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
33.70-2 8/28/09
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.
33.70-2 8/28/09
(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
33.70-2 8/28/09 CHAPTER 3. “SOFT TIME INSPECTION” ROTOR LIFE MANAGEMENT
1. Approach . The overall life management process encompasses a wide spectrum of design, manufacturing, and product support issues. This section addresses only one facet of that process, the assurance of structural integrity using inspection techniques and intervals derived from a damage tolerance (fracture mechanics-based) assessment. The inspection philosophy is intended solely to protect against anomalous conditions. It is not intended to allow operation beyond the safe-life limit specified in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness.
a. When probabilistic assessment indicates risk levels greater than the desired target, many strategies can be used to reduce the predicted risk to the appropriate level. However, only the inservice inspection option is addressed here.
b. The industry data on uncontained fracture experience summarized in SAE reports AIR 1537 (1959 through 1975), AIR 4003 (1976 through 1983), and SP1270 (1984 through 1989) was used to guide the development of the inspection philosophy. These reports indicate that maintenance-induced uncontained failure rates were comparable to the failure rates for anomalous conditions (material and manufacture). This data suggests that additional inspection requirements, if not properly integrated into the normal maintenance scheduled for the engine, would have no net benefit to reduce uncontained failure rates.
c. The inspection philosophy presented here evolved from the desire to easily integrate inspections into the operation of the engine yet still achieve measurable reduction in the uncontained failure rates. The inspection philosophy advocates the use of opportunity inspections rather than forced inspections at ‘not to exceed’ intervals. These opportunity inspections occur due to the ‘on-condition’ maintenance practices used by operators today. Although opportunity inspections occur at random intervals, they can be treated statistically and used effectively to lower the calculated risk of an uncontained event.
d. Opportunity inspection refers to those instances when the hardware in question is available in a form such that the specified inspection can be performed. This condition is generally viewed as being reduced to the piece part; however; opportunity inspections can be performed on assembled modules. For example, an ECI of a disk bore may be specified on an assembled module whenever the module is available. This inspection is an opportunity inspection based upon module availability rather than piece part availability.
e. Whenever possible, the designs should use opportunity inspections to meet the DTR levels. However, in some instances, the probabilistic analysis may indicate an unacceptable risk level when using just opportunity inspections and some additional action may be required to meet the DTR. One of the options to mitigate this risk is to force inspection opportunities by specifying disassembly of modules or engines when a cyclic life interval has been exceeded.
There are many options on how to implement forced disassembly. Options range from mandatory engine removal and subsequent teardown at not to exceed cyclic limits (“hard-time” limits) to mandatory module teardown when the naturally occurring module availability exceeds the specified cyclic life inspection interval of one of the parts contained within that module (“soft-time” limits). This AC advocates the use of the piece part soft-time inspection option and 10
33.70-2 8/28/09 only suggests that forced disassembly of modules be considered when it is required to meet the DTR levels.
f. The soft-time inspection philosophy retains the “on-condition” maintenance practice and minimizes the impact of additional module disassembly. The inspection requirement exists only after the engine has been removed from the aircraft for a reason other than the inspection itself and is in a sufficient state of disassembly to allow access to the module containing the component in question. An available module containing a part with Cycles Since Last Inspection (CSLI) in excess of the soft-time interval must be disassembled to a condition that allows inspection by the procedure specified by the engine manufacturer. The engine manufacturer must evaluate the risk associated with parts that become available for inspection before the soft-time interval to determine if the CSLI can be reset.
g. The maintenance impact of the soft-time intervals should be considered during the design phase. Use the probabilistic analysis, summarized in Chapter 2, along with the anticipated engine removal rate and the module and piece part availability to develop designs that not only achieve the design target but also result in acceptable soft-time intervals and procedures, if such action is required.
h. When invoked, the soft-time inspection approach establishes interval limits beyond which rotor components must be inspected when the rotors are available in modular form. The soft-time inspection requirement is not intended to affect the current practice of forced inspection programs to address safety of flight concerns that arise in the course of engine operation and maturation. These safety of flight concerns should continue to be addressed through aggressive inspection programs, which are communicated using service bulletins.
i. Applicants should recognize that the inspection assumptions made in the probabilistic risk assessment must be communicated and implemented accurately to the field by using the ALS of the ICA. These assumptions must also be validated by the review of engine removal rates and module and piece part availability data. For example, the ALS must call out an eddy current inspection if that was an assumption in setting the original soft-time interval. Similarly, the amount of inspected material should correspond to the analysis assumptions. Likewise, if field experience suggests that the opportunity inspection intervals are in excess of the assumed rates in the probabilistic risk assessment, then appropriate corrective action, such as a modified inspection plan, is required.
j. The soft-time inspection interval and reference to the corresponding inspection procedures must be specified in the ALS of the ICA. This information must be provided for all rotor parts with specified retirement life limits that require any inspection plans beyond opportunity inspections to meet the DTR levels. The required inspection information should also be included in the individual ALS of the ICA with the other rotor inspection requirements. The manufacturer must also provide necessary information to focus the prescribed inspections to those areas of highest relative risk.
2. Inspection Scenarios . The following scenarios clarify th e action that should be taken at a maintenance inspection opportunity. Note that the inspection plans may vary for each part, depending on the outcome of the probabilistic assessments.