[Federal Register Volume 68, Number 236 (Tuesday, December 9, 2003)]
[Proposed Rules]
[Pages 68563-68573]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-30449]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM270; Notice No. 25-03-08-SC]


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

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This notice proposes special conditions for the Boeing Model 
747-100/200B/200F/200C/SR/SP/100B SUD/400/400D/400F series airplanes. 
These airplanes, as modified by Boeing Commercial Airplanes, will 
incorporate a new flammability reduction system that uses a nitrogen 
generation system to reduce the oxygen content in the center wing fuel 
tank so that exposure to a combustible mixture of fuel and air is 
substantially minimized. This system is intended to reduce the average 
flammability exposure of the fleet of airplanes with the system 
installed to a level equivalent to 3 percent of the airplane operating 
time. The applicable airworthiness regulations do not contain adequate 
or appropriate safety standards for the design and installation of this 
system. These proposed special conditions contain the additional safety 
standards that the Administrator considers necessary to ensure an 
acceptable level of safety for the installation of the system and to 
define performance objectives that the system must achieve to be 
considered an acceptable means for minimizing the development of 
flammable vapors in the fuel tank installation.

DATES: Comments must be received on or before January 23, 2004.

ADDRESSES: Comments on this proposal may be mailed in duplicate to: 
Federal Aviation Administration, Transport Airplane Directorate, Attn: 
Rules Docket (ANM-113), Docket No. NM270, 1601 Lind Avenue SW., Renton, 
Washington, 98055-4056; or delivered in duplicate to the Transport 
Airplane Directorate at the above address. Comments must be marked: 
Docket No. NM270. Comments may be inspected in the Rules Docket 
weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.

FOR FURTHER INFORMATION CONTACT: Mike Dostert, Propulsion and 
Mechanical Systems Branch, FAA, 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 [email protected].

SUPPLEMENTARY INFORMATION: 

Comments Invited

    The FAA invites interested persons to participate in this 
rulemaking by submitting written comments, data, or views. The most 
helpful comments reference a specific portion of the proposed special 
conditions, explain the reason for any recommended change, and include 
supporting data. We ask that you send us two copies of written 
comments.
    All comments received will be filed in the docket, as well as a 
report summarizing each substantive public contact with FAA personnel 
concerning these proposed special conditions. The docket is available 
for public inspection before and after the comment closing date. If you 
wish to review the docket in person, go to the address in the ADDRESSES 
section of this preamble between 7:30 a.m. and 4 p.m., Monday through 
Friday, except Federal holidays.
    We will consider all comments we receive on or before the closing 
date for comments. We will consider comments filed late if it is 
possible to do so without incurring expense or delay. We may change 
these proposed special conditions based on the comments we receive.
    If you want the FAA to acknowledge receipt of your comments on 
these proposed special conditions, include with your comments a pre-
addressed, stamped postcard on which the docket number appears. We will 
stamp the date on the postcard and mail it back to you.

Background

    Boeing Commercial Airplanes intends to modify Model 747 series 
airplanes to incorporate a new flammability reduction system that will 
inert the center fuel tanks with nitrogen-enriched air. Though the 
provisions of Sec.  25.981, as amended by Amendment 25-102, will apply 
to this design change, these special conditions are being proposed to 
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 airplane 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 aircraft 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 (tank vapor 
space). 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

    The flammability temperature range of jet engine fuel vapors varies 
with the type of jet fuel, the ambient pressure in the tank, and the 
amount of dissolved oxygen that may be present in the tank due to 
vibration and sloshing of the fuel that occurs within the tank.
    At sea level pressures and with no sloshing or vibration present, 
Jet A fuel, the most common commercial jet fuel in the United States, 
and Jet A1 used in most portions of the world, have flammability 
characteristics that tend to make the fuel vapor-air mixture too 
``lean'' to ignite at temperatures below approximately 100[deg]F, and 
too ``rich'' to ignite at temperatures above 175[deg]F. This range of 
flammability (100[deg]F to 175[deg]F) is reduced to cooler temperatures 
as the airplane gains altitude due to the corresponding reduction of 
pressure. For example, at an altitude of 30,000 feet the flammability 
temperature range is approximately 60[deg]F to 120[deg]F.
    The flammability range of Jet B (JP-4), another fuel approved for 
use on most commercial transport airplanes but not used as a primary 
fuel, is approximately 15[deg]F to 75[deg]F at sea level, and -20[deg]F 
to 35[deg]F at 30,000 feet. Because Jet B fuel flammable temperature 
ranges as a function of pressure altitude are more within normal 
temperatures at altitudes, airplane fuel tanks are flammable for a much 
larger portion of the flight.
    Most commercial transports are approved for operation at altitudes 
in the range of 30,000 to 45,000 feet. The FAA has always assumed that 
airplanes

[[Page 68564]]

would be operated with flammable fuel vapors in their fuel tank ullage. 
Commercial transports operated in the United States, and in most 
overseas locales, use Jet A or Jet A-1 fuel, which typically limits 
exposure to operation in the flammability range to warmer days.

