Appendix 3 APPENDIX 3. ACRONYMS USED IN THIS AC
AASA Aging Airplane Safety Act AASFR Aging Airplane Safety Final Rule AASIFR Aging Airplane Safety Interim Final Rule AAWG Airworthiness Assurance Working Group AC Advisory Circular ACO Aircraft Certification Office AD Airworthiness Directive ALI Airworthiness Limitation Items ALS Airworthiness Limitations Section ARAC Aviation Rulemaking Advisory Committee DSG Design Service Goal DT Data Damage Tolerance Data DTE Damage Tolerance Evaluation DTIs Damage Tolerance Inspections FAA Federal Aviation Administration FCBS Fatigue Critical Baseline Structure FCS Fatigue Critical Structure OIP Operator’s Implementation Plan PMI Principal Maintenance Inspector PSE Principal Structural Element RAG Repair Assessment Guideline RAP Repair Assessment Program REGs Repair Evaluation Guidelines SB Service Bulletins SRM Structural Repair Manual SSID Supplemental Structural Inspection Document SSIP Supplemental Structural Inspection Program STC Supplemental Type Certificate STG Structures Task Group TC Type Certificate
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Appendix 4 APPENDIX 4. BACKGROUND
a. Structural fatigue is recognized as a significant threat to the continued airworthiness of airplanes. This is because even small fatigue cracks can significantly reduce the strength of airplane structure. Consistent with this, the airworthiness standards for certification of new transport category airplanes for over 50 years have addressed fatigue with the intent of preventing catastrophic failures due to fatigue throughout the anticipated operational life of the airplane. However, these standards have not remained unchanged. They have evolved as the relevant knowledge base has increased. This knowledge includes service experience, specific incidents and accidents, and technological advances in designing, analyzing, testing, manufacturing, and inspecting airplanes.
b. One of the first significant changes in the standards occurred in March 1956 with the revision of the Fatigue Evaluation requirements contained in Civil Air Regulations (CAR) 4b.270. This revision added “Fail-safe strength” as an option to the “Fatigue strength” approach for addressing fatigue. Motivation for this change was the realization that precluding fatigue cracking from occurring might not always be possible and, therefore, as an option, the structure may be designed to survive cracking. The fatigue strength approach tries to achieve a design where fatigue cracking is not probable within the operational life of the airplane. The fail-safe approach assumes that cracking could occur, while maintaining a specified minimum strength after a “fatigue failure or obvious partial failure” had occurred. The efficacy of the fail-safe approach was not only dependent on the structure keeping the specified minimum strength with the fatigue damage present, but also on finding the damage during normal maintenance. As applied, the fail-safe approach emphasis is on redundancy as opposed to fatigue performance, and inspectability is assumed and not quantified. The fail-safe option was the predominate approach chosen for most large transport category airplanes certified in the 1960s and 1970s.
c. Another significant change in the airworthiness standards for fatigue occurred in October 1978 with Amendment 25-45, when § 25.571 was revised and § 25.573 was deleted. This change involved removing the fail-safe option entirely and establishing a new requirement to develop damage tolerance-based inspections wherever practical. The fatigue strength approach, as a default option, is used only if the damage tolerance approach is impractical.
The motivation for the 1978 change was a recognition, based on mounting evidence, that the fail-safe approach applied up to that point was not reliable and would not achieve the desired level of safety. Specific areas of concern with the fail-safe approach included the loss of fail-safety with age. This was because of the increased probability of cracking in the structure adjacent to the fatigue failure, or obvious partial failure, and the lack of directed inspections and quantification of residual life with the assumed damage present. It was agreed at the time that more emphasis was needed on where and how fatigue cracking could occur in the structure, and on quantifying crack growth and residual strength characteristics.
This includes damage tolerance characteristics and development of effective inspection protocols, such as where, when, how, and how often to inspect. The 1978 changes achieved this for certification of new transport category airplanes.
