U.S . Department of Transportation Federal Aviation Administration Advisory Circular Subject: Reliability Program Methods— Standards for Determining Time Limitations Date: 12/19/18 AC No: 120-17B Updated: 7/9/26 Change: (Editorial) Initiated by: AFS-300 This advisory circular (AC) provides guidance for developing and maintaining a reliability program as part of a Continuous Airworthiness Maintenance Program (CAMP). The guidance includes information for:
• Collecting and analyzing operational data,
• Developing operational performance standards,
• Identifying and correcting deficiencies,
• A reliability program’s data display and reporting function,
• Maintenance Schedule task and interval adjustments, and
• Reliability program management and administration.
This AC applies to you if you are a Title 14 of the Code of Federal Regulations (14 CFR) part 121 air carrier, § 135.411(a)(2) or § 135.411(b) air carrier, or a part 91 subpart K (part 91K) operator maintaining your aircraft under a CAMP (§ 91.1411), and you choose to use standards for determining your time limitations. Your reliability program is your standard, and provides compliance with 14 CFR §§ 119.49(a)(8) and 91.1015(a)(5), as applicable.
Ro bert C. Carty Deputy Executive Director, Flight Standards Service
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CONTENTS Paragraph Page
Chapter 1. General Information1-1
1.1 Purpose of This Advisory Circular (AC)1-1
1.2 Audience1-2
1.3 Where You Can Find This AC1-2
1.4 What This AC Cancels1-2
1.5 Source Basis1-2
1.6 Definitions1-3
1.7 Background1-5
Chapter 2. Reliability Program Evaluation and Review2-1
2.1 Program Creation and Revisions2-1
Figure 2-1. Reliability Program Management and Administration2-3
2.2 Regulatory Requirements2-5
2.3 Continuous Evaluation2-5
Chapter 3. Data Collection System3-1
3.1 Identification of Data Sources3-1
3.2 Identification of Data Types3-1
3.3 Data Quality3-2
Chapter 4. Performance Standards System4-1
4.1 Identification of Performance Standards4-1
4.2 Techniques for Determining Deviations From Standards4-1
4.3 Non-Performance-Standard-Driven Requests4-5
Chapter 5. Analysis and Recommendation5-1
5.1 Root Cause Analysis of Variations From Performance Standard5-1
5.2 An
alysis of Task Effectiveness5-2
Figure 5-1. Data Selection Example5-6
5.3 Recommendations5-11
Chapter 6. Maintenance Schedule Changes—Internal Approval and Implementation6-1
6.1 Approval Process6-1
6.2 Implementation6-1
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Chapter 7. Reporting and Display Format7-1
7.1 General7-1
Chapter 8. Administrative8-1
8.1 Regulatory References8-1
8.2 Guidance References (current editions)8-1
8.3 Other Source References8-2
8.4 AC Feedback Form8-2
Appendix A. Task/Interval Adjustments—Top-Level ChartA-1
Appendix B. F ailure Effect Category (FEC) 5/8 TasksB-1
Appendix C. Failure Effect Category (FEC) 6/7/9 TasksC-1
Appendix D. Lube/Filter TasksD-1
Appendix E. Zonal/Enhanced Zonal Analysis Procedure (EZAP)E-1
Appendix F. StructureF-1
Appendix G. Lightning/High Intensity Radiated Fields (L/HIRF) TasksG-1
Appendix H. Operator Programs and AppearanceH-1
Appendix I. Regulatory Sourced RequirementsI-1
List of Tables
Table 2-1. Recommended Competency Training2-4
Table 3-1. Data Types3-1
Table 4-1. Techniques for Determining Deviation From Performance Standards4-2
Table 5-1. Data Sources by Task Type5-9
Table 5-2. Review of Systems/Powerplant Tasks5-10
1-1 CHAPTER 1. GENERAL INFORMATION
1.1 Purpose of This Advisory Circular (AC). This publication provides guidance for developing a reliability program, which are your standards for determining time limitations. When accepted by the Federal Aviation Administration (FAA) and after receiving authorization for its use by operations specifications (OpSpecs), your reliability program gives you the authority and the means to adjust your maintenance schedule tasks and intervals without prior approval or acceptance by the FAA. However, these changes are subject to subsequent FAA evaluation for continued acceptability. This autonomous authority granted to operators by the FAA does not relieve the operator or the FAA of their responsibility for the program’s effect on safety, and is not without limitation.
