c. Rotor Grade Materials Other Than Titanium Alloy. At the present time, there are no default data distributions for use in FMBP assessments for rotors made of materials other than titanium alloy.
4. Default Input - POD by Nondestructive Evaluation (NDE).
a. 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 (such as its composition, grain size, conductivity, surface texture, etc.).
(2) The inspection materials or instrumentation (such as the specific penetrant and developer, the inspection frequency, instrument bandwidth and linearity, etc.).
(3) The inspection parameters (such as scan index).
(4) The inspector (such as visual acuity, attention span, training, etc.).
b. 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. It must be recognized 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 4). If exacting use is to be made of these data, skilled professional judgment as to their applicability will be necessary. POD curves are described in Appendix 5, FIGURES A5- 1-A5-5, as listed below:
(1) Appendix 5, FIGURE A5-1. Mean POD for Fluorescent Penetrant Inspection of Finish-Machined Surfaces.
S3-8
(2) Appendix 5, FIGURE A5-2. Mean POD for #1 FBH Ultrasonic Inspection of Field Components.
(3) Appendix 5, FIGURE A5-3. Mean POD for #2 FBH Ultrasonic Inspection of Field Components.
(4) Appendix 5, FIGURE A5-4. Mean POD for #3 FBH Ultrasonic Inspection of Field Components.
(5) Appendix 5, FIGURE A5-5. Mean POD for Eddy-Current Inspection of Finish Machined Surfaces.
NOTE: Refer to Appendix 1 for an example of the use of this data, and Appendix 4 for the NDE Applicability of these POD curves.
5. Design Target Risk (DTR).
a. The DTR is an agreed upon benchmark risk level selected to enhance the overall safety of high energy titanium 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. Representative “Component level DTRs” and an “engine level DTR” for titanium hard-alpha anomalies have been established based on improvements of the data presented in the SAE report. These DTRs represent consensus values, developed from assessments on representative component designs using the methodology and inputs (described in paragraphs 1-4 of Section 3). The “component level DTR” corresponds to the maximum allowable predicted component event rate. The “engine level DTR” corresponds to the maximum allowable (cumulative) component event rate for all critical titanium rotating parts in a given engine.
c. Designs must satisfy the component level DTR and the engine level DTR to be considered acceptable.
(1) Application. Default DTR values have been established for melt related (hard-alpha) anomalies. Calculated event rates should be assessed against the S3-9
appropriate DTR values. For multiple stage components, such as spools, each individual stage must satisfy the component level DTR value.
(2) Default DTR values for titanium alloys:
(a) Melt related (hard-alpha) anomalies:
component level DTR: 1 x 10-9 events/flight cycle engine level DTR: 5 x 10-9 events/flight cycle
(b) Manufacturing or Maintenance Induced Surface Anomalies: At the present time, there are no default DTR values for manufacturing or maintenance related surface anomalies.
(3) Default DTR Values for Other Than Titanium Alloy: At the present time, there are no default DTR values for other than titanium alloys.
S3-10
SECTION 4 “SOFT TIME INSPECTION” ROTOR LIFE MANAGEMENT
1. Approach. The overall life management process encompasses a wide spectrum of design, manufacture, and product support issues. This section addresses only one facet of that overall process, namely the assurance of structural integrity using inspection techniques and intervals derived from a damage tolerance (fracture-mechanic based) assessment. The inspection philosophy is solely intended 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. In instances where probabilistic assessment indicates risk levels greater than the desired target, many strategies can be utilized to reduce the predicted risk to the appropriate level. However, only the in-service 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 the 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 the uncontained failure rates.
c. The inspection philosophy presented here evolved from the desire to have inspections easily integrated into the operation of the engine yet 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 the 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 S4-1
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 unacceptable risk level when using just opportunity inspections and some additional action may be required to meet the DTR. One of the many 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 the forced disassembly. The 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 only advocates the use of the soft-time inspection option when forced disassembly of modules 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 comes into effect 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 afford 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 will be required to be disassembled to a condition that allows inspection by the procedure specified by the engine manufacturer. The risk associated with parts that become available for inspection before the soft-time interval must be evaluated by the engine manufacturer to determine if the CSLI can be reset.