d. The same events and reasoning that drove the changes to airworthiness standards for new airplanes also influenced the strategy adopted to ensure the continued airworthiness of
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Appendix 4 the existing fleet. There was increasing concern about existing older airplanes certified according to the fail-safe requirements of CAR 4b.270. Eleven large transport models were specifically identified as needing the most attention. The FAA determined a need to develop damage tolerance-based inspection programs. These inspections supplement existing maintenance inspections, so these programs were referred to as SSIPs. The inspection requirements were documented in supplemental structural inspection documents (SSIDs). It was also agreed that SSIDs would be developed by the original equipment manufacturers (OEM) on a voluntary basis and then mandated by AD. The Civil Aviation Authority (CAA) for the United Kingdom published guidance for developing the SSIPs in Airworthiness Notice No. 89, Continuing Structural Integrity of Transport Aeroplanes, dated August 23, 1978, and by the FAA guidance for developing the SSIPs in AC No. 91-56, Supplemental Structural Inspection Program for Large Transport Category Airplanes, dated May 6, 1981. Subsequently, SSIPs were developed and mandated by AD for the eleven aging model airplanes. In developing the SSIDs little or no consideration was given to the effect of the repairs and alterations on inspections required for supplemental inspections. However, later revisions to some SSID ADs addressed supplemental inspection requirements for repairs and alteration to varying degrees.
e. In April 1988 one of the eleven aging model airplanes suffered major structural damage to its pressurized fuselage structure because of undetected fatigue cracking of the baseline primary structure. That airplane had a SSIP that was mandated by AD. The accident was attributed, in part, to the aging of the airplane involved, and precipitated actions that culminated in regulations aimed at avoiding catastrophic failures from fatigue in existing and future airplanes.
f. In response to the April 1988 accident the FAA sponsored a conference on aging airplanes and established a task force representing the interests of the airplane operators, airplane manufacturers, regulatory authorities, and other aviation representatives. This task force specifically recommended consideration of damage tolerance for repairs. In direct response to the recommendations, the FAA adopted changes to parts 91, 121, 125 and 129 in April 2000. These changes required operators to incorporate damage tolerance-based inspections for existing and future repairs to the fuselage pressure boundary for the eleven aging model airplanes previously identified. This did not address other model airplanes or repairs to other structure.
g. The April 1988 accident also precipitated Congressional legislation. In October 1991, Congress enacted Title IV of Public Law 102-143, the Aging Airplane Safety Act of 1991 (AASA). Two key elements of the AASA are as follows:
(1) Required “the Administrator to make such inspections and conduct such reviews of maintenance and other records of each airplane used by an operator to provide air transportation as may be necessary to determine that such is in a safe condition and is properly maintained for operation in air transportation.”
(2) Specified that an operator must be able to demonstrate, as part of the inspection, “that maintenance of the airplane’s structure, skin, and other age sensitive parts and components have been adequate and timely enough to ensure the highest level of safety.”
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Appendix 4
h. Although the AASA did not define specifics of what had to be done, the one clear intent was to avoid catastrophic failures throughout the operational life of each affected airplane because of fatigue. Consistent with this, and the damage tolerance requirements adopted in 1978 for new transport category airplanes, FAA initiated rulemaking that would require broader implementation of damage tolerance-based structural inspection programs. This would apply to almost all multi-engine airplanes used in scheduled passenger service.
Additionally, the intent was to address all structure where fatigue cracking could result in catastrophic failure.
i. In response to the AASA, FAA rulemaking efforts eventually resulted in the issuance of the Aging Airplane Safety Interim Final Rule (AASIFR) on December 6, 2002. This rule required implementation of damage tolerance-based inspection programs for all airplanes operated under part 121 and 129 operations. The AASIFR was also applicable to all multiengine airplanes engaged in part 129 or 135 operations that were initially certificated with 10 or more passenger seats. Airplanes operated between any point within the State of Alaska and any other point within the State of Alaska were exempt from that rule.
j. The AASIFR was subsequently amended and finalized on February 2, 2005, as the Aging Airplane Safety Final Rule (AASFR). The revised rule requires implementation of damage tolerance-based inspection programs by December 20, 2010. This applies to airplanes engaged in part 121 or 129 operations with type certificated passenger seating capacity of 30 or more or a payload capacity of 7,500 pounds or more. Airplanes operated within Alaska remain exempt. Although the scope has been reduced, the AASFR still affects the majority of airplanes engaged in scheduled passenger service. Relative to damage tolerance-based inspection programs, the AASFR raises the level of safety on the existing fleet of affected airplanes to the same level required for current transport category airplane type design approvals.
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Appendix 5 APPENDIX 5. APPROVAL PROCESS FOR NEW REPAIRS
In the past, AC 25.1529-1, Instructions for Continued Airworthiness of Structural Repairs on Transport Airplanes, August 1, 1991, described a two-stage approach for approving repairs to principal structural elements. The two-stage approach consisted of:
• Evaluating type design strength requirements per § 25.305 before return to service.
• Performing a damage tolerance evaluation and developing DT Data to demonstrate compliance with § 25.571 within 12 months of return to service.
