Appendix B A PPENDIX B. SOFT-TIME INSPECTION EXAMPLE B.1 Overview of Soft-time Inspection Example.
This appendix provides an example of an acceptable process to set the opportunity inspection requirements that will be specified in the airworthiness limitations (AL) section of the instructions for continued airworthiness (ICA). As discussed in chapter 5, the application of the opportunity inspection is one of several options available to reduce the predicted probability of failure (POF) in the event that a component design does not meet the DTR criteria.
Chapter 5 introduced the following three scenarios for opportunity inspections to clarify the actions that technicians could take at a maintenance opportunity. The scenarios are as follows:
1. Hardware available for opportunity inspection.
2. Module below soft-time interval.
3. Module above soft-time interval.
B.2 Note: The FAA presents examples of the first and third scenarios in this appendix. The second scenario could be analyz ed in a similar manner to the third scenario.
The key elements to determine opportunity inspection requirements, given any scenario, are as follows:
• Type of inspection method and associated level of sensitivity.
• Maintenance interval at which time the hardware will be exposed for inspection.
• Cyclic threshold or soft-time interval for module exposure at which time the inspections wi ll be invoked.
Given each scenario, details of an inspection plan can take many forms. Figure B-1 shows the decision process to select the appropriate inspection requirements. The FAA will reference this flowchart throughout this section to guide the discussion.
Examples of Scenario 1: Hardware Available for Opportunity Inspection.
1. Figure B-1 shows that the predicted POF for the simple ring disk, without the benefit of in-service inspection, is 1.9 E-09 events/cycle. Therefore, the ring disk design does not meet the 1.0 E-09 event/cycle DTR (a “No” answer at block 3). If the ring disk POF were less than the DTR, the design would be considered acceptable (block 4), and no in-service inspection would be required.
2. Assuming a design change is not possible (for example, stress reduction, material change, or manufacturing inspection enhancement), then the decision is made (block 5) to explore the opportunity inspection option to reduce the component risk below the DTR.
3. With the decision made to pursue the inspection route, the level of maintenance opportunity is selected for the study. The options available are piece part, module, B-1
Appendix B eng ine, or some combination of these opportunities. The desire is to select an exposure level or combination of levels that minimize the impact on the operator, yet has a high potential to reduce the component risk level. It is anticipated that the applicant will use trial and error to arrive at the best solution. Working with these damage tolerance criteria will give the applicant experience to make good initial selections, which will reduce the amount of analytical effort in future analyses. For the initial pass, a one-time ultrasonic inspection at first piece part exposure (block
6) and an inspection threshold of zero cycles (block 7) will be evaluated. Figure B-2 shows the piece part, maintenance exposure distribution for the ring disk.
4. An ultrasonic inspection that rejects indications equal to or greater than a #3 FBH is selected. The solid line in figure E-4 is the POD for this inspection (block 8).
5. The POF calculations (block 9) are performed in the same manner as in paragraph A.4.7; except, instead of a fixed inspection at 10,000 cycles, inspections are assumed to occur as the piece parts are exposed. The piece part, exposure distribution is treated as a random variable in the probabilistic analysis.
6. The calculated probability of failure is 1.3E-9 events/cycle, which is still greater than the DTR (a “No” answer at block 10). On a second pass, a more sensitive ultrasonic inspection is assumed, which rejects indications equal to or greater than one-half the response from a #3 FBH. The dotted line in figure E-5 represents the associated POD for this inspection. The resulting POF is 9.9E-10 events/cycle, meeting the DTR (a “Yes” answer at block 10).
7. The design would be considered acceptable (relative to the damage tolerance criteria), and the following inspection requirements would be placed in step 12 of the AL section of the ICA.
•
Inspect at first piece part exposure.
•
Ultrasonic inspection calibrated to a #3 FBH.
• Reject indications equal to or greater than one-half the response from a #3 FBH calibration.
• Include reference to detailed ultrasonic inspection procedures.
B.3 Example of Scenario 3:
Module above Soft-Time Interval.
1. For this example, piece part exposure of the ring disk is expected to occur at a lower rate than in the previous scenario. Figure B-3 depicts this change.
2. The predicted POF is 1.2E-09 events/cycle, assuming this new exposure distribution and the same UT inspection and sensitivity as in scenario 1. Since the predicted POF exceeds the DTR, additional action is necessary.
3. Assuming that it is not reasonable to use a more sensitive field ultrasonic inspection (for example, calibration to a smaller flat bottom hole), then the module exposure distribution is evaluated (figure B-4).
B-2
Appendix B
4. The resulting predicted POF is 8.3E-10 events/cycle, clearing the DTR with margin.
However, specifying an ultrasonic inspection of the ring disk upon module exposure requires removal of the disk from the module, increasing the burden on the operator. Since there is margin between the predicted POF and the DTR, consider another inspection plan to alleviate some of the burdens of forcing modules to piece part level. This approach implements the soft- time, inspection interval scenario. Instead of going to the module exposure, the inspections would be performed at piece part exposure, at a specified cyclic interval, and then change to inspections at module exposure. The cyclic interval, before imposing an inspection based on module exposure, is the soft-time inspection interval.
