Special Conditions: Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 Series Airplanes; Flammability Reduction Means (Fuel Tank Inerting), 73561-73576 [05-23936]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 70, Number 238 (Tuesday, December 13, 2005)]
[Rules and Regulations]
[Pages 73561-73576]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 05-23936]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No NM309; Special Conditions No. 25-308-SC]


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: Final special conditions.

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SUMMARY: These special conditions are issued 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 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: The effective date of these special conditions is December 5, 
2005.

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

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 special conditions address 
novel design features. These special conditions are similar to those 
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

[[Page 73562]]

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; 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

[[Page 73563]]

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 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 
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

[[Page 73564]]

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 (737 Classics) and December 2, 2005 (737 NG), 
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 pounds 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 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(a) and (b) 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, special conditions are prescribed under the provisions of 
Sec.  21.16.
    In addition to the applicable airworthiness regulations and 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.
    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 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 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. Oxygen-enriched air (OEA) that is generated in this process is 
dumped overboard. The FRM also includes modifications to the fuel tank 
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 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 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 special conditions, the tank is considered 
inert

[[Page 73565]]

when the 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 special conditions. Therefore, the effect 
of the definition of ``inert'' within these special conditions is that 
the average oxygen concentration 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 
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 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 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 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 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 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 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 special 
conditions, are excluded from consideration in the Monte Carlo model 
used for demonstrating compliance with these 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

[[Page 73566]]

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 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 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 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 these 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 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 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 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 
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 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 indicate that certain systems on the 
airplane are routinely repaired prior to the maximum allowable 
interval. These 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

[[Page 73567]]

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 analysis, 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 
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 these 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 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 these 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 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 provides additional guidance regarding 
markings and placards.

Effect 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 by the ASM is ducted and dumped overboard. Special 
requirements are included in these 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, these special conditions (per Sec.  21.16) are needed to address 
the unusual design features of an FRM. These 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.

[[Page 73568]]

Discussion of Comments

    Notice of Proposed Special Conditions No. 25-05-06-SC for the 
Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 series 
airplanes was published in the Federal Register on June 15, 2005 (70 FR 
34702). Five commenters responded to the notice.

General Comments

    Comment: The commenter disagrees with the premise in the proposed 
special conditions that wing fuel tanks offer an acceptable minimum 
level of flammability exposure and is therefore concerned about using 
this minimum level for development of inerting systems. The commenter 
believes that the flammability exposure in the fuel tanks should be 
reduced to the lowest level technically feasible.
    FAA Reply: We do not concur. These special conditions address fuel 
tank flammability for Boeing Model 737 airplanes currently in service. 
Although technical advancements have made it practical to incorporate 
FRM into existing airplanes, it is not practical at this time to reduce 
fuel tank flammability exposure below the levels identified in these 
special conditions because airplane systems needed to support the 
current technology that utilizes inerting were not sized to provide an 
optimized pressurized air source. Compliance with the average fuel tank 
flammability requirement and the warm day requirement in these special 
conditions results in a significant reduction in fuel tank 
flammability, to a level below that of an unheated aluminum wing fuel 
tank, and improved airplane safety. No changes were made as a result of 
this comment.
    Comment: The commenter requests that the long-term goal for the 
definition of ``inert'' at sea level be established as 9 percent oxygen 
concentration. The commenter believes that the 12 percent value used in 
the definition of ``inert'' in the proposed special conditions, should 
be considered as a ``level of reduced flammability.'' The commenter 
states that past research conducted to support development of military 
aircraft inerting systems has shown that fuel vapors are combustible at 
12 percent oxygen concentration. These military systems, designed to 
protect against high-energy (intentional) ignition threats, have 
established 9 percent as an acceptable oxygen concentration to prevent 
ignition.
    FAA Reply: We do not concur. The special condition requirement of 
12 percent maximum oxygen concentration at sea level is based on FAA 
fuel vapor ignition testing at various oxygen contents and review of 
other test data, such as Navy live gunfire tests using 30 mm incendiary 
ammunition. These data are provided in Naval Weapons Center document 
NWC TP 7129, ``The Effectiveness of Ullage Nitrogen-Inerting Systems 
Against 30 mm High-Explosive Incendiary Projectiles,'' dated May 1991, 
that is available in the docket file for these special conditions. 
These data show that 12 percent oxygen concentration will prevent a 
fuel tank explosion for airplane system failure and malfunction-
generated ignition sources. No changes were made as a result of this 
comment.