g. The maintenance impact of the soft-time intervals should be considered during the design phase. The probabilistic analysis summarized in Section 3 should be used along with the anticipated engine removal rate and the module and piece part availability to develop designs that achieve the design target, but also result in acceptable soft-time intervals and procedures, should such action be 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 impact or modify S4-2
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 would continue to be addressed through aggressive inspection programs which are mandated through Airworthiness Directives (ADs).
i. It is important to recognize that the inspection assumptions made in the probabilistic risk assessment must be communicated and implemented accurately to the field by using the Airworthiness Limitations Section of the Instructions for Continued Airworthiness, and be validated by the review of engine removal rates and module and piece part availability data. For example, the Airworthiness Limitations Section must call out an immersion ultrasonic 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 the 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 will be specified in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness. This information is to 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 Airworthiness Limitations Section of the Instructions for Continued Airworthiness with the other rotor inspection requirements. The manufacturer will also provide necessary information to focus the prescribed inspections to those areas of highest relative risk.
2. Inspection Scenarios.
a. The following scenarios clarify the action that would 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.
(1) Maintenance Opportunity - Hardware Available For Opportunity Inspection: Hardware available in the condition to perform the specified opportunity inspection must be inspected by the procedures specified in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness. This would be a mandatory inspection.
S4-3
(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 Airworthiness Limitations Section of the Instructions for Continued Airworthiness. The CSLI may be reset to zero, provided the engine manufacturer has assessed the risk impact associated with this action. This would be a discretionary inspection.
(3) Maintenance Opportunity - Module Above Soft Time Interval: Hardware listed in Airworthiness Limitations Section of the Instructions for Continued Airworthiness must be made available for nondestructive inspection, using the procedures that are specified. This inspection must be performed whenever the module is available and the CSLI for any contained hardware that exceeds the inspection cycle limit. This would be a mandatory inspection.
S4-4
SECTION 5
REFERENCES
1. SAE/FAA Committee On Uncontained Turbine Engine Rotor Events, Report No. AIR 1537, Data Period 1962-1975.
2. SAE/FAA Committee On Uncontained Turbine Engine Rotor Events, Report No. AIR 4003, Data Period 1976-1983.
3. SAE/FAA Committee On Uncontained Turbine Engine Rotor Events, Report No. SP- 1270, Data Period 1984-1989.
4. FAA Advisory Circular 33.15-1, “Manufacturing Process of Premium Quality Titanium Alloy Rotating Engine Components”.
5. “Development of Anomaly Distributions for Aircraft Engine Titanium Disk Alloys” Technical paper was presented by the AIA Rotor Integrity Sub-Committee at the American Institute of Aeronautics and Astronautics (AIAA) Conference in April 1997.
S5-1
Appendix 1 APPENDIX 1
CALIBRATION TEST CASE
1. This Appendix provides a self contained package for calibration of a probabilistic risk assessment methodology. The package includes all required input data for the test case, analysis guidelines, and a test case analysis section that permits manufacturers to estimate the level of acceptability of their risk calculations and gain insights on intermediate results.
2. Test Case Input data
a. Anomaly distribution curve. The anomaly distributions are provided in Appendix 3.
For this test case we will use the anomaly distribution presented in FIGURE A3-7 of Appendix 3 titled “Post 1995 Triple Melt Hard-Alpha Distribution with #3 FBH billet inspection and #3 FBH forging inspection”. Billet and forging manufacturing inspections are fully accounted for in this curve. No additional modifications are necessary.
(1) It is assumed that anomalies are spherically shaped and uniformly distributed throughout the part.
(2) The anomaly distribution should be linearly extrapolated when anomaly sizes are required outside the range of data provided. The curve is shown in FIGURE A1-11.
b. POD. The POD curve used to determine the effect of an in-service inspection is contained in Appendix 5. The default curve to be used is the mean POD for ultrasonic inspection of field components, with reject indications equal to or greater than those from a 3/64 inch (1.19 mm) diameter FBH. For the test case, it is assumed that this curve applies to the whole volume, including the near surface volume of the component.
c. Maintenance exposure interval. It should be assumed that 100-percent of the fleet is ultrasonically inspected at 10,000 cycles, which represents 50-percent of the certified part life (20,000 cycles).
