Document ID: FAA-2014-0383-0001
Agency: faa
Document Type: Proposed Rule
Title: Special Conditions: Bombardier Aerospace, Models BD-500-1A10 and BD-500-1A11 Series Airplanes; Alternate Fuel Tank Structural Lightning Protection Requirements
Posted Date: 2014-07-25T04:00Z

[Federal Register Volume 79, Number 143 (Friday, July 25, 2014)]
[Proposed Rules]
[Pages 43318-43322]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2014-17517]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2014-0383; Notice No. 25-14-05-SC]

Special Conditions: Bombardier Aerospace, Models BD-500-1A10 and 
BD-500-1A11 Series Airplanes; Alternate Fuel Tank Structural Lightning 
Protection Requirements

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed special conditions.

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SUMMARY: This action proposes special conditions for the Bombardier 
Aerospace Models BD-500-1A10 and BD-500-1A11 series airplanes. These 
airplanes will have a novel or unusual design feature that will 
incorporate a nitrogen generation system (NGS) for all fuel tanks that 
actively reduce flammability exposure within the fuel tanks 
significantly below that required by the fuel tank flammability 
regulations. Among other benefits, the NGS significantly reduces the 
potential for fuel vapor ignition caused by lightning strikes. The 
applicable airworthiness regulations do not contain adequate or 
appropriate safety standards for this design feature. 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.

DATES: Send your comments on or before September 8, 2014.

ADDRESSES: Send comments identified by docket number FAA-2014-0383 
using any of the following methods:
     Federal eRegulations Portal: Go to http://www.regulations.gov/ and follow the online instructions for sending 
your comments electronically.
     Mail: Send comments to Docket Operations, M-30, U.S. 
Department of Transportation (DOT), 1200 New Jersey Avenue SE., Room 
W12-140, West Building Ground Floor, Washington, DC, 20590-0001.
     Hand Delivery or Courier: Take comments to Docket 
Operations in Room W12-140 of the West Building Ground Floor at 1200 
New Jersey Avenue SE., Washington, DC, between 9 a.m. and 5 p.m., 
Monday through Friday, except federal holidays.
     Fax: Fax comments to Docket Operations at 202-493-2251.
    Privacy: The FAA will post all comments it receives, without 
change, to http://www.regulations.gov/, including any personal 
information the commenter provides. Using the search function of the 
docket Web site, anyone can find and read the electronic form of all 
comments received into any FAA docket, including the name of the 
individual sending the comment (or signing the comment for an 
association, business, labor union, etc.). DOT's complete Privacy Act 
Statement can be found in the Federal Register published on April 11, 
2000 (65 FR 19477-19478), as well as at http://DocketsInfo.dot.gov/.
    Docket: Background documents or comments received may be read at 
http://www.regulations.gov/ at any time. Follow the online instructions 
for accessing the docket or go to the Docket Operations in Room W12-140 
of the West Building Ground Floor at 1200 New Jersey Avenue SE., 
Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, 
except federal holidays.

FOR FURTHER INFORMATION CONTACT: Margaret Langsted, FAA, Propulsion and 
Mechanical Systems Branch, ANM-112, Transport Airplane Directorate, 
Aircraft Certification Service, 1601 Lind Avenue SW., Renton, 
Washington, 98057-3356; telephone 425-227-2677; facsimile 425-227-1149.

SUPPLEMENTARY INFORMATION:

Comments Invited

    We invite interested people to take part in this rulemaking by 
sending written comments, data, or views. The most helpful comments 
reference a specific portion of the special conditions, explain the 
reason for any recommended change, and include supporting data.
    We will consider all comments we receive on or before the closing 
date for comments. We may change these special conditions based on the 
comments we receive.

Background

    On December 10, 2009, Bombardier Aerospace applied for a type 
certificate for their new Models BD-500-1A10 and BD-500-1A11 series 
airplanes (hereafter collectively referred to as ``CSeries''). The 
CSeries airplanes are swept-wing monoplanes with a composite wing fuel 
tank structure and an aluminum alloy fuselage sized for 5-abreast 
seating. Passenger capacity is designated as 110 for the Model BD-500-
1A10 and 125 for the Model BD-500-1A11. Maximum takeoff weight is 
131,000 pounds for the Model BD-500-1A10 and 144,000 pounds for the 
Model BD-500-1A11.

