Advisory U.S. Department of Transportation Federal Aviation Circular Administration Subject: Damage Tolerance for High Date: 3/7/17 AC No: AC 33.14-1 Energy Turbine Engine Rotors Initiated By: ANE-111 Change: 1
1. Purpose. This advisory circular (AC) change clarifies the default anomaly distribution curves and updates the associated figures. This AC provides guidance for demonstrating compliance with design and life management of high energy rotating parts requirements of Title 14, Code of Federal Regulations (14 CFR) 33.14, Amendment 33-10, Effective 3/26/84.
2. Principal Changes.
a. Paragraphs 1.a., 2.a. - c.; Section 5, item 4.; Appendix 1, Figure A1-1; Appendix 1, paragraph 3.c.(1)(c); Appendix 1, Figures A1-2, A1-6, A1-9, A1-10; Appendix 3, paragraphs
1.a., 1.a.(1) - (6)(b); Appendix 3, Figures A3-1 - A3-4; Appendix 3, Table A3-1; Appendix 4, paragraph 3.c., and Appendix 4, paragraphs 5.(1) and (2) are changed.
b. The AC change number and the date of the change are shown at the top of each applicable page. The change bar in the right or left margin indicates where the change is located.
The changes described may shift the original text.
Page Control Chart Removed Pages Dated Inserted Pages Dated Cover 1/8/01 Cover 3/7/17 S5-1 1/8/01 S5-1 3/7/17 A1-4 1/8/01 A1-4 3/7/17 A1-5 1/8/01 A1-5 3/7/17 A1-6 1/8/01 A1-6 3/7/17 A1-10 1/8/01 A1-10 3/7/17 A1-17 1/8/01 A1-17 3/7/17 A1-19 1/8/01 A1-19 3/7/17 A3-1 1/8/01 A3-1 3/7/17 A3-2 1/8/01 A3-2 3/7/17
Removed Pages Dated Inserted Pages Dated A3-3 1/8/01 A3-3 3/7/17 A3-4 1/8/01 A3-4 3/7/17 A3-5 1/8/01 A3-5 3/7/17 A3-6 1/8/01 A3-6 3/7/17 A3-7 1/8/01 A3-7 3/7/17 A3-8 1/8/01 A3-8 3/7/17 A4-2 1/8/01 A4-2 3/7/17 A4-3 1/8/01 A4-3 3/7/17
3. Website Availability. To access this AC electronically, go to the AC library at http://www.faa.gov/regulations_policies/advisory_circulars/.
Acting Manager, Engine & Propeller Directorate, ANE-100 Aircraft Certification Service 2
Subject: Damage Tolerance for High Energy Date: 3/7/17 AC No: AC 33.14-1 Turbine Engine Rotors Initiated By: ANE-110 Change: 1
1. PURPOSE.
a. This advisory circular (AC) describes an acceptable means for showing compliance with the requirements of Title 14, Code of Federal Regulations (14 CFR) 33.14, Start-stop cyclic stress (low-cycle fatigue). Section 33.14 contains requirements applicable to the design and life management of high energy rotating parts of aircraft gas turbine engines certified to Amendment 33-10, effective March 26, 1984, or prior amendments.
b. Like all advisory material, this AC is not, in itself, mandatory, and does not constitute a regulation. It is issued to describe an acceptable means, but not the only means, for demonstrating compliance applicable to the design and life management of titanium alloy high energy rotating parts of aircraft gas turbine engines. Terms such as "shall" and "must" are used only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance described in this document is used.
c. This AC does not change, create any additional, authorize changes in, or permit deviations from the existing regulatory requirements.
2. RELATED READING MATERIAL.
a. FAA Advisory Circular: AC 33.2, titled “General Type Certification Guidelines for Turbine Engines”.
b. FAA Advisory Circular: AC 33.15-1, titled “Manufacturing Process of Premium Quality Titanium Alloy Rotating Engine Components”.
c. FAA Advisory Circular: AC 33.70-1, titled “Guidance Material for Aircraft Engine Life-Limited Parts Requirements”.
3. DEFI NITIONS.
a. Component Event Rate. The number of events for a given titanium rotor component stage for each flight cycle, calculated over the projected life of the component.
b. Critical Rotating Parts. Rotor structural parts (such as disks, spools, spacers, hubs, and shafts), the failure of which could result in a hazardous condition. In this context a hazardous condition should be interpreted as the con ditions described in § 33.75.
c. Damage Tolerance. This is an element of the life management process that recognizes the potential existence of component imperfections. The potential existence of component imperfections are the result of inherent material structure, material processing, component design, manufacturing or usage. Damage tolerance addresses this situation through the incorporation of fracture resistant design, fracture-mechanics, process control, or nondestructive inspection.
