Subject: GUIDANCE FOR 30-SECOND AND 2- Date: 6/11/09 AC No: 33-7A MINUTE ONE-ENGINE-INOPERATIVE (OEI) Initiated By: ANE RATINGS FOR ROTORCRAFT TURBINE 111
ENGINES
1. Purpose. This advisory circular (AC) provides guidance and acceptable means, but not the only means, for demonstrating compliance with the specific requirements applicable to 30-second and 2-minute One-Engine-Inoperative (OEI) rotorcraft turbine engine ratings in part 33 of Title 14 of the Code of Federal Regulations (14 CFR part 33).
2. Applicability.
a. The guidance provided in this document is directed to engine designers, manufacturers, and modifiers.
b. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The Federal Aviation Administration (FAA, we) will consider other methods of demonstrating compliance that an applicant may elect to present. Terms such as “should,” “shall,” “may,” and “must” are used only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance in this document is used. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. On the other hand, if the FAA becomes aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation as the basis for finding compliance.
c. This material does not change, create any additional, authorize changes in, or permit deviations from existing regulatory requirements.
3. Related Regulations and References.
a. 14 CFR part 21, Certification Procedures for Products and Parts.
b. 14 CFR part 27, Airworthiness Standards: Normal category rotorcraft, § 27.45(f).
c. 14 CFR part 29, Airworthiness Standards: Transport category rotorcraft, § 29.45(f).
d. 14 CFR part 33, Airworthiness Standards: Aircraft Engines.
e. 14 CFR part 91, General operating and flight rules, § 91.3.
f. Society of Automotive Engineering, Aerospace Recommended Practice, Certification Considerations for Highly-integrated or Complex Aircraft Systems, SAE ARP4754.
g. Policy memorandum PS-ANE100-1997-00004 “Engine Overtemperature Test;
Governing Temperature Location,” October 7, 1997.
4. Background .
a. The 30-second OEI rating provides a short burst of high power particularly needed by a rotorcraft if one engine fails or is shut down during a critical flight configuration such as takeoff, rejected takeoff, or landing. The power levels associated with the 30 second and 2-minute OEI ratings would allow the rotorcraft to complete the takeoff or rejected takeoff with the remaining engine(s) and continue into climbout to a safe altitude or airspeed. Similarly, these ratings are intended to provide adequate power to the rotorcraft to execute a safe landing with the remaining engine(s) or a balked landing followed by a climbout.
b. The applicant selects the ratings from the OEI ratings specified in § 33.7. The 30 second and 2-minute OEI ratings differ from the other OEI ratings because of their higher power and shorter duration; their limitation to three usages during one flight; and their requirement for mandatory inspection and maintenance after use.
c. The 30-second OEI power level is considered reached when one or more of the operating limitations that apply to the 2-minute OEI power are exceeded. The 2-minute OEI power level is considered reached when one or more of the operating limitations that apply to the next lower OEI or engine rating are exceeded.
d. The rulemaking background for the 30-second and 2-minute OEI ratings is provided in Appendix 1.
5. Guidance.
a. Section 33.5 Instruction manual for installing and operating the engine.
(1) For rotorcraft engines having 30-second and 2-minute OEI ratings, the applicant must provide in the installation manuals the data on engine performance characteristics and variability required under § 33.5(b)(4). This data will enable the installer to meet the power availability requirements under §§ 27.45(f) and 29.45(f). The engine performance characteristics and variability should consider the power losses due to engine installation that can be defined at the engine level. For example, such installation losses should account for the effects of customer bleed and customer power extraction up to and including the highest rated power.
(2) Section 33.5(b)(4) requires that the engine be capable of obtaining and sustaining the OEI rated power while operating within the associated operating limitations. The applicant should use the required engine data to establish a procedure for trending individual engine performance. This data should support maintenance procedures, intervals, and specifications that apply to the engine, its sensors, and indicating systems.
(3) The data should also support procedures for detection of latent or dormant failure conditions of engine systems that may not be detectable through the recommended power assurance procedures or because the procedures do not include a topping check to the highest OEI rated power. For example, such latent or dormant conditions may involve the sensors and indicating systems of the fuel control maximum flow capability, measured turbine temperature, and speed limits. The applicant should analyze the dormant failures, which could lead to non-availability of the OEI rated power, and provide the results of this review for compliance with § 33.5(b)(4).