A1-1
Appendix 1
d. Incubation. No anomaly incubation life should be assumed.
e. Stress. The limiting operational principal stress is the hoop stress.
f. Material Data. Two sets of material data are provided:
(1) Physical properties. Data required:
Density: 4,450 kg/m3 or 0.161 lb./in3 Young modulus: 120,000 MPa or 17.4E3 ksi Poisson's ratio: 0.361
(2) Crack Growth. Assume the following data represents both air and vacuum crack propagation. Crack propagation rate:
da/dN = 9.25 E-13 (K)3.87 (da/dN in m/cycle and K in MPam)
- or - da/dN = 5.248 E-11 (K)3.87 (da/dN in in/cycle and K in ksiin) K threshold = 0.0 MPam or 0 ksiin Fracture toughness = 64.5 MPam or 58.7 ksiin Yield = 834 MPa or 121.0 ksi UTS = 910 MPa or 132.0 ksi
(a) The above data applies at the test case component temperature.
(b) Crack propagation data are for a stress ratio of zero, therefore, no stress ratio correction is required.
(c) These data were taken from MCIC-HB-01R, Damage Tolerant Design Handbook, A Compilation of Fracture and Crack Growth Data for High Stress Alloys, vol. 1, dated December 1983 (page 411.257, Figure 4.113.104). It represents generic Ti 6-4 Paris fit data. These data are provided for example purposes only, and do not constitute a recommendation for analyzing actual components.
A1-2
Appendix 1
3. Test Case Analysis Guidelines
a. Analytical guidelines for the probabilistic assessments are provided with the intent to minimize the variations of the applicants results due to analytical assumptions.
The practice presented is based on a typical embedded anomaly probabilistic fracturemechanics approach. The component is subdivided into zones, the relative risk or probability of fracture (POF) is calculated for each zone, and results for each zone summed statistically to arrive at the total component POF or relative risk.
(1) This analytical approach can be broken down into five basic steps:
(a) Stress analysis.
(b) Zone definition and volume calculation.
(c) Crack growth model definition.
(d) Crack growth calculation.
(e) Zone and total part POF calculation.
(f) Paragraph 4 of this Appendix provides a systematic example for the calibration test case.
b. General Analytical Guidelines:
(1) Stress Analysis. The level of mesh refinement of the part model is left up to the individual applicant’s discretion. However, steps should be taken to ensure that the final answer does not change by a significant amount (5-percent on relative risk or POF) if a finer mesh is chosen.
(2) Zone Definition. Zones are defined as regions of the component (typically made up of a number of finite elements) where life is approximately constant for a given initial crack size. Grouping elements into zones based on stress intervals of 5 A1-3
Appendix 1 ksi (34.5 Mpa) is a suggested practice for initial zone definition. Further zone refinements may be required for analytical convergence. FIGURE A1-1 provides a general description of the typical types of zones.
FIGURE A1-1
(3) Crack Growth Calculation. The crack growth life assumed for each zone should be based on the minimum life location in the zone. This conservative assumption may require that regions of the component that make a significant contribution to the total part POF be broken down into multiple zones. This subdivision process is carried out until convergence of the risk calculation is reached.
NOTE: The effect of surface enhancement, such as shot-peen, is not considered in the test case analysis. However, once the test case has been performed and the analysis process successfully calibrated, the manufacturer may consider these effects when analyzing actual hardware.
(4) POF/Risk Calculation. The POF of the part is calculated by statistically combining the POF of each zone (surface and subsurface). The POF of each zone can be calculated in either of two ways:
(a) An integrated probabilistic method.
(b) A ‘’Monte - Carlo’’ method. The number of simulations required is related to the computed risk. The general rule is that the number of simulations should be at least 2 orders of magnitude higher than the computed risk, for example, if risk is 1 failure in 104 parts, the number of samples required is 106. This ensures that about 100 “failed” parts are involved in the assessment.
A1-4