AC 33.70-5 CHAPTER 3. DA MAGE TOLERANCE ASSESSMENTS
3.1 Approach.
3.1.1 Introduction to PDTRA Methodology.
As described in AC 33.70-1, probabilistic damage tolerance risk assessment (PDTRA) is an acceptable method to assess the ability of a component to tolerate anomalies. The results of these assessments form the basis for evaluating the relative damage tolerance capabilities of candidate part designs, helping the engine manufacturer to balance its designs for both enhanced reliability and customer impact. The results will be compared against DTR values (see paragraph 3.6 in this chapter).
3.1.2 Evaluating and Achieving Compliance with DTR.
The need for further risk-reducing actions by the engine manufacturer depends on whether the design under consideration meets the DTR value. If the target is met, the design complies with § 33.70. To meet the DTR, manufacturers can use various methods, including parametric studies that analyze the effects of key variables like inspection methods, inspection frequency, hardware geometry, hardware processing, material selection and life limit reduction. The manufacturer may make changes to the design, field management of the part, or both, to achieve the DTR value.
3.1.3 Applying PDTRA in the Design Phase.
PDTRA will usually be performed during the engine component detail design phase.
Paragraph 3.2 defines a probabilistic surface damage tolerance (PSDT) assessment methodology applicable to manufacturing and service-induced anomalies in axial blade slot features and provides a process to refine a design to achieve the desired DTR value.
3.2 Methodology.
3.2.1 Probabilistic Risk Assessments.
Probabilistic risk assessments can be performed using various methods, including Monte Carlo simulation or numerical integration techniques. To ensure consistent assessment results when conducting a PDTRA, use the standardized inputs and default data presented in this AC.
3.2.2 Standardized Inputs.
A list of standardized inputs is provided under paragraph 3.2.3 with default input data outlined in paragraphs 3.3 and 3.4. No further validation is required for applicability or accuracy when using default data in probabilistic assessments. However, when this data is used, applicants should follow the guidelines accompanying this data to ensure applicability. When using inputs other than these default values, additional validation may be necessary to verify applicability, adequacy, and accuracy.
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3.2.3 Inputs for Blade Slot Risk Assessments.
Probabilistic risk assessments for blade slots should incorporate the following inputs as part of a basic analysis.
3.2.3.1 Stress and Temperature.
• These variables influence crack propagation life predictions. Engine cycles for design low cycle fatigue (LCF) certification, or actual usage (if known), should be used to establish the stress and temperature distributions. Surface assessments should incorporate the appropriate surface and near surface stress distributions, including the effects of stress concentrations.
• The method described in this AC has been calibrated based on industry experience but does not consider the effect of shot peening or the resulting beneficial residual stresses on the crack propagation lives. No crack growth credit should be taken for beneficial surface residual stresses.
3.2.3.2 Number of Blade Slots.
A higher number of blade slots raises the chances of surface damage in axial blade slots and raises the probability of component failure.
3.2.3.3 Material Cyclic Crack Growth Rate Data.
Use the average crack growth rate properties of the base material as the default condition to calculate crack propagation life.
3.2.3.4 Crack Propagation Life Predictions.
• The predicted number of cycles for a given size crack to grow to a critical size is based on predicted stresses, temperatures, geometry, stress gradient, crack orientation, and material properties. Linear elastic fracture mechanics should be used for the calculation of propagation life. Default conditions should assume anomalies sampled from the distribution in paragraph 3.2.3.5 behave like sharp propagating cracks and are oriented in the worst orientation relative to the stress field. Crack growth rates depend on stress-temperature conditions.
• The method described in this AC has been calibrated assuming a “prescribed TMF” (thermo-mechanical fatigue) approach (see figure A-14 and table A-6 in appendix A), which should be used for consistency with the TMF method. This approach prescribes:
• Cycle pairs are extracted using a standardized stress rainflow extraction method (see ASTM E 1049-85, paragraph 1.3.4).
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• The crack growth rate at the maximum temperature of the paired cycle is used to compute crack growth life.
3.2.3.5 Anomaly Distribution.
The method described in this AC has been calibrated using the default anomaly distribution in paragraph 3.3 of this chapter.
3.2.3.6 Manufacturing Methods.
The method described in this AC is calibrated predominantly for axial blade slots manufactured using broaching methods, with some milled slots also included. Controls and techniques specific to blade slot forming operations and blade slot edge formation and finishing can influence damage tolerance assessments. Since the PSDT methodology in this AC xmay be relevant to slot formation methods other than broaching or milling, refer to appendix F of this AC for definitions and additional guidance.
