33.70-2 8/28/09 engine internal parts because the module or parts have been disassembled as a result of normal maintenance activity.
q. Mean POD. The 50-percent confidence level POD versus anomaly size curve.
r. Module Available. An individual module removed from the engine.
s. Module. A combination of assemblies, subassemblies, and parts contained in one package, or arranged to be installed in one maintenance action.
t. Operationally-Induced Anomaly. A surface imperfection introduced during service operations (includes flight operations and maintenance) that is potentially detrimental to the structural integrity of the life-limited rotating part.
u. Part Available. A part that can be inspected, as required by the ALS of the ICA, 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.
v. Probabilistic Risk Assessment. A fracture-mechanics base d simulation procedure that uses statistical techniques to mathematically model and combine the influence of two or more variables to estimate the likelihood of various 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. Results from these analyses are typically used for design optimization to meet a predefined target or to conduct parametric studies. This type of procedure differs 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 with safety and reliability consequences.
w. Probability of Detection. A quantitative statistical measure for detecting a particular type of anomaly (flaw) over a range of sizes using a specific nondestructive inspection technique under specific conditions. Typically, the mean POD curve is used.
x. Rotor Life-Limited Parts. Engine rotor parts whose primary failure is likely to result in a hazardous engine effect, which is any of the conditions listed in § 33.75.
y. Safe Life. A low cycle fatigue-based process in which life-limited components are designed, manufactured, and substantiated to have a specified service life or life limit, which is stated in operating flight cycles, operating hours, or both. The “safe life approach” requires that parts be removed from service prior to the development of an unsafe condition (i.e., crack initiation). When a component reaches its published life limit, it is retired from service. The safe life approach only applies to parts which define crack initiation as the limit of the useful life such as rotating parts.
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z. Soft Time Inspection Interval. The number of engine cycles since new, or the most recent inspection, after which a rotor part in an available module must receive the inspection specified in the ALS of the ICA.
aa. Stage. The rotor structure that supports and is attached to a singl e aerodynamic blade row.
6. Background.
a. As required by § 33.70(a), applicants must perform appropriate damage tolerance assessments to address the potential for failure from material, manufacturing, and service induced anomalies within the approved life of the part. This is necessary because service experience with gas turbine engines has demonstrated that material, manufacturing and serviceinduced anomalies do occur, and they can potentially degrade the structural integrity of rotor life-limited parts. Several approaches are currently available to address this requirement and have been introduced by AC 33.70-1 and AC 33.14-1. 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.
b. The complex and time-consuming nature of the effort required to develop the technical elements associated with this method—for example, data collection, analysis, methodology development, acceptable design targets, and validation—has led to a decision to introduce the surface damage tolerance assessment methodology for manufacturing and in-service anomalies in phases. Phase 1 of this methodology, introduced in this AC, presents a damage tolerance approach which can be used to address manufacturing and operationally-induced anomalies in circular hole features (see Appendix 5) in rotor parts. We will introduce other phases in future ACs that will address the remaining surface features.
Francis A. Favara Manager, Engine & Propeller Directorate Aircraft Certification Service vi
TABLE OF CONTENTS
CHAPTER 1. ENHANCED LIFE MANAGEMENT PROCESS 1
CHAPTER 2. DAMAGE TOLER ANCE ASSESSMENTS 3
CHAPTER 3. “SOFT TIME INSPECTION” ROTOR LIFE MANAGEMENT 10
APPENDIX 1. MANUFACTURING-I NDUCED ANOMALY CALIBRATION TEST CASE 13
APPENDIX 2. DEFAULT ANOM ALY DISTRIBUTION CURVES 21
APPENDIX 3. DEFAULT POD APPLICABILITY 25
APPENDIX 4. DEFAULT POD CURVES 28
APPENDIX 5. DEFINITION OF A HOLE FOR MANUFACTURING ANOMALY ASSESSMENTS 30
APPENDIX 6. MANUFACTURING CREDITS 31
APPENDIX 7. SURFACE (MANUFACT URING INDUCTED ANOMALY) DAMAGE TOLERANCE ASSESSMENT 34
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CHAPTER 1. ENHANCED LIFE MANAGEMENT PROCESS
1. Based on historical experience, the safe-life philosophy has worked well and provides a solid base for further enhancement. Modifications to the industry standard life management procedures should augment, not replace, current approaches, which are based on the safe-life philosophy.