The guidance material in AC 25.1529-1 is now embodied in this AC, and is modified to describe a three-stage approach now commonly used in the aviation industry. The three-stage approach is in lieu of the two-stage approach discussed above.
The DT Data include inspection requirements, such as inspection threshold, inspection method, and inspection repetitive interval, or may specify a time limit when a repair or alteration needs to be replaced or modified. The required data may be submitted all at once, prior to the airplane return to service, or it may be submitted in stages. The following three-stage approval process is available, which involves incremental approval of engineering data to allow an airplane to return to service before all the engineering data previously described are submitted. The three stages are described as follows:
a. The first stage is approval of the static strength data and the schedule for submittal of the DT Data. This approval is required prior to returning an airplane to service.
b. The second stage is approval of the DT Data. Unless the FAA agrees to a longer period, the DT Data should be submitted no later then 12 months after the airplane was returned to service. The DT Data might only contain the threshold where inspections are required to begin as long as a process is in place to develop the required inspection method and repetitive intervals before the threshold is reached. In this case, the submittal and approval of the remaining DT Data may be deferred to the third stage.
c. The third stage is approval of the DT Data not submitted and approved in the second stage. This would typically involve the inspection method and the repetitive intervals. This data would need to be submitted and approved prior to the inspection threshold being reached. Operation beyond the threshold would not be allowed unless the data are submitted to and approved by the FAA.
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Appendix 6 APPENDIX 6. ASSESSMENT OF EXISTING REPAIRS
A Damage Tolerance Inspection (DTI) assessment process consists of the following three steps:
1. Airplane Repair Survey. A survey will be used to identify existing repairs and repair configurations on FCBS. The survey would apply to all affected airplanes in an operator’s fleet, as defined in the OIP, using the process contained in the Compliance Document. An example of how to establish the timeline for conducting the survey can be found in Appendix 8 of this AC.
The procedure to identify repairs that require a DTE should be developed using AC 25.571-1C (using the AC revision level that corresponds to the airplane certification level for § 25.571), together with additional guidance specific to repairs, such as:
a. Size of the repair,
b. Repair configuration,
(1) SRM standards
(2) Other
c. Proximity to other repairs, and
d. Potential effect on FCBS
(1) Inspectability (access and method)
(2) Load distribution.
2. Identification and Disposition of Repairs Requiring Immediate Action. Certain repairs may not meet minimum requirements because of cracking, corrosion, dents, or inadequate design. The operator should use the guidance provided in the Compliance Document to identify these repairs and, once identified, take appropriate corrective action. In some cases, modifications may need to be made before further flight. The operator should consider establishing a fleet campaign if similar repairs may have been installed on other airplanes.
3. Damage Tolerance Inspection Development. This includes the development of DTI for the repair under consideration. During this step determine the inspection method, threshold, and repetitive interval. Obtain the necessary DTI from existing applicable DT Data or from the results of an individual damage tolerance evaluation performed using the guidance in AC 25.571-1C. If the DTI is practical, incorporate the DTI into the individual airplane maintenance program. If the DTI is either impractical or impossible for a particular structural detail, incorporate a replacement time for the repair into the individual airplane maintenance program. The three-stage approach discussed in Appendix 5 of this AC may be used, if appropriate.
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Appendix 7 APPENDIX 7.
REPAIRS AND ALTERATIONS TO REMOVABLE STRUCTURAL
COMPONENTS
This appendix provides guidance for developing and implementing Damage Tolerance (DT) Data for existing and new repairs and alterations to fatigue critical structure (FCS) on removable structural components. In summary, the guidance covers:
• Methods of determining or assigning the age (in flight hours, flight cycles, or both) to a removable structural component when its original life history is unknown.
• Guidance for tracking removable components that contain FCS.
• Methods for developing DT Data for repairs and alterations to removable components that contain FCS and implementing schedules for incorporating Damage Tolerance Inspections (DTIs) into maintenance programs.
• Options for implementing DT Data for removable components that contain FCS.
For determining the age of a component or tracking structural components, methods other than those given below may be used if approved by the operator’s Principal Maintenance Inspector as part of the Operator’s Implementation Plan.
a. Determining the Age of a Removable Structural Component. Determining an actual component age or assigning a conservative age provides flexibility and reduces operator burden when implementing DT Data for repairs and alterations to structural components. In some cases, the actual component age may be determined from records. If the actual age cannot be determined this way, the component age may be conservatively assigned using one of the following fleet leader concepts, depending upon the origin of the component:
(1) If component times are not available, but records indicate that no part changes have occurred, airplane flight cycles or flight hours can be used.