(4) The applicant should validate the adequacy of the procedures, intervals, and specifications applied to maintaining the engine, its sensors, and indicating systems on the basis of the engine and the engine system’s failure modes and effects analysis (FMEA). The engine data should include a thermodynamic model, and when applicable, should incorporate the experience gained during development and certification testing, as well as field experience gained with this engine type or engine of similar design.
(5) The data required under § 33.5(b)(4) should enable the installer to establish the power assurance procedures in which the extrapolation of power assurance results can be achieved from a lower power level to the highest OEI rated power. The performance extrapolation may be accomplished by comparing the performance characteristics with the minimum acceptable performance for the deteriorated engine. Establishing the minimum acceptable engine performance characteristics depends on the applicant’s experience with that engine and the size of its reliability database. For example:
(a) In a mature engine program, the applicant may use the new production engine acceptance test data, engine-to-engine variation, and also testing on engines prior 3
to overhaul to determine the effects of deterioration. This program would maintain up-to-date minimum engine performance characteristics.
(b) For a new engine design or a remote derivative of an existing design, it may be more difficult for an applicant to establish the initial database. The applicant, however, could use the experience gained from the engine’s development and certification tests. This experience usually includes several thousand hours of running time to schedules which are often far more rigorous than normal commercial service. The information gathered from these tests could provide a sufficient database for the assessment of in-service engines including the rate of deterioration. Testing engines in production will eventually establish engine-to-engine variation, but the applicant should assume initially an estimated worst variation based on similar experience with other engines.
(6) The applicant should also provide information on methods to ensure that engine limiter settings will not prevent the engine from reaching 30-second or 2-minute OEI power. These engine limiter settings may include engine speed, measured gas temperature, fuel flow, and torque.
b. Section 33.7 Engine ratings and operating limitations.
(1) The 30-second and 2-minute OEI power ratings for rotorcraft engines are two separate ratings defined in § 1.1. These ratings are optional and the applicant may specifically request them from the available ratings under § 33.7. In the worst-case scenario identified in the test requirements, these ratings are associated in a combined structure of 2.5 minutes duration, composed of 30 seconds at the 30-second OEI rated power immediately followed by 2 minutes at the 2-minute OEI rated power.
(2) These power ratings are intended for completing the flight after the failure or shutdown of one engine in multi-engine rotorcraft during takeoff, climb, or landing. The 30-second OEI rated power provides a short burst of high power to conduct a continued or rejected takeoff, if an engine failure occurs at the critical decision point, so the rotorcraft can lift clear of any obstructions in the flight path and climb out or reject the takeoff. Similarly, this rated power provides adequate power for rotorcraft to carry out a safe landing or a balked landing if an engine fails at any point up to and including the landing decision point. The 2-minute OEI rated power provides a further period of increased power for the rotorcraft to climb out from takeoff to a safe altitude and airspeed or to perform a balked landing.
(3) While the 30-second and 2-minute OEI ratings were originally conceived as high power ratings using the available margins in the engine design and followed by a mandatory engine overhaul, experience has shown that the manufacturers provide engines that have different capabilities and use different amount of the margins.
Therefore, some flexibility is possible in defining the mandatory maintenance actions, provided the applicant appropriately validates the actions during certification (see also paragraph 5.l. of this guidance).
(4) In some circumstances, the highest power used during a 2½ minute duration event might be lower than the 30-second OEI rating power band, but still within the 2 minute OEI rating power band. In this case, the use of the 2-minute OEI rated power may be extended to 2½ minutes. However, the additional 30-second period is considered as usage of the 30-second OEI rating, and the mandatory inspection and maintenance actions prescribed for the 30-second OEI rating should be used. See paragraph 5.l.(1)(f) for additional guidance.
(5) If necessary, engine operating limitations associated with the use of the 30 second and 2-minute OEI power ratings may be established to mitigate engine failure or component deterioration observed during the applicable portion of the 2-hour supplementary test of § 33.87(f). Refer to paragraph 5.l.(1) of this AC for additional guidance.
(6) Although the certification requirements have been defined around the worstcase scenario involving the possible use of these ratings three times in one flight, these ratings are intended for one usage per flight in an emergency during takeoff or landing.
