~ I ~ C AdvisoryU.S. Department of Transportation Federal Aviation Circular Administration Subject: Damage Tolerance for Material AC No: 33.70-3 Anomalies in Titanium Life-Limited Date: 4/17/23 Initiated By: AIR-624 Turbine Eng ine Rotors 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.
Digitally signed by
VICTOR W WICKLUND Date: 2023.04.17 17:23:31 -04'00' Victor Wicklund Aviation Safety Acting Director, Policy & Standards Division Aircraft Certification Service
CONTENTS Paragraph Page
Chapter 1. Introduction1-1
1.1 Purpose1-1
1.2 Applicability1-1
1.3 Related Reading Materia
ls1-1
1.4 Definitions1-3
1.5 Background1-3
Chapter 2. Conventional (“Safe-Life
”) Life Management Process2-1
2.1 Safe-life Philosophy2-1
2.2 Addition of Damage Tolerance Assessment2-1
Chapter 3. Enhanced Life Management Process3-1
3.1 Damage Tolerance Assessment for Critical Titanium Rotating Parts3-1
3.2 Options to Reduce the POF3-1
Chapter 4. Damage Tolerance Assessments4-1
4.1 Approach4-1
4.2 Further Risk Reduction Actions4-1
4.3 Methodology4-2
4.4 Anomaly Distributions4-6
4.5 Default Input - POD by Nondestructive Evaluation4-7
4.6 Design Target Risk4-8
Chapter 5. “Soft Time Inspection” Rotor Life Management5-1
5.1 Approach5-1
5.2 Inspection Scenarios5-3
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CONTENTS (CONTINUED)
FIGURES Number Page Figu
re 3-1. Life Management Process3-2
Fi
gure 4-1. Typical Elements of a Titanium Melt-Related Anomaly Risk Assessment4-10
Fig
ure A-1. Typical Zone TypesA-6
F
igure A-2. Guidelines for Crack Growth AnalysisA-6
F igure A-3. Geometry...............................................................................................................
A-10 F
igure A-4. Room Temperature Test CycleA-10
F
igure A-5. Component Stress Model and Component Principal Stress Contour PlotA-11
F
igure A-6. Component Stress Model/Stress Volume DefinitionA-11
F
igure A-7. Zone Crack LocationA-13
F
igure A-8. Crack Growth CalculationA-14
F
igure A-9. Overall Probabilistic Assessment Process – No In-Service InspectionA-15
F
igure A-10. Overall Probabilistic Assessment Process – With In-Service InspectionA-16
F
igure A-11. Hard Alpha Anomaly Distribution for Test CaseA-17
F igure A-12. Results .................................................................................................................
A-20 F igure B-1. In-Service Inspection Decision Process ..................................................................
B-4 Fig
ure B-2. Ring Disk Overhaul First Piece Part Exposure DistributionB
-5 F
igure B-3. New Ring Disk First Exposure DistributionB
-5 F
igure B-4. New Ring Disk First Exposure DistributionsB
-6 Fig
ure B-5. Combination of First Piece Part and Module Exposure DistributionsB
-6 F igure C-1. Hard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (>10 In. Billet)
Calibrated to #3 FBH / Forging Conventional #3 FBHC-3
F igure C-2. Hard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (>10 In. Billet)
Calibrated to #3 FBH / Forging Conventional #2 FBHC-4
F igure C-3. Hard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (>10 In. Billet)
Calibrated to #3 FBH / Forging Conventional #1 FBHC-5
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AC 33.70-34/17/23
CONTENTS (CONTINUED) Figure C-4. H ard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (5-10 In. Billet)
Calibrated to #2 FBH / Forging Conventional #3 FBHC-6
Figure C-5. H ard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (5-10 In. Billet)
Calibrated to #2 FBH / Forging Conventional #2 FBHC-7
Figure C-6. H ard Alpha Exceedance Curves - Billet Zoned AMS2628 Class A (5-10 In. Billet)
Calibrated to #2 FBH / Forging Conventional #1 FBHC-8
Figure E-
1. POD for Fluorescent Penetrant Inspection of Finish-Machined SurfacesE-2
Figure E-
2. Mean (50 Percent Confidence) POD for Ultrasonic Inspection of Field Components,
#1 FBH (1/64 In. Diameter) CalibrationE-3
Figure E-
3. Mean (50 Percent Confidence) POD for Ultrasonic Inspection of Field Components,
#2 FBH (2/64 In. Diameter) CalibrationE-4
Figure E-
4. Mean (50 Percent Confidence) POD for Ultrasonic Inspection of Field Components,
#3 FBH (3/64 In. Diameter) CalibrationE-5
Figure E-
5. Mean (50 Percent Confidence) POD for Eddy Current Inspection of Finish Machined
ComponentsE-6
TABLES Number Page
Table 4-1. Billet Inspection Standards4-3
Table A-1. Failure Risk DataA-9
Table A-2. Zone Volume DataA-12
Table A-3. Tabular Data – Anomaly Distribution Curves for Test Case (Figure A-11)A-18
Table C-1. Default CurvesC-2
Table C-2. Tabular Data – Anomaly Distribution Curves (Figure C-1 through Figure C-6)C-9
