AC 33.70-5

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Damage Tolerance of Axial Blade Slot Date: AC No: 33.70-5 Feat ures in High-Energy Turbine Engine Rotors Initiated By: AIR-625

This advisory circular (AC) describes an acceptable means for demonstrating compliance with the requirements of Title 14, Code of Federal Regulations (14 CFR) 33.70, Engine Life-Limited Parts.

If you have suggestions for improving this AC, you may use the Advisory Circular Feedback form at the end of this AC.

Daniel J. Elgas Aviation Safety Director, Policy and Standards Division, Aircraft Certification Service

AC 33.70-5

CONTENTS Paragrap h Page

Chapter 1. Introduction1-1

1.1 Purpose1-1

1.2 Applicability.1-1

1.3 Related Reading Material.1-2

1.4 Definition of Key Terms1-3

1.5 Background1-4

Chapter 2. Enhanced Life Management Process2-1

2.1 Overview2-1

2.2 Showing Compliance to an Acceptable Design Target Risk (DTR).2-1

Chapter 3. Damage Tolerance Assessments3-1

3.1 Approach3-1

3.2 Methodology3-1

3.3 Default Input - Anomaly Distributions3-5

3.4 Manufacturing Credits for Manufacturing-Induced Anomaly Distributions3-6

3.5 Default Input – Nondestructive Evaluation (NDE) POD3-6

3.6 Design Target Risk.3-7

Chapter 4. “Soft Time Inspection” Rotor Life Management4-1

4.1 Approach4-1

4.2 Inspection Scenarios4-3

Appendix A. Manufacturing-Induced Anomaly Calibration Test Case for Axial Blade Slots ...................................................................................................................................... A–1 A.1 Introduction. ................................................................................................................. A–1 A.2 Test Case Input Data. ................................................................................................... A–2 A.3 Test Case Analysis Guidelines................................................................................... A–16 A.4 Test Case Results. ...................................................................................................... A–21 Appendix B. Default Anomaly Distribution Curves. ........................................................... B–1 B.1 Manufacturing Anomaly Distribution Curve for Disk Axial Blade Slots. .................. B–1 Appendix C. Default Probability of Detection (POD) Applicability .................................. C–1 ii

AC 33.70-5

CONTENTS (CONTINUED) Par agraph Page C.1 Use of Default Probability of Detection (POD) Data. ................................................. C–1 C.2 Demonstrating Inspection Capability. ......................................................................... C–1 C.3 General Applicability and Restrictions. ....................................................................... C–2 C.4 ECI: Applicability and Restrictions. ............................................................................ C–3 C.5 Penetrant Inspection (PT/FPI): Applicability and Restrictions. .................................. C–3 C.6 Crack Characteristics and Limitations ......................................................................... C–3 Appendix D. Default POD Curves ........................................................................................ D–1 D.1 Overview. ..................................................................................................................... D–1 D.2 [Reserved.] ................................................................................................................... D–6 Appendix E. Definition of an Axial Blade Slot for Manufacturing Anomaly Assessment ............................................................................................................................................... E–1 E.1 Definition and Description of Axial Blade Slots. ........................................................ E–1 E.2 [Reserved.] ................................................................................................................... E–3 Appendix F. Manufacturing Credits ..................................................................................... F–1 F.1 Overview of Manufacturing Credits. ........................................................................... F–1 F.2 Calculation of Effective Number of Slots. ................................................................... F–1 F.3 Rules for Applying Credits. ......................................................................................... F–7 F.4 Credit Application Process. ......................................................................................... F–8 F.5 Slot Surface Credit Examples. ..................................................................................... F–8 F.6 Slot Edge Credit Examples. ......................................................................................... F–9 Appendix G. Surface (Manufacturing Induced Anomaly) Damage Tolerance Assessment Flowcharts .................................................................................................... G–1 G.1 Overview. ..................................................................................................................... G–1 G.2 Process. ........................................................................................................................ G–1 Appendix H. Definitions ......................................................................................................... H–1 H.1 Axial Blade Slot. .......................................................................................................... H–1 H.2 Component. .................................................................................................................. H–1 H.3 Component Feature. ..................................................................................................... H–1 H.4 Damage Tolerance. ...................................................................................................... H–1 iii

AC 33.70-5

CONTENTS (CONTINUED) Paragraph Page H.5 Default Probability of Detection (POD) Values. ......................................................... H–1 H.6 Design Target Risk (DTR). .......................................................................................... H–1 H.7 Event. ........................................................................................................................... H–1 H.8 Exceedance Curve. ....................................................................................................... H–2 H.9 Focused Inspection....................................................................................................... H–2 H.10 Full Field Inspection. ................................................................................................... H–2 H.11 Hard Time Inspection Interval. .................................................................................... H–2 H.12 Hazardous Engine Effects. ........................................................................................... H–2 H.13 Inspection Opportunity. ............................................................................................... H–2 H.14 Linear Elastic Fracture Mechanics. .............................................................................. H–2 H.15 Low Cycle Fatigue (LCF). ........................................................................................... H–2 H.16 Maintenance Exposure Interval. .................................................................................. H–3 H.17 Manufacturing Anomaly. ............................................................................................. H–3 H.18 Mean Probability of Detection (POD). ........................................................................ H–3 H.19 Module. ........................................................................................................................ H–3 H.20 Operationally-Induced Anomaly. ................................................................................ H–3 H.21 Part Available............................................................................................................... H–3 H.22 Probability of Detection. .............................................................................................. H–3 H.23 Probabilistic Risk Assessment. .................................................................................... H–3 H.24 Rotor Life-Limited Parts. ............................................................................................. H–4 H.25 Stage. ............................................................................................................................ H–4 H.26 Safe Life. ...................................................................................................................... H–4 H.27 Soft Time Inspection Interval. ..................................................................................... H–4 iv

