Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/ 100B SUD/400/400D/400F Airplanes; Flammability Reduction Means (Fuel Tank Inerting), 7800-7827 [05-2752]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 70, Number 30 (Tuesday, February 15, 2005)]
[Rules and Regulations]
[Pages 7800-7827]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 05-2752]



[[Page 7799]]

-----------------------------------------------------------------------

Part II





Department of Transportation





-----------------------------------------------------------------------



Federal Aviation Administration



-----------------------------------------------------------------------



14 CFR Part 25



Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/
100B SUD/400/400D/400F Airplanes; Flammability Reduction Means (Fuel 
Tank Inerting); Final Special Conditions; Rule

Federal Register / Vol. 70, No. 30 / Tuesday, February 15, 2005 / 
Rules and Regulations

[[Page 7800]]


-----------------------------------------------------------------------

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM270; Special Conditions No. 25-285-SC]


Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/
100B/300/ 100B SUD/400/400D/400F Airplanes; Flammability Reduction 
Means (Fuel Tank Inerting)

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

-----------------------------------------------------------------------

SUMMARY: These special conditions are issued for the Boeing Model 747-
100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F 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 March 17, 
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 Model 747 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.
    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

[[Page 7801]]

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 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

[[Page 7802]]

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 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 November 15, 2002, Boeing Commercial Airplanes applied for a 
change to Type Certificate A20WE to modify Model 747-100/200B/200F/
200C/SR/SP/100B/300/100B SUD/400/400D/400F series airplanes to 
incorporate a new FRM that inerts the center fuel tanks with NEA. These 
airplanes, approved under Type Certificate No. A20WE, are four-engine 
transport airplanes with a passenger capacity up to 624, depending on 
the submodel. These airplanes have an approximate maximum gross weight 
of 910,000 lbs with an operating range up to 7,700 miles.

Type Certification Basis

    Under the provisions of Sec.  21.101, Boeing Commercial Airplanes 
must show that the Model 747-100/200B/200F/200C/SR/SP/100B/300/100B 
SUD/400/400D/400F series airplanes, as changed, continue to meet the 
applicable provisions of the regulations incorporated by reference in 
Type Certificate No. A20WE, 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 A20WE include 14 CFR part 25, dated 
February 1, 1965, as amended by Amendments 25-1 through 25-70, except 
for special conditions and exceptions noted in Type Certificate Data 
Sheet A20WE.
    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 747-100/200B/200F/200C/SR/SP/100B/300/
100B SUD/400/400D/400F 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 747-100/200B/200F/200C/SR/SP/
100B/300/100B SUD/400/400D/400F 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 747-100/200B/

[[Page 7803]]

200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F 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, the 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 747-
100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F series 
airplanes. Boeing also plans to seek approval of this system on Boeing 
Model 737, 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 originally proposed that the system be operated only during 
flight and that the center tank would continue to be inert on landing 
and remain inert during normal ground procedures. Boeing has more 
recently stated that the FRM design may include the capability to be 
operated on the ground.
    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 
originally proposed that there be no cockpit or maintenance indication 
onboard for the NGS, and that periodic maintenance, using ground 
service equipment, be performed to verify system operation. More 
recently 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. Since amendment 25-102 was adopted, significant advancements in 
inerting technology have reduced the size and complexity of inerting 
systems. Developments in inerting technology have made it practical to 
significantly reduce fuel tank flammability below the levels required 
within the rule. However, due to factors such as the limited 
availability of bleed air and electrical power, it is not considered 
practical at this time to develop systems for retrofit into existing 
airplane designs that can maintain a non-flammable tank ullage in all 
fuel tanks or during all operating conditions. 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 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 special conditions. Therefore, the effect 
of the definition of ``inert'' within these 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 special conditions (12 
percent or less at sea level) to be considered inert.

[[Page 7804]]

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 JAA 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 special conditions, the FAA and the 
European Joint Aviation Authorities (JAA) 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 the Joint Airworthiness Authorities 
(JAA) 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, takeoff, and 
climb phases of

[[Page 7805]]

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 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, it is considered a safety 
system that 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. At a 
minimum, proper function of essential features of the system should be 
validated once per day by maintenance review of indications or 
functional checks, possibly prior to the first flight of the day. 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 indicates 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 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 
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 special conditions requires the applicant to provide 
markings to emphasize the potential hazards associated with confined 
spaces and areas where a hazardous atmosphere

[[Page 7806]]

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 the 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 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 install 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 
fuel tank volume of the 747 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 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, 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.

Discussion of Comments

    Notice of Proposed Special Conditions No. 25-03-08-SC for the 
Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/
400F series airplanes was published in the Federal Register on December 
9, 2003 (68 FR 68563). Thirteen commenters responded to the notice.