3.2.3.7 Inspection Probability of Detection (POD).
Inspection at time of manufacture and during maintenance in service can impact component risk. The default POD information used to calibrate the method in this AC, along with instructions, are provided in paragraph 3.5.
The default POD information is only appropriate for in-service inspections, because the sampled anomalies are treated as actively growing cracks from the first cycle.
3.2.3.8 Maintenance Exposure Interval.
When assessing inspection benefits, the exposure intervals for the engine, module, or specific component can be considered in the analysis.
3.2.4 Calibration.
3.2.4.1 The applicant should calibrate its analytical prediction tools by performing the industry test case detailed in appendix A of this AC.
3.2.4.2 The test case in appendix A consists of a probabilistic analysis of a simulated nickel turbine disk with a two-lobed firtree slot, using specified inputs and scenarios. Table A-6 in appendix A provides acceptable ranges for test case results. Test case results outside of these ranges may indicate deficiencies with either the probabilistic assessment technique or the assumptions made in the stress analysis, considering that the speed and temperature distribution are s pecified.
3.2.5 Output.
Surface-induced anomaly assessments for axial blade slots.
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3.2.5.1 Applicability.
This procedure was specifically developed for field cracking or failures, or both, of axial blade slots, including but not limited to:
• Axial blade slots in nickel and titanium life-limited parts,
• Broached and milled axial blade slots,
• Slot bottom surface and edge features as shown in figures E-2 and E-3 in appendix E.
Note: While the methodology discussed in this AC was developed based on titanium and nickel axial blade slots, it can also apply to axial blade slots made from other materials (such as steel) subject to the determination of appropriate input data.
Note: This procedure may also apply to axial blade slots manufactured by other means than broaching or milling subject to the determination of appropriate input data.
Note: This procedure can be extended to other lower-risk locations within the axial blade slot not highlighted in figures E-2 and E-3 in appendix E.
For the purposes of this AC, lower-risk locations are those where a crack would not grow into the disk but would instead cause the release of the blade post and two blades. These other locations would use the same exceedance curve, frequency, manufacturing credits, and DTR. If the original equipment manufacturer (OEM) has experience to show a reduced number of cracking or fracture events in the lower-risk locations, it may be acceptable to lower the frequency accordingly.
3.2.5.2 Blade Slot Assessment.
The probabilistic assessment for a set of axial blade slots should be calculated over the entire anticipated service life of the component. This result, expressed as the probability of fracture per component, is called the predicted “blade slot event rate.” This event rate should be compared to the blade slot DTR value to determine whether the design is acceptable.
3.2.6 Long-Term Risk Management, Inspection, and Manufacturing Practices.
3.2.6.1 The FAA encourages engine manufacturers to incorporate fractureresistant design concepts where possible. Credit may be given for such design features if they clearly demonstrate, through both analysis and testing, a reduction in the risk of rotor failure due to the presence of unanticipated anomalies. These credits can be determined using the probabilistic risk assessment methods outlined in this AC, supported by data that validates the effectiveness of the fracture-resistant features.
3.2.6.2 Because the design of an aircraft turbine engine rotor is a lengthy process involving numerous iterations, each of which can substantially alter the AC 33.70-5 3-4
AC 33.70-5 initial design risk value, it is important that the design features satisfy the appropriate DTR value at the time of engine certification.
3.2.6.3 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.
3.2.6.4 In conjunction with probabilistic assessments and fracture-resistant designs, surface and edge inspections of axial blade slots play a critical role in ensuring the ongoing integrity of rotor parts. These inspections, which evaluate geometry, surface condition, and material quality, are widely used throughout the industry. Existing inspections should not be discontinued, even when advanced damage tolerance analysis is employed.
3.2.6.5 Established best practices define how to conduct process validation and prepare the associated manufacturing control plan (see paragraph 1.3.3).
These process validation and manufacturing control plans are mandatory, regardless of whether you apply manufacturing credits in the PSDT to meet the DTR.
3.3 Default Input - Anomaly Distributions.
3.3.1 Overview of Anomaly Distributions.
A key input associated with PDTRA assessments is the size distribution of potential anomalies. This type of information is statistical in nature and can be presented in a form that specifies the number, or rate, of anomalies that exceed a particular size.
3.3.2 Anomaly Distributions for Axial Blade Slots.
3.3.2.1 Anomaly distributions that apply to axial blade slots in disks have been developed to characterize the size (depth) distribution and frequency of potential anomalies (see appendix B). The distributions apply to axial blade slots machined in titanium or nickel rotor components.