d. Default Probability of Detection (POD) Values. Values representing mean probabilities of detecting anomalies of various types and sizes, under specified inspection conditions consistent with good industry practice.
e. Design Target Risk (DTR) Value. This is the standard against which probabilistic assessment results (stated in terms of component event rates and engine level event rates, as defined below) are compared.
f. Engine Event Rate. The cumulative number of events for each flight cycle, for all critical titanium rotating parts in a given engine, calculated over the projected life of those components.
g. Event. A rotor structural part separation, failure, or burst with no regard to the consequence.
h. Focused Inspection. A term used to describe inspections in which any necessary specialized processing instructions have been provided, and the inspector has been instructed to pay attention to specific critical features.
i. Full Field Inspection. A term used to describe the general inspection of a component without special attention to any specific features.
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j. Hard-Alpha or High Interstitial Defect (HID). An interstitially stabilized alpha phase region of substantially higher hardness than the surrounding material. This comes from very high local nitrogen, oxygen, or carbon concentrations, which increase the beta transus and produce the high hardness, often brittle, alpha phase; also commonly called a Type I defect, low density inclusion (LDI), or a hard-alpha; often associated with voids and cracks.
k. Hard Time Inspection Interval. The number of flight cycles since new or the most recent inspection, after which a rotor part must be made available and receive the inspection specified in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness.
l. Inspected Material. That portion of the total volume of a component that is actually inspected under the described conditions. Inspected material does not guarantee anomaly free material.
m. Inspection Opportunity. An occasion when an engine is disassembled to at least the modular level and the hardware in question is accessible for inspection, whether or not the hardware has been reduced to the piece part level.
n. Low Cycle Fatigue (LCF) Initiation. The process of progressive and permanent local structural deterioration occurring in a material subject to cyclic variations, in stress and strain, of sufficient magnitude and number of repetitions. The process will culminate in detectable crack initiation typically within 105 cycles. For the purposes of this AC, a detectable crack initiation is defined as 0.030 inches in length by 0.015 inches in depth.
o. Maintenance Exposure Interval. Distribution of shop visits (in-flight cycles) since new or last overhaul that an engine, module, or component is exposed to as a function of normal maintenance activity.
p. Mean POD. The 50-percent confidence level POD versus anomaly size curve.
q. Module Available. An individual module removed from the engine.
r. Module. A combination of assemblies, subassemblies, and parts contained in one package, or arranged to be installed in one maintenance action.
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s. Part Available. A part that can be inspected, as required by the Airworthiness Limitations Section of the Instructions for Continued Airworthiness, without any further disassembly. Depending on the inspection requirements, some parts may require a fully disassembled “piece part” condition, while other parts may be available for inspection while still in the assembled module.
t. Probabilistic (Relative Risk) Assessment. A fracture-mechanics based simulation procedure that uses statistical techniques to mathematically model and quantitatively combine the influence of two or more variables to estimate a most likely outcome or range of outcomes for a product. Since not all variables may be considered or may not be capable of being accurately quantified, the numerical predictions are used on a comparative basis to evaluate various options having the same level of inputs. Results from these analyses are typically used for design optimization to meet a predefined target, or to conduct parametric studies. This type of procedure is distinctly different from an absolute risk analysis, which attempts to consider all significant variables, and is used to quantify, on an absolute basis, the predicted number of future events having safety and reliability ramifications.
u. POD. A quantitative statistical measure of detecting a particular type of anomaly over a range of sizes using a specific nondestructive inspection technique under specific conditions. Typically, the mean POD curve is used.
v. Safe-Life. A LCF based process where components are designed and substantiated to have a specified service life, which is stated in operating cycles or operating hours, or both.
Continued safe operation, up to the stated life limit is not contingent upon each unit of a given design receiving interim inspections. When a component reaches its published life limit, it is retired from service.
w. Soft Time Inspection Interval. The number of flight cycles since new or the most recent inspection, after which a rotor part in an available module must receive the inspection specified in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness.
x. Stage. The rotor structure which supports, and is attached to a single aerodynamic blade row.
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4. BACKGROUND.
a. 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 of the existence of nominal material variations and manufacturing conditions. Consequently, the methodology does not explicitly address the occurrence of such anomalies, although some level of tolerance to anomalies is implicitly built-in using design margins, factory and field inspections, etc.
b. Under nominal conditions, this safe-life methodology provides a structured process for the design and life management of high energy rotors, which results in the assurance of structural integrity throughout the life of the rotor. Undetectable material processing and manufacturing induced anomalies, therefore, represent a departure from the assumed nominal conditions. In 1990, to quantify the extent of such occurrences the FAA requested that the Society of Automotive Engineers (SAE) reconvene the ad hoc committee on uncontained events. The statistics pertaining to uncontained rotor events are reported in the SAE committee report Nos. AIR 1537, AIR 4003, and SP-1270. While no adverse trends were identified, the committee expressed concern that the projected 5-percent increase in airline passenger traffic each year would lead to a noticeable increase in the number of aircraft accidents from uncontained rotor events. Uncontained rotor events have the potential to cause catastrophic aircraft accidents.
