U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Operator Information for Incorporating Fuel Tank Flammability Reduction Requirements into a Maintenance or Inspection Program Date: 6/22/12 Initiated by: AFS-300 AC No: 120-98A Change:
On July 21, 2008 the Federal Aviation Administration (FAA) published the Reduction of Fuel Tank Flammability in Transport Category Airplanes, final rule. It is referred to as the Fuel Tank Flammability Reduction (FTFR) rule. This rule amends Title 14 of the Code of Federal Regulations (14 CFR) parts 25, 26, 121, 125, and 129, § 129.14, U.S. registered airplanes. The FTFR rule requires manufacturers and operators of certain transport category airplanes to take steps that in combination with the Transport Airplane Fuel Tank System Design Review, Flammability Reduction, and Maintenance and Inspection Requirements (Special Federal Aviation Regulation (SFAR) 88, Fuel Tank System Fault Tolerance Evaluation Requirements) will greatly reduce the chances of a catastrophic fuel tank explosion. The purpose of these rules is to help ensure the continued safety of transport category airplanes by reducing fuel tank ignition sources and the flammability exposure in fuel tanks that are most at risk. The FTFR requires either a flammability reduction means (FRM) such as nitrogen inerting, or an ignition mitigation means (IMM) like polyurethane foam be incorporated into airplanes with high flammability fuel tanks to prevent fuel tank explosions.
ORIGINAL SIGNED by /s/ Raymond Towles for
John M. Allen Director, Flight Standards Service
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CONTENTS Paragraph Page
CHAPTER 1. GENERAL1
1-1. Pupose1
1-2. Applicability1
1-3. Background1
1-4. Related Reading Materi als (current editions)2
1-5. Related CFRs3
CHAPTER 2. FTFR REQUIREMENTS5
2-1. Design Approval Holder (DAH) Requirements5
2-2. Excluded Models5
2-3. Operator Requirements–New Production6
2-4. FTFR Rule Exclusions6
2-5. Operator Requirements–Retrofit6
2-6. Operator Requirements-Pre-1992 Airplane7
2-7. Models Excepted From The FTFR7
2-8. Operator Retrofit Compliance Times7
2-9. An Operator’s Airplane Retrofit Fleet Defined8
2-10. Use of Ground Conditioned Air9
2-11. Service Instructions10
2-12. Airworthiness Limitations10
2-13. Operator Maintenance/Inspection Program Requirements12
2-14. Maintenance Program Revisions After Alterations12
APPENDIX 1. ACRONYMS (2 pages)1
APPENDIX 2. DEFINITIONS (4 pages)1
APPENDIX 3. FAA OVERSIGHT OFFICES (1 page)1
Par 1-1 Page 1 CHAPTER 1. GENERAL
1-1. PURPOSE. This advisory circular (AC) provides information and describes an acceptable means of compliance (AMC) with the Reduction of Fuel Tank Flammability in Transport Category Airplanes, final rule. It is commonly called the Fuel Tank Flammability Reduction (FTFR) rule. This AC describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The Federal Aviation Administration (FAA) will consider other methods of demonstrating compliance that an applicant may elect to present.
While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. If the FAA becomes aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, the FAA may require additional substantiation or design changes as a basis for finding compliance. This material does not change or create any additional regulatory requirements, nor does it authorize changes in or permit deviations from existing regulatory requirements.
NOTE: Terms such as “will” or “must” are used in this AC only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance described here is used.
1-2. APPLICABILITY. This AC applies to operators of transport-category turbine-powered airplanes with a type certificate (TC) issued after January 1, 1958, that as a result of original type certification or later increase in capacity, have a maximum TC’d passenger capacity of 30 or more, or a maximum payload capacity of 7,500 pounds or more. In addition, this AC applies to operators of transport-category airplanes of any size that include Title 14 of the Code of Federal Regulations (14 CFR) part 25, § 25.981, Amendment 25-102, or later in the original certification basis of the airplane or in the certification basis of modifications to the airplane.