1.1.1 Not all maintenance schedule tasks and intervals are subject to the authority granted within your program. Operators should be knowledgeable of the task’s source and must operate within each task’s restrictions. Examples where restrictions exist include, but are not limited to:
• Airworthiness Directives (AD),
• Certification Maintenance Requirements (CMR),
• Airworthiness limitations (AL),
• Fuel tank safety (FTS) tasks,
• Maximum intervals for tasks “tagged” in the Maintenance Review Board Report (MRBR) (or equivalent) as precluding a candidate CMR (refer to AC 25-19, Certification Maintenance Requirements),
• Structural sampling periods imposed by the Maintenance Review Board (MRB),
• Critical design configuration control limitations (CDCCL), and
• Deleting Maintenance Steering Group - 3rd Task Force (MSG-3) Failure Effect Category (FEC) 5 and 8 tasks.
1.1.2 Title 14 CFR parts 91, 91 subpart K (part 91K), 119, 121, and 135 are the regulatory basis for this AC. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, of complying with 14 CFR.
Because the method in this AC is not mandatory, the term “should” applies only when you choose to follow this method. You may elect to follow an alternative method provided your method is accepted by the FAA. When we use “we,” “us,” or “our” in this AC, we mean the FAA. When we use “you,” “your,” or “yours,” we mean you, the air carrier or operator. When we use the term “person,” it has the same meaning as that in 14 CFR § 1.1.
Note: This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty.
Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
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1.2 Audience. This AC applies to air carriers conducting operations under 14 CFR part 121;
§ 135.411(a)(2) or (b); or part 91K operators choosing to maintain program aircraft under a Continuous Airworthiness Maintenance Program (CAMP) and use standards for determining time limitations.
Note: Operators who are not authorized by OpSpecs to use a reliability program may apply the standards in this AC when proposing adjustments to their time limitations. However, these operators must follow established procedures prior to implementing any adjustments to their time limitations.
1.3 Where You Can Find This AC. You can find this AC on the FAA’s website at https://www.faa.gov/regulations_policies/advisory_circulars and the Dynamic Regulatory System (DRS) at https://drs.faa.gov.
1.4 What This AC Cancels. AC 120-17A, Maintenance Control by Reliability Methods, dated March 27, 1978, is cancelled.
1.5 Source Basis.
1.5.1 Title 49 of the United States Code (49 U.S.C.). Title 49 U.S.C. § 44701 is the primary authority for Federal aviation regulations. Title 49 U.S.C. § 44701 instructs the FAA to promote the safe flight of civil aircraft in air commerce by prescribing regulations and standards in the interest of safety.
1.5.2 Title 14 CFR. The 14 CFR regulatory requirements supported by this AC include:
• Title 14 CFR §§ 91.1015(a)(5) and 91.1109(b)(5).
• Title 14 CFR § 119.49(a)(8), (a)(14), (b)(8), and (c)(8).
• Title 14 CFR § 121.135(a)(1) and (b)(18).
• Title 14 CFR §§ 135.23(s) and 135.427(b).
1.5.3 Methods of Compliance.
1.5.3.1 This AC describes processes, techniques, and procedures that will lead to an effective reliability program, and will help to ensure a level of safety appropriate to the type of operations conducted. None of the information in this AC is mandatory or constitutes a regulation. This AC does not impose, reduce, or change regulatory requirements.
1.5.3.2 Each operator is unique, and therefore a single means of compliance for developing, implementing, and maintaining a reliability program applicable to all cannot be provided.
1.5.3.3 By design, this AC is highly dependent upon the assumption that all elements of your CAMP are established, documented, and effective. The standards described in this AC are in no way intended to account for deficiencies in
1-3 other areas of your CAMP, and as such, can be considered a minimum standard. The FAA’s determination that your proposed reliability program is acceptable may differ from this AC to account for deficiencies in other areas of your CAMP.
1.6 Definitions. For the purposes of this AC, the following definitions are applicable:
1. Acceptable Level of Reliability. Maintaining operational performance equal to or higher than a value determined by the operator that supports the operator’s operational and economic objectives. Following MSG guidelines, operational failures or a loss of function having a direct effect on safety are never considered acceptable.
2. Airworthiness Limitation (AL). Instructions for mandatory replacement items, inspection intervals, related inspection procedures, and/or configuration control limitations.
3. Certification Maintenance Requirements (CMR), Candidate Certification Maintenance Requirements (CCMR). Refer to AC 25-19.
4. Data Quality. In a reliability program, data is generally considered high quality if it is fit for its intended uses in operation, analysis, decision making, and planning. Data Quality also includes form, format, and accuracy standards necessary to enable the program’s intended uses.
5. Effective. Capable of achieving the desired result. An indicator of scheduled maintenance effectiveness is the availability of your aircraft for flight or operations.