Type Certification Basis

    Under the provisions of Title 14, Code of Federal Regulations (14 
CFR) 21.17, Bombardier Aerospace must show that the CSeries airplanes 
meet the applicable provisions of part 25 as amended by Amendments 25-1 
through 25-129.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the CSeries airplanes because of a 
novel or unusual design feature, special conditions are prescribed 
under the provisions of Sec.  21.16.
    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, the special conditions would also 
apply to the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and special 
conditions, the CSeries airplanes must comply with the fuel vent and 
exhaust emission requirements of 14 CFR part 34 and the noise 
certification requirements of 14 CFR part 36, and the FAA must issue a 
finding of regulatory adequacy under section 611 of Public Law 92-574, 
the ``Noise Control Act of 1972.''
    The FAA issues special conditions, as defined in 14 CFR 11.19, in 
accordance with Sec.  11.38, and they become part of the type 
certification basis under Sec.  21.17.

Novel or Unusual Design Features

    The CSeries airplanes will incorporate the following novel or 
unusual design features: A fuel tank nitrogen generation system (NGS) 
that is intended to control fuel tank flammability for all fuel tanks. 
This NGS is designed to provide a level of performance that applies the 
more stringent standard for warm day flammability performance 
applicable to normally emptied tanks within the fuselage contour from 
Sec.  25.981(b) and appendix M to part 25 to all fuel tanks of the 
CSeries airplanes. This high level of NGS performance for all fuel 
tanks is

[[Page 43319]]

a novel or unusual design feature compared to the state of technology 
envisioned in the airworthiness standards for transport category 
airplanes.

Discussion

    The certification basis of the CSeries airplanes includes Sec.  
25.981, as amended by Amendment 25-125, as required by 14 CFR 26.37. 
This amendment includes the ignition prevention requirements in Sec.  
25.981(a), as amended by Amendment 25-102, and it includes revised 
flammability limits for all fuel tanks and new specific limitations on 
flammability for all fuel tanks as defined in Sec.  25.981(b), as 
amended by Amendment 25-125.

Ignition Source Prevention

    Section 25.981(a)(3) requires applicants to show that an ignition 
source in the fuel tank system could not result from any single 
failure, from any single failure in combination with any latent failure 
condition not shown to be extremely remote, or from any combination of 
failures not shown to be extremely improbable. This requirement was 
originally adopted in Amendment 25-102, and it requires the assumption 
that the fuel tanks are always flammable when showing that the 
probability of an ignition source being present is extremely remote. 
(Amendment 25-102 included Sec.  25.981(c) that required minimizing 
fuel tank flammability, and this was defined in the preamble as being 
equivalent to unheated aluminum fuel tanks located in the wing.) This 
requirement defines three types of scenarios that must be addressed in 
order to show compliance with Sec.  25.981(a)(3). The first scenario is 
that any single failure, regardless of the probability of occurrence of 
the failure, must not cause an ignition source. The second scenario is 
that any single failure, regardless of the probability of occurrence, 
in combination with any latent failure condition not shown to be at 
least extremely remote, must not cause an ignition source. The third 
scenario is that any combination of failures not shown to be extremely 
improbable must not cause an ignition source. Demonstration of 
compliance with this requirement would typically require a structured, 
quantitative safety analysis. Design areas that have latent failure 
conditions typically would be driven by these requirements to have 
multiple fault tolerance, or ``triple redundancy.'' This means that 
ignition sources are still prevented even after two independent 
failures.

Flammability Limits

    Section 25.981(b) states that no fuel tank fleet average 
flammability exposure may exceed 3 percent of the flammability exposure 
evaluation time calculated using the method in part 25, Appendix N, or 
the fleet average flammability of a fuel tank within the wing of the 
airplane being evaluated, whichever is greater. If the wing is not a 
conventional unheated aluminum wing, the analysis must be based on an 
assumed equivalent conventional construction unheated aluminum wing. In 
addition, for fuel tanks that are normally emptied during operation and 
that have any part of the tank located within the fuselage contour, the 
fleet average flammability for warm days (above 80[emsp14][deg]F) must 
be limited to 3 percent as calculated using the method in part 25, 
Appendix M.

