Appendix 6
(b) Reduction of SMP. If an inspection program for WFD is in place, and if cracks are found in the structure during an inspection (or during a modification or replacement), the SMP should be reevaluated to determine if it provides sufficient freedom from WFD. If it does not, the point in the life of the airplane at which it is accomplished should be shortened, and you should revise the existing service information accordingly.
Figure 6-3 MSD/MED Residual Strength Curve (Including WFD Inspections) * REDUCTION FROM AVERAGE BEHAVIOR
TO PROVIDE EQUIVALENT PROTECTION PROBABILITY DENSITY
TO A TWO-LIFETIME FATIGUE TEST
FUNCTION FOR CYCLES
TO WFD CONDITION
REQUIRED ACTUAL
STRENGTH RESIDUAL
RESIDUAL STRENGTH
SPECIAL MSD/MED
INSPECTIONS
• WFD I WED IWFD, I WFD NOTE: No Scale Implied. CRACK LENGTH * crit WFD For Definition Use Only
- - det
ISP DETECTABLE MSD/MED SMP (AVERAGE BEHAVIOR) WFD
FLIGHT CYCLES/HOURS A6-8
Appendix 7 A7-1 APPENDIX 7
EXAMPLE OF HOW TO ESTABLISH AN LOV This appendix provides a simplified example of how to establish an LOV for a specified airplane structural configuration. The process for establishing an LOV involves four steps— Step 1. Identifying a candidate LOV (see subparagraph 204a of this AC) for the airplane structural configuration (see paragraph 203).
Step 2. Identifying WFD-susceptible structure (see subparagraph 204b). For this evaluation example, it was determined that the airplane structural configuration had six areas with WFD-susceptible structure.
Step 3. Performing a WFD evaluation of each of the six areas of WFD-susceptible structure (see subparagraph 204c and Appendix 6) to determine whether there are ISPs and SMPs for the candidate LOV identified. This allows evaluation of the candidate LOV.
Figure 7-1, shown below, shows the WFD behavior for one WFD-susceptible area. The figure also shows three different candidate LOVs. Candidate LOV1 is at a point that occurs significantly before the WFD(average behavior) line. This LOV won’t require any maintenance actions. Candidate LOV2 occurs before the WFD(average behavior) line, but closer to it. As a result, inspection will need to start before the LOV. Although candidate LOV3 occurs before the WFD(average behavior) line, with this LOV the probability of WFD in the fleet is unacceptable and an inspection and subsequent modification or replacement is required before the airplane reaches LOV3. Note that for LOV2 and LOV3, if inspections were determined to be unreliable, then the SMP would occur at the point on the chart where the ISP is. Using this example, this decision process needs to be repeated for all six WFD-susceptible areas.
Evaluate candidate LOVs and results of WFD evaluations for each susceptible area.
Appendix 7 A7-2 Figure 7-1 Comparison of WFD-Susceptible Structure to Airplane LOV
S tep 4. Finalizing the LOV. Once all susceptible areas have been evaluated, the final step is to determine where to establish the LOV that you will propose for compliance (see subparagraph 204d). Figure 7-2 shows the results of the WFD evaluation of the six WFD-susceptible areas. As shown, there are inspections and modifications or replacements that must be performed over time to preclude WFD. Any LOV can be valid as long as it is demonstrated that, based on its inherent fatigue characteristics and any required maintenance actions, the airplane model will be free from WFD up to the LOV. The example in Figure 7-2 includes three LOVs that could be proposed for compliance.
• LOV1: Maintenance actions are not required to address WFD.
• LOV2: Inspection and modification or replacement of area four are required to address WFD.
Appendix 7 A7-3
• LOV3: The DAH may propose an LOV that is greater than LOV2. However, as shown in Figure 7-2, that would result in more maintenance actions than identified for LOV2. Operators would be required to perform maintenance actions in four out of the six WFD-susceptible areas. Areas 1, 2, and 4 would have to be inspected prior to the LOV. Areas 3 and 5 would be free from WFD maintenance actions. Area 4 would be required to be inspected and modified, and then the modification would be required to be inspected prior to the LOV. Area 6 would require only modification prior to reaching the LOV because it was determined that inspections in this area would not be reliable. Some of the maintenance actions required for the LOV may have already been issued in a service bulletin and mandated by AD. For the rest, ADs would need to be issued.