1-3. BACKGROUND.
a. Accident. Since 1959 there have been 18 fuel tank explosions on transport category airplanes. Most notably, on July 17, 1996, a 25-year old Boeing 747-100 series airplane, operating as TWA Flight 800, was involved in an in-flight breakup after takeoff from Kennedy International Airport in New York, resulting in 230 fatalities. The National Transportation Safety Board (NTSB) determined the probable cause of the TWA 800 accident was an explosion of the center wing fuel tank (CWT) due to ignition of the flammable fuel vapor and air mixture in the tank. The source of ignition energy for the explosion could not be determined conclusively, though the NTSB determined the most likely was a combination of failures involving the fuel quantity indicating system (FQIS). In addition, their accident report concluded that “a fuel tank design and certification philosophy that relies solely on elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated.”
b. Fuel Tank Safety (FTS) Rule. On June 6, 2001, the Transport Airplane Fuel Tank System Design Review, Flammability Reduction, and Maintenance and Inspection Requirements rule became effective. It is referred to as the 2001 FTS rule. It amended 14 CFR part 21 and created Special Federal Aviation Regulation (SFAR) 88. SFAR 88 is in part 21 and was the only
change made by the 2001 FTS rule to part 21. SFAR 88 required TC and Supplemental Type Certificate (STC) holders to conduct a safety review of the applicable airplanes identified in the rule to determine if their design meets the FTS ignition prevention requirements described in §§ 25.901 and 25.981(a) and (b) and develop service information necessary for FAA to issue airworthiness directives (AD) to correct unsafe conditions identified by the reviews.
c. Fuel Tank Flammability Reduction (FTFR) Rule. On July 21, 2008 the FAA published the Reduction of Fuel Tank Flammability in Transport Category Airplanes, final rule.
It is referred to as the FTFR rule. The FTFR rule contains new requirements for design approval holders (DAH) and operators of certain transport airplanes. Given that ignition sources have and will continue to develop in spite of the effort of the FTS rule to mitigate the risk, the chances of a fuel tank explosion naturally correlate with the exposure of the tank to flammable vapors. The requirements in FTFR rule mitigate the effects of such flammability exposure and limit it to acceptable levels by mandating the installation of either a flammability reduction means (FRM) or an ignition mitigation means (IMM) for fuel tanks that are most at risk. The design and operational requirements in these rules, in combination, should greatly reduce the chances of a catastrophic fuel tank explosion.
1-4. RELATED READING MATERIALS (current editions).
a. ACs. You can find these ACs on the FAA’s Web site at http://www.faa.gov/regulations_policies/advisory_circulars/.
• AC 25-8, Auxiliary Fuel System Installations;
• AC 25.981-1, Fuel Tank Ignition Source Prevention Guidelines;
• AC 25.981-2, Fuel Tank Flammability Reduction Means;
• AC 120-16, Air Carrier Maintenance Programs;
• AC 120-97, Incorporation of Fuel Tank System Instructions for Continued Airworthiness into Operator Maintenance or Inspection Programs;
• AC 120-102, Incorporation of Electrical Wiring Interconnection Systems Instructions for Continued Airworthiness into an Operator’s Maintenance Program;
and
• AC 121-22, Maintenance Review Board Report Maintenance Type Board, and OEM/TCH Inspection Program Procedures.
b. FAA Policy Statements. An electronic copy of the following Policy Statements can be downloaded at http://www.airweb.faa.gov/rgl.
• PS-ANM112-05-001, Process for Developing SFAR 88-related Instructions for Maintenance and Inspection of Fuel Tank Systems, October 6, 2004; and
• PS-ANM110-7-12-2005, Safety—A Shared Responsibility—New Direction for Addressing Airworthiness Issues for Transport Airplanes, July 12, 2005.
c. Industry Reference. See the Airlines for America (A4A), Maintenance Steering Group-3rd Task Force (MSG-3): Operator/Manufacturer Scheduled Maintenance Development.