(2) If no records are available, and the parts could have been switched from one or more older airplanes under the same maintenance program, it should be assumed that the time on any component is equal to the oldest airplane in the program. If this is unknown, the time should be assumed equal to the same model airplane that is the oldest or has the most flight cycles or flight hours in the world fleet.
(3) A manufacturing date marked on a component may also be used to establish the component’s age. This can be done by using the above reasoning and comparing it to airplanes in the affected fleet with the same or older manufacturing date.
If none of these options can be used to determine or assign a component age or total number of flight cycles or flight hours, a conservative implementation schedule can be established
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Appendix 7 by using the guidelines applied in paragraph c of this appendix, for the initial inspection, if required by the DT Data.
b. Tracking. An effective, formal, control or tracking system should be established for removable structural components that are identified as FCBS or that contain FCS. This will help ensure compliance with maintenance program requirements specific to repairs and alterations installed on an affected removable structural component. Paragraph d of this appendix provides options that could be used to alleviate some of the burdens associated with tracking all repairs to affected removable structural components.
c. Developing and Implementing DT Data:
(1) Existing Repairs and Alterations – Components Installed Prior to December 20, 2010. Do the initial inspection on an affected component according to the schedule in paragraphs c(1)(a), c(1)(b) and c(1)(c) of this appendix.
Repairs: Accomplish the initial repair assessment of the affected structural component at the same time as the airplane level repair survey for the airplane on which the component is installed. Develop the DT Data per the process given in step 3 of Appendix 6 and incorporate the DTI into the maintenance program.
Alterations: Accomplish the initial alteration assessment of the affected structural component at the same time as the airplane level alteration assessment for the airplane on which the component is installed. Develop the DT Data and incorporate the DTI into the maintenance program.
(a) If the actual age of the repairs or alterations installation, or the total number of flight cycles or flight hours is known, use that information to establish when the initial inspection of the component should be performed. Repeat the inspection at the intervals provided by the TC or STC Holder for the repair or alteration installed on the component.
(b) If the actual age of the repairs or alterations installation, or the total number of flight cycles or flight hours is unknown, but the component age or total number of flight cycles or flight hours is known, or can be assigned conservatively, use the component age, or total number of flight cycles or flight hours, to establish when the initial inspection of the component should be performed. Repeat the inspection at the intervals provided by the TC or STC Holder for the repairs and alterations against the component.
(c) As an option, accomplish the initial inspection on the affected component at the next C-check (or equivalent interval) following the repair assessment. Repeat the inspection at the intervals provided by the TC or STC Holder for the repairs and alterations against the component.
(2) Existing Repairs and Alterations – Components Retrieved from Storage after December 20, 2010. For components retrieved from storage and
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Appendix 7 installed after December 20, 2010, that have not previously had DTE performed and DTI implemented, develop and implement DT Data as follows:
(a) If the time on the component (in flight cycles or flight hours) is known, or can be conservatively assigned, perform the following:
1 Survey the component,
2 Disposition the repairs and alterations,
3 Implement any DTI in accordance with the schedule in Chapter 2 or 3 of this AC, using the component’s age, and
4 Accomplish the initial inspection using the actual age of the repairs or alterations, or total number of flight cycles or flight hours, if known. If the age of the repairs or alterations is not known, use the component age.
Repeat the inspection at the intervals given for the repairs or alterations against the component.
(b) If the time on the component (in flight cycles or flight hours) is unknown and cannot be conservatively assigned, perform the initial repair or alteration assessment of the affected component prior to installation, and take the following actions:
1 Develop the DT Data per the process given in Chapter 2 or 3 of this AC, as applicable.
2 Incorporate any DTI into the maintenance program.
3 Accomplish the first inspection on the affected component at the next C-check (or equivalent interval) following the repair or alteration assessment.
4 Repeat the inspection at the intervals given for the repair or alteration against the component.
(3) New Repairs and Alterations. New repairs and alterations to affected removable structural components installed after December 20, 2010, must have a DTE performed and DTIs implemented according to an approved process. For repairs, that process is described in Appendix 5 of this AC. Perform the initial and repetitive inspections at the intervals given for the repair or alteration against the component.
d. Implementation Options to Help Reduce Tracking Burden. The following implementation techniques may be used to alleviate some of the burdens associated with tracking repairs to affected removable structural components. These techniques, if used, would need to be included in the OIP and may require additional FAA approval and TC or STC Holder input for DTIs.
