AC 33.70-5 CHAPTER 1. I NTRODUCTION
1.1 Purpose.
1.1.1 Purpose and Scope.
This AC provides definitions, guidance, and acceptable methods, but not the only methods, that may be used to demonstrate compliance with requirements in § 33.70 related to the integrity of engine life-limited parts. Section 33.70 contains requirements applicable to the design and life management of propulsion system life-limited parts, including high-energy rotating parts.
1.1.2 Damage Tolerance Approach.
1.1.2.1 This AC presents a damage tolerance approach to address manufacturingand operationally induced anomalies in turbine engine rotating-part axial blade slot features. Applicants can integrate this approach with the existing “safe-life” process for high-energy rotors to create an enhanced life-management process. This approach supplements, rather than replaces, the existing safe-life approach and methodology (see paragraph
2.1.1).
1.1.2.2 In the context of damage tolerance, this AC does not permit operation beyond the life limits specified in the component manual under the existing safe-life approach, which limits useful rotor life to the minimum number of flight cycles required to initiate a crack. The procedures in this AC exclude rotor failure modes that demonstrate full containment of highenergy debris; applicants do not need to include those failure modes in the overall risk assessment.
1.2 Applicability.
1.2.1 The guidance provided in this AC is for aircraft engine manufacturers, modifiers, foreign regulatory authorities, Federal Aviation Administration (FAA) certification engineers, and FAA designees.
1.2.2 If this document is utilized by persons other than FAA employees or the Administrator’s designees, it 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.
1.2.3 The FAA will consider other means of demonstrating compliance that an applicant may elect to present. Terms such as “should,” “may,” and “must” are used only in the sense of ensuring the applicability of this particular method of compliance when the acceptable method of compliance in this document is used. If the FAA becomes aware 1-1
AC 33.70-5 of circumstances in which 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.
1.2.4 The material contained in this AC does not change or create any additional regulatory requirement, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
1.3 Related Reading Material.
1.3.1 Title 14, Code of Federal Regulations (14 CFR).
The following 14 CFR regulations are related to this AC. You can download the full text of these regulations from the Federal Register website at eCFR.
• Section 33.4, Instructions for Continued Airworthiness.
• Section 33.15, Materials.
• Section 33.19, Durability.
• Section 33.27, Turbine, compressor, fan, and turbosupercharger rotor overspeed.
• Section 33.63, Vibration.
• Section 33.70, Engine life-limited parts.
• Section 33.75, Safety analysis.
1.3.2 FAA Advisory Circulars.
The following ACs are related to the guidance in this AC. The latest version of each AC referenced in this document is available on the FAA website at FAA Advisory Circulars and on the Dynamic Regulatory System.
• AC 33-3, Turbine and Compressor Rotor Type Certification Substantiation Procedures.
• AC 33.14-1, Change 1, Damage Tolerance for High Energy Turbine Engine Rotors.
• AC 33.70-1, Change 1, Guidance Material for Aircraft Engine Life-Limited Parts Requirements.
• AC 33.70-2, Damage Tolerance of Hole Features in High-Energy Turbine Engine Rotors.
• AC 33.70-3, Damage Tolerance for Material Anomalies in Titanium Life-Limited Turbine Engine Rotors.
• AC 33.70-4, Fatigue Life of Nickel Powder Rotating Life-Limited Parts.
1.3.3 FAA Report and Test Case.
DOT/FAA/TC-23/8, Guidelines to Minimize Manufacturing Induced Anomalies in Critical Rotating Parts – 2022 Revision, is related to the guidance in this AC. The latest 1-2
AC 33.70-5 version of this report is available at the FAA website and the US Department of Transportation ROSA P website. You can access the neutral files containing the test case geometry at AC33.70-5 Axial Blade Slot Test Case Geometry.
1.3.4 ASTM.
ASTM International (ASTM) E 1049-85, Standard practices for cycle counting in fatigue analysis, is related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. The documents are available online at E1049 Standard Practices for Cycle Counting in Fatigue Analysis.