General Comments

    Comment: One commenter supports the special conditions but states 
that ignition source prevention must still be provided. The commenter 
believes that the combination of flammability reduction and ignition 
source prevention is the most effective means to prevent fuel tank 
explosions.
    FAA Reply: The safety assessment required by Special Federal 
Aviation Regulation (SFAR) No. 88, Fuel Tank System Fault Tolerance 
Evaluation, identifies design and maintenance changes that are needed 
to prevent ignition sources in transport category airplanes. The FAA is 
developing a number of airworthiness directives (ADs) to address 
ignition sources resulting from single failures in all fuel tanks and 
combinations of failures in tanks that have been classified as high 
flammability. We will not issue ADs to address combinations of failures 
in high flammability tanks if the FRM is installed because of the 
significant improvement in fuel tank safety offered by the FRM required 
by this special condition. We are not considering a change to the 
current ignition prevention analysis requirements that include assuming 
a flammable ullage. No changes were made as a result of this comment.
    Comment: Two commenters believe the special conditions for the FRM 
are

[[Page 7807]]

not appropriate because the special conditions are written to fit the 
applicant's proposed design of an inerting system to reduce 
flammability of fuel tanks and are therefore considered ``prejudiced.'' 
One of these commenters adds that regulatory guidance should be 
unprejudiced and available before development of any design.
    FAA Reply: We do not concur. As stated earlier in this document, 
these special conditions are specific to certification of an FRM based 
on inerting technology. As discussed in AC 25.981-2, inerting, as well 
as other technologies such as cooling, is an acceptable means of 
compliance with Sec.  25.981(c). No changes were made as a result of 
this comment.
    Comment: Two commenters believe the limited FRM, as described in 
the special conditions, would not comply with the requirements of 
Sec. Sec.  25.981(c) and 25.1309 for new airplane designs (post 
amendment 25-102) with high flammability fuel tanks.
    FAA Reply: As stated earlier, these special conditions apply 
specifically to certification of an FRM for applicable Boeing Model 747 
series airplanes and do not apply to new airplane designs. However, we 
have determined that an FRM that complies with these special conditions 
would meet the intent of Sec.  25.981(c). No changes were made as a 
result of this comment.
    Comment: One commenter would support rulemaking to investigate 
amending Sec.  25.981 (and revising AC 25.981-2) to:
     Clarify that ``minimization of flammable vapors'' in 
accordance with Sec.  25.981(c) is to be accomplished through design 
features ensuring the tank will have inherent low flammability (e.g. 
venting, cooling, control of heat transfer, etc.); and
     Eliminate the possibility of compliance for future 
airplane designs through the installation of a limited FRM.
    FAA Reply: On February 17, 2004, the FAA Administrator announced 
plans to issue a notice of proposed rulemaking that will require 
approximately 3,800 Airbus and Boeing planes be fitted with systems 
that reduce the presence of flammable vapors in fuel tanks. This 
proposal could require airlines to install new systems to reduce fuel 
tank flammability on existing and newly produced larger passenger jets. 
We are also considering amending Sec.  25.981(c) and revising AC 
25.981-2 to further limit fuel tank flammability. No changes were made 
as a result of these comments.
    Comment: The commenter requests that before proceeding with any 
further regulatory activities, the FAA should provide additional 
detailed information on whether SFAR 88 changes are sufficient to cover 
the requirements of Sec.  25.981. The commenter believes that ``SFAR 88 
meets the requirement of Sec.  25.981(c)(2) and does not understand the 
need to also address Sec.  25.981(c)(1).'' This commenter also states 
that harmonization with the European Aviation Safety Agency (EASA) on 
these special conditions is essential for industry.
    FAA Reply: We do not concur with the commenter's first statement. A 
direct relationship between SFAR 88 and Sec.  25.981(c)(1), or Sec.  
25.981(c)(2), does not exist. SFAR 88 addresses ignition source 
prevention, while Sec.  25.981(c)(1) acknowledges an ignition source 
may be present under some remote circumstances. Section 25.981(c)(2) 
assumes that an ignition can occur--in essence that SFAR 88 was not 
successful and also flammable vapors are present--and requires that the 
resulting ignition of flammable vapor will not prevent continued safe 
flight and landing. The FAA has fully coordinated these special 
conditions with the JAA/EASA. No changes were made as a result of these 
comments.
    Comment: One commenter notes that although the special condition 
requirements for system reliability and performance are very specific, 
they do not address the qualification standards that the system will 
have to meet. Additional guidance on this subject would be appropriate. 
Another commenter expresses concern about use of the terms ``intended'' 
and ``expected'' in the special conditions when relating to an FRM. It 
is the commenter's opinion that the use of these terms indicates that 
the applicant is not confident that their design ``will'' or ``shall'' 
contribute to the overall safety of the airplanes.
    FAA Reply: We do not concur. In the preamble to the special 
conditions, we state that the applicant is required to show compliance 
with the applicable airworthiness regulations and special conditions. 
In part, the applicable regulations, Sec.  25.1301 and Sec.  25.1309, 
require the applicant to show that the equipment ``functions properly 
when installed'' and ``is designed to ensure that they perform their 
intended functions under any foreseeable operating condition.'' 
Irrespective of any wording in the preamble to the special conditions, 
the special conditions include requirements to address foreseeable 
specific safety issues that are not addressed by the current 
regulations. Any airplane that meets the requirements of the special 
conditions will maintain the level of safety intended by the applicable 
requirements of the Code of Federal Regulations (CFR). No changes were 
made as a result of these comments.
    Comment: One commenter states that there are various statements 
made throughout the special conditions that refer to reliability and 
maintenance of the system. It is the commenter's opinion that these 
statements are specific to implementation, and the actual approach 
should be derived using standard methodology used for certification of 
the airplane.
    FAA Reply: To achieve the desired safety level of the FRM, we 
believe the special condition requirements for determining reliability 
and maintainability of the FRM are necessary. This is to ensure that 
the FRM is an acceptable means by which the development of flammable 
vapors in the center wing tank is minimized as required by Sec.  
25.981. No changes were made as a result of this comment.
    Comment: One commenter notes that ``inert'' is not defined 
consistently throughout the special conditions. The commenter suggests 
the use of only one definition and proposes the definition used in 
special condition paragraph I. Definitions. The same commenter also 
requests clarification if linear extrapolation of oxygen concentration 
can be used for aircraft ceilings above 40,000 feet, and clarification 
of the difference between the terms ``bulk'' and ``bulk average.''
    FAA Reply: We concur that the definition of inert needs to be 
consistent throughout the special conditions and have therefore 
modified the definition of inert in the preamble to incorporate the 
definition of inert provided in paragraph I. Definitions of the special 
conditions. With respect to aircraft altitudes above 40,000 feet, we 
have added that linear extrapolation can continue for oxygen 
concentration from 14.5 percent at 40,000 feet to the required 
operating altitude. Concerning the use of bulk and bulk average in the 
special conditions, we have modified the preamble and special 
conditions to consistently use the term ``bulk average'' when referring 
to the fuel temperature or oxygen concentration within the fuel tank.
    Comment: The commenter requests that the FAA clarify if the FRM is 
a safety enhancement system or a safety system. The commenter notes 
that in the preamble discussion of the ``Inerting System Indication,'' 
the FAA states that the applicant may propose master minimum equipment 
list (MMEL) relief