Figure 7-2 Airplane Maintenance Actions
Appendix 8 A8-1 APPENDIX 8—SAMPLE COMPLIANCE PLAN This appendix provides an example of what the compliance plan required by § 26.21(d) might look like. It tracks the steps necessary to develop a compliance plan and shows how the AAC Airplane Company prepares the compliance plan for its AAC1 airplanes. DAHs are not required to use the same format as the one used in this example.
References to compliance/completion/meeting dates are tied to the effective date of the rule, and some may differ based on the DAH’s schedules.
Appendix 8 A8-2 Limit of Validity Widespread Fatigue Damage 14 CFR Part 26 Compliance Plan Document No. AAC1-26.21-CP-01 AAC Airplane Company Airplane Model: AAC1
REVISION X (DATE)
Appendix 8 A8-3 Table of Contents I. Design Approval Holder (DAH): AAC Airplane Company II. Airplane Model Applicability [§ 26.21(a)]
a. Airplane Model: AAC1
b. Airplane Model Description
c. Certification Basis (amendment level for § 25.571) III. Applicable Part 26 Requirements IV. Airplane Structural Configurations [§ 26.21(b)(1)(i) and (ii)] V. Establishing the Limit of Validity [§ 26.21(b)(1)]
a. Candidate LOV
b. WFD-Susceptible Structure
c. WFD Evaluation of Susceptible Structure
d. Finalizing LOV for Compliance VI. Maintenance Actions and Service Information [§ 26.21(b)(2)] VII. Airworthiness Limitations Section (ALS) [§ 26.21(b)(3) and (4)] VIII. Certification Compliance Matrix IX. Deviations to Guidance Material X. Compliance Documents and Supporting Data XI. Schedule XII. Model AAC1 Part 26 Compliance Team XIII. Communication and Coordination Plan XIV. Delegation Plan XV. Operators’ Involvement
Appendix 8 A8-4 List of Tables Table 1 – List of Applicable Part 26 Regulations Table 2 – Certification Compliance Matrix Table 3 – Compliance Document Summary Table 4 – Certification Compliance Team Matrix
Appendix 8 A8-5 I. Design Approval Holder (DAH).
If you are preparing the compliance plan for § 26.21 on behalf of a DAH, you must first identify the DAH and provide a brief description of the company’s background as it relates to the certification of transport category airplane structure.
Example: The AAC Airplane Company (AAC) is a well-established airplane manufacturer that has been designing and manufacturing airplane model AAC1 and its derivatives since 1965. We have a long history of optimizing the structural design features of our aircraft. We have developed the capability and expertise to use state–of- the-art structural analysis tools and techniques and we have extensive experience in structural analysis, structural validation testing, and in-service evaluations. We have performed partial teardown of our oldest, retired airplanes to support structural validation work. We have also developed numerous repairs and modifications/alterations for our airplanes.
AAC employs a staff of 357 structures engineers responsible for new designs and continued operational safety for our existing fleet, as well as customer requests. Thirty-two of these engineers are FAA Designated Engineering Representatives (DER). Internal company oversight of our DERs is provided by our Regulatory and Government Affairs organization, AAC-001.
II. Airplane Model Applicability [§ 26.21(a)].
a. Airplane Model—Reference the type certificate (TC) data sheet.
b. Airplane Model Description—Provide a brief description of all model types.
[At a minimum, provide sufficient information about the product to show part 26 applicability (i.e., maximum takeoff gross weight (MTGW), date certificated). A sketch of the model, with different views, may be a way to further describe it.] Example: The Model AAC1 is a turbine-powered transport category airplane with two wing-mounted turbine powered engines. The Model AAC1 has a metallic airframe, constructed with aluminum skin and stringer and frame construction. The Model AAC1 also uses aluminum floor beams. The wing, the vertical fin, and the horizontal stabilizer all use an aluminum rib spar configuration.
Only Models AAC1-P2 and AAC1-C are affected by § 26.21. Those two models fit the rule criteria and are not included in the exception list contained in § 26.21(g). Model AAC1-P1 is not affected by § 26.21 because of its MTGW. A summary of all three models is listed for reference.
Appendix 8 A8-6 AAC1-P1: TC approved February 8, 1968. This passenger airplane model has two variations–
(1) A single-class seating configuration with a capacity of 70 passengers and MTGW of 72,500 lbs.
(2) A three-class seating configuration with a capacity of 60 passengers and a MTGW of 68,000 lbs.