We recognize that the ratings may be inadvertently used in some unexpected, non-critical conditions, such as an engine failure in a rotorcraft flying at a high-speed cruise. In all cases, the required mandatory maintenance actions apply after any use of the OEI rating powers.
c. Section 33.27 Turbine, compressor, fan, and turbosupercharger rotors. We approved special conditions in accordance with § 21.14 for certain turboshaft engines having 30-second OEI and 2-minute OEI ratings. Applicants who seek approval of these ratings may either comply with the existing requirements, or request a special condition with appropriate justifications. See Appendix 2 for the technical content of previously issued special conditions amending § 33.27 that applies to the 30-second OEI and 2 minute OEI ratings.
d. Section 33.28 Engine control systems.
(1) The automatic controls required by § 33.28(k) should be active when the 30 second OEI rated power is used and should prevent the engine from exceeding the operating limitations associated with the rating, except for the time duration discussed in paragraph 5.d.(2) below. These limitations are established under the requirements of § 33.7 and declared in the type certificate data sheet (TCDS). They may include parameters such as the output shaft torque or power, output shaft speed, gas producer speed, and gas path temperature. Exceeding these limits could result in engine failure since the engine operation at 30-second OEI rated power could have already used most or all available margins in the engine design. An engine failure would be unacceptable in this OEI flight configuration when one engine has already failed. The automatic control of the 30-second OEI rated power is intended to free the pilot from monitoring the associated engine parameters and ensure they remain within their limits. This is because the pilot may need to operate in a high workload environment and perform simultaneous 5
actions to maintain a safe flight during the use of 30-second OEI rating.
(2) The applicant’s design should not limit the time duration at which the 30 second OEI power is used. This allows the pilot to exceed the time limit if needed to safely land the rotorcraft in an emergency as permitted by § 91.3(b).
(3) When, instead of the means, the applicant provides the provision for a means for automatic controls, the installation manuals should declare all necessary information for the installer to develop the automatic controls system. The means, or the provision for a means, for the automatic control of the engine within its operating limitations should be effective whether the engine operation is considered normal or abnormal.
(4) The required means or the provision for a means for automatic control should not prevent the engine from reaching and maintaining its 30-second OEI rated power.
e. Section 33.29 Instrument connection.
(1) For compliance with § 33.29(c), the 30-second OEI power level is considered reached when one or more of the operating limitations that apply to the 2-minute OEI power is exceeded. The 2-minute OEI power level is considered reached when one or more of the operating limitations that apply to the next lower OEI or engine rating are exceeded.
(2) The applicant must provide either a means or a provision for a means to comply with the requirements of § 33.29(c)(1) through (c)(4). For example, the applicant may satisfy “a means” by providing a recorder to record the entry into the OEI power bands. Alternatively, the applicant may fulfill “a provision for a means” by specifying that the installer provide a recorder to record entry into the OEI power bands. The applicant may find that a “provision for a means” is more suitable to satisfy the pilot alert requirement when a portion of the pilot alert system is certified with the rotorcraft. In this case, the applicant should identify this provision in the installation manual, enabling the installer to develop the required pilot alert system.
(3) The automatic recording of data required under § 33.29(c)(2) should provide the information needed to conduct the mandatory inspection and prescribed maintenance per Appendix A33.4. At a minimum, it should record each rating’s occurrences per flight;
the duration for each occurrence or accumulated time; and any exceedence of the operating limitations or time limits associated with each OEI rating. See paragraph
5.b.(4) for guidance when the 2-minute time limit at 2-minute OEI rated power is exceeded.
(4) The applicant should ensure the overall development assurance level of the recording and retrieval system is consistent with its classification of at least hazardous based on failure condition classifications defined in SAE ARP4754; see reference in paragraph 3.f. The development assurance level(s) for the components of systems that record the 2-minute and 30-second OEI power usages and retrieve the recorded data 6
should be based on the criticality of the function(s) performed within the recording and retrieval system as determined by a system safety assessment (SSA). The overall system assurance level can be achieved based on an appropriate combination of system architecture and component assurance levels. The objectives are to ensure the information needed for the mandatory maintenance is available after the use of OEI powers and to avoid continued operation of the engine in a potentially unsafe condition.
(5) If the recording or retrieval system is not part of the engine, the applicant must specify in the installation instructions its required failure condition classification, system design features, and interface requirements (e.g., reliability, development assurance level, software level, lightning, and high energy radiated frequency). If software is used for data recording and retrieval, the specifications must comply with the requirements of § 33.28(g).