Application of Existing Regulations Inappropriate Due to Impracticality

    Since the issuance of Sec.  25.981(a)(3), as amended by Amendment 
25-102, the FAA has conducted certification projects in which 
applicants found it impractical to meet the requirements of that 
regulation for some areas of lightning protection for fuel tank 
structure. Partial exemptions were issued for these projects. These 
same difficulties exist for the CSeries project.
    The difficulty of designing multiple-fault-tolerant structure, and 
the difficulty of detecting failures of hidden structural design 
features in general, makes compliance with Sec.  25.981(a)(3) uniquely 
challenging and impractical for certain aspects of the electrical 
bonding of structural elements. Such bonding is needed to prevent 
occurrence of fuel tank ignition sources from lightning strikes. The 
effectiveness and fault tolerance of electrical bonding features for 
structural joints and fasteners is partially dependent on design 
features that cannot be effectively inspected or tested after assembly 
without damaging the structure, joint, or fastener. Examples of such 
features include a required interference fit between the shank of a 
fastener and the hole in which the fastener is installed, metal foil or 
mesh imbedded in composite material, a required clamping force provided 
by a fastener to pull two structural parts together, and a required 
faying surface bond between the flush surfaces of adjacent pieces of 
structural material such as in a wing skin joint or a mounting bracket 
installation. In addition, other features that can be physically 
inspected or tested may be located within the fuel tanks; therefore, it 
is not practical to inspect for failures of those features at short 
intervals. Examples of such failures include separation or loosening of 
cap seals over fastener ends and actual structural failures of internal 
fasteners. This inability to practically detect manufacturing errors 
and failures of structural design features critical to lightning 
protection results in degraded conditions that occur and remain in 
place for a very long time, possibly for the remaining life of the 
airplane.
    Accounting for such long failure latency periods in the system 
safety analysis required by Sec.  25.981(a)(3) would require multiple 
fault tolerance in the structural lightning protection design. As part 
of the design development activity for the CSeries, Bombardier has 
examined possible design provisions to provide multiple fault tolerance 
in the structural design to prevent ignition sources from occurring in 
the event of lightning attachment to the airplane in critical 
locations. Bombardier has concluded from this examination that 
providing multiple fault tolerance for some structural elements is not 
practical. Bombardier has also identified some areas of the CSeries 
design where it is impractical to provide even single fault tolerance 
in the structural design to prevent ignition sources from occurring in 
the event of lightning attachment after a single failure. The FAA has 
reviewed this examination with Bombardier in detail and has agreed that 
providing fault tolerance beyond that in the proposed CSeries design 
for these areas would be impractical.
    As a result of the CSeries and other certifications projects, the 
FAA has now determined that compliance with Sec.  25.981(a)(3) is 
impractical for some areas of lightning protection for fuel tank 
structure, and that application of Sec.  25.981(a)(3) to those design 
areas is therefore inappropriate. The FAA plans further rulemaking to 
revise Sec.  25.981(a)(3). As appropriate, the FAA plans to issue 
special conditions or exemptions, for certification projects 
progressing before the revision is complete. This is discussed in FAA 
Memorandum ANM-112-08-002, Policy on Issuance of Special Conditions and 
Exemptions Related to Lightning Protection of Fuel Tank Structure, 
dated May 26, 2009.

Application of Existing Regulations Inappropriate Due to Compensating 
Feature That Provides Equivalent Level of Safety

    Section 25.981(b) sets specific standards for fuel tank 
flammability as discussed above under ``Flammability

[[Page 43320]]