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1-5. RELATED CFRs. The Regulatory and Guidance Library (RGL) is a set of searchable databases that contain regulatory, guidance, and aviation product information. The RGL contains certain CFRs and SFARs from 14 CFR in their current version as well as historical versions. The following sections of 14 CFR apply:
• Part 21, SFAR, Special Federal Aviation Regulation No. 88, Fuel Tank System Fault Tolerance Evaluation Requirements;
• Part 25, § 25.981, Fuel Tank Explosion Prevention;
• Part 25, § 25.1529, Instructions for Continued Airworthiness;
• Part 25 Appendix H, Instructions for Continued Airworthiness;
• Part 26, Continued Airworthiness and Safety Improvements for Transport Category Airplanes;
• Part 43, § 43.13, Performance Rules (General);
• Part 43, § 43.16. Airworthiness Limitations;
• Part 91, § 91.403(c), General;
• Part 121, § 121.1113, Fuel Tank System Maintenance Program;
• Part 125, § 125.507, Fuel Tank System Inspection Program; and
• Part 129, § 129.113, Fuel Tank System Maintenance Program.
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Par 2-1 Page 5 CHAPTER 2. FTFR REQUIREMENTS
2-1. DESIGN APPROVAL HOLDER (DAH) REQUIREMENTS. The Fuel Tank Flammability Reduction (FTFR) rule requires DAHs and operators increase the level of safety of certain fuel tanks. It accomplishes this by requiring incorporation of either flammability reduction means (FRM) such as nitrogen inerting or ignition mitigation means (IMM) such as polyurethane foam to reduce the fuel tank explosion risk to an acceptable level. The fuel tank flammability safety standards in 14 CFR part 26, subpart D, require DAHs to conduct flammability assessments, incorporate FRM or IMM designs in new production airplanes, and develop service instructions for retrofit installation of FRM or IMM for those affected airplane models with high flammability exposure time fuel tanks.
a. Possible Adverse Effects on the Design Approval. The DAHs for auxiliary fuel tanks in the airplanes where FRM or IMM are required must assess any possible adverse effects of their design approval on the effectiveness of the fuel tank safety (FTS) improvements. If an operator installed an auxiliary fuel tank pursuant to a field approval in an airplane affected by part 26, § 26.33, the operator is the auxiliary fuel tank DAH. Any adverse effects on the DAH’s approved design must be mitigated by design changes and service instructions (refer to § 26.35(d)) identified as Flammability Impact Mitigation Means (FIMM). FIMM are modifications developed by DAH for auxiliary fuel tanks when a flammability exposure analysis per § 26.35 determines that an auxiliary fuel tank installed by STC or field approval could have an adverse impact on the performance of existing fuel system FRM or IMM of other tanks.
b. Example of FIMM. The CWT vent system is part of the type design of the airplane.
The installation of an auxiliary fuel tank such as in the forward cargo compartment may reduce the effectiveness of the Nitrogen Gas System (NGS) resulting in an increase in flammability exposure of the CWT. The NGS installation is designed to provide the proper level of nitrogen to the CWT in order to reduce the flammability exposure time to levels required by the certification regulations. Auxiliary fuel tank installations typically transfer fuel and ullage gases to and from the CWT. The concern here is that the auxiliary fuel tank that allows transfer of ullage directly to or from the CWT might increase the flammability exposure time of the CWT above the certification limits. Therefore, the DAH for the auxiliary fuel tank system must comply with § 26.35 and determine if the auxiliary fuel tank system reduces the effectiveness of the NGS resulting in an increase in flammability exposure of the CWT and therefore FIMM is necessary.
c. Field Approval Auxiliary Fuel Tanks. Section 121.1117(c) and the similar provisions in parts 125 and 129 require that, after the applicable date stated in § 121.1117(e), no operator may operate any airplane subject to § 26.33 that has an auxiliary fuel tank installed by a field approval unless that operator complies with § 26.35 by the applicable date specified in that section and, if required, installs a FIMM approved by the FAA Oversight Office. The operator may also choose to deactivate or remove the auxiliary fuel tank but must use FAA Oversight Office approved data to accomplish that.