AAC1-P2: TC approved July 12, 1974. This passenger airplane model has a three-class seating configuration with a capacity of 100 passengers. The MTGW is 80,000 lbs.
AAC1-C: TC approved October 4, 1984. This cargo model has a MTGW of 95,000 lbs.
c. Certification Basis (amendment level for § 25.571)—identify the certification basis of the airplane model for § 25.571.
[It may be necessary to provide certification basis information by airplane model, airplane serial numbers, and by sections of the airplane where appropriate. Some manufacturers have modified portions of an airplane model on amended type certification programs, and have different parts of the airplane certified to different amendment levels of § 25.571 (reference § 21.101 Designation of Applicable Regulations).] Example: The Model AAC1 has the following certification basis for § 25.571:
• AAC1-P2. There is a supplemental structural inspection document for the airplane (Document No. AAC1-P2-SSID) which identifies damage-tolerance-based inspections. These inspections are based on the guidance in AC 91-56 (as revised) and mandated by Airworthiness Directive (AD) 88-xx-yy.
• AAC1-C—Amendment 25-54, for the fuselage from the forward pressure bulkhead to the aft pressure bulkhead. There is a supplemental structural inspection document (Document No. AAC1-C-SSID) for the fuselage, which identifies damage-tolerance-based inspections for that structure. These inspections are based on AC 91-56 guidance (as revised) and mandated by AD 98-xy-zz.
III. Applicable Part 26 Requirements.
Identify the applicable part 26 requirements.
Appendix 8 A8-7 Example:
Table 1 List of Applicable Part 26 Regulations
PART 26—CONTINUED AIRWORTHINESS AND SAFETY IMPROVEMENTS
FOR TRANSPORT CATEGORY AIRPLANES Regulation Requirement
26.21 Limit of Validity
26.21(a) Applicability
26.21(b) Limit of validity
26.21(c) Persons who must comply and compliance dates
26.21(d) Compliance plan
26.21(e) Compliance dates for compliance plans
26.21(f) Compliance plan implementation IV. Airplane Structural Configurations [§ 26.21(b)(1)(i) and (ii)].
Describe the proposed means for defining the airplane structural configurations for each model’s LOV. The configuration comprises all model variations and derivatives approved under the TC, and any structural modifications or replacements mandated by ADs as of [insert effective date of the final rule].
Example: AAC proposes to establish a separate LOV for each model variation (passenger model AAC1-P2 and cargo model AAC1-C). AAC will follow the guidance in AC 120-104 (as revised) to identify the structural configurations of each model airplane. The configuration will include the basic model (including all variations) and all structural modifications and replacements mandated by AD for that model.
For the airplane structural configuration section of AAC1-P2-WFD and AAC1-C-WFD documents, AAC will include the following information:
• Overall picture of the airplane with dimensions (span, length, etc.).
• View of each airplane section—wing, fuselage sections, horizontal stabilizer, vertical stabilizer, etc.—with a description of relevant details. These figures will also include a definition or identification of airplane sections, airplane zones, details, dimensions, etc. as necessary to define the locations of structure susceptible to WFD and such identifiers as body/wing stations, frame/stringer locations, and door numbers/locations.
Appendix 8 A8-8
• Identification of airplane structural configurations (by model and dash numbers) and rationale for grouping airplane models, as applicable, for which the LOV will apply.
• List of all structural modifications and replacements mandated by AD.
V. Establishing the Limit of Validity [§ 26.21(b)(1)].
Describe the proposed means of compliance (MOC) for establishing an LOV.
[Once you have defined the airplane structural configuration, you can assess its fatigue characteristics to determine the LOV for that configuration. The process for establishing an LOV typically involves four steps—
(1) Identifying a “candidate LOV.”
(2) Identifying WFD-susceptible structure.
(3) Performing a WFD evaluation of all susceptible structure.
(4) Finalizing the LOV.] Example: AAC proposes to establish one LOV for the passenger model AAC1-P2 and one for the cargo model AAC1-C. AAC will provide compliance documents AAC1-P2-WFD and AAC1-C-WFD for the passenger and cargo models respectively. The document for each model will follow the format of the Model AAC1 maintenance planning data and include—
• An LOV.
• The airplane structural configurations.
• The substantiation data.
• A list of all WFD maintenance actions that support the LOV.
• A proposed binding schedule for developing any service information that has not yet been issued.