Novel or Unusual Design Features

    Comment: The commenter requests that the sentence ``The OEA from 
the ASM is mixed with cooling air from the heat exchanger to dilute the 
oxygen concentration and then exhausted overboard'' be deleted. The 
commenter states this does not apply to the 737 FRM design.
    FAA Reply: We concur in part with the commenter. We have removed 
this sentence from the second to the last paragraph under this section 
in the final special conditions but have modified the previous sentence 
to state ``The cooled air flows through a filter into an air separation 
module (ASM) that generates NEA, which is supplied to the center fuel 
tank. Oxygen-enriched air (OEA) which is generated in this process is 
dumped overboard.'' We have also modified the sentence regarding how 
OEA will be disposed, under the Disposal of Oxygen-Enriched Air (OEA) 
section, to state ``The OEA produced by the ASM is ducted and dumped 
overboard'' to be consistent with how the system has been designed.

Inerting System Indications

    Comment: The commenter requests that alternative options to daily 
maintenance checks of the FRM system be provided in the instructions 
for continued airworthiness for operators that would have difficulty in 
meeting a daily maintenance requirement. The commenter states that a 
daily maintenance check of the FRM system does not fit into their 
current maintenance programs and would be a burden to their operation. 
The preamble to the proposed special conditions states that ``if system 
maintenance indication is not provided for features of the system 
essential for proper system operation, system functional checks will be 
required for these features.''
    FAA Reply: We recognize the concern stated by the commenter and 
provide clarification of the intent of these special conditions. We 
agree that daily maintenance checks could be burdensome to operators of 
the affected airplanes. The preamble discussion was not intended to 
mandate daily checks by maintenance personnel. However, in order to 
comply with the special conditions, the applicant must demonstrate that 
the FRM meets specific performance and reliability requirements. 
Various design methods to ensure the reliability and performance is 
provided may include a combination of system integrity monitoring and 
indication, redundancy of components, and maintenance actions. The need 
for system functional checks and the interval between the checks will 
be established based on the level of ``system maintenance indication 
provided for features of the system essential for proper system 
operation'' and the reliability of the system. If continual system 
monitoring is provided or features of the system have high inherent 
reliability, daily checks would not be needed to meet the reliability 
requirements in these special conditions. As we stated in the preamble, 
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. The time 
interval between system health checks and maintenance will be 
established by the reliability analysis, any airworthiness limitations, 
and the FOEB. No changes were made as a result of this comment.
    Comment: The commenter states that these special conditions propose 
that the MMEL permit operation with an inoperative flammability 
reduction system (FRS) for up to 10 days/60 flight hours. The commenter 
agrees that the system should be operational to the maximum extent 
practical and therefore, as stated in the preamble, ``the shortest 
practical MMEL relief interval should be proposed.'' The commenter 
believes that 10 days is an excessive MMEL relief interval for the FRS 
and states that a 3-day interval, such as adopted for other inoperative 
safety systems such as flight data recorders, would be a more 
appropriate interval.
    FAA Reply: We do not concur with the commenter regarding setting a 
specific MMEL interval in the special conditions. The applicant has 
proposed a 10-day MMEL relief period, but the FOEB will determine and 
approve the appropriate MMEL intervals based on data the applicant 
submits to the FAA. The applicant must show that the fleet average 
flammability exposure of a tank with an FRM installed is equal to or 
less than 3 percent, including any time

[[Page 73569]]

when the system is inoperative. Setting a prescriptive limit on the 
MMEL interval such as 3 days would not allow the designer to use the 
more objective performance based criteria that are currently in these 
special conditions. No changes were made as a result of this comment.

Special Conditions

I. Definitions
    Comment: The commenter requests ``bulk average'' be removed from 
the definition of inert. The commenter requests this change in order 
that the FAA and EASA FRM special conditions for the Boeing 737 series 
airplanes remain harmonized.
    FAA Reply: We concur with the commenter. We have modified the 
definition to read as follows:
    Inert. For the purpose of these special conditions, the tank is 
considered inert when the 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.
II. System Performance and Reliability
    Comment: The commenter would like to know why the takeoff phase of 
flight was not included in the warm day requirements in paragraphs 
II(b) and II(b)(2). The commenter states the 747 FRM Special Conditions 
25-285-SC included this phase.
    FAA Reply: Although the takeoff phase of flight is not specifically 
called out in these special conditions, it remains one portion of the 
flight that must be included in the warm day requirements. We changed 
paragraph II(b)(2) to define the climb portion of the flight to include 
the short time interval of takeoff. The ground phase of operation is 
differentiated from the climb phase (that includes takeoff) by aircraft 
rotation. This was done to simplify the flammability analysis by 
eliminating the need to conduct a separate warm day flammability 
analysis for the takeoff phase of flight. No changes were made as a 
result of this comment.
III. Maintenance
    Comment: The commenter requests that the requirements in paragraphs 
III(a) and III(b) of the FAA 737 FRM Special Conditions be revised to 
align with the following maintenance requirement in the EASA 747 FRM 
Special Condition RP747-E-01 (the maintenance requirement proposed for 
the EASA 737 FRM Special Conditions is identical):

