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.
4-2
AC 33.70-5
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.
4-3
AC 33.70-5
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.
4-4
AC 33.70-5 APPENDIX A. MA NUFACTURING-INDUCED ANOMALY CALIBRATION TEST CASE FOR
AXIAL BLADE SLOTS A.1 Introduction.
A.1.1 Purpose of the Test Case.
This appendix provides instructions for calibrating a probabilistic risk assessment methodology for manufacturing-induced anomalies in axial blade attachment slots. This includes damage associated with blade insertion and removal during assembly and overhaul. It does not specifically include damage in terms of size or frequency unique to the service operation of the engine.
A.1.2 Scope of Input Data.
This appendix includes all the input data required to complete the test case, such as analysis guidelines, component geometry, boundary conditions, material property data, and inspection efficacy. The input data should be used to compute the POF of a feature within the defined cyclic life interval. An applicant must demonstrate the acceptability of their probabilistic process by meeting the POF criteria defined in table A-8.
A–1
AC 33.70-5 A.2 Test Case Input Data.
A.2.1 Problem Description and Geometry.
The test case geometry, shown in figure A-1, consists of two components: a turbine disk and a blade. Both parts are made from a nickel-based superalloy, with the blade assumed to be solid and isotropic. The blade applies centripetal forces to the disk, but no aerodynamic or pressure loads are applied. In the finite element analysis, the blade should be constrained to prevent axial movement out of the attachment slot.
Figure A-1. Representation of Test Case Blade and Disk Geometry A–2
Figure A-2. Test Case Geometry Definition: Isometric View of 5 Degree Sector Showing Disk and Blade AC 33.70-5 A.2.2 Test Case G eometry Details.
An isometric view and drawings of the disk and blade are shown in figures A-2 through A-8. Dimensions are provided in both Imperial and International System of Units (SI units). The disk features a two-lobe firtree, with 72 equally spaced slots.
A.2.2.1 Figure A-2 provides an isometric view of a 5-degree sector of the test case geometry, showing the disk and blade for the component under assessment.
A–3
AC 33.70-5 A.2.2.2 Figure A-3 provides an axisymmetric profile view of the disk, with dimensions specified in both inches and millimeters.
1.000 -(25.40) 51*22 - 72 SLOTS EQ SP
- 93) ฿18.000
(457.20) @16.300 _R .250
(414.02) (6.34)
•0"13'9" 305
(7.75) ฿12.055
(306.20) . 250
(6.35 ) R 2.000 (50,80) ฿5.994
(152.25) #3.996
1.500 (101.50)
(38.10)
ALL DIM APPLY BOTH SIDES UOS Figure A-3. Test Case Geometry Definition: Disk Axisymmetric Profile in Inches and (Millimeters) A-4
AC 33.70-5 A.2.2.3 Figure A-4 shows a radially inward view of the test case geometry, highlighting an 8-degree broach angle. This view provides critical details of the angular orientation, ensuring a precise understanding of the broaching direction for axial blade slot geometry shown in figure A-5.
Figure A-4. Test Case Geometry Definition: View Looking Radially Inward Showing 8.0 Degree Broach Angle A–5
AC 33.70-5 A.2.2.4 Figure A-5 presents the A -A view of the disk broach slot profile, displaying key dimensions in inches and millimeters.
Figure A-5. Test Case Geometry Definition: View A-A of Disk Broach Slot Profile in Inches and (Millimeters) Not to Scale A–6