Appendix 1 FIGURE A1-9
A1-17
Appendix 1
(6) Step 6 - Relative Risk Calculations – With a Single In-Service Inspection.
The “with inspection” probability of fracture calculations are performed in the same manner as in step 5, except the ultrasonic technique (UT) inspection POD data and cycles to inspection are included in the risk integration (FIGURE A1-10). The calculated probability of fracture with a mid-life inspection is 1.4 E - 09 events/cycle.
(a) Assumptions:
1. The UT inspection POD curve is applicable for 100-percent of the component volume (surface connected and subsurface).
2. Inspection performed at 10,000 cycles.
3. Assume the anomaly area in the inspection plane is equivalent to the anomaly area in the stress plane.
A1-18
Appendix 1 Anomaly Distribution Remaining Life Data Volume Data REMAINING LIFE DATA Volume Data lost 1995 Triple Me Colt Hearth • VAR VOLUME NO.
CYCLES Service Life RISK → Events/Cycle CALCULATOR = 1.4E-9 Ultrasonic Insp. POD
PERCENT POD 1000 ANOMALY AREA (mlls*2) FIGURE A1-10 A1-19
Appendix 1
(7) Step 7 - Results. This test case has been performed by a number of manufacturers. A statistical analysis of the results, given in failure risk for each cycle, was performed and demonstrated the following statistical values.
Failure risk for each cycle Mean value : m m – 1.65 s m + 1.65 s Without in-service inspection With an in-service inspection
1.57E-09
1.13E-09
1.27E-09
8.36E-10
1.93E-09
1.53E-09 All results in the range of (m-1.65s, m+1.65s) for both conditions are considered acceptable. A graphical representation is shown in FIGURE A1-12.
NOTE: This range is defined as the interval, centered on the mean value, covering 90-percent of the result population assuming a log-normal distribution.
A1-20
I0/8/1 AC 33.14-1 Appendix 1 FIGURE A1-11 Post 1995 Triple Melt/Cold Hearth + VAR Hard Alpha Inclusion Distribution Billet/Forging Inspections #3#3 FBH
1.0E+02
• 1.0E+01 · 1.0E+00
1.0E-01
1.0E-02 +
1.0E+02 1.0E+03 1.0E+04 1.0E+05 INCLUSION AREA (sq mils) A1-21
Appendix 1 FIGURE A1-12: Results m - 1.65 s = 1.27E-09 TOTAL FAILURE RISK
WITHOUT INSPECTION MEAN VALUE It = 1.57E-09 GAUSS FUNCTION I - 1.65 s = 1.27E-09 I + 1.65 s = 1.93E-09
1.0E-10 1.0E-09 1.0E-08
FAILURE RISK PER CYCLE
TOTAL FAILURE RISK
WITH AN IN-SERVICE INSPECTION MEAN VALUE 11 = 1.13E-09 GAUSS FUNCTION I1 - 1.65 s = 8.36E-10 It + 1.65 ≤ = 1.53E-09
1.0E-10 1.0E-09 1.0E-08
FAILURE RISK PER CYCLE A1-22
Appendix 2 APPENDIX 2
SOFT TIME INSPECTION EXAMPLE
1. This Appendix provides an example of an acceptable process for setting the opportunity inspection requirements that will be specified in Airworthiness Limitations Section of the Instructions for Continued Airworthiness. As discussed in Section 4, the application of the opportunity inspection is one of a number of options available to reduce the predicted POF in the event that a component design does not meet the DTR criteria.
a. Section 4 introduced the following three scenarios for opportunity inspections to clarify the actions that could be taken at a maintenance opportunity. They are (1) Hardware available for opportunity inspection, (2) Module below soft time interval, (3) Module above soft time interval.
Examples of the first and third scenarios will be presented in this Appendix, and the second scenario would be analyzed in a similar fashion to the third scenario.
b. Key elements in determining opportunity inspection requirements given any scenario are the type of inspection method and associated level of sensitivity, the maintenance interval at which the hardware will be exposed for inspection, and the cyclic threshold or soft time interval for module exposure at which time the inspections will be invoked. Given a scenario, details of an inspection plan can take many forms. FIGURE A2-1 shows the decision process for selecting the appropriate inspection requirements. This flowchart will be referenced throughout this section to guide the discussion.