3.3.2.2 The anomaly distributions contained in appendix B may be used to determine compliance with paragraph 3.6 of this chapter. The anomaly distributions in appendix B were used to calibrate this PDTRA method for a broad range of OEM field experience in terms of both product type and commercial usage over several decades.
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3.4 Manufacturing Credits for Manufacturing-Induced Anomaly Distributions.
3.4.1 Manufacturing Process Validation and Controls.
To encourage the use of best practices when manufacturing axial blade slots in rotor life-limited parts, a process has been established to take credit for manufacturing process validation and controls that reduce the risk of blade slot surface and blade slot edge damage during manufacture. This process quantifies the benefits of:
• Process validation to modern standards (see paragraph 1.3.3).
• Process validation and control plan, machine condition, fluid and fluid condition, tooling and setup validation, fluid monitoring, and process monitoring during the processes used to produce the axial blade slot surface.
• Process validation and control plan, semi-automated and automated edge processing of the axial blade slot edges.
• Inspection of axial blade slot surface and edge geometry.
• Non-destructive inspection of axial blade slot surfaces and periodic metallographic cutups and etch inspection.
3.4.2 The allowable manufacturing credit is a function of which of the above available actions are employed during blade slot manufacture. Calculation of manufacturing credit is outlined in appendix F. The manufacturing-induced probabilistic damage tolerance approach has been calibrated including the benefits of enhanced manufacturing validation and process controls. Hence, the credit is applied as a reduction factor to the results of the manufacturing-induced probabilistic assessments prior to comparing with the axial blade slot feature DTR.
3.5 Default Input – Nondestructive Evaluation (NDE) POD.
3.5.1 Detection of Geometric vs. Non-Geometric Anomalies.
3.5.1.1 Detection of non-geometric anomalies, such as highly distorted grain structure, white or amorphous layer, and bent grains, 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 component is inspected at a shop visit.
3.5.1.2 These methods, however, may be of very limited benefit at new manufacture. If credit for non-geometric anomaly inspection at new manufacture is claimed, applicants must justify the detection capability of the technique on the target anomaly types. The default POD data supplied in this AC in appendix D only refers to the detection of cracks and is not necessarily applicable to non-geometric anomalies.
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3.5.2 Parameters Influencing NDE Detection Capabilities.
NDE process capabilities, 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 anomaly’s size, shape, orientation, location, and chemical or metallurgical character. In addition, the following parameters should be considered when assessing the capabilities of an NDE process:
• Material being inspected (e.g., composition, grain size, conductivity, surface condition—such as cleanliness, peened vs. unpenned, damaged vs. undamaged— and surface texture).
• Inspection materials or instrumentation (e.g., specific penetrant and developer, inspection frequency, instrument bandwidth, and linearity).
• Inspection parameters (e.g., scan index).
• Inspector attributes (e.g., his or her visual acuity, attention span, and training).
3.5.3 Applicability of Default POD Data.
The default POD data in this AC represent inspection capabilities measured under typical, well-controlled conditions. These values are provided to assist in selecting NDE techniques for damage-tolerant inspections. However, though properly applied inspections should result in capability similar to these default values, these values only apply under the conditions in which they were obtained (see appendix C). POD curves are provided in appendix D:
3.5.3.1 Appendix D, Figure D-1. Mean POD for Fluorescent Penetrant Inspection of Finished-Machined Surfaces.
3.5.3.2 Appendix D, Figure D-2. Mean POD for Eddy Current Inspection of Finish Machined Surfaces.
3.5.4 POD Curves and Figures.
For more information on using POD data, refer to appendix A for an example and appendix C for NDE applicability guidelines.
3.6 Design Target Risk.
3.6.1 Discussion of DTR.
The DTR is a benchmark relative risk level selected to enhance the overall safety of high-energy rotating components.
3.6.2 Application to Manufacturing-Induced Anomalies.
Designs must meet the specified feature DTR for manufacturing-induced anomalies to be considered acceptable under the methods set forth in this AC. Manufacturinginduced anomalies include damage during blade insertion and removal in assembly and AC 33.70-5 3-7
AC 33.70-5 overhaul. Manufacturing-induced anomalies do not cover damage size or frequency unique to engine service operations.
3.6.3 DTR Values.
The DTR values for axial blade slots are specified in table 3-1.