2-2. EXCLUDED MODELS. The airplane models listed under Exclusions in 14 CFR parts 121, § 121.1117(o), 125, § 125.509(m), and 129, § 129.117(o) of the FTFR rules are excluded from the retrofit requirements. All the airplanes produced for these models were issued an original Certificate of Airworthiness before January 1, 1992. These are also airplane
models that, because of their advanced age and small numbers, would likely make compliance economically impractical even if used in passenger service beyond the 100 percent retrofit compliance dates.
2-3. OPERATOR REQUIREMENTS–NEW PRODUCTION. For §§ 121.1117, 125.509, and 129.117, no operator may operate a new production airplane identified in Table 1 in §§ 121.1117, 125.509, or 129.117 below (including all-cargo airplanes) for which the State of Manufacture issued the original Certificate of Airworthiness or export airworthiness approval after December 27, 2010, unless a FRM or IMM meeting the requirements of § 26.33 is operational. Table 1 contains current production airplanes at the time the FTFR rule was issued.
NOTE: Section 26.37 requires affected pending new transport airplane type certification projects comply with the 14 CFR part 25, § 25.981 fuel tank flammability type certification requirements adopted by the FTFR rule.
NOTE: Table 1 does not include the Boeing 757 or Airbus A300 or A310 because they were out-of-production when the FTFR rule was adopted.
Table 1 Model—Boeing Model—Airbus 747 Series A318, A319, A320, A321 Series 737 Series A330, A340 Series 777 Series 767 Series
2-4. FTFR RULE EXCLUSIONS. The FTFR rule does not apply to any airplane for which the State of Manufacture issued the original Certificate of Airworthiness or export airworthiness approval before January 1, 1992, unless they are operated in passenger service beyond the date by which the operator is required to modify 100 percent of its affected fleet. After that date all pre-January 1, 1992 airplanes listed in Table 2 in §§ 121.1117, 125.509 or 129.117, that will still operate in passenger service will have to be § 26.33(c) compliant. The 100 percent retrofit date is contingent on whether the operator is using “Ground Conditioned Air” identified in paragraph 2- 10 of this chapter.
NOTE: The retrofit requirements of the FTFR rule do not apply to airplanes designed solely for all-cargo operations (§26.33(a)).
2-5. OPERATOR REQUIREMENTS–RETROFIT. Section 121.1117, paragraphs (d), (e) and (m) and similar provisions in parts 125 and 129 require that if FRM, IMM, or FIMM is required by §§ 26.33 or 26.35, each operator must complete the FAA Oversight Office-approved installation of FRM, IMM, or FIMM. The retrofit compliance dates require each operator to complete retrofit on a percentage of its fleet by specific dates. The DAH is required by §§ 26.33 or 26.35 to develop retrofit service instructions for the airplane models manufactured on or after January 1, 1992, if those part 26 regulations require FRM, IMM, or FIMM.
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2-6. OPERATOR REQUIREMENTS–PRE-1992 AIRPLANE. If an operator wishes to operate in passenger service a pre-1992 airplane listed in Table 2 past their 100 percent fleet retrofit compliance date, § 121.1117(m) and similar requirements in §§ 125.509 and 129.117 require the operator modify the airplane to comply with § 26.33(c) before that date. However, there is no requirement in §§ 26.33 or 26.35 for DAHs to develop retrofit service information for pre-1992 airplanes. Therefore, the operator of pre-1992 airplanes must comply with the part 26 requirements.