Limits.'' Under that regulation, the fleet average flammability 
exposure of all fuel tanks on the CSeries airplanes may not exceed 3 
percent of the flammability exposure evaluation time calculated using 
the method in part 25, Appendix N, or the fleet average flammability of 
a wing main tank within an equivalent construction conventional 
unheated aluminum wing fuel tank, whichever is greater. The typical 
fleet average fuel tank flammability of fuel tanks located in the wing 
ranges between 1 and 5 percent. If it is assumed that a CSeries 
equivalent conventional unheated aluminum wing fuel tank would not 
exceed a fleet average flammability time of 3 percent, the actual 
composite airplane wing fuel tank design would be required to comply 
with the 3 percent fleet average flammability standard, and therefore a 
means to reduce the flammability to 3 percent would be required. 
However, the proposed CSeries design includes NGS for all fuel tanks 
that will also be shown to meet the additional, more stringent warm day 
average flammability standard in part 25, Appendix M, which is only 
required for normally emptied fuel tanks with some part of the tank 
within the fuselage contour. Fuel tanks that meet this requirement 
typically have average fuel tank flammability levels well below the 
required 3 percent.
    Since the proposed NGS for all fuel tanks on the CSeries provides 
performance that meets part 25, Appendix M, the FAA has determined that 
the risk reduction provided by this additional performance will provide 
compensation for some relief from the ignition prevention requirements 
of Sec.  25.981(a)(3) while still establishing a level of safety 
equivalent to that established in the regulations.
    In determining the appropriate amount of relief from the ignition 
prevention requirements of Sec.  25.981(a), the FAA considered the 
original overall intent of Amendment 25-102, which was to ensure the 
prevention of catastrophic events due to fuel tank vapor explosion. 
These proposed special conditions are intended to achieve that 
objective through a prescriptive requirement that fault tolerance (with 
respect to the creation of an ignition source) be provided for all 
structural lightning protection design features where providing such 
fault tolerance is practical, and through a performance-based standard 
for the risk due to any single failure vulnerability that exists in the 
design. In addition, for any structural lightning protection design 
features for which Bombardier shows that providing fault tolerance is 
impractical, these proposed special conditions would require Bombardier 
to show that a fuel tank vapor ignition event due to the summed risk of 
all non-fault-tolerant design features is extremely improbable. 
Bombardier would be required to show that this safety objective is met 
by the proposed design using a structured system safety assessment 
similar to that currently used for demonstrating compliance with 
Sec. Sec.  25.901 and 25.1309.
    Given these novel or unusual design features, and the compliance 
challenges noted earlier in this document, the FAA has determined that 
application of Sec.  25.981(a)(3) is inappropriate in that it is 
neither practical nor necessary to apply the ignition source prevention 
provisions of Sec.  25.981(a)(3) to the specific fuel tank structural 
lightning protection features of the Bombardier CSeries airplanes. 
However, without the Sec.  25.981(a)(3) provisions, the remaining 
applicable regulations in the CSeries certification basis would be 
inadequate to set an appropriate standard for fuel tank ignition 
prevention. Therefore, in accordance with provisions of Sec.  21.16, 
the FAA has determined that, instead of Sec.  25.981(a)(3), alternative 
fuel tank structural lightning protection requirements be applied to 
fuel tank lightning protection features that are integral to the 
airframe structure of the CSeries airplanes. These proposed alternative 
requirements are intended to provide the level of safety intended by 
Sec.  25.981(a)(3), based on our recognition, as discussed above, that 
a highly effective NGS for the fuel tanks makes it unnecessary to 
assume that the fuel tank is always flammable. As discussed previously, 
the assumption that the fuel tanks are always flammable was required 
when demonstrating compliance to the ignition prevention requirements 
of Sec.  25.981(a)(3).
    One resulting difference between these proposed special conditions 
and the Sec.  25.981(a)(3) provisions they are meant to replace is the 
outcome being prevented--fuel vapor ignition versus an ignition source. 
These proposed special conditions acknowledge that the application of 
fuel tank flammability performance standards will reduce fuel tank 
flammability to an extent that it is appropriate to consider the 
beneficial effects of flammability reduction when considering design 
areas where it is impractical to comply with Sec.  25.981(a)(3).
    One of the core requirements of these proposed special conditions 
is a prescriptive requirement that structural lightning protection 
design features must be fault tolerant. (An exception wherein 
Bombardier can show that providing fault tolerance is impractical, and 
associated requirements, is discussed below.) The other core 
requirement is that Bombardier must show that the design, manufacturing 
processes, and Airworthiness Limitations section of the Instructions 
for Continued Airworthiness include all practical measures to prevent, 
and detect and correct, failures of structural lightning protection 
features due to manufacturing variability, aging, wear, corrosion, and 
likely damage. The FAA has determined that, if these core requirements 
are met, a fuel tank vapor ignition event due to lightning is not 
anticipated to occur in the life of the airplane fleet. This conclusion 
is based on the fact that a critical lightning strike to any given 
airplane is itself a remote event, and on the fact that fuel tanks must 
be shown to be flammable for only a relatively small portion of the 
fleet operational life.
    For any non-fault-tolerant features proposed in the design, 
Bombardier must show that eliminating these features or making them 
fault tolerant is impractical. The requirements and considerations for 
showing it is impractical to provide fault tolerance are described in 
FAA Memorandum ANM-112-08-002. This requirement is intended to minimize 
the number of non-fault tolerant features in the design.
    For areas of the design where Bombardier shows that providing fault 
tolerant structural lightning protection features is impractical, non-
fault-tolerant features will be allowed provided Bombardier can show 
that a fuel tank vapor ignition event due to the non-fault-tolerant 
features is extremely improbable when the sum of probabilities of those 
events due to all non-fault-tolerant features is considered. Bombardier 
will be required to submit a structured, quantitative assessment of 
fleet average risk for a fuel tank vapor ignition event due to all non-
fault-tolerant design features included in the design. This will 
require determination of the number of non-fault tolerant design 
features, estimates of the probability of the failure of each non-
fault-tolerant design feature, and estimates of the exposure time for 
those failures. This analysis must include failures due to 
manufacturing variability, aging, wear, corrosion, and likely damage.
    It is acceptable to consider the probability of fuel tank 
flammability, the probability of a lightning strike to the airplane, 
the probability of a lightning strike to specific zones of the airplane 
(for example, Zone 2 behind the nacelle, but not a specific location or 
feature), and a distribution of lightning strike amplitude in 
performing