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1.1
1.2
1.2.1
1.2.2
1.2.3
1.3
1.3.1 CHA PTER 1. INTRODUCTION Purpose.
This advisory circular (AC) describes an acceptable means for demonstrating compliance with the r equirements of title 14, Code of Federal Regulations (14 CFR)
33.70, Engine Life-Limited Parts. Section 33.70 contains requirements applicable to engine life-limited parts of turbine aircraft engines, including rotating parts made of titanium.
Applicability.
The guidance in this AC is for en gine manufacturers, modifiers, and Federal Aviation Administration (FAA) employees and designees.
The contents of this AC do not have the force and effect of law and are not meant to bind the public in any way, and this AC is intended only to provide information to the public regarding existing requirements under the law or agency policies. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, for showing compliance with § 33.70. When the method of compliance in this AC is used, terms such as “should,” “may,” and “must” are used only in the sense of ensuring applicability to this particular method of compliance. The FAA will consider other means of showing compliance that an applicant may elect to present.
While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. If, however, the FAA becomes aware of circumstances that convince the agency that following this AC would not result in compliance with the applicable regulations, the agency will not be bound by the terms of this AC, and may require additional substantiation as a basis for finding compliance.
The material in this AC does not change or create any additional regulatory requirements, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
Related Reading Materials.
The following materials are related to the guidance in this AC. Unless otherwise indicated, you should use the current edition if following the method of compliance set forth in this AC.
Title 14, Code of Federal Regulations (CFRs).
• Section 33.4, Instructions for Continued Airworthiness.
• Section 33.15, Materials.
• Section 33.19, Durability.
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• 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 Publications.
• AC 33.14-1, Damage Tolerance for High Energy Turbine Engine Rotors.
• AC 33.70-1, Guidance Material for Aircraft Engine Life-Limited Parts Requirements.
• AC 33.70-2, Damage Tolerance of Hole Features in High-Energy Turbine Engine Rotors.
• ANE-2002-33.15-R0, 14 CFR § 33.15, Materials.
• DOT/FAA/AR-06/3, Guidelines to Minimize Manufacturing Induced Anomalies in Critical Rotating Parts (available here DOT/FAA/AR-06/3).
• DOT/FAA/AR-07/63, Update of Default Probability of Detection Curves for the Ultrasonic Detection of Hard Alpha Inclusions in Titanium Alloy Billets (available here DOT/FAA/AR-07/63).
1.3.3 Industr y Publications.
• AIAA-97-1068, The Development of Anomaly Distributions for Aircraft Engine Titanium Disk Alloys. Technical paper, presented by the AIA Rotor Integrity Sub- Committee at the American Institute of Aeronautics and Astronautics (AIAA) Conference.
• ASME 2000-GT-0421, A Probabilistically-Based Damage Tolerance Analysis Computer Program for Hard Alpha Anomalies in Titanium Rotors. Proc. 45th ASME International Gas Turbine & Aero-Engine Technical Conference, Munich, Germany.
• ASME GT2006-90843, The Development of Anomaly Distributions for Machined Holes in Aircraft Engine Rotors. Technical paper, presented at ASME Turbo Expo in Barcelona, Spain.
• ASME GT2012-68987, Review of Probabilistic Damage Tolerance Methodology for Hard Alpha Anomalies. Technical paper, presented at ASME Turbo Expo in Copenhagen, Denmark.
• MCIC-HB-01R, Damage Tolerant Design Handbook: A Compilation of Fracture and Crack-Growth Data for High-Strength Alloys.
• SAE AIR1537, Report on Aircraft Engine Containment. Technical report, SAE International.
• SAE AIR4003, Report on Aircraft Engine Containment. Technical report, SAE International.
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1.3.4
1.4
1.5
1.5.1
1.5.2
• SAE SP-1270, Uncontained turbine engine rotor events: data period 1984 through 1989. Technical report, S AE International.