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. In 1996 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) 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 
http://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, 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 operational time 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 fuel tank operation 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

[[Page 68565]]

into and out of fuel tanks such that heating of the fuel would not 
occur. 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 the flammability 
on new type designs, the FAA developed and published Amendment No. 25-
102 in the Federal Register on May 7, 2001 (66 FR 23085). The amendment 
includes changes to Sec.  25.981 that require minimization of fuel tank 
flammability to address both reduction in the time fuel tanks contain 
flammable vapors, (new Sec.  25.981(c)), and additional changes 
regarding prevention of ignition sources in fuel tanks. The new Sec.  
25.981(c) is 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 
there was reference 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 membranes act as a molecular sieve, using the size difference 
between the nitrogen and oxygen molecules to separate the nitrogen-
enriched air (NEA) from the oxygen. In airplane applications NEA is 
produced when pressurized air from the airplane 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: http://www.fire.tc.faa.gov/pdf/tno2-79.pdf and will be 
published in 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 nitrogen-enriched air that is needed to inert 
commercial airplane fuel tanks was lessened so that an effective 
flammability reduction system 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, turbulent flow flame 
propagation, 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 flammability reduction system 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. It is expected that by 
combining these two approaches, particularly for tanks with high 
flammability exposures, 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 flammability reduction system that inerts the center 
fuel tanks with nitrogen-enriched air. The Model 747-100/200B/200F/
200C/SR/SP/100B/300/100B SUD/400/400D/400F series airplanes, approved 
under

[[Page 68566]]

Type Certificate No. A20WE, are four-engine transport airplanes with a 
passenger capacity up to 624 depending upon 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 FRS 
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/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 a flammability reduction system 
(FRS) to minimize the development of flammable vapors in the center 
fuel tanks of Model 747-100/200B/200F/200C/SR/SP/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 FRS does not introduce any additional potential 
sources of ignition into the fuel tanks.
    The proposed FRS 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 will be located in the air conditioning pack bay below the 
center wing fuel tank. Engine bleed air from the existing engine 
pneumatic bleed source will flow through a control valve into an ozone 
converter and then through a heat exchanger, where it will be cooled 
using outside cooling air. The cooled air will flow through a filter 
into an air separation module (ASM) that will generate nitrogen-
enriched air (NEA), which will be supplied to the center fuel tank, and 
also discharge oxygen-enriched air (OEA). The OEA from the ASM will be 
mixed with cooling air from the heat exchanger to dilute the oxygen 
concentration and then exhausted overboard. The FRS will also include 
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. 
Certain features of the FRS may introduce a hazard to the airplane if 
not properly addressed.
    Boeing originally proposed that the system be operated only during 
flight and that the center tank would continue to be inert upon landing 
and remain inert during normal ground procedures. Boeing has more 
recently stated that the FRS may 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 system reliability and 
availability objectives, built-in test functions would be included and 
system status indication of some kind would be provided but the 
indication would not be provided in the cockpit. 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 proposed FRS design will result in application of Sec. Sec.  
25.981(a) and (b), Amendment 25-102, for the proposed 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 FRS system 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 FRS 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. The FAA also

[[Page 68567]]

recognizes that fuel tank flammability reduction systems could be 
developed that would meet the flammability requirements of Sec.  
25.981(c), Amendment 25-102, but may not preclude fuel tanks from 
routinely being flammable under the specific operating conditions 
present when recent accidents occurred.

Definition of ``Inert''

    The definition of ``inert'' within these proposed special 
conditions provides that all portions of the tank under evaluation, 
including the bulk average of individual compartments, are equal to or 
less than the 12 percent oxygen limit at sea level. This 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 values used to establish the 12 
percent limit. Therefore, ignition in other compartments of the tank 
may be possible. Technical discussions with the applicant indicate the 
pressure rise in a fuel tank that was at or near the 12 percent oxygen 
concentration level would likely be well below the value that would 
rupture a typical transport airplane fuel tank. While this may be 
possible to show, it is not within the scope of these proposed special 
conditions. Therefore, the effect of the definition of ``inert'' within 
these proposed special conditions is that the bulk average of each 
individual compartment or bay of the tank be evaluated and shown to 
meet the oxygen concentration limits specified in the definitions 
section of these proposed special conditions (12 percent or less at sea 
level) to be considered inert.

Determining Flammability

    The methodology for determining fuel tank flammability defined for 
use in these proposed special conditions is based on that used by ARAC 
to compare the flammability of unheated aluminum wing fuel tanks to 
that of tanks that are heated by adjacent equipment. The ARAC evaluated 
the relative flammability of airplane fuel tanks using a statistical 
analysis commonly referred to as a ``Monte Carlo'' analysis that 
considered a number of factors affecting formation of flammable vapors 
in the fuel tanks. The Monte Carlo analysis calculates values for the 
parameter of interest by randomly selecting values for each of the 
uncertain variables from distribution tables. This calculation is 
conducted over and over to simulate a process where the variables are 
random within defined distributions. The results of 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 included in 
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, a definition of the conditions when the tank 
in question will be considered flammable, and those 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 
three variables. 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 is calculated and used as the parameter 
that established whether 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 the 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 loaded for a given flight, which is randomly 
selected from a database consisting of worldwide data. The criteria in 
the model is 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).