[[Page 43321]]

the assessment provided the associated assumptions are acceptable to 
the FAA. The analysis must account for any dependencies among these 
factors, if they are used. The assessment must also account for 
operation with inoperative features and systems, including any proposed 
or anticipated dispatch relief. This risk assessment requirement is 
intended to ensure that an acceptable level of safety is provided given 
the non-fault-tolerant features in the proposed design.
    Part 25, Appendix N, as adopted in Amendment 25-125, in conjunction 
with these proposed special conditions, constitutes the standard for 
how to determine flammability probability. In performing the safety 
analysis required by these special conditions, relevant Sec.  
25.981(a)(3) compliance guidance is still applicable. Appropriate 
credit for the conditional probability of environmental or operational 
conditions occurring is normally limited to those provisions involving 
multiple failures, and this type of credit is not normally allowed in 
evaluation of single failures. However, these special conditions would 
allow consideration of the probability of occurrence of lightning 
attachment and flammable conditions when assessing the probability of 
structural failures resulting in a fuel tank vapor ignition event.
    The FAA understands that lightning protection safety for airplane 
structure is inherently different from lightning protection for 
systems. We intend to apply these proposed special conditions only to 
structural lightning protection features of fuel systems. We do not 
intend to apply the alternative standards used under these proposed 
special conditions to other areas of the airplane design evaluation.

Requirements Provide Equivalent Level of Safety

    In recognition of the unusual design feature discussed above, and 
the impracticality of requiring multiple fault tolerance for lightning 
protection of certain aspects of fuel tank structure, the FAA has 
determined that a level of safety that is equivalent to direct 
compliance with Sec.  25.981(a)(3) will be achieved for the CSeries by 
applying these proposed requirements. The FAA considers that, instead 
of only concentrating on fault tolerance for ignition source 
prevention, significantly reducing fuel tank flammability exposure in 
addition to preventing ignition sources is a better approach to 
lightning protection for the fuel tanks. In addition, the level of 
average fuel tank flammability achieved by compliance with these 
proposed special conditions is low enough that it is not appropriate or 
accurate to assume in a safety analysis that the fuel tanks may always 
be flammable.
    Section 25.981(b), as amended by Amendment 25-125, sets limits on 
the allowable fuel tank flammability for the CSeries airplanes. 
Paragraph 2(a) of these proposed special conditions applies the more 
stringent standard for warm day flammability performance applicable to 
normally emptied tanks within the fuselage contour from Sec.  25.981(b) 
and part 25, Appendix M, to all of the fuel tanks of the CSeries 
airplanes.
    Because of the more stringent fuel tank flammability requirements 
in these proposed special conditions, and because the flammability 
state of a fuel tank is independent of the various failures of 
structural elements that could lead to an ignition source in the event 
of lightning attachment, the FAA has agreed that it is appropriate in 
this case to allow treatment of flammability as an independent factor 
in the safety analysis. The positive control of flammability and the 
lower flammability that is required by these proposed special 
conditions exceeds the minimum requirements of Sec.  25.981(b). This 
offsets a reduction of the stringent standard for ignition source 
prevention in Sec.  25.981(a)(3), which assumes that the fuel tank is 
flammable at all times.
    Given the stringent requirements for fuel tank flammability, the 
fuel vapor ignition prevention and the ignition source prevention 
requirements in these proposed special conditions will prevent ``. . . 
catastrophic failure . . . due to ignition of fuel or vapors'' as 
stated in Sec.  25.981(a). Thus, the overall level of safety achieved 
by these proposed special conditions is considered equivalent to that 
which would be required by compliance with Sec.  25.981(a)(3) and (b).