Industry Standards.
• SNT-TC-1A, Personnel Qualification and Certifica tion in Nondestructive Testing.
• ASTM E1417, Standard Practice for Liquid Penetrant Testing.
• ATA-105, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods.
• NAS410, Certification & Qualification of Nondestructive Test Personnel.
• SAE AMS2628 Class A, Ultrasonic Immersion Inspection Titanium and Titanium Alloy Billet Pre mium Grade.
• SAE AMS2644, Inspection Material, Penetrant.
• SAE AMS2647, Fluorescent Penetrant Inspection, Aircraft and Engine Component Ma intenance.
Def initions.
See appendix F for a list of definitions that apply to this AC.
Bac kground.
Section 33.70 contains requirements applicable to the design and life management of propulsion system life-limited parts, including high-energy rotating parts. This AC presents a damage tolerance approach that can be used to address inherent material anomalies in rotating life-limited engine parts made of titanium. This approach can be integrated with the existing life management process (safe-life). Section 33.70 and AC
33.70-1 define the safe-life approach and the basic damage tolerance requirements, while this AC is specific to titanium life-limited rotating parts. This approach does not replace the existing safe-life methodology but supplements it by addressing risks not addressed by the safe-life approach. The use of damage tolerance procedures specified in this AC is not intended to allow operation beyond the component manual limit set using the existing safe-life approach. The existing safe-life approach limits the useful rotor life to the minimum number of flight cycles needed to initiate a crack. Rotor failure modes, for which full containment of high-energy debris can be demonstrated, are excluded from the procedures outlined in this AC.
Material and Manufacturing Anomalies.
Service experience with gas turbine engines has demonstrated that material and manufacturing anomalies do occur. These anomalies can potentially degrade the structural integrity of high-energy rotors. Conventional rotor, life management methodology (safe-life method) is founded on the assumption that abnormal material variations and manufacturing conditions can be eliminated. Therefore, the methodology 1-3
_______________________ does not ex plicitly address the occurrence of such anomalies, although some level of tolerance to anomalies is essentially built-in using design margins, as well as factory and field inspections.
1.5.3 Safe-Life Methodology.
Under nominal (anomaly-free) conditions, the safe-life methodology provides a structured process for the design and life management of high-energy rotors, assuring structural integrity throughout the life of the rotor. Undetectable material processing and manufacturing-induced anomalies represent a departure from the assumed nominal conditions. To measure the extent of such occurrences, the FAA requested that the Society of Automotive Engineers (SAE) convene several ad hoc committees to document the number of uncontained events. The statistics pertaining to uncontained rotor events are reported in SAE committee reports AIR1537, AIR4003, and SP-1270.
While no adverse trends were identified during the 1984-1989 reporting period, the committee expressed concern that the projected 5 percent increase in airline passenger traffic each year would lead to an increase in the number of aircraft accidents from uncontained rotor events. Uncontained rotor events have the potential to cause catastrophic aircraft accidents.
1.5.4 Probabilistic Damage Tolerance Approach.
Following the Sioux City event in 19891, the gas turbine engine industry collaborated with the FAA to develop a probabilistic damage tolerance approach to address hard alpha melt anomalies in rotating life-limited parts made of titanium. The FAA later published a methodology in AC 33.14-1. With the release of AC 33.14-1, engine manufacturers can ensure future rotor parts will achieve a reduced event rate associated with the occurrence of hard alpha anomalies. This methodology was based on the best information available in 1996.
1.5.5 Titanium Melt Anomalies Knowledge Base.
Substantial progress has been made in expanding the industry knowledge base associated with melt anomalies in titanium. Through the collective efforts of engine manufacturers, melters, and forgers, there has been a significant improvement in the cleanliness of cast and wrought titanium rotor grade materials. In addition, FAA-funded research and development work performed through programs such as the Turbine Rotor Material Design at Southwest Research Institute has led to a deeper understanding of the fundamental material characteristics of hard alpha. This effort has led to the development and widespread availability of risk prediction design tools, enhanced inspection methods for billets and forgings, and an improved understanding and range of inspection capabilities.
1 NTSB Aircraft Accident Report - United Airlines Flight 232 1-4
AC 33.70-34/17/23
1.5.6 Anomal y Distribution Curves.
The knowledge gained between 1996 and 2012 has enabled the construction of new default hard alpha size and frequency distributions (anomaly distribution curves). The new curves provided in appendix C apply to billet materials that have been inspected using zoned ultrasonic inspection. The new curves benefit from the following:
1. Improved assumptions regarding the size and frequency of hard alpha anomalies present at the ingot stage.
2. An improved understanding of the capability of the titanium billet zoned ultrasonic inspection reflected in the revised probability of detection (POD) curves.
