Notice of proposed special conditions.
CFR Part: "14 CFR Part 25"
Citation: "79 FR 43318"
Document Number: "Docket No.
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.
EFFECTIVE DATE: Send your comments on or before
ADDRESSES: Send comments identified by docket number
* 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,
* Hand Delivery or Courier: Take comments to Docket Operations in Room W12-140 of the West Building Ground Floor at
* Fax: Fax comments to Docket Operations at 202-493-2251.
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
FOR FURTHER INFORMATION CONTACT:
SUPPLEMENTARY INFORMATION: 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.
Type Certification Basis
Under the provisions of Title 14, Code of Federal Regulations (14 CFR) 21.17,
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
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
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
The FAA issues special conditions, as defined in 14 CFR 11.19, in accordance with
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
The certification basis of the CSeries airplanes includes
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
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 [degrees] 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
The difficulty of designing multiple-fault-tolerant structure, and the difficulty of detecting failures of hidden structural design features in general, makes compliance with
Accounting for such long failure latency periods in the system safety analysis required by
As a result of the CSeries and other certifications projects, the
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 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
In determining the appropriate amount of relief from the ignition prevention requirements of
Given these novel or unusual design features, and the compliance challenges noted earlier in this document, the
One resulting difference between these proposed special conditions and the
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
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 the assessment provided the associated assumptions are acceptable to the
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
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
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
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
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
As discussed above, these special conditions are applicable to the Models BD-500-1A10 and BD-500-1A11 series airplanes. Should
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
Alternate Fuel Tank Structural Lightning Protection Requirements
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
(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
(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 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
(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
(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.
[FR Doc. 2014-17517 Filed 7-24-14;
BILLING CODE 4910-13-P
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