(6) The recording system should be designed so that only the maintenance personnel can reset it. This should prevent further engine operation without completion of the mandatory post-flight inspection and maintenance actions.
f. Section 33.70 Engine life-limited parts. For rotorcraft engines for which the 30 second OEI and 2-minute OEI ratings are desired, the applicant should provide a method to account for low cycle fatigue effects from the use of the two OEI ratings on engine life-limited parts. This may be done by adding to the expended lives of components a finite number of cycles associated with the use of each of the two OEI power ratings.
Alternatively, life reduction factor(s) associated with the use of each OEI power ratings may be applied to the lives of these components. Applicants with engines that were certified to § 33.14, which was removed by Amendment No. 33-22, should follow the guidance in this paragraph when applying for OEI ratings.
g. Section 33.83 Vibration test. Section 33.83(b) prescribes the required ranges of power and both the physical and corrected rotational speeds for vibration survey. For the 2-minute OEI rating, the test speed should be from the minimum rotational speed up to 103 percent of the maximum physical and corrected rotational speed permitted for the rating. For the 30-second OEI rating, the test speed should be from the minimum rotational speed up to 100 percent of the maximum physical and corrected rotational speed permitted for the rating. All other § 33.83(b) requirements apply to both OEI ratings.
h. Section 33.85 Calibration tests.
(1) Engines having 30-second and 2-minute OEI ratings are not required to perform calibration tests, but compliance with the requirements of § 33.85 must be satisfied based on the measurements taken during the endurance test, as specified under § 33.85(d). This is to minimize the damage to the engine and its hardware during the certification program, while taking full advantage of the data obtained during the endurance test.
(2) The data acquired during testing in compliance with the requirements of §§ 33.87(f), 33.88, and 33.90 should also be used to establish the engine characteristics throughout the engine’s operating envelope. In particular, the rated power for the 30 second and 2-minute OEI ratings must reflect the power deterioration observed from the test calibration prior to the endurance test of § 33.87(f) up to and including the third application of 30-second rated power. The power deterioration through the third application should be the best indicator of the worst-case power deterioration that could occur during actual use of the rating. The applicant should, therefore, include this information on power deterioration in the data given to the engine installer to enable them to define the rotorcraft performance characteristics. If power deterioration exceeds 10 percent at the 30-second rating over the course of the 2-hour test, the applicant must evaluate the mode of deterioration to ensure that the availability of 30-second rated power in service will not be compromised by deterioration variability.
i. Section 33.87 Endurance Test.
(1) For rotorcraft engines for which 30-second OEI and 2-minute OEI ratings are desired, the applicant must conduct a 2-hour supplementary test as required under § 33.87(f), in addition to the basic 150-hour endurance test. We recommend the applicant disassemble the tested engine and inspect the engine components at the conclusion of the 150-hour test, but prior to the supplementary test, using the criteria specified in § 33.93(a). In this case, the applicant must reassemble the engine using the same parts that completed the 150-hour endurance test, except for those defined as consumable. The applicant must then perform the supplementary test required in § 33.87(f) followed by a second teardown inspection of the engine using the criteria specified in § 33.93(b). The level of component cleaning to facilitate inspection prior to rebuild for the test in § 33.87(f) must be acceptable to the FAA. The applicant must show that any cleaning during the teardown inspection, or replacement of consumable parts, will not enhance the engine’s ability to meet § 33.93(b) requirements of the additional endurance test.
Note: If the applicant elects not to disassemble and inspect the engine prior to starting the supplementary test in § 33.87(f), then the teardown inspection requirements of § 33.93(a) apply on completion of the test; see § 33.93(c).
(2) The applicant must run four test sequences continuously for the required 2 hour test duration without stoppage. In the event of a stoppage, the interrupted sequence needs to be repeated in full or the sequence can be restarted from the interrupt point if there are technical justifications acceptable to the FAA. If it is determined that the sequence need not be repeated in its entirety, the test should be re-started from a point where the engine thermal condition would be the same as at the time of interruption. If an excessive number of interruptions occur, the applicant should repeat the entire test. In changing the power setting at the beginning and the end of each test sequence, the power control lever should be moved from one position to the other in not more than one second.
(3) To reduce test complexity, and to improve flexibility needed to attain the key 8
parameters (speed, temperature and torque) during the test in § 33.87(f), maximum air bleed for engine and aircraft services under § 33.87(a)(5) need not be used if the applicant can show by test, or analysis based on testing, that the engine’s ability to meet the teardown inspection requirements of § 33.93 is not enhanced. The analysis should include (1) the effect of the bleed air extraction to the engine secondary air system which provides cooling air to various engine components; and (2) the thermodynamic cycle effects of bleed (for example, core speed to output shaft speed changes).