A2-1
Appendix 2 FIGURE A2-1 In-service Inspection Decision Process YES 4 1 2 Acceptable Candidate Design Assess POF DTR? Meet Design NO 12 Incorporate - Method - Threshold 5 inspection requirements in - Exposure Apply Chapter 5 of the Opportunity engine manual Inspection YES 6 - Piece Part Select Component - Module - Engine 11 Exposure Level - Combination Acceptable YES Design NO (select 6, 7 or 8) 10 7 3000 Cycles Meet Select 2000 Cycles DTR? Inspection 1000 Cycles Threshold 9 8 - UT, #3 FBH, 1:1 POF Assess Inspection and Select - FPI, Full Field - EC, 1:1 - UT, #3 FBH, 2:1 Sensitivity A2-2
Appendix 2
2. Example of Scenario (1), Hardware Available for Opportunity Inspection.
a. It was shown in Appendix 1 that the predicted POF for the simple ring disk, without the benefit of in-service inspection is 1.9 E-09 events/cycle (Block 2 of FIGURE A2-1). 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 was less than the DTR, the design would be considered acceptable (Block 4) and no in-service inspection would be required.
b. Assuming a design change is not possible (for example, a reduction in stress, change in material, or enhance manufacturing inspection), the decision is made (Block 5) to explore the opportunity inspection option to reduce the component risk below the DTR.
c. With the decision made to pursue the inspection route, the level of maintenance opportunity is selected for study. The options available are piece part, module, engine, or some combination of these opportunities. The desire is to select an exposure level or combination of levels that minimizes the impact on the operator, yet has a high potential of reducing the component risk level. It is anticipated that the applicant will use trial and error to arrive at the optimum solution.
However, working with this damage tolerance criteria will give the applicant experience for making good initial selections reducing the amount of analytical effort in future analyses. For the initial pass, a one time ultrasonic inspection (UT) at first piece part exposure (Block 6), and an inspection threshold of zero cycles (Block 7) will be evaluated. The piece part maintenance exposure distribution for the ring disk is shown in FIGURE A2-2 below.
FIGURE A2-2 A2-3
Appendix 2
d. A UT inspection rejecting indications equal to or greater than a number 3 FBH is selected. The solid line in Appendix 5, FIGURE A5-4 is the POD for this inspection (Block 8).
e. The probability of fracture calculations are performed (Block 9) in the same manner as in step 6 of Appendix 1, 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.
f. The calculated probability of fracture is 1.3 E-9 events/cycle, still greater than the DTR (A “No” answer at Block 10). On a second pass a more sensitive UT inspection is assumed, rejecting indications equal to or greater than ½ the response from a number 3 FBH. The associated POD for this inspection is represented by the dotted line in FIGURE A5-5. The resulting POF is 9.9 E-10 events/cycle, meeting the DTR (A “Yes” answer at Block 10).
g. The design would be considered acceptable (relative to damage tolerance criteria) and the following inspection requirements would be placed in Airworthiness Limitations Section of the Instructions for Continued Airworthiness (Step 12):
(1) Inspect at first piece part exposure.
(2) UT inspection calibrated to a #3 FBH.
(3) Reject indications equal to or greater than ½ the response from a number 3 FBH calibration.
(4) Include reference to detailed UT inspection procedures.
3. Example of Scenario (3), Module Above Soft Time Interval
a. For this example, exposure of the ring disk piece parts is expected to occur at a lower rate than in the previous scenario. This change is depicted in FIGURE A2-3 below.
A2-4
Appendix 2 FIGURE A2-3: New Ring Disk First Part Exposure Distribution
b. The predicted POF, assuming this new exposure distribution, and the same UT inspection and sensitivity as in scenario (1), is 1.2E-09 events/cycle. Since the predicted POF exceeds the DTR, additional action is warranted.
c. Assuming that it is not reasonable to use a more sensitive UT field inspection (for example, calibration to a smaller flat bottom hole), the module exposure distribution is evaluated (see FIGURE A2-4 below).
A2-5
Appendix 2 FIGURE A2-4: New Ring Disk First Exposure Distributions
d. The resulting predicted POF is 8.3 E-10 events/cycle, clearing the DTR with margin.
However, specifying UT inspection of the ring disk at module exposure requires that the disk be removed from the module increasing the burden on the operator. Since there is margin between the predicted POF and the DTR, an alternative inspection plan will be considered that will alleviate some of the burden of forcing modules to piece part level. This approach implements the soft time inspection interval scenario. Instead of going to just the module exposure, the inspections would be performed at piece part exposure for a specified cyclic interval, then change to inspections at module exposure. The cyclic interval before imposing inspection based on module exposure is the soft time inspection interval.
A2-6