6. Failure. The inability of an item to perform within previously specified limits.
7. Failure Cause. The fundamental mechanism leading to a failure mode.
8. Failure Condition. The effect on the aircraft and its occupants, both direct and consequential, caused or contributed to by one or more failures, considering relevant adverse operational or environmental conditions.
9. Failure Effect. The result of a functional failure. (See also Failure Condition.)
10. Failure Effect Category (FEC) Code. A scheduled maintenance task classification as defined in the MRBR or equivalent manufacturer’s documents. Refer to Airlines for America (A4A) MSG-3, Operator/Manufacturer Scheduled Maintenance Development, paragraph 2-3-6.
11. Failure Mode. The way in which an item ceases to perform its intended function.
12. Function. The operation of an item in an intended or particular way to achieve a desired state or result.
13. Functional Failure. The inability of an item to perform its intended function within specified limits.
14. Hidden Failure Mode. A failure mode which is not detected through routine flightcrew operations or in-flight monitoring systems.
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15. Hidden Function.
• A function that is active when a system is used but where there is no indication to the operating crew when that function ceases to exist.
• A function that is normally inactive and the state of readiness to perform will not be evident before a demand for use.
16. Inherent Level of Reliability. The theoretical level of reliability established by the design and manufacturing criteria of an item. This is the highest level of reliability that can be expected from a unit, system, or aircraft. Achieving higher levels of reliability generally requires modification or redesign. (Refer to the A4A MSG-3.)
17. Item. Any level of hardware (e.g., a system, subsystem, module, accessory, component, unit, part, appliance, or structure).
18. Limitation. A binding limit (calendar, hours, or cycles) for scheduled maintenance task intervals.
19. Maintenance Program. Refers to the CAMP as described in AC 120-16, Air Carrier Maintenance Programs.
20. Maintenance Schedule. An element of the CAMP as described in AC 120-16; also called Time Limitations.
21. Operational Reliability. The reliability of an item calculated from operational data for a specific set of usage conditions, parameters, or element of exposure, such as number of departures, flight-hours, etc.
22. Operational Data. Any data generated as a result of aircraft operations. Examples of operational data are nonroutine events, Extended Operations (ETOPS) event reports, maintenance log items, delays or cancellations, mechanical interruption summaries, service difficulty reports, repeat items reports, unscheduled parts removals, component “teardown” reports, findings from scheduled maintenance tasks and inspections, etc.
23. Optimization. Substantiated revisions to the maintenance schedule such as a task revision, addition or deletion, or interval adjustment (up or down).
24. Performance Standards. The operational goals or standards developed by an operator to define an acceptable level of operational reliability. A performance standard may be defined within various areas of the operator’s CAMP or business objectives as it relates to fleet performance.
25. Predicted Reliability. The estimated reliability of an item.
26. Reliability. The probability that an item will perform a required function, under specified conditions, without failure, for a specified period of time (calendar, hours, or cycles).
27. Routine Task Findings. Data generated that document failures, defects, or degradation that are identified during the execution of a scheduled maintenance task. This data is usually in the form of nonroutine work cards or maintenance findings recorded in the
1-5 aircraft logbook, and may be used to determine the effectiveness of a routine task or to substantiate an interval adjustment.
28. Scheduled Maintenance. The defined set of maintenance tasks performed at stated intervals which, as a whole, comprises the maintenance schedule element of the operator’s CAMP. Refer to AC 120-16.
29. Scheduled Maintenance Task. An action performed at defined intervals, with the objective of retaining or restoring an item to a serviceable condition, or to discover a hidden failure, or to ensure a function is available.
30. Serviceable Condition. The subject item is capable of supporting or performing its intended function, resulting in continued airworthiness.
31. Task Interval. The specified parameter between consecutive occurrences of a maintenance task expressed in flight-hours, flight cycles, calendar-time, engine/auxiliary power unit (APU) hours/cycles, or defined opportunities where a specific task scope of work is required to be completed.
32. Task Scope. The defined procedures appropriate for satisfying the objective of an effective scheduled maintenance task.
33. Task Type. Standard MSG process classifications for a specific task scope
(e.g., inspection, lube, calibration, detailed visual inspection, or functional check).
34. Task Yield. A measure of the actual interval at which a task is accomplished, relative to the maximum allowed interval as defined in the operator’s maintenance schedule.
Yield is expressed as a percentage of the task’s maximum allowable interval since the task was last performed.