Table 2 Model—Boeing Model—Airbus 747 Series A318, A319, A320, A321 Series 737 Series A300, A310 Series 777 Series A330, A340 Series 767 Series 757 Series
2-7. MODELS EXCEPTED FROM THE FTFR. Table 2 does not include the airplane models listed in the following Table 3, Airplane Models Excepted from the Fuel Tank Flammability Reduction Rule, which is not included in the FTFR operating rules. These models are not listed under Exclusions in §§ 121.1117(o), 125.509(m), and 129.117(o) of the FTFR operating rules either. However, they were issued the original Certificate of Airworthiness or export airworthiness approval before January 1, 1992. They also do not include high flammability exposure fuel tanks that would require FRM or IMM in order to comply with § 26.33(c). Therefore, they are not required to comply with § 26.33(c) or the retrofit requirements in the FTFR rule even if an airplane listed in the Table 3 below were to operate in passenger service past the 100 percent retrofit compliance date.
NOTE: Table 3 contains airplane models excluded from the FTFR rule.
Table 3. Airplane Models Excepted from the Fuel Tank Flammability Reduction Rule Model Year of Last Delivery L-1011 1984 DC-10 1990 B-727 1984 DC-9 1982
2-8. OPERATOR RETROFIT COMPLIANCE TIMES. Section 121.1117, paragraphs (e) and (m), and similar provisions in parts 125 and 129, require the retrofit installations be phased in over the period specified in the rules. Below is a summary of the retrofit compliance date requirements.
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a. Fifty Percent. Fifty percent of each operator’s fleet must be modified no later than December 26, 2014. A one year extension may have been granted by using the “Ground Conditioned Air” provision in § 121.1117(k) or § 129.117(k). There is no similar provision in § 125.509.
b. One Hundred Percent. One hundred percent of each operator’s fleet must be modified no later than December 26, 2017, unless a one year extension has been granted by using the “Ground Conditioned Air” provision in the FTFR rule.
c. One Airplane Fleet. For those operators that have only one airplane of a model identified in Table 1 in § 121.1117(b) (“New Production Airplanes”) in their affected fleet, the airplane must be modified no later than December 26, 2017.
d. Compliance Plan. The operator should work with the principal inspector (PI) to develop a compliance plan for the 50 percent and 100 percent retrofit requirements in § 121.1117(e) and the similar provisions in parts 125 and 129. The compliance plan should contain a complete list/schedule of airplanes that require retrofit. The list should be updated as airplanes are added/completed. The operator should apprise the PI of any delays, addition, or deletion of airplanes from the list/schedule. Operators that have only one airplane must modify that airplane no later than December 26, 2017.
2-9. AN OPERATOR’S AIRPLANE RETROFIT FLEET DEFINED. For the purposes of the FTFR rule, an operator’s airplane retrofit fleet is defined as the total number of affected airplanes of all affected model types listed on operation specification (OpSpec)/LOA D085, Aircraft Listing on the retrofit compliance dates.
NOTE: “New Production Airplanes” that are FRM compliant (§121.1117(b)) either manufactured on or prior to the December 27, 2010 or those manufactured after December 27, 2010 cannot be counted as part of the 50 percent or 100 percent retrofit mix.
a. Retrofit Mix Example. For example, an operator has a fleet comprised of Boeing 737 and Airbus A320 airplanes. The “affected fleet” is the total number of the affected Boeing 737 airplanes plus the total number of affected Airbus A320 airplanes. The operating rules require the operator retrofit 50 percent of its “affected fleet” of airplanes by the 50 percent retrofit compliance date of December 26, 2014. Airplanes in long term storage and listed on OpSpec D106, Aircraft in Long-Term Maintenance or Storage, cannot not be counted in the 50 percent or 100 percent retrofit mix for the purposes of the FTFR rule. However, any airplane(s) brought back into service from storage and added to OpSpec/LOA D085 become part of the operator’s 50 percent or 100 percent retrofit mix. The operator must recalculate the retrofit mix based on the number of airplanes added to OpSpec/LOA D085. In addition, any newly acquired airplanes added to the operator’s fleet that are not FTFR retrofit compliant and added to OpSpec/LOA D085 become part of the operator’s 50 percent or 100 percent retrofit mix. If an operator has more than one affected airplane listed on its OpSpecs/LOA D085 on the 50 percent retrofit compliance date, they are required to have retrofitted 50 percent of those airplanes by that date.