1.3.5 Society of Automotive Engineers (SAE) International.
The following SAE documents are related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. The documents are available online at the SAE website.
• SAE/FAA Committee on Uncontained Turbine Engine Rotor Events, Report No.
AIR 1537, Data Period 1962-1975.
• SAE/FAA Committee on Uncontained Turbine Engine Rotor Events, Report No.
AIR 4003, Data Period 1976-1983.
• SAE/FAA Committee on Uncontained Turbine Engine Rotor Events, Report No.
SP-1270, Data Period 1984-1989.
1.3.6 Presentations.
• “Development of Anomaly Distributions for Aircraft Engine Titanium Disk Alloys.” Technical paper presented by the Aerospace Industries Association (AIA) Rotor Integrity Sub-Committee at the American Institute of Aeronautics and Astronautics Conference, April 1997.
• “Development of Anomaly Distributions for Machined Holes in Aircraft Engine Rotors.” Technical paper presented at the American Society of Mechanical Engineers (ASME) Turbo Expo, Barcelona, Spain, May 2006.
1.4 Definition of Key Terms.
See appendix H for a list of definitions applicable to this AC.
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1.5 Background.
1.5.1 Damage Tolerance Requirements.
1.5.1.1 As required by § 33.70(a), applicants must perform appropriate damage tolerance assessments to evaluate the potential for failure from material, manufacturing, and service-induced anomalies within the approved life of the component. Service history with gas turbine engines has shown that such material, manufacturing, and service-induced anomalies can occur and may weaken the structural integrity of rotor life-limited parts.
1.5.1.2 Several methods to meet this requirement have been introduced in AC 33.70-1 Change 1, AC 33.70-2, and AC 33.70-3. The methodology described in chapter 2 of this AC was first introduced in AC 33.14-1 to address the potential presence of hard alpha anomalies in titanium rotors.
1.5.2 Introduction of Damage Tolerance Methodology.
Because developing the necessary technical elements, such as data collection, analysis, and methodology development, is complex and time-consuming, the FAA is introducing the surface damage tolerance assessment methodology for manufacturing and in-service anomalies in phases. This AC presents a damage tolerance approach for addressing manufacturing and operationally induced anomalies in axial blade slot features in rotor parts (see appendix E for information about the axial blade slot features).
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AC 33.70-5 CHAPTER 2. E NHANCED LIFE MANAGEMENT PROCESS
2.1 Overview.
2.1.1 Safe-Life Approach.
The safe-life approach has been effective based on historical experience and continues to provide a solid foundation for further improvement. Modifications to industrystandard life management procedures should enhance, not replace, the current safe-life approach.
2.1.2 Enhanced Life Management Process.
The enhanced life management process incorporates damage tolerance, building upon the existing conventional life management system. This enhanced life management process is expected to further reduce the occurrence of uncontained rotor failures, improving flight safety (see figure 2-1).
2.2 Showing Compliance to an Acceptable Design Target Risk (DTR).
2.2.1 Damage Tolerance Assessment.
The enhanced life management process, shown in figure 2-1 of this AC, requires damage tolerance assessments for rotating, life-limited part designs. These will be in the form of fracture-mechanics-based probabilistic risk assessments, with the results compared to specified DTR values (see paragraph 3.6 for more information about DTR values). Designs that meet these DTR values comply with the damage tolerance requirements of § 33.70 (see chapter 3 for more information about damage tolerance assessments).
2.2.2 Options for Meeting DTR.
For designs that do not meet the DTR requirement, engine manufacturers have a variety of options to show compliance with an acceptable DTR value, including:
• Component Redesign: Modifying the design to improve durability and resistance to failure.
• Material Changes: Switching to stronger or more durable materials that reduce the likelihood of crack initiation or propagation.
• Material Process Improvements: Enhancing how materials are processed to reduce the introduction of defects.
• Manufacturing Process Improvements: Upgrading manufacturing techniques to improve consistency and reduce the risk of defects.
• Manufacturing Inspection Improvements: Implementing better inspections during manufacturing to catch and correct defects early.
• Enhanced In-Service Inspections: Increasing the frequency or thoroughness of inspections during service to identify and address issues before they lead to failure.