AC 33.70-5

FIGURES Number Page

Figure 2-1. Enhanced Life Management Process2-2

Figure A-1. Representation of Test Case Blade and Disk Geometry .......................... A–2 Figure A-2. Test Case Geometry Definition: Isometric View of 5 Degree Sector Showing Disk and Blade ............................................................................................................ A–3 Figure A-3. Test Case Geometry Definition: Disk Axisymmetric Profile in Inches and (Millimeters) ................................................................................................................. A–4 Figure A-4. Test Case Geometry Definition: View Looking Radially Inward Showing 8.0 Degree Broach A ngle .................................................................................................. A–5 Figure A-5. Test Case Geometry Definition: View A-A of Disk Broach Slot Profile in Inches and (Millimeters) Not to Scale .......................................................................... A–6 Figure A-6. Test Case Geometry Definition: Axisymmetric View of Blade Showing Height and Width in Inches and (Millimeters) .............................................................. A–7 Figure A-7. Test Case Geometry Definition: View of Blade Looking Radially Inward Showing Broach Angle and Blade Thickness in Inches and (Millimeters) ................... A–8 Figure A-8. Test Case Geometry Definition: View B-B Showing Blade Attachment Profile in Inches and (Millimeters) Not to Scale ...................................................................... A–9 Figure A-9. Disk Speed and Temperature over Test Case Flight Profile ................... A–10 Figure A-10. Example of Test Case Linear Thermal Gradient at t=642s ................... A–11 Figure A-11. Test Case Conventions Used for Test Case Inspection Scenario ........ A–16 Figure A-12. Test Case Anomaly Placement within Test Case ................................. A–17 Figure A-13. Test Case TMF Mission Example ......................................................... A–20 Figure B-1. Distribution for Manufacturing Induced Anomalies in Axial Blade Slots .... B–2 Figure C-1. Crack Dimension Conventions to be Used with POD Curves................... C–4 Figure D-1. POD for FPI of Finish Machined Surfaces ................................................ D–1 Figure D-2. POD for ECI of Finish Machined Surfaces ............................................... D–4 Figure E-1. Example of Axial Blade Slot Component Features ................................... E–2 Figure E-2. Example Dovetail Axial Blade Slot Geometry and Definition of the Features Covered by this AC ..................................................................................................... E–2 Figure E-3. Example Firtree Axial Blade Slot Geometry and Definition of the Features Covered by this AC ..................................................................................................... E–3 Figure G-1. Surface Damage Risk Assessment Work Flow ....................................... G–1 v

AC 33.70-5 Figure G-2. Crack Propagation Analysis .................................................................... G–2 Figure G-3. Feature Risk Assessment (Part 1 of 2).................................................... G–3 Figure G-4. Blade Slot Set Risk Assessment (Part 2 of 2) ......................................... G–4 Figure G-5. Determination of the Number of Effective Slots in the Blade Slot Set ..... G–5 Figure G-6. Probability of Fracture Calculation .......................................................... G–6 TABLES Number Page

Table 3-1. Allowable Design Target Risk for Axial Blade Slots3-8

Table A-1. Tabulated Disk Speeds and Temperatures for Test Case Flight Profile .. A–10 Table A-2. Test Case Physical Material Properties ................................................... A–12 Table A-3. Test Case Cyclic Crack Growth Rate Parameters (deg F) ...................... A–13 Table A-4. Test Case Cyclic Crack Growth Rate Parameters (deg C) ...................... A–14 Table A-5. Test Case Material Constants to be Used for Considerations of Plasticity (deg F) ....................................................................................................................... A–14 Table A-6. Test Case Material Constants to be Used for Considerations of Plasticity(deg

C)A-15

Table A-7. Test Case Prescribed TMF Method Results ............................................ A–20 Table A-8. Test Case Acceptable Component POF Results for the Slot Locations .. A–21 Table B-1. Tabular Data for Figure B-1 ....................................................................... B–3 Table D-1. Tabulated FPI POD (Directed Inspection) ................................................. D–2 Table D-2. Tabulated FPI POD (Full Field Inspection) ................................................ D–3 Table D-3. Tabulated ECI POD (Rejection at Half Calibration Threshold) .................. D–5 Table D-4. Tabulated ECI POD (Rejection at Calibration Threshold) .......................... D–6 Table F-1. Slot Surfaces – Process Controls, Control Definitions, and Credit Factor .. F–2 Table F-2. Slot Edges – Process Controls, Control Definitions, and Credit Factor ...... F–4 vi

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.

1-3

AC 33.70-5

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).

1-4

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.

2-1

AC 33.70-5

• 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