Definition of Transport Effects

    The effects of mass loading and the effects of fuel vaporization 
and condensation with time and temperature changes, referred to as 
``transport effects'' in these proposed special conditions, are 
excluded from consideration in the Monte Carlo model used for 
demonstrating compliance with these proposed special conditions. These 
effects have been excluded because they were not considered in the 
original ARAC analysis, which was based on a relative measure of 
flammability. For example, the 3 percent flammability value established 
by the ARAC as the benchmark for fuel tank safety for wing fuel tanks 
did not include the effects of cooling of the wing tank surfaces and 
the associated condensation of vapors from the tank ullage. If this 
effect had been included in the wing tank flammability calculation, it 
would have resulted in a significantly lower wing tank flammability 
benchmark value. The ARAC analysis also did not consider the effects of 
mass loading which would significantly lower the calculated 
flammability value for fuel tanks that are routinely emptied, e.g., 
center wing tanks. The FAA and JAA have determined that using the ARAC 
methodology provides a suitable basis for determining the adequacy of 
an FRS system.

Flammability Limit

    The FAA, in conjunction with the Joint Airworthiness Authorities 
(JAA) and Transport Canada, has developed criteria within these 
proposed special conditions that require overall fuel tank flammability 
to be limited to 3 percent of the fleet average operating time. This 
overall average flammability limit consists of times when the system

[[Page 68568]]

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 FRS is inoperative due to failures of 
the system and dispatch with the system inoperative.

Specific Risk Flammability Limit

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

Inerting System Indications

    Fleet average flammability exposure involves several elements, 
including--
    [sbull] The time the FRS is working properly and inerts the tank or 
when the tank is not flammable;
    [sbull] The time when the FRS is working properly but fails to 
inert the tank or part of the tank, because of mission variation or 
other effects;
    [sbull] The time the FRS is not functioning properly and the 
operator is unaware of the failure; and
    [sbull] The time the FRS 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 FRS unavailable; however, it is considered a safety 
system that should be operational to the maximum extent practical. 
Therefore, these proposed special conditions include reliability and 
reporting requirements to enhance system reliability so that dispatch 
of airplanes with the FRS inoperative would be very infrequent. Cockpit 
indication of the system function that is accessible to the flightcrew 
is not an explicit requirement, but may be required if the results of 
the Monte Carlo analysis show the system cannot otherwise meet the 
flammability and reliability requirements defined in these proposed 
special conditions. Flight test demonstration and analysis will be 
required to demonstrate that the performance of the inerting system is 
effective in inerting the tank during those portions of ground and the 
flight operations where inerting is needed to meet the flammability 
requirements of these proposed special conditions.
    Various means may be used to ensure system reliability and 
performance. These may include: system integrity monitoring and 
indication, redundancy of components, and maintenance actions. A 
combination of maintenance indication and/or maintenance check 
procedures will be required to limit exposure to latent failures within 
the system, or high inherent reliability is needed to assure the system 
will meet the fuel tank flammability requirements. The inerting system 
proposed by the applicant 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 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 FRS will result in fuel system airworthiness 
limitations as 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 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 has proposed that the FRS be eligible for a 10-day MMEL 
dispatch interval. The approved interval will be established by the 
Flight Operations Evaluation Board (FOEB) based on data submitted by 
the applicant 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 FRS, prior to FAA approval of the MMEL. 
Boeing has requested that the authorities agree to a MMEL inoperative 
dispatch interval to be used for design of the system. Data presented 
by Boeing indicates that certain systems on the airplane are routinely 
repaired prior to the maximum allowable interval. These proposed 
special conditions require an MMEL dispatch inoperative interval of 60 
hours to be used in the analysis as representative of the mean time for 
which an inoperative condition may occur for the 10-day MMEL maximum 
interval requested, and that Boeing include actual dispatch inoperative 
interval data in the quarterly reports required by these special 
conditions. 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 FRS achieved in service meets the 
levels used in the analysis.
    Appropriate maintenance and operational limitations with the FRS 
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 possible effects of 
the risk associated with portions of the fleet that operate in warmer 
climates where the fuel tanks are flammable a significant portion of 
the operational time when not inert. While the system reliability 
analysis may show that even with an MMEL allowing very long inoperative 
intervals, it is possible to achieve an overall average fleet exposure 
equal to or less than that of a typical unheated aluminum wing tank, 
the intent of the rule is to minimize flammability and the shortest 
practical MMEL relief interval should be proposed. To ensure limited 
airplane operation with the system inoperative and to meet the 
reliability requirements of these proposed special conditions, 
appropriate level messages that are needed to comply with any dispatch 
limitations of the MMEL must be provided.

[[Page 68569]]

Confined Space Hazard Markings

    Introduction of the FRS will result in NEA within the fuel tanks 
and the possibility of NEA in compartments adjacent to the fuel tanks 
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. These proposed special conditions introduce 
requirements to address this issue.

Affect of FRS on Auxiliary Fuel Tank System Supplemental Type 
Certificates

    Boeing plans to offer a service bulletin that will install the FRS 
on existing in-service airplanes. Some in-service airplanes have 
auxiliary fuel tank systems installed that interface with the center 
wing tank. The Boeing FRS 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 FRS 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 FRS.

Disposal of Oxygen-Enriched Air

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

Applicability

    As discussed above, these proposed special conditions are 
applicable to the Boeing Model 747-100/200B/200F/200C/SR/SP/100B SUD/
400/400D/400F series airplanes. Should the type certificate be amended 
later to include any other model that incorporates the same or similar 
novel or unusual design feature, or should any other model already 
included on the same type certificate be modified to incorporate the 
same or similar novel or unusual design feature, the special conditions 
would also apply to the other model under the provisions of Sec.  
21.101.