(4) If, during testing, the accessory drives and mounting attachments are not loaded, the equivalent power must be added (see § 33.87(a)(6)) to the required power at the output drive so that the power turbine rotor assembly is operated at or above the same levels it would be if the accessory drives and mounting attachments were loaded.
(5) The engine operating limitations of 30-second and 2-minute OEI ratings must be based on the minimum values obtained during the applications of the 2-hour test of § 33.87(f) and must be identified in the TCDS. Due allowance should be made for stabilization time and the limits of accuracy for the instrumentation or automatic controlling system declared in accordance with §§ 33.29(c) and 33.28(k).
(6) The power level of test condition § 33.87 (f)(4) is intended to demonstrate the highest en route, OEI, or non-OEI power.
j. Section 33.88 Engine overtemperature test.
(1) For the purpose of the test required under § 33.88(b), the maximum power-on
r.p.m. is the steady state rotor speed associated with the 30-second OEI rating. However, if the transient speed stabilization to steady state exceeds 3 seconds, then the higher transient rotor speed achieved during the transition to 30-second OEI power should be used instead.
(2) The gas temperature increase of 35 degrees F for conducting the test per § 33.88 (b) is based on the turbine inlet gas temperature at the location immediately in front of the first stage high pressure turbine rotor; see reference in paragraph 3.g.
(3) After the overtemperature test, the turbine assembly may show distress beyond serviceable limits provided that no burst, blade failure, or other significant failure of any engine component would occur, or become evident, during the test, shutdown, or subsequent teardown inspection. If a potential failure becomes evident, the applicant should analyze the failure and establish by analysis or test that the cause would not prevent the OEI rating power from being satisfactorily achieved in service.
k. Section 33.93 Teardown inspection.
(1) After the endurance test of § 33.87(f), the applicant should show that no failure of any significant engine component is evident during the test, shutdown, or subsequent teardown inspection. If a failure is evident, the applicant should analyze the failure and 9
establish by test or analysis, or both, that the cause is corrected, or certain limitations are imposed to the engine as appropriate. For this section, the engine parts that are deemed significant are those that affect structural integrity, including, but not limited to mounts, cases, bearing supports, shafts, and rotors.
(2) For components that are distressed beyond serviceable limits by this test, the applicant must show that the inspections and mandatory maintenance actions for these components specified in the Instructions for Continued Airworthiness (ICA) are adequate for maintaining continued airworthiness. The instructions should include means for proper identification of the structural and functional condition of these components and the appropriate maintenance actions. The applicant should identify component deterioration during the test and the effects on the engine performance. The component distress seen as a result of the test in § 33.87(f) should not indicate any potentially hazardous condition. The visible and non-visible physical damage should be identified and assessed, including the effects of creep, stress rupture, metallurgical effects, and life usage. The results of such evaluation should then be considered when defining and justifying the inspections and mandatory maintenance actions for the ICA.
l. Section 33.4 Instructions for Continued Airworthiness and Appendix A33.4 Airworthiness Limitations Section.
(1) Inspection and maintenance requirements for rotorcraft engines with 30 second and 2-minute OEI power ratings.
(a) For rotorcraft engines with 30-second and 2-minute OEI ratings, the Airworthiness Limitations Section (ALS) of the ICA is required to prescribe the mandatory post-flight inspection and maintenance actions following the use of either, or both, of these two ratings, regardless of the frequency, and prior to next flight. The ALS should also include the definition of data to be provided by the operator to support the applicant in completing the engine in service evaluation program (see paragraph 5.l.(3)).
(b) The applicant should determine the mandatory post-flight inspection and maintenance actions based on findings during certification tests, such as the endurance, overspeed, overtemperature, and maintenance tests. The applicant may also use data and information acquired during engine development tests or service experience with engines of the same or similar design. The inspection and maintenance actions required under A33.4(b)(1) should account for component deterioration and damage to ensure they are properly identified during the post-flight inspection and that adequate repairs, maintenance, or component replacement actions are prescribed. The adequacy of these inspections and maintenance actions must be validated as required under A33.4(b)(2) and their continued adequacy must be established based on an in-service program required under A33.4(b)(3). The mandatory inspection and maintenance actions may be modified after the engine enters service, based on its service experience.