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b. Retrofit Mix Adjustment. The 50 percent or 100 percent retrofit mix can be adjusted by the operator to suit its operational needs. The operator can change the retrofit mix of airplanes at will and at any time as long as the retrofit percentages and dates are complied with. For instance, an operator has 50 airplanes that need to be FTFR retrofit compliant by December 26, 2014. The operator sells, or puts in storage 25 of those airplanes and purchases 25 that are FTFR retrofit compliant (not new production airplanes). The operator’s 50 percent retrofit requirement remains the same, but now 25 are retrofit compliant and only 25 of the original 50 percent fleet need to be retrofitted to meet its 50 percent requirement by December 26, 2014. The fact that the operator purchased 25 FTFR retrofit compliant airplanes rather than retrofitting them itself has no bearing on its original 50 percent retrofit requirement.
For the purposes of the FTFR, the operator would meet the 50 percent requirement. This also applies to the operator’s 100 percent mix.
2-10. USE OF GROUND CONDITIONED AIR.
a. One Year Extension. In order to give the operators additional time to incorporate FRM at a time other than during a heavy check, the FTFR rule has a provision in §§ 121.1117(k) and
129.117(k) to allow a one year extension to the retrofit requirements in §§ 121.1117(e) and
129.117(e) if the operator uses ground con ditioned air for all airplanes with high flammability tanks for “actual gate times” exceeding 30 minutes when ground air is available at the gate and operational, and the ambient temperature exceeds 60 degrees Fahrenheit. Operators were required to apply for amendment to their OpSpec no later than March 26, 2009 in order to obtain this one year extension. This provision does not apply to § 125.507 because these airplanes are not typically parked at gates where ground conditioned air is available. In addition, these operators have few airplanes subject to this rule.
Table 4. Airplanes Authorized by OpSpec A570 to Use “Ground Conditioned Air” Boeing Airbus 747 Series A300, A310 737 Series A318, A319, A320, A321 Series 777 Series A330, A340 Series 767 Series 757 Series
b. OpSpec A570 Authorized Airplanes. If the operator applied for the “Ground Conditioned Air” provision it is controlled on OpSpec A570, one year Extension of compliance times in §§ 121.1117(e) and 129.117. The operator’s manual must contain any applicable airplanes in Table 4 that are authorized on OpSpec A570 to use “Ground Conditioned Air”. This must include a listing by N registration number and fleet type of airplanes that require retrofit with FRM or IMM. As airplanes are retrofitted they should be removed from the list. The OpSpec A570 authorization terminates on December 26, 2018 or when all of the operator’s airplanes listed in its manual are retrofitted, whichever occurs first.
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2-11. SERVICE INSTRUCTIONS.
a. Install FRM or IMM. In order for the operators to install FRM or IMM the DAHs are required to develop airplane model specific design change service instructions (Service Bulletins (SB)) that are FAA Oversight Office-approved. Operators will use these service instructions along with installation kits to install FRM or IMM. The FAA Oversight Office-approved FRM/IMM SBs are part of the airplane type design. Therefore, certain changes must be FAA Oversight Office-approved in accordance with the following procedures.
b. Deviation/Changes to FAA Oversight Office-approved FRM SBs. The retrofit requirements contained in 14 CFR part 121, § 121.1117 require installation of an IMM or FRM, if required by §§ 26.33, 26.35, or 26.37, that is “approved by the FAA Oversight Office”. Unlike an AD that typically requires accomplishment of corrective actions “in accordance with” a manufacturer’s SB, § 121.1117 does not require strict adherence (“in accordance with”) to the manufacturer’s SB. As with any SB that implements a change to type design, the manufacturer’s SBs for the NGS have been FAA-approved. Although the entire SB is identified as approved, only the technical data (type design) associated with the SB is FAA-approved. Therefore, deviations to the SBs that revise the technical data contained within the SB would have to be approved. Major changes to the technical data include, but are not limited to, any deviation/change to the SB that could affect the performance/reliability/operation of the NGS.