Conclusion

    This action affects only certain novel or unusual design features 
on Boeing Model 747-100/200B/200F/200C/SR/SP/100B SUD/400/400D/400F 
series airplanes. It is not a rule of general applicability and affects 
only the applicant who applied to the FAA for approval of these 
features on the airplane.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

    The authority citation for these special conditions is as follows:

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

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions as part of the type certification basis 
for Boeing Model 747-100/200B/200F/200C/ SR/SP/100B SUD/400/400D/400F 
series airplanes, modified by Boeing Commercial Airplanes, to include a 
flammability reduction system (FRS) that uses a nitrogen generation 
system to inert the center wing tank with nitrogen-enriched air (NEA).
    Compliance with these proposed special conditions does not relieve 
the applicant from compliance with the existing certification 
requirements.

I. Definitions.

    (a) Flammable. With respect to a fluid or gas, flammable means 
susceptible to igniting readily or to exploding (14 CFR part 1, 
Definitions). A non-flammable ullage is one where the gas mixture is 
too lean or too rich to burn and/or is inert per the definition below. 
For the purposes of these special conditions, a fuel tank is considered 
flammable when the bulk fuel temperature within any compartment of the 
tank is within the flammable range for the fuel type being used.
    (b) Flash Point. The flash point of a flammable fluid is defined as 
the lowest temperature at which the application of a flame to a heated 
sample causes the vapor to ignite momentarily, or ``flash.'' The test 
for jet fuel is defined in the ASTM specification, D56, ``Standard Test 
Method for Flash Point by Tag Close Cup Tester.''
    (c) Ignition Energy. The minimum amount of energy required to 
ignite fuel vapors. The inert oxygen concentration levels, described 
below in the definition for inert, were established using approximately 
a 0.5 Joule spark.
    (d) Inert. For the purpose of these special conditions, the tank is 
considered inert when the bulk 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.
    (e) Inerting. A process where a noncombustible gas is introduced 
into the ullage of a fuel tank so that the ullage becomes inert.
    (f) Monte Carlo Analysis. An analytical tool that provides a means 
to assess the degree of flammability exposure time for a fuel tank. See 
Appendices 1 and 2 of these special conditions for specific 
requirements for conducting the Monte Carlo analysis.
    (g) Operational Time. For the purpose of these special conditions, 
the time from the start of preparing the airplane for flight (that is, 
starting and connecting the auxiliary or ground power unit to the 
aircraft electrical system) through securing all power sources 
following flight termination.
    (h) Ullage, or Ullage Space. The volume within the tank not 
occupied by liquid fuel at the time interval under evaluation.
    (i) Hazardous atmosphere: An atmosphere that may expose employees 
to the risk of death, incapacitation, impairment of ability to self-
rescue (that is, escape unaided from a space), injury, or acute 
illness.

II. System Performance and Reliability

    The FRS, for the airplane model under evaluation, must comply with 
the performance and reliability requirements as follows:
    (a) The applicant must submit a Monte Carlo analysis, as defined in 
Appendices 1 and 2 of these special conditions that--
    (1) Demonstrates that the overall fleet flammability exposure of 
each fuel tank with an FRS installed is equal to or less than 3 percent 
of operational time; and
    (2) Demonstrates that neither the performance (when the FRS is 
operational) nor reliability (including all periods when the FRS is 
inoperative) contributions to the 3 percent overall fleet flammability 
exposure of a tank with an FRS installed are more than 1.8

[[Page 68570]]

percent (this will establish appropriate maintenance inspection 
procedures and intervals as required in paragraph III(a) of these 
special conditions).
    (b) The applicant must submit a Monte Carlo analysis that 
demonstrates that the FRS, when functional, reduces the overall fleet 
flammability exposure of each fuel tank with an FRS installed for warm 
day ground and climb phases to a level equal to or less than 3 percent 
of operational time in each of these phases for the following 
conditions--
    (1) The analysis must use the subset of 80[deg] F and warmer days 
from the Monte Carlo analyses done for overall performance; and
    (2) The flammability exposure must be calculated by comparing the 
time during ground and climb phases for which the tank was flammable 
and not inert, with the total time for the ground and climb phases.
    (c) The applicant must provide data from ground testing, and flight 
testing that--
    (1) Validate the inputs to the Monte Carlo analysis needed to meet 
paragraphs II(a), (b), and (c) of these special conditions; and
    (2) Substantiate that the NEA distribution is effective at inerting 
all portions of the tank where the inerting system is needed to show 
compliance with these paragraphs.
    (d) The applicant must validate that the FRS meets the requirements 
of paragraphs II(a), (b), and (c) of these special conditions with any 
combination of engine model, engine thrust rating, fuel type, and 
relevant pneumatic system configuration approved for the airplane.
    (e) Sufficient accessibility for maintenance personnel, or the 
flightcrew, must be provided to FRS status indications that are 
necessary to meet the reliability requirements of paragraph II(a) of 
these special conditions.
    (f) The access doors and panels to the fuel tanks (including any 
tanks that communicate with an inerted tank via a vent system), and to 
any other enclosed areas that could contain NEA in the event of a 
system failure, must be permanently stenciled, marked, or placarded as 
appropriate to warn maintenance crews of the presence of a potentially 
hazardous atmosphere.
    (g) Oxygen-enriched air produced by the nitrogen generation system 
must not create a hazard during normal operating conditions. It must be 
established that no single failure or malfunction or probable 
combination of failures will jeopardize the safe operation of the 
airplane.