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• Life Limit Reductions: Reducing the life limit of components to ensure they are replaced before critical defects develop.
Safe Life Prediction Life Management Process Development & Validation Te s t s Methods & Material Data Service Life & Product Assurance Damage T olerance Assessment New Element Added to Existing Safe Life Process Figure 2-1. Enhanced Life Management Process 2-2
AC 33.70-5 CHAPTER 3. DA MAGE TOLERANCE ASSESSMENTS
3.1 Approach.
3.1.1 Introduction to PDTRA Methodology.
As described in AC 33.70-1, probabilistic damage tolerance risk assessment (PDTRA) is an acceptable method to assess the ability of a component to tolerate anomalies. The results of these assessments form the basis for evaluating the relative damage tolerance capabilities of candidate part designs, helping the engine manufacturer to balance its designs for both enhanced reliability and customer impact. The results will be compared against DTR values (see paragraph 3.6 in this chapter).
3.1.2 Evaluating and Achieving Compliance with DTR.
The need for further risk-reducing actions by the engine manufacturer depends on whether the design under consideration meets the DTR value. If the target is met, the design complies with § 33.70. To meet the DTR, manufacturers can use various methods, including parametric studies that analyze the effects of key variables like inspection methods, inspection frequency, hardware geometry, hardware processing, material selection and life limit reduction. The manufacturer may make changes to the design, field management of the part, or both, to achieve the DTR value.
3.1.3 Applying PDTRA in the Design Phase.
PDTRA will usually be performed during the engine component detail design phase.
Paragraph 3.2 defines a probabilistic surface damage tolerance (PSDT) assessment methodology applicable to manufacturing and service-induced anomalies in axial blade slot features and provides a process to refine a design to achieve the desired DTR value.
3.2 Methodology.
3.2.1 Probabilistic Risk Assessments.
Probabilistic risk assessments can be performed using various methods, including Monte Carlo simulation or numerical integration techniques. To ensure consistent assessment results when conducting a PDTRA, use the standardized inputs and default data presented in this AC.
3.2.2 Standardized Inputs.
A list of standardized inputs is provided under paragraph 3.2.3 with default input data outlined in paragraphs 3.3 and 3.4. No further validation is required for applicability or accuracy when using default data in probabilistic assessments. However, when this data is used, applicants should follow the guidelines accompanying this data to ensure applicability. When using inputs other than these default values, additional validation may be necessary to verify applicability, adequacy, and accuracy.
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3.2.3 Inputs for Blade Slot Risk Assessments.
Probabilistic risk assessments for blade slots should incorporate the following inputs as part of a basic analysis.
3.2.3.1 Stress and Temperature.
• These variables influence crack propagation life predictions. Engine cycles for design low cycle fatigue (LCF) certification, or actual usage (if known), should be used to establish the stress and temperature distributions. Surface assessments should incorporate the appropriate surface and near surface stress distributions, including the effects of stress concentrations.
• The method described in this AC has been calibrated based on industry experience but does not consider the effect of shot peening or the resulting beneficial residual stresses on the crack propagation lives. No crack growth credit should be taken for beneficial surface residual stresses.
3.2.3.2 Number of Blade Slots.
A higher number of blade slots raises the chances of surface damage in axial blade slots and raises the probability of component failure.
3.2.3.3 Material Cyclic Crack Growth Rate Data.
Use the average crack growth rate properties of the base material as the default condition to calculate crack propagation life.
3.2.3.4 Crack Propagation Life Predictions.
• The predicted number of cycles for a given size crack to grow to a critical size is based on predicted stresses, temperatures, geometry, stress gradient, crack orientation, and material properties. Linear elastic fracture mechanics should be used for the calculation of propagation life. Default conditions should assume anomalies sampled from the distribution in paragraph 3.2.3.5 behave like sharp propagating cracks and are oriented in the worst orientation relative to the stress field. Crack growth rates depend on stress-temperature conditions.