c. As a result of the accident at Sioux City in 1989, the FAA requested the turbine engine manufacturers, through the Aerospace Industries Association (AIA), to review available techniques to determine if a damage tolerance approach could be introduced which, if appropriately implemented, could reduce the occurrence of uncontained rotor events.
d. The industry working group concluded that the technology was now available to implement additional enhancements to the conventional rotor life management process, which would explicitly address anomalous conditions. Furthermore, these enhancements could be structured to enforce design and life management adaptations, which would enhance rotor integrity under anomalous material or manufacturing conditions. The additional guidelines regarding damage tolerance provided in this AC explicitly address these anomalous conditions.
In addition, the damage tolerance approach will also implicitly enhance damage tolerance to other anomalous conditions related to design, manufacture, maintenance and operation.
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David A. Downey Assistant Manager, Engine & Propell Aircraft Certification Service
1/8/01 AC 33.14-1
5. APPLICABILITY.
a. This material has been prepared to present a generic damage tolerance approach which can be readily integrated with the existing “safe-life” process for high energy rotors to produce an enhanced life management process. However, the data included i n this AC are only ap plicable to titanium alloy rotor c omponents.
b. This material is intended to be applicable to all “new design” critical titanium alloy rotor components on a “go forward basis.” It is not intended to apply retroactively to existing hardware.
c.
The new generic damage tolerance appr oach does not replace existing safe-life me thodology, but expands upon it.
d.
In the context of damage tolerance, it is not intended to allow operation beyond the co mponent manual limit set using the existing safe-life approach.
e . Rotor failure modes for which full containment can be demonstrated are excluded fr om the procedures outlined in this document and need no t b e accounted for in the overall risk assessment.
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CONTENTS Page No.
Section 1 Conventional (“Safe-Life”) Life Management Process S1-1 Section 2 Enhanced Life Management Process S2-1 Section 3 Damage Tolerance Implementation S3-1
- Approach
- Methodology
- Default Input - Anomaly Distributions
- Default Input - Nondestructive Evaluation (NDE) Probability of Detection (POD)
- Design Target Risk (DTR) Section 4 “ Soft-Time Inspection” Rotor Life Management Approach S4-1 Section 5 References S5-1
APPENDICES
Appendix 1 Calibration Test Case & Example A1-1 Damage Tolerance Assessment Appendix 2 Soft Time Inspection Interval Example A2-1 Appendix 3 Default Anomaly Distribution Curves A3-1 Appendix 4 Default POD Applicability A4-1 Appendix 5 Default POD Curves A5-1
SECTION 1 CONVENTIONAL (“Safe-Life”) LIFE MANAGEMENT PROCESS
1. The conventional gas turbine engine critical rotor life management methodology (the “safelife” method) consists of comprehensive processes and technologies related to the design, manufacture, test validation, certification and field management of high energy rotors. FIGURE S1-1 illustrates the basic elements of the safe-life approach to rotor life management. Taken together, these processes and technologies constitute a procedure approved by the FAA as fulfilling the requirements of § 33.14 for the type certification of aircraft engines under part 33.
a. Analytical and empirical engineering processes are applied to determine statistically quantified LCF-based minimum rotor service lives. The minimum lives are reflective of service usage profiles associated with specific applications.
b. Structured component and engine testing is conducted to confirm rotor operating conditions and to enhance confidence in the predicted minimum service lives.
c. Controlled and fixed processes are established for the manufacture of critical rotors as part of the safe-life process. These manufacturing processes are formalized in addition to the product definition data. Periodic quality validation testing is used to assure product quality consistent with intended design.
d. Certification of declared lives is established as a fraction of the minimum LCF life, with the fractional ratio dependent upon the approved safety factors associated with the particular application. The declared lives and supporting engineering data are submitted to the FAA for review and approval as part of the overall type certification process. Upon FAA approval, the certificated critical rotor life limits are documented in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness, and becomes an operational limit.
After introduction into operational service, the FAA may approve increases to the critical rotor certificated life limits if the engine manufacturer can validate the proposed increases with sufficient supporting data. The FAA may also order decreases to the certificated life under part 39 when warranted.
e. Critical rotors are retired through scheduled operator maintenance action, before or upon reaching the certificated life limit.
S1-1
f. Opportunity inspections, as part of the normal maintenance practices, are another element of the conventional life management process. Even though the approval of certificated life limits is not contingent upon interim or even periodic inspections, most of the critical rotors do become available for inspection several times as part of normal engine maintenance practices.
S1-2