(c) To ensure the engine is maintained in a condition so that the 30-second and 2-minute OEI rating powers can be attained and sustained, the power assurance 10
procedures and instructions for continued airworthiness should include the following:
1 Recording parameters and times in maintenance records 2 Visual inspection(s) and results 3 Power assurance/trend check 4 Reduction in time to overhaul or component change 5 Addition of special inspection and intervals 6 Module change/overhaul 7 Engine removal/overhaul; and 8 Other maintenance details regarding servicing points, inspections, adjustments, tests, and replacement of components if required.
(d) The 30-second and 2-minute OEI ratings are intended to safely use the available engine design margins for brief periods of exposure with component deterioration permitted beyond serviceable limits and not available for further use. The extent of component damage or life reduction depends on the severity and duration of the operating conditions associated with the use of these ratings, the engine design margins, and prior hardware condition.
(e) Because the use of these ratings requires that engine operation conditions and time durations be recorded, the prescribed inspection and maintenance actions may be directly related to the actual documented usage level, time, and, if applicable, to a known condition prior to the rating(s) usage (for example, hours/cycles/prior rating exposure). Depending on the actual operating parameters, such as temperature and time exposure recorded during the use of these ratings in accordance with § 33.29, it is possible to predefine a maintenance action that reduces the remaining time interval to the overhaul or component replacement. If the inspection mandated for the use of these ratings results in no further maintenance, then the minimum requirement would be the interpretation of recorded event data and documentation of the data in the maintenance log(s).
(f) When usage of 2-minute OEI rating is extended to 2½ minutes, the additional 30-second period is considered as usage of the 30-second OEI rating, and the inspection and maintenance actions prescribed for the 30-second OEI rating should be used. Alternately, for the extended use of the 2-minute OEI power rating to 2½-minute duration, the applicant may seek approval for different inspection and maintenance actions, also subject to the requirements of A33.4(b)(2) and (b)(3). The substantiation and validation of these actions may be based on similar engines having an approved 2½-minute OEI power rating. The applicant must show that the extended use of the 2 minute OEI power rating to 2½-minutes is equivalent to the 2½-minute OEI power rating approved for the other engine. If the applicant records only the accumulated usage time under § 33.29(c)(2), then the inspection and maintenance actions must be based on the total recorded time duration regardless of the number of application at the ratings used in one flight.
(2) Validation of mandatory post-flight inspection and maintenance.
(a) The mandatory inspection and maintenance following the use of 30-second or 2-minute OEI ratings should identify and correct any component distress that could significantly reduce subsequent engine reliability or prevent the engine from achieving or sustaining further application of the OEI ratings. The applicant should provide evidence by endurance test results, analysis based on test data of the endurance tests, and/or certification tests and service experience of similar engine types and designs to show that power at 30-second and 2-minute OEI ratings is achievable and can be sustained for the respective duration during the intervals between overhauls or major maintenance of the engine.
(b) The certification procedures for 30-second and 2-minute OEI ratings demonstrate the design adequacy by endurance testing and by specific margin tests for turbine temperature and rotor speeds. Essential to establishing mandatory inspection and maintenance instructions is a thorough knowledge of the potential damage incurred with use of the 30-second and 2-minute OEI ratings. The applicant should demonstrate a good understanding of the engine operating margins to various failure modes when using the 30-second and 2-minute OEI ratings. These failure modes and margins should be determined and validated by appropriate methods or experience. These may include but are not limited to:
1 Design analytical predictions 2 Service experience of identical or similar design 3 Test or service failure experience 4 Results of OEI certification tests 5 Dedicated engine and component tests; and 6 FMEA predictions.
(c) Understanding of failure modes may come from service experience in which hardware distress or failure was caused by exceeding an operating limitation associated with the rating. Alternatively, either component or engine level exceedence testing could be useful in evaluating failure mode margins, indications, power decay characteristics and severity. Existing methods to establishing failure margins are specific to a certain engine design and experience and, therefore, vary greatly between engine types. One test method that illustrates the objective of assessing the failure margin(s) from the 30-second OEI power rating condition is to progressively increase engine fuel flow until either an abrupt failure occurs or power begins to decay due to component degradation. Results of such a test could establish margins to and consequences of component failure. This could be useful in establishing appropriate maintenance instructions. Potential failure modes are design dependent, however, most are related to excessive turbine temperature or engine overtorque. Some potential failure modes could include but are not limited to:
1 Blade stress rupture 2 Vane distortion/area change 3 Case distortion 12