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Appendix 7
(1) Upgrading Existing Repairs.
(a) As an option, existing repairs may be removed and replaced to zero time the DTI requirements of the repair and establish an initial tracking point for the repair.
Normally, this would be done at or before the survey for maximum benefit. The initial and repetitive inspections for the upgraded repair would then be accomplished at the intervals given for the repair against the component.
(b) A repair could also be upgraded to one whose inspection requirements and methods are already fulfilled by a part 121 or part 129 operator’s maintenance or inspection program (see paragraph 406 of this AC). That repair would then be repetitively inspected at each routine inspection interval applicable to the repair.
Specific tracking would not be required because that area of the airplane would already be normally inspected on each airplane in the fleet as part of the existing approved maintenance program. If the operator’s program intervals were changed, the effect on requirements for specific tracking would have to be re-evaluated.
(2) Special Initial and/or Routine Inspections.
(a) As an option, existing repairs may have special initial inspections accomplished during the component survey. This initial inspection establishes an initial tracking point for the repair. Following this initial inspection, the DTI requirements (e.g., repetitive inspections) of the repair would be implemented.
(b) In addition, special routine inspections could be defined for typical repairs that could be applied at a normal interval. In this case, an operator could check the affected components on each airplane for this type of a repair at the defined interval. If the repair was found, the special inspection would be applied to ensure its airworthiness until the next scheduled check. This alleviates the need to specifically track affected components for every repair, especially typical ones.
(c) The development of inspection processes, methods, applicability and intervals would most likely require the assistance of the TC or STC Holder for the FCS in question. In all circumstances, the data must be approved by the FAA Oversight Office.
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Appendix 8 APPENDIX 8. PROGRAM IMPLEMENTATION EXAMPLES
The following are provided to assist the operator in understanding the timeline for when the program should be implemented. Two examples are given, one covers airplanes below 75 percent of the design service goal (DSG) on December 18, 2009, and the other is for airplanes beyond the DSG on December 18, 2009.
a. Airplane Below 75 Percent of the DSG on December 18, 2009. Operators should complete a survey at the first heavy maintenance check (time limit equivalent to a “D-check”) after the airplanes reach 75 percent of the DSG, and not to exceed the DSG.
Consider the following:
(1) Airplane Total Flight Cycles on December 18, 2009, – 55,000
(2) DSG = 75,000 Cycles, 75percent DSG – 56,250 Flight Cycles
(3) Time of last “D”-Check Equivalent – 53,000 Flight Cycles
(4) 8-Year “D”-Check Equivalent – 365 Days/Year, 4 cycles/day = 11,680 Flight Cycles
The survey would be performed after the airplane has reached 56,250 total flight cycles (75% DSG) and would be due before the airplane has accumulated 11,680 flight cycles (D- check equivalent) beyond the time of the last D-check (53,000 flight cycles). Therefore, the survey would be due before the airplane has reached 64,680 total flight cycles, but in any case would be required before the airplane has reached 75,000 total flight cycles (DSG).
b. Airplane Beyond the DSG on December 18, 2009. Operators complete a survey of these airplanes at or before the next heavy maintenance check (equivalent to a D-check) after December 20, 2010, not to exceed 6 years.
Consider the following:
(1) Airplane Total Flight Cycles on December 18, 2009, – 80,000
(2) DSG = 75,000 Cycles, 75percent DSG – 56,250 Flight Cycles
(3) Time of last “D”-Check Equivalent – 78,540 Flight Cycles, conducted in Jan 2009
(4) 8-Year “D”-Check Equivalent – 365 Days/Year, 4 cycles/day = 11,680 Flight Cycles
(5) Assume maximum total flight cycles accumulated by December 20, 2010, to be 81,460 (80,000 flight cycles on 12/18/2009 + 1,460 flight cycles/year).
Consider an airplane that has 80,000 total flight cycles as of December 18, 2009, and a DSG of 75,000 total flight cycles. The airplane is currently on an 8-year D-check equivalent and the last D-check was performed in January 2009 at 78,540 total flight cycles. The survey would need to be performed prior to the airplane accumulating 11,680 flight cycles (D-check equivalent) beyond the time of the last D-check (78,540 flight cycles) or 6 years,
whichever occurs first. Therefore, the survey would be due before the airplane has reached 90,220 total flight cycles or 6 years, whichever occurs first, based on the airplane utilization
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Appendix 8 of 4 cycles/day, a 365-day year, and a maximum total of 81,460 flight cycles as of December 20, 2010.
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