III. Maintenance

    (a) Airworthiness Limitations must be identified for all 
maintenance and/or inspection tasks required to identify failures of 
components within the FRS that are needed to meet paragraphs II(a), 
(b), and (c) of these special conditions.
    (b) The applicant must provide the maintenance procedures that will 
be necessary and present a design review that identifies any hazardous 
aspects to be considered during maintenance of the FRS that will be 
included in the instructions for continued airworthiness (ICA) or 
appropriate maintenance documents.
    (c) To ensure that the implications of component failures affecting 
the FRS are adequately assessed on an on-going basis, the applicant 
must--
    (1) Demonstrate effective means to ensure collection of FRS 
reliability data. The means must provide data affecting FRS 
availablity, such as component failures, and the FRS inoperative 
intervals due to dispatch under the MMEL;
    (2) Provide a report to the FAA on a quarterly basis for the first 
five years of service introduction. After that period, continued 
quarterly reporting may be replaced with other reliability tracking 
methods found acceptable to the FAA or eliminated if it is established 
that the reliability of the FRS meets, and will continue to meet, the 
exposure requirements of paragraphs II(a) and II(b) of these special 
conditions;
    (3) Provide a report to the validating authorities for a period of 
at least two years following introduction to service; and
    (4) Develop service instructions or revise the applicable airplane 
manual, per a schedule agreed upon by the FAA, to correct any failures 
of the FRS that occur in service that could increase the fleet 
flammability exposure of the tank to more than 3 percent.

Appendix 1

Monte Carlo Analysis

    (a) A Monte Carlo analysis must be conducted for the fuel tank 
under evaluation to determine fleet average flammability exposure 
for the airplane and fuel type under evaluation. An analysis for a 
fuel tank is defined in Appendix 2 of these special conditions and 
must be used as the basis for development of the Monte Carlo 
analysis to satisfy these special conditions. Parameters used in the 
Monte Carlo analysis must include:
    (1) FRS Performance--as defined by system performance.
    (2) Cruise Altitude--as defined by airplane performance.
    (3) Cruise Ambient Temperature--as defined in Appendix 2 of 
these special conditions.
    (4) Overnight Temperature Drop--as defined in Appendix 2 of 
these special conditions.
    (5) Flash Point--as defined in Appendix 2 of these special 
conditions.
    (6) Fuel Burn--as defined by airplane performance.
    (7) Fuel Load--as defined by airplane performance.
    (8) Fuel Transfer--as defined by airplane performance.
    (9) Fueling--as defined by airplane performance.
    (10) Ground Temperature--as defined in Appendix 2 of these 
special conditions.
    (11) Mach Number--as defined by airplane performance.
    (12) Mission Distribution--the applicant must either provide 
their own data with substantiation or use what is defined in 
Appendix 2 of these special conditions for mission distribution.
    (13) Oxygen Evolution--as defined by airplane performance or as 
defined in Appendix 2 of these special conditions.
    (14) Range--as defined by airplane performance.
    (15) Tank Thermal Characteristics--as defined by airplane 
performance.
    (16) Descent Profile Distribution--the applicant must either 
provide its own fleet representative distribution with 
substantiation or use a fixed 2500 feet per minute descent rate.
    (b) The assumptions for the analysis must include--
    (1) Predicted system performance;
    (2) Vent losses due to crosswind effects and airplane 
performance;
    (3) Periods when the system is operating properly but fails to 
inert the tank;
    (4) Expected system reliability; and
    (5) The MMEL/MEL dispatch inoperative period assumed in the 
reliability analysis, (60 flight hours must be used for a 10-day 
MMEL dispatch limit unless an alternative period has been approved 
by the FAA), including action to be taken when dispatching with the 
FRS inoperative (Note: The actual MMEL dispatch inoperative period 
data must be included in the engineering reporting requirement of 
paragraph III(c)(1) of these special conditions);
    (6) Possible periods of system inoperability due to latent or 
known failures, including airplane system shut-downs and failures 
that could cause the FRS to shut down or become inoperative; and
    (7) Affects of failures of the FRS that could increase the 
flammability of the fuel tank.
    (c) The variation assumed in the analysis on each of the 
parameters (as identified under paragraph (a) of this appendix) that 
affect flammability must be stated and substantiating data must be 
included.

Appendix 2

I. Monte Carlo Model

    The FAA has developed a Monte Carlo model that can be used to 
calculate fleet average flammability exposure for a fuel tank in an 
airplane. The program requires the user to enter the airplane 
performance data specific to the airplane model being evaluated, 
such as maximum range, cruise mach number, typical step climb 
altitudes,

[[Page 68571]]

tank thermal characteristics specified as exponential heating/
cooling time constants, and equilibrium temperatures for various 
fuel tank conditions. Single flights may be studied, or a multi-
flight Monte Carlo analysis may be run. This model is intended to 
provide comparison trends and not absolute numbers. The general 
methodology for conducting a Monte Carlo model is described in AC 
25.981-2.
    The FAA has developed a specific model for calculating fleet 
average flammability exposure using the Monte Carlo methodology. The 
FAA model, or one with modifications approved by the FAA, must be 
used as the means of compliance with these special conditions. The 
accepted model can be downloaded from the Web site http://qps.airweb.faa.gov/sfar88flamex. On this Web site, the model is 
located under the page ``Flam Ex Resources,'' and is titled ``Monte 
Carlo Model Version 6a.'' The ``6a'' represents Version 6A. Only 
version 6A or later of this model can be used. The following 
procedures, input variables, and data tables must be used in the 
analysis if the applicant develops a unique model to determine fleet 
average flammability exposure for a specific airplane type.