• The method described in this AC has been calibrated assuming a “prescribed TMF” (thermo-mechanical fatigue) approach (see figure A-14 and table A-6 in appendix A), which should be used for consistency with the TMF method. This approach prescribes:
• Cycle pairs are extracted using a standardized stress rainflow extraction method (see ASTM E 1049-85, paragraph 1.3.4).
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• The crack growth rate at the maximum temperature of the paired cycle is used to compute crack growth life.
3.2.3.5 Anomaly Distribution.
The method described in this AC has been calibrated using the default anomaly distribution in paragraph 3.3 of this chapter.
3.2.3.6 Manufacturing Methods.
The method described in this AC is calibrated predominantly for axial blade slots manufactured using broaching methods, with some milled slots also included. Controls and techniques specific to blade slot forming operations and blade slot edge formation and finishing can influence damage tolerance assessments. Since the PSDT methodology in this AC xmay be relevant to slot formation methods other than broaching or milling, refer to appendix F of this AC for definitions and additional guidance.
3.2.3.7 Inspection Probability of Detection (POD).
Inspection at time of manufacture and during maintenance in service can impact component risk. The default POD information used to calibrate the method in this AC, along with instructions, are provided in paragraph 3.5.
The default POD information is only appropriate for in-service inspections, because the sampled anomalies are treated as actively growing cracks from the first cycle.
3.2.3.8 Maintenance Exposure Interval.
When assessing inspection benefits, the exposure intervals for the engine, module, or specific component can be considered in the analysis.
3.2.4 Calibration.
3.2.4.1 The applicant should calibrate its analytical prediction tools by performing the industry test case detailed in appendix A of this AC.
3.2.4.2 The test case in appendix A consists of a probabilistic analysis of a simulated nickel turbine disk with a two-lobed firtree slot, using specified inputs and scenarios. Table A-6 in appendix A provides acceptable ranges for test case results. Test case results outside of these ranges may indicate deficiencies with either the probabilistic assessment technique or the assumptions made in the stress analysis, considering that the speed and temperature distribution are s pecified.
3.2.5 Output.
Surface-induced anomaly assessments for axial blade slots.
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3.2.5.1 Applicability.
This procedure was specifically developed for field cracking or failures, or both, of axial blade slots, including but not limited to:
• Axial blade slots in nickel and titanium life-limited parts,
• Broached and milled axial blade slots,
• Slot bottom surface and edge features as shown in figures E-2 and E-3 in appendix E.
Note: While the methodology discussed in this AC was developed based on titanium and nickel axial blade slots, it can also apply to axial blade slots made from other materials (such as steel) subject to the determination of appropriate input data.
Note: This procedure may also apply to axial blade slots manufactured by other means than broaching or milling subject to the determination of appropriate input data.
Note: This procedure can be extended to other lower-risk locations within the axial blade slot not highlighted in figures E-2 and E-3 in appendix E.
For the purposes of this AC, lower-risk locations are those where a crack would not grow into the disk but would instead cause the release of the blade post and two blades. These other locations would use the same exceedance curve, frequency, manufacturing credits, and DTR. If the original equipment manufacturer (OEM) has experience to show a reduced number of cracking or fracture events in the lower-risk locations, it may be acceptable to lower the frequency accordingly.
3.2.5.2 Blade Slot Assessment.
The probabilistic assessment for a set of axial blade slots should be calculated over the entire anticipated service life of the component. This result, expressed as the probability of fracture per component, is called the predicted “blade slot event rate.” This event rate should be compared to the blade slot DTR value to determine whether the design is acceptable.
3.2.6 Long-Term Risk Management, Inspection, and Manufacturing Practices.
3.2.6.1 The FAA encourages engine manufacturers to incorporate fractureresistant design concepts where possible. Credit may be given for such design features if they clearly demonstrate, through both analysis and testing, a reduction in the risk of rotor failure due to the presence of unanticipated anomalies. These credits can be determined using the probabilistic risk assessment methods outlined in this AC, supported by data that validates the effectiveness of the fracture-resistant features.