Minor deviations/changes to non-AD FAA-approved SBs should be handled in accordance with the procedures in the operator’s manual acceptable to the PI. Operators should refer to the current edition of AC 120-77, Maintenance and Alteration Data, for guidance regarding considerations in making major/minor decisions and the needed analysis/justification.
(1) Proposed Deviations. Operators should coordinate with the PI of their FAA certificate management office (CMO) or local Flight Standards District Office (FSDO) in requesting and implementing approval of major deviations/revisions directly by the FAA Oversight Office to the FRM SBs.
(2) Procedures That Address Major and Minor Changes. The operator must have procedures in its manual that address both major and minor changes to FAA Oversight Office-approved FRM/IMM service instructions/bulletins.
2-12. AIRWORTHINESS LIMITATIONS.
a. DAH Developed Airworthiness Limitations. The DAH is required to develop FAA Oversight Office-approved airworthiness limitations which include critical design configuration control limitations (CDCCL), inspections, or other procedures to prevent increasing the flammability exposure of any tanks equipped with FRM or FIMM above the certification limits, or to prevent degradation of the performance of any IMM installed throughout the operational life of the airplane. The DAH is required to include these airworthiness limitations in the Airworthiness Limitations Section (ALS) of the instructions for continued airworthiness (ICA).
These airworthiness limitations are part of the type design of the modified airplane. This requirement is similar to that contained in § 25.571. Airplane fuel tank system airworthiness limitations are intended to be treated the same as airplane structure airworthiness limitations.
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Part 25 appendix H, requires including fuel tank system airworthiness limitations in the airworthiness limitations of the ICA.
(1) Sections 26.33 and 26.35 requires the DAH to develop airworthiness limitations to prevent increasing the flammability exposure of any tanks equipped with FRM or FIMM and to prevent degradation of the performance of any IMM.
(2) Section 121.1117 and similar provisions in parts 125 and 129 require the operator to incorporate the airworthiness limitations required by §§ 26.33 and 26.35, if required, into their maintenance or inspection program.
b. Types of FRM/IMM Limitations. There are three types of fuel system airworthiness limitations.
(1) An airworthiness limitation item (ALI) inspection which has a specific task and interval, such as 10 years.
(2) An ALI procedure that could have specific task intervals.
(3) A CDCCL which has no interval but establishes configuration limitations to maintain and to protect the “critical design feature” identified in the CDCCL. CDCCLs can also include requirements to have placards on the airplane with information about critical features.
(See AC 25.981-1 or AC 25.981-2 for additional information on CDCCLs.)
FIGURE 1. EXAMPLE OF A FRM/IMM CDCCL TITLE: Center Fuel Tank Vent System – Auxiliary Tanks The Concern: The addition of an auxiliary fuel tank system via a Supplemental Type Certificate (STC) may reduce the effectiveness of the Nitrogen Gas System (NGS) resulting in an increase in flammability exposure of the center tank.
Reason for Concern: The NGS installation is designed to provide the proper level of nitrogen to the center wing fuel tank (CWT) in order to reduce the fleet wide average flammability exposure to levels equal to that of wing fuel tanks. Auxiliary fuel tank installations typically transfer fuel and ullage gases to and from the CWT.
The Critical Design Feature: Any auxiliary fuel tank system that allows transfer of ullage directly to or from the CWT must not significantly affect the performance of the NGS.
c. Critical Design Feature. Section 121.1117 and similar provisions in parts 125 and 129 require the operator to incorporate the airworthiness limitations required by §§ 26.33 and 26.35, if required, into their maintenance or inspection program. The FAA Oversight Office-approved FTFR airworthiness limitations make the CDCCL the primary source for the “critical design feature” information. While the “critical design feature” may also be in the Aircraft Maintenance Manual (AMM) reference, it is not to be used as the primary source of information for the CDCCL “critical design feature”. The primary source is the CDCCL.
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