1.5.7 Default-Zoned Ultrasonic Inspection POD Curves.
The new default-zoned ultrasonic inspection POD curves and the method used to develop the POD curves are contained in FAA Report, DOT/FAA/AR-07/63. This report specifies that the new zoned POD accounts for a “dual reject criteria based on amplitude and signal-to-noise,” consistent with AMS2628 Class A. The new anomaly distribution curves do not reflect the improvements in material cleanliness that have occurred since 1996. The hard alpha find rate, per million pounds, remains unchanged and consistent with the 1996 information provided in AC 33.14-1. The intent is to provide margin to address other major anomaly types found in titanium and accommodate fluctuations in the annual hard alpha find rate.
1.5.8 Default Hard Alpha Size and Frequency Curves.
Future updates to the default hard alpha size and frequency curves should be based on the updated ingot size distribution (see GT2012-68987). Future default hard alpha size and frequency curve updates should maintain a margin between the hard alpha find rate used in the development of the curves and the prevailing 5-year moving average. This margin will allow for fluctuations in the annual rate and other major anomaly types found in titanium.
1.5.9 Enhanced Life Management Process.
The Enhanced Life Management Process first introduced in AC 33.14-1 is retained.
The probabilistic fracture mechanics assessment methodology and calibration test case described in AC 33.14-1 is also retained and unchanged. It is included in this document for convenience purposes.
1.5.10 Fracture Mechanics-Based Methodology.
The applicant should assess titanium rotor designs using the fracture mechanics-based methodology contained in this AC. Anomaly distribution curves should be selected from appendix C unless an FAA-approved company-specific curve is available.
Anomaly distribution curve selection should be consistent with the production inspection process for the design in question. Designs satisfying the design target risk (DTR) values will be considered compliant with the damage tolerance requirements required by § 33.70 for titanium melt anomaly only. AC 33.70-1 and AC 33.70-2 define other damage tolerance requirements that apply when using this method.
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AC 33.70-34/17/23
1.5.11 Results of Gained Industry Experience.
Industry experience resulting from improvements in material cleanliness, billet and forging processes ultrasonic inspections, and the publication of AC 33.14-1 has been positive. Since 1990, there have not been any incidents of a cracked or fractured titanium rotor component related to hard alpha anomalies.
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AC 33.70-34/17/23 CHA PTER 2. CONVENTIONAL (“SAFE-LIFE”) LIFE MANAGEMENT PROCESS
2.1 Safe-life Philosophy.
2.1.1 The traditional safe-life philosophy has served the turbine engine industry and flying public well. It provides a solid foundation, which can be enhanced to address the threat from anomalies that cannot be eliminated by the best available manufacturing and inspection processes.
2.1.2 AC 33.14-1 originally introduced the “Enhanced Life Management Process” to the conventional life management procedure. This process was designed to expand the safelife philosophy, not replace it.
2.2 Addition of Damage Tolerance Assessment.
2.2.1 In 2007, the FAA added a new requirement, “Damage Tolerance,” with the introduction of section 33.70.
2.2.2 The new element, Damage Tolerance Assessment, is designed to minimize the occurrence of uncontained rotor failures due to material and manufacturing induced anomalies, therefore, improving flight safety (see figure 3-1).
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AC 33.70-34/17/23 C HAPTER 3. ENHANCED LIFE MANAGEMENT PROCESS
3.1 Damage Tolerance Assessment for Critical Titanium Rotating Parts.
Section 33.70 requires applicants to perform appropriate damage tolerance assessments (see figure 3-1). Damage tolerance assessments are fracture-mechanics based probabilistic risk assessments that predict the relative probability of failure (POF) for each part. The predicted POF is compared to an allowable DTR. Designs that satisfy the allowable DTR will be considered compliant with the titanium melt anomaly damage tolerance requirements as set forth in § 33.70.
3.2 Options to Reduce the POF.
Engine manufacturers have several options available to reduce the POF to meet the allowable DTR. They include, but are not limited to, the following options:
• Component redesign.
• Material changes.
• Material process improvements.
• Manufacturing inspection improvements.
• In-service inspections.
• Life limit reductions.
Chapter 4 presents an overview of the methodolog y for conducting the fracturemechanics based probabilistic analyses mentioned above.
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