a. A new element, damage tolerance, has been added to the existing conventional life management process to define the enhanced life management process. We expect enhanced life management to further minimize the occurrence of uncontained rotor failures and thus improve flight safety (see figure 1-1).
b. Use of the enhanced life management process, shown in figure 1-1, will result in damage tolerance assessments being conducted on critical rotating part designs. These will be in the form of fracture-mechanics-based probabilistic risk assessments, the results of which will be compared to specified design target risk values. Designs that satisfy these DTR values will be considered in compliance with the damage tolerance requirements of § 33.70. Engine manufacturers will have a variety of options available to achieve the DTR. These options include, but are not limited to:
• component redesign,
• material changes,
• material process improvements,
• manufacturing process improvements,
• manufacturing inspection improvements,
• enhanced in-service inspections, and
• life limit reductions.
33.70-2 8/28/09 Figure 1-1: Enhanced Life Management Process Safe Life Prediction Life Management Process Development & Validation Tes t s Methods & Material Data Service Life & Product Assurance Damage Tol er an c e Assessment New Element
Added to
Existing Safe
Life Process
33.70-2 8/28/09 CHAPTER 2. DAMAGE TOLERANCE ASSESSMENTS
1. Approach .
a. As described in AC 33.70-1, Probabilistic Damage Tolerance Risk Assessments (PDTRA) is an acceptable method to assess the ability of a part to tolerate anomalies. The results of these assessments will provide the basis for evaluating the relative damage tolerance capabilities of candidate part designs and will also allow the engine manufacturer to balance its designs for both enhanced reliability and customer impact. The results will be compared against design target risk values (see paragraph 6, “Design Target Risk,” in this chapter).
b. The need for further risk reducing actions on the part of the engine manufacturer is based on whether the design under consideration satisfies the desired DTR values. If the targets are met, then the design is in compliance with § 33.70. The manufacturer has a variety of options available to achieve the required DTR and may conduct quantitative parametric studies to determine the influence of key variables such as inspection methods, inspection frequency, hardware geometry, hardware processing, material selection and life limit reduction. The manufacturer may make changes to the design, or field management of the part, or both, to achieve the DTR values.
c. PDTRA will usually be performed during the engine component detail design phase.
Paragraph 2 of this chapter defines an assessment methodology applicable to manufacturing and service-induced anomalies and provides a process to refine a design to achieve the desired DTR values.
2. Methodology.
a. Probabilistic risk assessments may be conducted using a variety of methods, such as Monte Carlo simulation or numerical integration techniques. When performing a PDTRA, use of the standardized inputs and default data presented in this AC will achieve consistent assessment results.
b. A list of standardized inputs is provided under paragraph “c” below. Default input data is described in paragraphs 3 and 4 of this chapter. Use of this default data in probabilistic assessments requires no further demonstration of applicability or accuracy. However, when this data is used, the applicant should follow the guidelines accompanying this data to ensure applicability. Use of inputs other than these default values may require additional validation to verify applicability, adequacy, and accuracy.
c. Probabilistic risk assessments should incorporate the following inputs as part of a basic analysis: component stress, surface area, material properties, crack growth (propagation) life, anomaly distribution, design service life, inspection PODs, and maintenance exposure rate.
(1) Input. The following data sets are defined as basic input elements.
(a) Stress. This variable influences crack propagation life. Data input should encompass the most limiting operational principal stresses. Engine cycle(s) for design LCF certification, or actual usage (if known), should be used to establish the stress distribution.