II. Monte Carlo Variables and Data Tables

    Fleet average flammability exposure is the percent of the 
mission time the fuel ullage is flammable for a fleet of an airplane 
type operating over the range of actual or expected missions and in 
a world-wide range of environmental conditions and fuel properties. 
Variables used to calculate fleet flammability exposure must include 
atmosphere, mission length (as defined in Special Condition I(g), 
Definitions, as Operational Time), fuel flash point, thermal 
characteristics of the fuel tank, overnight temperature drop, and 
oxygen evolution from the fuel into the ullage. Transport effects, 
including mass loading, flammability lag time, and condensation of 
vapors due to cold surfaces, are not to be allowed as parameters in 
the analysis.

Atmosphere

    In order to predict flammability along a given flight, the 
variation of ground ambient temperatures, cruise ambient 
temperatures, and a method to compute the transition from ground to 
cruise and back again must be used. The variation of the ground and 
cruise temperatures and the flash point of the fuel can be defined 
by a Gaussian curve, given by the 50 percent value and a +/- 1-sigma 
value.
    The ground and cruise temperatures are linked by a set of 
assumptions on the atmosphere. The temperature versus altitude 
follows a standard lapse rate from the ground temperature until the 
cruise temperature is reached. Above this altitude, the temperature 
is fixed at the cruise temperature. This gives a variation in the 
tropopause altitude. For cold days, an inversion is applied up to 
10,000 feet, and then the standard lapse rate is applied.
    The analysis must be able to execute a number of flights, and 
for each flight a separate random number must be generated for each 
of the three parameters (i.e., ground ambient temperature, cruise 
ambient temperature, and fuel flash point) using the Gaussian 
distribution defined in Table 1. The applicant can verify the output 
values from the Gaussian distribution using Table 2. Table 2 is 
based on typical use of Jet A type fuel. If an airplane is approved 
for use of lower flash point fuels such as JP-4, Russian, and 
Chinese fuels, and it is expected to be used for more than 1 percent 
of the fleet operating time, then the Monte Carlo analysis must 
include fuel property variation acceptable to the FAA for the 
approved fuels.

               Table 1.--Gaussian Distribution for Ground Ambient, Cruise Ambient, and Flash Point
----------------------------------------------------------------------------------------------------------------
                                              Temperature in Deg F
-----------------------------------------------------------------------------------------------------------------
                                                                                                    Flash point
                            Parameter                               Ground amb.     Cruise amb.        (FP)
----------------------------------------------------------------------------------------------------------------
Mean Temp.......................................................           59.95             -70             120
neg 1 std dev...................................................           20.14               8               8
pos 1 std dev...................................................           17.28  ..............  ..............
----------------------------------------------------------------------------------------------------------------


                                                            Table 2.--Verification of Table 1
--------------------------------------------------------------------------------------------------------------------------------------------------------
% probability of temps & flash point being below the    Ground amb.     Cruise amb.     Flash point     Ground amb.     Cruise amb.    Flash Point (FP)
                    listed values                          Deg F           Deg F           Deg F           Deg C           Deg C             Deg C
--------------------------------------------------------------------------------------------------------------------------------------------------------
1...................................................            13.1           -88.6           101.4           -10.5           -67.0                38.5
5...................................................            26.8           -83.2           106.8            -2.9           -64.0                41.6
10..................................................            34.1           -80.3           109.7             1.2           -62.4                43.2
15..................................................            39.1           -78.3           111.7             3.9           -61.3                44.3
20..................................................            43.0           -76.7           113.3             6.1           -60.4                45.1
25..................................................            46.4           -75.4           114.6             8.0           -59.7                45.9
30..................................................            49.4           -74.2           115.8             9.7           -59.0                46.6
35..................................................            52.2           -73.1           116.9            11.2           -58.4                47.2
40..................................................            54.8           -72.0           118.0            12.7           -57.8                47.8
45..................................................            57.4           -71.0           119.0            14.1           -57.2                48.3
50..................................................            59.9           -70.0           120.0            15.5           -56.7                48.9
55..................................................            62.1           -69.0           121.0            16.7           -56.1                49.4
60..................................................            64.3           -68.0           122.0            18.0           -55.5                50.0
65..................................................            66.6           -66.9           123.1            19.2           -55.0                50.6
70..................................................            69.0           -65.8           124.2            20.6           -54.3                51.2
75..................................................            71.6           -64.6           125.4            22.0           -53.7                51.9
80..................................................            74.5           -63.3           126.7            23.6           -52.9                52.6
85..................................................            77.9           -61.7           128.3            25.5           -52.1                53.5
90..................................................            82.1           -59.7           130.3            27.8           -51.0                54.6
95..................................................            88.4           -56.8           133.2            31.3           -49.4                56.2
99..................................................           100.1           -51.4           138.6            37.9           -46.3                59.2
--------------------------------------------------------------------------------------------------------------------------------------------------------


[[Page 68572]]

Flight Mission Distribution

    The mission length is determined from an equation that takes the 
maximum mission length for the airplane and creates multiple flight 
lengths based on typical airline usage.
    The mission length is also used to define the time on the ground 
prior to takeoff, and the type of flight profile to be followed. 
Table 3 must be used to define the mission distribution, unless the 
applicant has more appropriate data for the specific airplane model 
under evaluation, together with substantiation for the data. A 
linear interpolation may be used between the table values.