Table 3-1. Allowable Design Target Risk for Axial Blade Slots Component Feature DTR (events per component published service lifetime) Axial Blade Slots Set 2.0 E-05 3-8
AC 33.70-5 CHAPTER 4. “ SOFT TIME INSPECTION” ROTOR LIFE MANAGEMENT
4.1 Approach.
4.1.1 Overview of the Life Management Process.
The 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 goal of the inspection philosophy described in this AC is to address anomalous conditions without allowing operation beyond the safe-life limit specified in the Airworthiness Limitations Section (ALS) of the Instructions for Continued Airworthiness (ICA).
4.1.2 Managing Higher-Than-Desired Risk Levels.
If probabilistic assessments indicate risk levels higher than the desired target, there are many strategies to reduce the predicted risk to the appropriate level. However, this chapter addresses only the in-service inspection option.
4.1.3 Use of Industry Data on Uncontained Fracture Experience.
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 by the FAA 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 manufacturing). This data suggests that additional inspection requirements, if not properly integrated into the regular engine maintenance schedule, would have no net benefit in reducing uncontained failure rates.
4.1.4 General Inspection Philosophy.
4.1.4.1 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 incorporates the use of opportunity inspections rather than mandatory inspections at “not to exceed” intervals.
4.1.4.2 These opportunity inspections occur due to the “on-condition” maintenance practices used by operators today. Though opportunity inspections occur at random intervals, they can be treated statistically and used effectively to lower the calculated risk of an uncontained event.
4.1.5 Opportunity Inspections.
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 eddy current inspection 4-1
AC 33.70-5 (ECI) of a disk bore may be specified on an assembled module whenever the module is available. This inspection is an opportunity inspection based on module availability rather than piece part availability.
4.1.6 Forced Inspections Through Disassembly.
4.1.6.1 The methods provided by this AC may 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 mandate inspections by specifying the disassembly of modules or engines when a cyclic life interval has been exceeded.
4.1.6.2 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 includes the use of the piece part soft-time inspection option and only suggests that forced disassembly of modules be considered when necessary to meet the DTR levels.
4.1.7 On-Condition Maintenance Practice Philosophy.
4.1.7.1 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.
4.1.7.2 An available module containing a component 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 components that become available for inspection before the soft-time interval to determine if the CSLI can be reset.
4.1.8 Maintenance Impact of Soft-Time Intervals.
The maintenance impact of the soft-time intervals should be considered during the design phase using probabilistic analysis summarized in chapter 2, along with the anticipated engine removal rate and the module and piece part availability. This approach ensures that designs not only meet the design target but also result in acceptable soft-time intervals and procedures if required.
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4.1.9 Establishing Interval Limits with Soft-Time Inspection.
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 and Airworthiness Directives.
4.1.10 Communication and Implementation of Inspection Assumptions.
4.1.10.1 Applicants should recognize that the inspection assumptions made in the probabilistic risk assessment must be communicated to and implemented accurately in 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 specify an eddy current inspection if that was an assumption in setting the original soft-time interval.
4.1.10.2 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.
4.1.11 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 the necessary information to focus the prescribed inspections to those areas of highest relative risk.
4.1.12 Non-destructive inspection(s) at manufacture cannot be accounted for in the analysis to lower the probability of failure/fracture (POF) value before the application of manufacturing credits (see restriction 5 in appendix F).
4.2 Inspection Scenarios.
The following scenarios clarify the actions that should be taken during a maintenance inspection opportunity (see appendix H.13). Note that the inspection plans may vary for each component based on the outcome of the probabilistic assessments.
4.2.1 Maintenance Opportunity - Hardware Available for Opportunity Inspection.
Hardware available in the condition to perform a specified opportunity inspection must be inspected according to the procedures specified in the ALS of the ICA. This inspection is mandatory.
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4.2.2 Maintenance Opportunity - Module Below Soft Time Interval.
Hardware accessible in the assembled or partially disassembled module may be nondestructively inspected by the procedures specified in the ALS of the ICA. The CSLI may be reset to zero, provided the engine manufacturer has assessed the risk impact associated with this action. This is a discretionary inspection.
4.2.3 Maintenance Opportunity - Module Exceeding Soft Time Interval.
Hardware listed in the ALS of the ICA must undergo nondestructive inspection whenever the module is accessible, particularly when the CSLI for any hardware exceeds the inspection cycle limit. This inspection is mandatory.
4.2.4 Soft Time Inspection Interval Example.
An example of how to develop a soft time inspection interval is provided in chapter 5 of AC 33.70-3.
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