    The FRS [flammability reduction system] shall be subject to 
analysis using conventional processes and methodology to ensure that 
the minimum scheduled maintenance tasks required for securing the 
continuing airworthiness of the system and installation are 
identified and published as part of the CS 25.1529 compliance. 
Maintenance tasks arising from either the Monte Carlo analysis or a 
CS 25.1309 safety assessment shall be dealt with in accordance with 
the principles laid down in FAA AC 25.19. The applicant shall 
prepare a validation program for the associated continuing 
airworthiness maintenance tasks, fault finding procedures, and 
maintenance procedures.

The commenter agrees that conventional procedures should be used to 
identify necessary maintenance tasks. The FAA wording implies that 
limitations must be identified for all maintenance tasks, whereas 
detailed development of the Model 747 FRM maintenance procedures has 
identified that this is not appropriate for some tasks (i.e., the daily 
inspection of status messages on the Engine Indication and Crew 
Alerting System (EICAS)). Airworthiness limitations in the form of 
maintenance tasks, inspections, or Critical Design Configuration 
Control Limitations (CDCCL) were defined by SFAR 88 to address unsafe 
conditions resulting from ignition source risks. The proposed FRM is 
intended as an additional layer of safety above ignition source 
prevention measures. The FRM will be allowed to be inoperative and on 
the Minimum Equipment List (MEL). Therefore, no feature of the FRM 
affects the airworthiness of the airplane.
    FAA Reply: We agree in part regarding the comment that 
Airworthiness Limitations, in the form of maintenance tasks, 
inspections, or CDCCLs were defined by SFAR 88 to address unsafe 
conditions resulting from ignition source risks and that the FRM is 
seen as an additional layer of protection to the ignition source 
prevention measures. However, the performance and reliability of the 
FRM, are critical to providing that additional layer of safety for the 
center wing tank and as such, there must be limitations established to 
ensure that maintenance actions and installations of auxiliary fuel 
tanks do not increase the overall fleet average flammability exposures 
above that permitted by these special conditions. Airworthiness 
Limitations for the FRM system are only required for:
    (1) those FRM components that, if failed, would affect the 
performance and/or reliability of the FRM system as dictated by the 
requirements in paragraphs II(a) and (b); and
    (2) any critical features of a fuel tank system needed in order to 
prevent an auxiliary fuel tank installation from increasing the 
flammability exposure in the center wing fuel tank above that required 
under paragraphs II(a)(1), II(a)(2), and II(b) or degrading the 
performance or reliability of the FRM.
    No changes have been made as a result of this comment.
    Comment: This commenter requests that the FAA revise paragraphs 
III(c) and III(c)(1) in the final 737 FRM Special Conditions to align 
with the EASA 747 FRM Special Condition RP747-E-01 requirement for In-
Service monitoring which states ``Following introduction to service the 
applicant must introduce an event monitoring program, accruing data 
from a reasonably representative sample of global operations, to ensure 
that the implications of component failures affecting the FRS are 
adequately assessed on an on-going basis.'' The In-service monitoring 
requirement proposed for the EASA 737 FRM Special Condition is the 
same. The commenter states that the sampling approach in the EASA 
requirement will be sufficient to verify whether the FRM is operating 
within the expected failure rates, or if changes are necessary to 
improve reliability. Requirements harmonized with EASA will facilitate 
consistent requirements for all manufacturers and operators.
    FAA Reply: We do not concur with changing the special conditions. 
The reporting requirements defined in these special conditions allow 
the design approval holder (DAH) the latitude to develop a reporting 
system for approval by the authorities based on data obtained through 
business agreements with certain operators. Since the special 
conditions do not require data be collected from all operators and 
allows the DAH to propose a reporting system that does not require data 
from all operators, the requirements already allow for sampling to some 
degree. Since the FRS may only be installed on a relatively small 
number of airplanes operated in distinct portions of the globe, it may 
not be possible to provide data for ``reasonably representative sample 
of global operations'' as stated in the EASA proposed special 
conditions. No changes were made as a result of this comment.

Appendix 1: Monte Carlo Analysis

    Comment: The commenter requests that the phrase ``fleet average 
flammability exposure'' be changed to ``fleet average or warm day 
flammability exposure'' in paragraph (c) of Appendix 1. The commenter 
requests this change be made in order that the FAA and

[[Page 73570]]

EASA 737 FRM special requirements remain harmonized.
    FAA Reply: We concur with the commenter. We intend that paragraph 
(c) of Appendix 1 requir