33.70-2 8/28/09 Surface assessments should incorporate the appropriate surface and near surface stress distributions, including the effects of stress concentrations and vibratory stresses.
Note: The method described in this AC has been calibrated against industry experience without considering the effect of shot peening and the resulting beneficial residual stresses on the crack propagation lives. As a result, with this method it is inappropriate to include the beneficial effects of shot peening.
(b) Surface Area. This variable influences the probability of having a manufacturing-induced anomaly on the surface of a specific component/component feature. For hole features, this corresponds to the total surface area of one hole in a given feature.
(c) Material Cyclic Crack Growth Rate (CCGR) Data. Use average crack growth rate properties of the base material as the default condition to calculate crack propagation life.
(d) Propagation Life. Nu mber of cycles for a given size anomaly to grow to a critical size. This is based on knowledge of part stresses, including the vibratory stresses, temperature, geometry, stress gradient, anomaly orientation, and material properties. Linear elastic fracture mechanics should be used for the calculation of propagation life. Default conditions should assume anomalies to be in the worst orientation to the stress field.
(e) Anomaly Distribution. For manufacturing-induced assessments, use the default anomaly distributions outlined in paragraph 3 of this chapter or FAA-approved companyspecific data. Company-specific data should be developed using the same process employed to derive the default anomaly distributions. For manufacturing-induced anomaly assessments, this process is described in a technical paper titled “Development of Anomaly Distributions for Machined Holes in Aircraft Engine Rotors” - see paragraph b.(2) in “References and Related Reading.” The paper also describes the process required to establish the potential benefits (e.g., reduced anomaly size/frequency distributions) derived from the imposition of enhanced surface inspections during the manufacturing process. Note: In all assessments, anomalies will be treated as sharp propagating cracks from the first stress cycle.
(f) POD. Default PODs and instructio ns on use of individual company values are contained in paragraph 5 of this chapter. For surface assessments, consider the effects of fluorescent penetrant inspection (FPI) and eddy current inspection (ECI) as appropriate.
(g) Maintenance Exposure Interval. For the purpose of assessing inspection benefits, model exposure interval curves for the engine, module, or component in question in the analysis.
(2) Calibration.
(a) Each engine manufacturer should calibrate its analytical prediction tools by conducting the industry test case detailed in Appendix 1.
(b) The manufacturing-induced anomaly test case in Appendix 1 consists of a probabilistic analysis of a simulated titanium ring disk with a row of ½" diameter holes, using specified inputs and scenarios. Test case resu lts in the ranges from 2.58E-04 to 3.22E-04 events 4
33.70-2 8/28/09 within the service life of the part (for the “no inspection” case) and from 7.29E-05 to 2.25E-04 (for the “with in-service inspection” case) are considered acceptable. Test case results outside of these ranges may indicate deficiencies with either the probabilistic assessment technique or the assumptions made.
(3) Output: Manufacturing-Induced Anomaly Assessments. Holes.
(a) Applicability. This pr ocedure applies to circular holes in rotor life-limited parts with the exception of the bore hole. The bore hole is specifically exempted because the machining anomalies observed in field events originating at the bore hole do not resemble the anomaly types observed in drilled hole features, which suggests different damage mechanisms.
(b) Hole Feature Assessments. The probabilistic assessments and prediction of event potential for a given hole feature should be calculated over the entire anticipated service life of the part. This result should be expressed as the number of predicted events for each cycle and should be designated as the predicted “hole feature event rate.” The predicted hole feature event rate should then be compared to the hole feature level DTR value to assess design acceptability.
(4) General Comments.
(a) The FAA encourages engine manufacturers to incorporate fracture resistant design concepts where possible. Credit may be given for fracture resistant or burst resistant engine design features, which clearly demonstrate, through both analysis and test, a reduction in the potential for rotor failure due to the presence of unanticipated anomalies. Credit may be determined using the probabilistic risk assessment methods contained with this AC and by data which supports the fracture resistant features of the design.