[[Page 7808]]

be provided for airplane operation with the FRM unavailable. The 
system, however, is considered a safety system that should be 
operational to the maximum extent practical. If this system is 
considered a safety system, then a form of redundancy will have to be 
built in. At this time, the applicant's design does not show any 
redundancy.
    FAA Reply: The FRM is a safety system designed to provide an 
additional layer of protection to the ignition prevention means already 
in place. The system by itself is not intended to be fully redundant 
since it provides a second layer of protection. The FRM is intended to 
be a safety enhancement system that provides an additional layer of 
protection by reducing the exposure to flammable vapors in the heated 
center wing fuel tank. This protection, when added to ignition 
prevention measures, will substantially reduce the likelihood of future 
fuel tank explosions in the fleet. The applicant has proposed a 10-day 
MMEL relief period, but the Flight Operations Evaluation Board (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 when the system is 
inoperative. No changes were made as a result of these comments.
    Comment: One commenter says the cost of the FRM is substantial and 
justification for it is debatable. The commenter believes the FRM will 
put a heavy economic burden on the slowly recovering airline industry 
and only supports the adoption of an FRM on new type designs and newly 
built airplanes as an improvement in fuel system safety. This commenter 
also says that considering the potential affects of this subject on the 
European airline industry, joint European position activity is critical 
to ensure that decisions are based on safety grounds and not on 
political motivations.
    FAA Reply: We do not concur with the commenter regarding the impact 
of cost associated with the issuance of the special conditions. These 
special conditions are unique to the applicant's certification of an 
FRM for the applicable Boeing Model 747 series airplanes and do not 
mandate that an FRM must be added to an operator's 747 fleet. They have 
been fully harmonized with EASA. The FAA announcement of issuance of a 
notice of proposed rulemaking that would propose retrofit and 
production incorporation of FRM into U.S-registered airplanes is a 
separate rulemaking effort that will require a cost benefit analysis 
and will be published for public comment. No changes were made as a 
result of this comment.
    Comment: One commenter notes that the applicant has planned a 3-
month, in-service evaluation (ISE) of the FRM. It is the opinion of two 
other commenters that a 4,000-hour (12 month) ISE should be specified 
before certification of the FRM because--
     It adds complexity,
     It has not yet been retrofitted in an in-service airplane,
     It has no proven track record for reliability, and
     Ground and flight tests are not sufficient to demonstrate 
overall reliability of the system.

    The commenters say that maintenance and performance features of the 
system were designed to support a 10-day relief under the MMEL program. 
If the demonstrated performance and reliability of the system meet 
design objectives, then the FAA should support the planned relief. 
Another commenter recommends a one-year in-service evaluation (ISE) 
program following the first installation of an FRM and prior to FRM 
installation on a production airplane. This commenter says that past 
experience has shown reliability and system degradation by oil 
contamination scenarios, with the engine and APU being the source, and 
carbon particle buildup on components similar to those required by the 
proposed FRM, due to airport and airplane turbine exhausts. This 
commenter believes that one year would be an adequate time for the 
manufacturer to develop and provide corrective actions for 
discrepancies or reliability issues with the FRM that are identified 
during the ISE program.
    FAA Reply: We do not concur with the commenters. The industry 
commonly conducts ISE through coopera