5. Thi s strategy essentially accelerates the piece part exposure rate, as shown in figure B-5. By iterating on the length of the soft-time interval, a 12,300 cycle value is found to yield a POF of 1.0E-09 events for each cycle, satisfying the DTR criteria.
The design would be considered acceptable, relative to the damage tolerance criteria, and the following inspection requirements would be placed in step 12 of the AL section of the ICA.
•
Inspect at first piece part exposure.
• For parts not previously inspected before 12,300 cycles, inspect at first module exposure above 12,300 cycles, soft-time inspection interval.
•
Ultrasonic inspection calibrated to a #3 FBH.
• Reject indications equal to or greater than one-half the response from a #3 FBH calibration.
•
Include reference to detailed ultrasonic inspection procedures.
6. The information contained in this section is provided for ex ample purposes only.
Each individual component design and engine maintenance practice may require different solutions than those presented here. The AL section of the ICA requirements should reflect actions consistent with the analytical assumptions made to meet the DTR criteria.
B-3
Appendix B CANDIDATE DESIGN 1 ASSESS POF 2 MEET DTR? YES → ACCEPTABLE DESIGN 4 NO 5 APPLY
NO OPPORTUNITY
INSPEC- TION?
YES 6 7 8
SELECT SELECT SELECT COMPONENT EXPOSURE -PIECE PART LEVEL THRESHOLD INSPECTION THRESHOLD INSPECTION SENSITIVITY -UT, #3 FBH, 1:1 ASSESS POF 9 -MODULE -3000 CYCLES -UT, #3 FBH, 2:1 -2000 CYCLES -ENGINE -FPI. FULL FIELD -COMBINATION - 1000 CYCLES -EC, 1:1 NO (SELECT 6, 7, OR 8) 10 MEET DTR?
YES 12
INCORPORATE ACCEPTABLE DESIGN 11 • REQUREMENTS IN CHAPTER 5 OF INSPECTION
ENGINE MANUAL AC33.70-3_15.TIF Figure B-1. In-Service Inspection Decision Process B-4
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Appendix C AP PENDIX C. DEFAULT ANOMALY DISTRIBUTION CURVES C.1 Use of Anomaly Distribution Curves.
The anomaly distribution curves associated with hard alpha inclusions in titanium engine rotors are shown in figures C-1 through C-6. The following text provides additional information associated with using these distributions.
1. The distributions apply only to hard alpha inclusions in rotor grade (premium) titanium, melted using triple VAR or CHM plus VAR processes.
2. It is crucial to use the appropriate distribution curve that accurately reflects the inspection sensitivities performed at the billet and forging stages of the manufacturing process. The distributions in this appendix are applicable to components that were inspected at the billet stage using a zoned, ultrasonic inspection performed in accordance with the ultrasonic titanium billet inspection policy found in the FAA’s Policy Memo ANE 2002-33.15-R0.
3. For example, the material must be inspected using ultrasonic inspection to at least a #3 FBH with the reject level set at one-half that of the calibration level (for additional instructions, see appendix E). Technicians must perform inspections at both the billet and sonic shape stages.
4. The distribution accounts for all steps required to manufacture a finished part, including the in-process billet and sonic shape, forging ultrasonic inspections.
Therefore, the distribution used should reflect the inspection sensitivities used in the billet and forging inspections and should not be altered.
C.2 Critical Elements of Anomaly Distribution Curves.
Applicants and technicians should consider the following elements when referring to figures C-1 through C-6:
1. The vertical axis represents the expected number of hard alpha inclusions for each million-pounds of titanium. This information should be treated as the probability of having an inclusion of a given size or larger (exceedance probability).
2. The horizontal axis is the inclusion cross-sectional area, including the hard alpha core and the surrounding diffusion zone. Technicians should assume a circular inclusion cross-section with the corresponding diameter used as the initial size in the crack growth analysis.
3. Default curves, listed in Table C-1, are available to demonstrate compliance with § 33.70.
C- 1
Appendix C T able C-1. Default Curves Billet Inspection Forging Inspection Figure AMS2628 Class A Zoned >10 in. Billet Diameter (#3 FBH) #3 FBH Conventional Figure C-1 AMS2628 Class A Zoned >10 in. Billet Diameter (#3 FBH) #2 FBH Conventional Figure C-2 AMS2628 Class A Zoned >10 in. Billet Diameter (#3 FBH) #1 FBH Conventional Figure C-3 AMS2628 Class A Zoned 5-10 in.
Billet Diameter (#2 FBH) #3 FBH Conventional Figure C-4 AMS2628 Class A Zoned 5-10 in.
Billet Diameter (#2 FBH) #2 FBH Conventional Figure C-5 AMS2628 Class A Zoned 5-10 in.
Billet Diameter (#2 FBH) #1 FBH Conventional Figure C-6 Note: For convenience, tabular data defining each curve is shown after the plots.
Manufacturers using forging input material < 5 inches in diameter should provide data to substantiate use of the default anomaly distribution selected.
C- 2
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