(b) Because the design of an aircraft turbine engine rotor is a lengthy process involving numerous iterations, each of which can substantially alter the initial design risk value, it is important that the design feature satisfy the appropriate DTR value at the time of engine certification. 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.
(c) Etch inspection is widely used in the industry to evaluate material quality on rotor part surfaces. While etch inspections are not yet included in the damage tolerance assessment, they have been identified as the foremost means of detecting non-geometric anomalies (worked layers) in holes by the FAA Report number DOT/FAA/AR-06/3 “Guidelines to Minimize Manufacturing Induced Anomalies in rotor life limited parts” – see paragraph a.(3) in “References and Related Reading.” As such, etch inspections currently in-place perform a vital role and should not be discontinued because of any damage tolerance analysis set forth in this AC.
(d) Crack propagation assessment guidelines for consistent treatment of out of phase stress vs. temperature history are included below. These guidelines apply only to the probabilistic damage tolerance analysis.
33.70-2 8/28/09 1 Crack propagation under out-of-phase stress vs. temperature transient loadings is a complex phenomenon. Compared to isothermal tests, non-isothermal experimental results suggest that the crack propagation rate can be higher than predicted at the temperature of the maximum stress when the maximum temperature during the flight is greater than the temperature at the maximum stress (see figure 2-1).
2 While some OEMs have developed models to predict this phenomenon, no industry standard is yet available.
3 The following design guidelines provide a default approach for the probabilistic analysis described in this AC.
4 Design Guidelines (Default Option). These are not intended to become a general industry standard. For the major and minor stress cycles over the full flight, the temperature to consider is the most life limiting between:
• Temperature at maximum stress during the cycle, and
• Maximum temperature during the flight 5 Design Guidelines (Alternative Option). A crack propagation model accounting for out of phase temperature vs. stress history for the full flight can be used if it is validated with the OEM laboratory and field experience on the considered material.
Figure 2-1. Stress and temperature transient loadings over a flight mission major cycle max stress Stress Temperature flight max temperature temperature at major cycle max stress time Stress Temperature major cycle min stress 6
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3. Anomaly Distributions .
a. A key input distribution associated with PDTRA assessments is the size and rate of occurrence of the anomalies. This type of information is statistical in nature and can be presented in a form that specifies the number of anomalies that exceed a particular size in a given amount of surface area.
b. Manufacturing-Induced Anomaly Distributions.
(1) Manufacturing-induced anomaly distributions that apply to circular hole features in disks have been developed to characterize the size (depth) distribution and frequency of said anomalies (see Appendix 2). The distributions apply to circular holes machined in titanium, steel, nickel, or powder nickel rotor components.
(2) The anomaly distributions contained in Appendix 2 may be used to determine compliance with paragraph 6 (“Design Target Risk”) of this chapter. The background pertaining to the development of these distributions is contained in the technical paper “Development of Anomaly Distributions for Machined Holes in Aircraft Engine Rotors.” The distributions were developed by modeling crack sizes detected during component/engine testing or through inservice experience using an EIFS (Effective Initial Flaw Size) approach. The final distributions were validated based on OEM field experience. This process resulted in a probability of exceedance curve (see figure A2-1) that can be used to determine a relative risk reduction but not an absolute level of risk.
(3) Individual engine manufacturers who want to use an alternate anomaly distribution or an improved inspection capability, or extend the PDTRA process to features other than circular holes should use a methodology similar to that contained in the paper “Development of Anomaly Distributions for Machined Holes in Aircraft Engine Rotors.” This paper contains instructions on how to develop an alternate distribution, which must be substantiated by the appropriate background data. Alternate manufacturing-induced anomaly distributions should:
(a) include fracture and crack find data,
(b) include inspection POD data, and
(c) be based upon substantial field experience.