Applicability

    As discussed above, these special conditions are applicable to the 
Models BD-500-1A10 and BD-500-1A11 series airplanes. Should Bombardier 
Aerospace apply at a later date for a change to the type certificate to 
include another model incorporating the same novel or unusual design 
feature, the special conditions would apply to that model as well.

Conclusion

    This action affects only certain novel or unusual design features 
on two model series of airplanes. It is not a rule of general 
applicability.

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 Bombardier Aerospace Models BD-500-1A10 and BD-500-1A11 series 
airplanes.

Alternate Fuel Tank Structural Lightning Protection Requirements

1. Definitions

    Most of the terms used in these proposed special conditions either 
have the common dictionary meaning or are defined in Advisory Circular 
25.1309-1A, System Design and Analysis, dated June 21, 1988. The 
following definitions are the only terms intended to have a specialized 
meaning when used in these proposed special conditions:
    (a) Basic Airframe Structure. Includes design elements such as 
structural members, structural joint features, and fastener systems 
including airplane skins, ribs, spars, stringers, etc., and associated 
fasteners, joints, coatings, and sealant. Basic airframe structure may 
also include those structural elements that are expected to be removed 
for maintenance, such as exterior fuel tank access panels and fairing 
attachment features, provided maintenance errors that could compromise 
associated lightning protection features would be evident upon an 
exterior preflight inspection of the airplane and would be corrected 
prior to flight.
    (b) Permanent Systems Supporting Structure. Includes static, 
permanently attached structural parts (such as brackets) that are used 
to support system elements. It does not include any part intended to be 
removed, or any joint intended to be separated, to maintain or replace 
system elements or other parts, unless that part removal or joint 
separation is accepted by the FAA as being extremely remote.
    (c) Manufacturing Variability. Includes tolerances and variability 
allowed by the design and production specifications as well as 
anticipated errors or escapes from the manufacturing and inspection 
processes.
    (d) Extremely Remote. Conditions that are not anticipated to occur 
to each airplane during its total life, but which

[[Page 43322]]

may occur a few times when considering the total operational life of 
all airplanes of one type. Extremely remote conditions are those having 
an average probability per flight hour on the order of 1 x 
10-7 or less, but greater than on the order of 1 x 
10-9.
    (e) Extremely Improbable. Conditions that are so unlikely that they 
are not anticipated to occur during the entire operational life of all 
airplanes of one type. Extremely improbable conditions are those having 
an average probability per flight hour of the order of 1 x 
10-9 or less.

2. Alternative Fuel Tank Structural Lightning Protection Requirements

    For lightning protection features that are integral to fuel tank 
basic airframe structure or permanent systems supporting structure, as 
defined in Special Condition No. 1, ``Definitions,'' for which 
Bombardier shows and the FAA finds compliance with Sec.  25.981(a)(3) 
to be impractical, the following requirements may be applied in lieu of 
the requirements of Sec.  25.981(a)(3):
    (a) Bombardier must show that the airplane design meets the 
requirements of part 25, appendix M, as amended by Amendment 25-125, 
for all fuel tanks installed on the airplane.
    (b) Bombardier must show that the design includes at least two 
independent, effective, and reliable lightning protection features (or 
sets of features) such that fault tolerance to prevent lightning-
related ignition sources is provided for each area of the structural 
design proposed to be shown compliant with these special conditions in 
lieu of compliance with the requirements of Sec.  25.981(a)(3). Fault 
tolerance is not required for any specific design feature if:
    (1) For that feature, providing fault tolerance is shown to be 
impractical, and
    (2) Fuel tank vapor ignition due to that feature and all other non-
fault-tolerant features, when their fuel tank vapor ignition event 
probabilities are summed, is shown to be extremely improbable.
    (c) Bombardier must perform an analysis to show that the design, 
manufacturing processes, and the airworthiness limitations section of 
the instructions for continued airworthiness include all practical 
measures to prevent, and detect and correct, failures of structural 
lightning protection features due to manufacturing variability, aging, 
wear, corrosion, and likely damage.

    Issued in Renton, Washington, on June 6, 2014.
Jeffrey E. Duven,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2014-17517 Filed 7-24-14; 8:45 am]
BILLING CODE 4910-13-P