1.7 Background.
1.7.1 Title 14 CFR Parts 121 and 135 Operators. Title 14 CFR requires air carriers operating under 14 CFR part 121 and § 135.411(a)(2) or (b) to establish and maintain time limitations or standards for determining their time limitations. These time limitations specify the tasks and intervals for scheduled maintenance: the what, how, and when of your scheduled maintenance effort. In the context of the air carrier’s CAMP, this is referred to as the maintenance schedule.
1.7.2 Title 14 CFR Part 91K Operators. Additionally, 14 CFR also requires 14 CFR part 91K operators who choose to maintain program aircraft under a CAMP to establish and maintain time limitations or standards for determining their time limitations.
1.7.3 Maintenance Schedule. Regardless of the type of operation, acceptance criteria is written broadly to allow you to establish and maintain your maintenance schedule by one of two ways:
• You may submit your time limitations to the FAA for review and acceptance, or
• You may develop standards for determining your time limitations and obtain FAA acceptance of those standards.
1-6 Note: After the FAA determines your time limitations or standards for determining time limitations are acceptable, you will be issued the appropriate OpSpec/management specification (MSpec) that authorizes its use.
1.7.4 Maintenance Steering Group - 3rd Task Force (MSG-3). The previous version of this AC integrated the Maintenance Steering Group - 2nd Task Force (MSG-2) maintenance program development process, which focused on individual part failure rates and the maintenance processes designed to control them. This revised AC integrates the MSG-3 process, which focuses on maintaining aircraft system functions, while also considering maintenance cost-effectiveness. In addition, the MSG-3 process identifies the consequences of functional failures, and allows a wider selection of maintenance task types than MSG-2. To the extent possible, this revised AC does not identify any specific Maintenance Steering Group (MSG) process as it relates to the operator’s processes, methods, and standards specified in a reliability program.
1.7.5 Continuing Analysis and Surveillance System (CASS). A properly designed and effective reliability program is a part of your CASS, and can fulfill a portion of your CASS requirements regarding operational data collection, analysis, and corrective action. As is the case for CASS, your reliability program is not intended to duplicate your 14 CFR part 5 Safety Management System (SMS) Safety Assurance (SA) requirements. Rather, all these systems should be designed to naturally interface and complement each other, resulting in little or no duplication of effort or responsibilities.
2-1 CHAPTER 2. RELIABILITY PROGRAM EVALUATION AND REVIEW
2.1 Program Creation and Revisions.
2.1.1 Main Parts of an Operator’s Reliability Program. The elements of an operator’s reliability program are typically:
• A data collection system,
• A performance standards system,
• Analysis and recommendation,
• Internal approval and implementation, and
• A reporting and display format.
Note: Figure 2-1, Reliability Program Management and Administration, is a closed-loop, process-oriented flow diagram of the reliability process described in this AC.
2.1.2 Documentation Within the Operator’s Manual. You should ensure your standards for determining time limitations contain the following:
1. A general description of the reliability program, including definitions of (or reference to) significant terms used in the reliability program.
Note: You may not use your reliability program as a means to change the meaning or intent of regulatory-based definitions, restrictions, limitations, or reporting requirements. When defining terms, you should be as specific as possible and when applicable, provide qualifying criteria to avoid ambiguity and confusion when executing program requirements.
2. The application of the reliability program by aircraft fleet type and model.
3. Procedures and standards for data collection and ensuring applicability/data quality.
4. The organizational responsibilities, training, and experience requirements.
5. Duties and responsibilities for personnel who perform program requirements. This includes personnel such as those who monitor performance standards or alert/event levels, analyze operational data, and board members or personnel who perform decision-making functions.
6. The procedures for monitoring and revising the performance standards.
7. Data analysis methods.
8. The corrective action recommendation process, including approval and implementation.
9. Reports used or generated, and the frequency.
10. Standards and procedures for adjusting time limitations.
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11. Description or references to forms unique to the reliability program.
12. Management and administration procedures, including reliability program revision process.
13. Requirements or references to self-audits and other monitoring procedures for the performance and effectiveness of the reliability program.
14. Description or organizational chart of reliability program organizational authority and delegation. The administration of a reliability program generally requires a board or other organizational body within an operator’s maintenance organization. This board should be comprised of experienced and competent personnel with decision-making authority to approve changes to the operator’s maintenance schedule.
2.1.3 Identifying Participants. You should identify in your manual those participants who have authority and responsibility for your reliability program, including:
1. The Director of Maintenance (DOM) or equivalent who has overall authority and responsibility for the entire program.
2. The person who has direct authority and responsibility for the reliability program process.
3. The individual positions in the operator’s organization and their role within the reliability program, including responsibilities and authority.
2-3 Figure 2-1. Reliability Program Management and Administration