                                                             Table 3.--Mission Distribution
                                                       Airplane Maximum Range--Nautical Miles (NM)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                          Range (NM)                                                          Distribution of missions (%)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                         From                             To      1000     2000     3000     4000     5000     6000     7000     8000     9000    10000
--------------------------------------------------------------------------------------------------------------------------------------------------------
0....................................................      200     11.7      7.5      6.2      5.5      4.7      4.0      3.4      3.0      2.6      2.3
200..................................................      400     27.3     19.9     17.0     15.2     13.2     11.4      9.7      8.5      7.5      6.7
400..................................................      600     46.3     40.0     35.7     32.6     28.5     24.9     21.2     18.7     16.4     14.8
600..................................................      800     10.3     11.6     11.0     10.2      9.1      8.0      6.9      6.1      5.4      4.8
800..................................................     1000      4.4      8.5      8.6      8.2      7.4      6.6      5.7      5.0      4.5      4.0
1000.................................................     1200      0.0      4.8      5.3      5.3      4.8      4.3      3.8      3.3      3.0      2.7
1200.................................................     1400      0.0      3.6      4.4      4.5      4.2      3.8      3.3      3.0      2.7      2.4
1400.................................................     1600      0.0      2.2      3.3      3.5      3.3      3.1      2.7      2.4      2.2      2.0
1600.................................................     1800      0.0      1.2      2.3      2.6      2.5      2.4      2.1      1.9      1.7      1.6
1800.................................................     2000      0.0      0.7      2.2      2.6      2.6      2.5      2.2      2.0      1.8      1.7
2000.................................................     2200      0.0      0.0      1.6      2.1      2.2      2.1      1.9      1.7      1.6      1.4
2200.................................................     2400      0.0      0.0      1.1      1.6      1.7      1.7      1.6      1.4      1.3      1.2
2400.................................................     2600      0.0      0.0      0.7      1.2      1.4      1.4      1.3      1.2      1.1      1.0
2600.................................................     2800      0.0      0.0      0.4      0.9      1.0      1.1      1.0      0.9      0.9      0.8
2800.................................................     3000      0.0      0.0      0.2      0.6      0.7      0.8      0.7      0.7      0.6      0.6
3000.................................................     3200      0.0      0.0      0.0      0.6      0.8      0.8      0.8      0.8      0.7      0.7
3200.................................................     3400      0.0      0.0      0.0      0.7      1.1      1.2      1.2      1.1      1.1      1.0
3400.................................................     3600      0.0      0.0      0.0      0.7      1.3      1.6      1.6      1.5      1.5      1.4
3600.................................................     3800      0.0      0.0      0.0      0.9      2.2      2.7      2.8      2.7      2.6      2.5
3800.................................................     4000      0.0      0.0      0.0      0.5      2.0      2.6      2.8      2.8      2.7      2.6
4000.................................................     4200      0.0      0.0      0.0      0.0      2.1      3.0      3.2      3.3      3.2      3.1
4200.................................................     4400      0.0      0.0      0.0      0.0      1.4      2.2      2.5      2.6      2.6      2.5
4400.................................................     4600      0.0      0.0      0.0      0.0      1.0      2.0      2.3      2.5      2.5      2.4
4600.................................................     4800      0.0      0.0      0.0      0.0      0.6      1.5      1.8      2.0      2.0      2.0
4800.................................................     5000      0.0      0.0      0.0      0.0      0.2      1.0      1.4      1.5      1.6      1.5
5000.................................................     5200      0.0      0.0      0.0      0.0      0.0      0.8      1.1      1.3      1.3      1.3
5200.................................................     5400      0.0      0.0      0.0      0.0      0.0      0.8      1.2      1.5      1.6      1.6
5400.................................................     5600      0.0      0.0      0.0      0.0      0.0      0.9      1.7      2.1      2.2      2.3
5600.................................................     5800      0.0      0.0      0.0      0.0      0.0      0.6      1.6      2.2      2.4      2.5
5800.................................................     6000      0.0      0.0      0.0      0.0      0.0      0.2      1.8      2.4      2.8      2.9
6000.................................................     6200      0.0      0.0      0.0      0.0      0.0      0.0      1.7      2.6      3.1      3.3
6200.................................................     6400      0.0      0.0      0.0      0.0      0.0      0.0      1.4      2.4      2.9      3.1
6400.................................................     6600      0.0      0.0      0.0      0.0      0.0      0.0      0.9      1.8      2.2      2.5
6600.................................................     6800      0.0      0.0      0.0      0.0      0.0      0.0      0.5      1.2      1.6      1.9
6800.................................................     7000      0.0      0.0      0.0      0.0      0.0      0.0      0.2      0.8      1.1      1.3
7000.................................................     7200      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.4      0.7      0.8
7200.................................................     7400      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.3      0.5      0.7
7400.................................................     7600      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.2      0.5      0.6
7600.................................................     7800      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.1      0.5      0.7
7800.................................................     8000      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.1      0.6      0.8
8000.................................................     8200      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.5      0.8
8200.................................................     8400      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.5      1.0
8400.................................................     8600      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.6      1.3
8600.................................................     8800      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.4      1.1
8800.................................................     9000      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.2      0.8
9000.................................................     9200      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.5
9200.................................................     9400      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.2
9400.................................................     9600      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.1
9600.................................................     9800      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.1
9800.................................................    10000      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.1
--------------------------------------------------------------------------------------------------------------------------------------------------------

