
conditions).
    (3) identifies critical features of the fuel tank system to prevent 
an auxiliary fuel tank installation from increasing the flammability 
exposure of the center wing tank above that permitted under paragraphs 
II(a)(1), II(a)(2), and II(b) of these proposed special conditions and 
to prevent degradation of the performance and reliability of the FRM.
    (b) The applicant must submit a Monte Carlo analysis that 
demonstrates that the FRM, when functional, reduces the overall 
flammability exposure of each fuel tank with an FRM installed for warm 
day ground and climb phases to a level equal to or less than 3 percent 
of the FEET in each of these phases for the following conditions--
    (1) The analysis must use the subset of 80[deg] F and warmer days 
from the Monte Carlo analyses done for overall performance; and
    (2) The flammability exposure must be calculated by comparing the 
time during ground and climb phases for which the tank was flammable 
and not inert, with the total time for the ground and climb phases.
    (c) The applicant must provide data from ground testing and flight 
testing that--
    (1) validate the inputs to the Monte Carlo analysis needed to show 
compliance with (or meet the requirements of) paragraphs II(a), (b), 
and (c)(2) of these proposed special conditions; and
    (2) substantiate that the NEA distribution is effective at inerting 
all portions of the tank where the inerting system is needed to show 
compliance with these paragraphs.
    (d) The applicant must validate that the FRM meets the requirements 
of paragraphs II(a), (b), and (c)(2) of these proposed special 
conditions, with any combination of engine model, engine thrust rating, 
fuel type, and relevant pneumatic system configuration approved for the 
airplane.
    (e) Sufficient accessibility for maintenance personnel, or the 
flightcrew, must be provided to FRM status indications necessary to 
meet the reliability requirements of paragraph II(a) of these proposed 
special conditions.
    (f) The access doors and panels to the fuel tanks with an FRM 
(including any tanks that communicate with an inerted tank via a vent 
system), and to any other confined spaces or enclosed areas that could 
contain NEA under normal conditions or failure conditions, must be 
permanently stenciled, marked, or placarded as appropriate to warn 
maintenance crews of the possible presence of a potentially hazardous 
atmosphere. The proposal for markings does not alter the existing 
requirements that must be addressed when entering airplane fuel tanks.
    (g) Any FRM failures, or failures that could affect the FRM, with 
potential catastrophic consequences must not result from a single 
failure or a combination of failures not shown to be extremely 
improbable.
    III. Maintenance.
    (a) Airworthiness Limitations must be identified for all 
maintenance and/or inspection tasks required to identify failures of 
components within the FRM that are needed to meet paragraphs II(a), 
(b), and (c)(2) of these proposed special conditions. Airworthiness 
Limitations must also be identified for the critical fuel tank system 
features identified under paragraph II(a)(3).
    (b) The applicant must provide the maintenance procedures that will 
be necessary and present a design review that identifies any hazardous 
aspects to be considered during maintenance of the FRM that will be 
included in the instructions for continued airworthiness (ICA) or 
appropriate maintenance documents.
    (c) To ensure that the effects of component failures on FRM 
reliability are dequately assessed on an on-going basis, the applicant 
must--
    (1) demonstrate effective means to ensure collection of FRM 
reliability data. The means must provide data affecting FRM 
availablity, such as component failures, and the FRM inoperative 
intervals due to dispatch under the MMEL;
    (2) provide a report to the FAA on a quarterly basis for the first 
five years after service introduction. After that period, continued 
quarterly reporting may be replaced with other reliability tracking 
methods found acceptable to the FAA or eliminated if it is established 
that the reliability of the FRM meets, and will continue to meet, the 
exposure requirements of paragraphs II(a) and (b) of these proposed 
special conditions;
    (3) provide a report to the validating authorities for a period of 
at least two years following introduction to service; and
    (4) develop service instructions or revise the applicable airplane 
manual, per a schedule agreed on by the FAA, to correct any failures of 
the FRM that occur in service that could increase the fleet average or 
warm day flammability

[[Page 34711]]

exposure of the tank to more than the exposure requirements of 
paragraphs II(a) and (b) of these proposed special conditions.

Appendix 1

Monte Carlo Analysis

    (a) A Monte Carlo analysis must be conducted for the fuel tank 
under evaluation to determine fleet average and warm day 
flammability exposure for the airplane and fuel type under 
evaluation. The analysis must include the parameters defined in 
appendices 1 and 2 of these proposed special conditions. The 
airplane specific parameters and assumptions used in the Monte Carlo 
analysis must include:
    (1) FRM Performance--as defined by system performance.
    (2) Cruise Altitude--as defined by airplane performance.
    (3) Cruise Ambient Temperature--as defined in appendix 2 of 
these proposed special conditions.
    (4) Overnight Temperature Drop--as defined in appendix 2 of 
these proposed special conditions.
    (5) Fuel Flash Point and Upper and Lower Flammability Limits--as 
defined in appendix 2 of these proposed special conditions.
    (6) Fuel Burn--as defined by airplane performance.
    (7) Fuel Quantity--as defined by airplane performance.
    (8) Fuel Transfer--as defined by airplane performance.
    (9) Fueling Duration--as defined by airplane performance.
    (10) Ground Temperature--as defined in appendix 2 of these 
proposed special conditions.
    (11) Mach Number--as defined by airplane performance.
    (12) Mission Distribution--the applicant must use the mission 
distribution defined in appendix 2 of these proposed special 
conditions or may request FAA approval of alternate data from the 
service history of the Model 737.
    (13) Oxygen Evolution--as defined by airplane performance and as 
discussed in appendix 2 of these proposed special conditions.
    (14) Maximum Airplane Range--as defined by airplane performance.
    (15) Tank Thermal Characteristics--as defined by airplane 
performance.
    (16) Descent Profile Distribution--the applicant must use a 
fixed 2500 feet per minute descent rate or may request FAA approval 
of alternate data from the service history of the Model 737.
    (b) The assumptions for the analysis must include--
    (1) FRM performance throughout the flammability exposure 
evaluation time;
    (2) Vent losses due to crosswind effects and airplane 
performance;
    (3) Any time periods when the system is operating properly but 
fails to inert the tank;

    Note: Localized concentrations above the inert level as a result 
of fresh air that is drawn into the fuel tank through vents during 
descent would not be consi