3.2.6.2 Because the design of an aircraft turbine engine rotor is a lengthy process involving numerous iterations, each of which can substantially alter the AC 33.70-5 3-4
AC 33.70-5 initial design risk value, it is important that the design features satisfy the appropriate DTR value at the time of engine certification.
3.2.6.3 Some risk assessments will be conducted several years after the engine enters service due to in-service problems, changes in engine service usage, improvements in analytical predictive tools, or changes in engine component materials. In these instances, it is important to note that the DTR values should be satisfied throughout the life of the hardware.
3.2.6.4 In conjunction with probabilistic assessments and fracture-resistant designs, surface and edge inspections of axial blade slots play a critical role in ensuring the ongoing integrity of rotor parts. These inspections, which evaluate geometry, surface condition, and material quality, are widely used throughout the industry. Existing inspections should not be discontinued, even when advanced damage tolerance analysis is employed.
3.2.6.5 Established best practices define how to conduct process validation and prepare the associated manufacturing control plan (see paragraph 1.3.3).
These process validation and manufacturing control plans are mandatory, regardless of whether you apply manufacturing credits in the PSDT to meet the DTR.
3.3 Default Input - Anomaly Distributions.
3.3.1 Overview of Anomaly Distributions.
A key input associated with PDTRA assessments is the size distribution of potential anomalies. This type of information is statistical in nature and can be presented in a form that specifies the number, or rate, of anomalies that exceed a particular size.
3.3.2 Anomaly Distributions for Axial Blade Slots.
3.3.2.1 Anomaly distributions that apply to axial blade slots in disks have been developed to characterize the size (depth) distribution and frequency of potential anomalies (see appendix B). The distributions apply to axial blade slots machined in titanium or nickel rotor components.
3.3.2.2 The anomaly distributions contained in appendix B may be used to determine compliance with paragraph 3.6 of this chapter. The anomaly distributions in appendix B were used to calibrate this PDTRA method for a broad range of OEM field experience in terms of both product type and commercial usage over several decades.
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3.4 Manufacturing Credits for Manufacturing-Induced Anomaly Distributions.
3.4.1 Manufacturing Process Validation and Controls.
To encourage the use of best practices when manufacturing axial blade slots in rotor life-limited parts, a process has been established to take credit for manufacturing process validation and controls that reduce the risk of blade slot surface and blade slot edge damage during manufacture. This process quantifies the benefits of:
• Process validation to modern standards (see paragraph 1.3.3).
• Process validation and control plan, machine condition, fluid and fluid condition, tooling and setup validation, fluid monitoring, and process monitoring during the processes used to produce the axial blade slot surface.
• Process validation and control plan, semi-automated and automated edge processing of the axial blade slot edges.
• Inspection of axial blade slot surface and edge geometry.
• Non-destructive inspection of axial blade slot surfaces and periodic metallographic cutups and etch inspection.
3.4.2 The allowable manufacturing credit is a function of which of the above available actions are employed during blade slot manufacture. Calculation of manufacturing credit is outlined in appendix F. The manufacturing-induced probabilistic damage tolerance approach has been calibrated including the benefits of enhanced manufacturing validation and process controls. Hence, the credit is applied as a reduction factor to the results of the manufacturing-induced probabilistic assessments prior to comparing with the axial blade slot feature DTR.
3.5 Default Input – Nondestructive Evaluation (NDE) POD.
3.5.1 Detection of Geometric vs. Non-Geometric Anomalies.
3.5.1.1 Detection of non-geometric anomalies, such as highly distorted grain structure, white or amorphous layer, and bent grains, which have dominated the experience of cracking in rotors, is quite distinct from detecting cracks and geometric anomalies such as nicks or dents. Cracks that may develop from non-geometric anomalies can be detected by conventional methods, such as eddy current, penetrant, or ultrasonic inspection when the component is inspected at a shop visit.
3.5.1.2 These methods, however, may be of very limited benefit at new manufacture. If credit for non-geometric anomaly inspection at new manufacture is claimed, applicants must justify the detection capability of the technique on the target anomaly types. The default POD data supplied in this AC in appendix D only refers to the detection of cracks and is not necessarily applicable to non-geometric anomalies.
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