Fuel Tank Thermal Characteristics

    The applicant must account for the thermal conditions of the 
fuel tank both on the ground and in flight. The Monte Carlo model, 
available on the Web site listed above, defines the ground condition 
using an equilibrium delta temperature (relative to the ambient 
temperature) that the tank will reach given a long enough time, with 
any heat inputs from airplane sources. Values are also input to 
define two exponential time constants (one for a near empty tank and 
one for a near full tank) for the ground condition. These time 
constants define the time for the fuel in the fuel tank to heat or 
cool in response to heat input. The fuel is assumed to heat or cool 
according to a normal exponential transition, governed by the 
temperature difference between the current temperature and the 
equilibrium temperature, given by ambient temperature plus delta 
temperature. Input values for this data can be obtained from 
validated thermal models of the tank based on ground and flight test 
data. The inputs for the inflight condition are similar but are used 
for inflight analysis.
    Fuel management techniques are unique to each manufacturer's 
design and variations in

[[Page 68573]]

fuel load within the tank for given points in the flight must be 
accounted for in the model. The model uses a ``tank full'' time, 
specified in minutes, that defines the time before touchdown when 
the fuel tank is still full. For a center wing tank used first, this 
number would be the maximum flight time, and the tank would start to 
empty at takeoff. For a main tank used last, the tank will remain 
full for a shorter time before touch down, and would be ``empty'' at 
touch down (i.e., tank empty at 0 minutes before touch down). In the 
case of a main tank with reserves, the term empty means at reserve 
level rather than totally empty. The thermal data for tank empty 
would also be for reserve level.
    The model also uses a ``tank empty'' time to define the time 
when the tank is emptying, and the program uses a linear 
interpolation between the exponential time constants for full and 
empty during the time the tank is emptying. For a tank that is only 
used for long-range flights, the tank would be full only on very 
long-range missions and would be empty a long time before touch 
down. For short flights, it would be empty for the whole flight. For 
a main tank that carried reserve fuel, it would be full for a long 
time and would only be down to empty at touch down. In this case, 
empty would really be at reserve level, and the thermal constants at 
empty should be those for the reserve level.
    The applicant, whether using the available model or using 
another analysis tool, must propose means to validate thermal time 
constants and equilibrium temperatures to be used in the analysis. 
The applicant may propose using a more detailed thermal definition, 
such as changing time constants as a function of fuel load, provided 
the details and substantiation information are acceptable and the 
Monte Carlo model program changes are validated.

Overnight Temperature Drop

    An overnight temperature drop must be considered in the Monte 
Carlo analysis as it may affect the oxygen concentration level in 
the fuel tank. The overnight temperature drop for these special 
conditions will be defined using:
    [sbull] A landing temperature that is a random value based on a 
Gaussian distribution; and
    [sbull] An overnight temperature drop that is a random value 
based on a Gaussian distribution.
    For any flight that will end with an overnight ground period 
(one flight per day out of an average of x flights per day, 
depending on utilization of the particular airplane model being 
evaluated), the landing outside air temperature (OAT) is to be 
chosen as a random value from the following Gaussian curve:

                          Table 4.--Landing OAT
------------------------------------------------------------------------
                                                            Landing
                      Parameter                       temperature [deg]F
------------------------------------------------------------------------
Mean Temp...........................................               58.68
neg 1 std dev.......................................               20.55
pos 1 std dev.......................................               13.21
------------------------------------------------------------------------

    The outside ambient air temperature (OAT) drop for that night is 
to be chosen as a random value from the following Gaussian curve:

                           Table 5.--OAT Drop
------------------------------------------------------------------------
                                                           OAT drop
                      Parameter                       temperature [deg]F
------------------------------------------------------------------------
Mean Temp...........................................                12.0
1 std dev...........................................                 6.0
------------------------------------------------------------------------

Oxygen Evolution

    Fuel contains dissolved gases, and in the case of oxygen and 
nitrogen absorbed from the air, the oxygen level in the fuel can 
exceed 30 percent, instead of the normal 21 percent oxygen in air. 
Some of these gases will be released from the fuel during the 
reduction of ambient pressure experienced in the climb and cruise 
phases of flight. The applicant must consider the effects of air 
evolution from the fuel on the level of oxygen in the tank ullage 
during ground and flight operations and address these effects on the 
overall performance of the FRS. The applicant must provide the air 
evolution rate for the fuel tank under evaluation, along with 
substantiation data.

Number of Flights Required in Analysis

    In order for the Monte Carlo analysis to be valid for showing 
compliance with the flammability requirements of these special 
conditions, the applicant must run the analysis for an appropriate 
number of flights to ensure that the flammability exposure for the 
fuel tank under evaluation meets the criteria defined in Table 6.

                      Table 6.--Flammability Limit
------------------------------------------------------------------------
                                                     Maximum acceptable
     Number of flights in Monte Carlo analysis            fuel tank
                                                      flammability (%)
------------------------------------------------------------------------
1,000.............................................                  2.73
5,000.............................................                  2.88
10,000............................................                  2.91
100,000...........................................                  2.98
1,000,000.........................................                  3.00
------------------------------------------------------------------------


    Issued in Renton, Washington, on November 28, 2003.
Kevin M. Mullin,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.

[FR Doc. 03-30449 Filed 12-8-03; 8:45 am]
BILLING CODE 4910-13-P