4. Manufacturing Credits Applicable to Manufacturing-Induced Anomaly Distributions.
a. To encourage the use of best practices when manufacturing circular holes in rotor lifelimited parts, a process has been established to take credit for available manufacturing process validation and controls that reduce the risk of hole damage during manufacture. This process includes:
(1) Process validation to a modern standard as described in FAA Report number DOT/FAA/AR-06/3.
33.70-2 8/28/09
(2) Single point boring as a method of finishing a hole. This is restricted to titanium alloys.
(3) Honing all alloys as a method of finishing a hole.
(4) The provision of a coolant monitor to ensure the flow of coolant towards the cutting edge.
(5) Power monitoring to a standard detailed in the FAA Report number DOT/FAA/AR-06/3.
(6) Other process monitoring, including feed force, vibration, etc., to a standard similar to that for power monitoring above.
(7) Inspection for detection of non-geometric anomalies.
b. The degree of allowable manufacturing credit is a function of which of the above available actions are actually employed during hole making. Calculation of manufacturing credit is outlined in Appendix 6. The manufacturing-induced probabilistic damage tolerance approach has been developed excluding the benefits of enhanced manufacturing validation and process controls. Hence, the credit is applied as a reduction factor to the results of the manufacturinginduced probabilistic assessments prior to comparing with the hole feature DTR.
5. Default Input - Nondestructive Evaluation (NDE) POD.
a. Detection of non-geometric anomalies, such as highly distorted grain structure, white or amorphous layer, and bent grain, 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 part is inspected at shop visit. These methods, however, may be of very limited benefit at new manufacture. If credit for inspection at new manufacture is claimed, applicants must demonstrate that the capability of the technique on the target anomaly types. The default POD data supplied in this AC in Appendix 4 only refers to the detection of cracks.
b. The capability of individual NDE processes, such as eddy current, penetrant, or ultrasonic inspection, for the detection of local material anomalies (discontinuities or potential anomalies), is a function of numerous parameters, including the size, shape, orientation, location, and chemical or metallurgical character of the anomaly. In addition, the following four parameters should be considered when assessing the capabilities of an NDE process:
(1) The material being inspected (for example, its composition, grain size, conductivity, and surface texture).
(2) The inspection materials or instrumentation (for example, the specific penetrant and developer, the inspection frequency, instrument bandwidth and linearity).
(3) The inspection parameters such as scan index.
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(4) The inspector (for example, his or her visual acuity, attention span, and training).
c. The “default” POD data supplied with this AC are characteristic of inspection capability that has been measured under typical, well-controlled conditions. These default POD values are provided primarily to facilitate selection of nondestructive inspection techniques that are best suited to support attainment of damage tolerant inspections. Manufacturers should recognize, however, that although properly applied inspections should result in capability similar to these default values, they are strictly applicable only under the conditions under which they were acquired (see Appendix 3). POD curves are described in Appendix 4, figures A4-1 and A4-2, as listed below:
(1) Appendix 4, Figure A4-1. Mean POD for Fluorescen t Penetrant Inspection of Finish-Machined Surfaces.
(2) Appendix 4, Figure A4-2. Mean POD for Eddy Current Inspection of Finish Machined Surfaces.
NOTE: Refer to Appendix 1 for an exam ple of the use of this data and to Appendix 3 for the NDE Applicability of these POD curves.
6. Design Target Risk.
a. The DTR is a benchmark relative risk level selected to enhance the overall safety of high-energy rotating components. Since no machine or device is 100-percent reliable, it is inappropriate to require a level that is technologically unachievable. Nevertheless, the goal is to achieve a significant and distinct improvement over and above current rotor designs.
b. For manufacturing-induced anomaly assessments, designs must meet the specified feature DTR to be considered acceptable. (Note: Only circular hole feature assessments and DTR’s are currently available.)
c. Design Target Risk values are specified in Table 2-1 below:
Table 2-1. Allowable Risk for Circular Holes Component Feature Design Target Risk (feature events per component published service lifetime) Circular Holes 2.0 E-05 9