3-7. Endurance Test Schedule, § 33.87(e), for Rotorcraft Engines for which a 2½-minute OEI Power Rating is Desired.
a. Explanation.
(1) The maximum gross weight of a multiengine rotorcraft is limited by the power available from the remaining operating engine(s) when one engine fails or is shutdown during flight. Flight performance analysis has shown that in the event of engine failure at the critical point during take-off or landing, a period of higher power, referred to as 2½-minute OEI power, is required to lift the aircraft, gain forward speed, clear obstructions in the flight path, and climb to a safe altitude.
(2) Once the aircraft has reached a safe altitude, a longer period at a lower power, which is equal to or higher than maximum continuous power, is required to continue the fight until able to reach a suitable landing site. This power level will be rated maximum continuous, 30-minute OEI, or continuous OEI power depending on the rating structure of the engine. For these reasons, the endurance test schedules for the 2½-minute OEI rating are structured to run under one of the following three combinations of ratings:
(a) Rated takeoff, maximum continuous, and 2½-minute OEI, or
(b) Rated takeoff, maximum continuous, 2½-minute OEI, and 30-minute OEI, or
(c) Rated takeoff, maximum continuous, 2½-minute OEI, and continuous OEI.
(3) See appendix 9 of this AC for more information on rotorcraft operation at the 2½ minute OEI condition.
(4) The schedules prescribed in §§ 33.87(e)(1) and (e)(2) are required for rotorcraft engines for which the applicant desires a 2½-minute OEI power rating. The applicant should select one of the three combinations of ratings in paragraphs 3-7.a.(2)(a), (2)(b) and (2)(c) above for the engine model being certified and apply the associated test schedules for compliance with endurance test requirements.
(5) See figures 7 through 11 in appendix 1 of this AC for a graph representation of the § 33.87(e) test schedules.
b. Guidance. The test must include the runs specified in §§ 33.87(e)(1) and (e)(2), unless we approve an alternate test sequence. The guidance described in paragraphs 3-4.a.(2) of this AC for the required minimum physical power and the general endurance test requirements prescribed in § 33.87(a), apply to this test schedule.
(1) Section 33.87(e)(1), Takeoff, 2½-minute OEI and idling. This guidance is the same as that given for § 33.87(b)(1) under “Guidance” in paragraph 3-4.b.(1), except that during the third and sixth takeoff power periods in each of the alternate 5-minute periods, only 2.5 minutes 26
need be conducted at rated takeoff power. The applicant should conduct the remaining 2.5 minutes at rated 2½-minute OEI power.
(2) Section 33.87(e)(2). In complying with this paragraph, the applicant should select one of the options in the following paragraphs, depending on the rating structure of the engine:
(a) For engines with rated takeoff, maximum continuous, and 2½-minute OEI power ratings, the guidance is the same as that given for §§ 33.87(b)(2) through (b)(6) under “Guidance” in paragraphs 3-4.b.(2) through b.(5) of this AC, except that in the last one of the 25 endurance test cycles under § 33.87(b)(2)(ii), the last 5 minutes must be run at 2½-minute OEI power rating.
(b) For engines with rated takeoff, maximum continuous, 2½-minute OEI, and 30-minute OEI ratings, the guidance is the same as that given for § 33.87(c)(2) through (c)(7) in paragraphs 3-5.c through 3-5.f of this AC, except that, during the last of the 25 endurance test cycles, the last 5 minutes of § 33.87(c)(4) 30 minutes run at the 30-minute OEI power must be run at the 2½-minute OEI power.
(c) For engines with rated takeoff, maximum continuous, 2½-minute OEI, and continuous OEI ratings, the guidance is the same as that given for § 33.87(d)(2) through (d)(7) described in paragraphs 3-6.b through 3-6.e of this AC, except that during the last of the 25 endurance test cycles, the last 5 minutes of § 33.87(d)(4) one hour run at continuous OEI must be run at 2½-minute OEI power rating.
(d) See appendix 10 of this AC for the accumulated testing times for 150 hours of testing to these three schedules.
3-8. Endurance Test Schedule, § 33.87(f), for Rotorcraft Engines for which 30-second OEI and 2-minute OEI Ratings are Desired.
a. The 30-second OEI and 2-minute OEI power ratings. When used in combination, t he 30-second OEI and 2-minute OEI power ratings enable rotorcraft to perform a mission very similar to that of the 2½-minute OEI power rating described in section 3-7.a, “Explanation,” of this AC. There are, however, significant differences between these ratings.
(1) The use of the 2½-minute OEI rating is not limited to a certain number of occurrences, while the use of the 30-second OEI and 2-minute OEI ratings is limited to three occurrences during a single flight. In service, use of the 30-second OEI and 2-minute OEI ratings is likely once per flight with a required mandatory inspection after that flight. However, we have defined certification requirements around the worst-case scenario involving the possible use of the rating three times in one flight; takeoff, balked landing, and final landing of a rotorcraft. The intent of the 30-second OEI and 2-minute OEI ratings is to safely use available engine design margins for brief periods of exposure.
(2) Inspection and maintenance actions are not required after a flight when the 2½-minute OEI rating is used. Inspection and maintenance actions, however, are required after a 27
flight when either the 30-second OEI or the 2-minute OEI rating is used. This difference is supported by the pass or fail criteria of the teardown inspection requirements in § 33.93.
Following the endurance test for the engine’s 2½-minute OEI rating, the teardown inspection must show that all engine parts are in a condition for safe operation in compliance with § 33.93(a)(2). Following the endurance test for the engine’s 30-second OEI and 2-minute OEI ratings, the teardown inspection must show that the engine maintains its structural integrity in compliance with § 33.93(b)(2). Note that use of the 30-second OEI and 2-minute OEI ratings may result in component or part deterioration beyond serviceable limits, making these components unavailable for further use. See appendix 9 of this AC for more information on use of 30-second OEI and 2-minute OEI ratings for rotorcraft.
b. Endurance test for 30-second and 2-minute OEI ratings. The endurance test comprises of the 150 hour test run under §§ 33.87(b), (c), (d), or (e), as applicable to the engine OEI ratings structure, followed by a supplementary 2-hour test under § 33.87(f). See figures 12 and 13 in appendix 1 of this AC for graph representations of the § 33.87(f) test schedules.
(1) Following the tests in § 33.87(b), (c), (d), or (e), the applicant may disassemble the tested engine to the extent necessary to show compliance with the requirements of § 33.93(a).
The applicant must then reassemble the tested engine using the same engine parts used for the 150-hour endurance test run under §§ 33.87(b), (c), (d), or (e), except for those parts that are defined as consumable in the ICA. The level of component cleaning to facilitate the inspection prior to rebuild for the test under § 33.87(f) should be acceptable to us. The applicant should show that any cleaning during the teardown inspection, or replacement of consumable parts, will not enhance the engine’s ability to meet the requirements under § 33.93(b).
(2) The applicant may elect to continue the 2-hour test under § 33.87(f) without an intervening disassembly and inspection. In this case, the applicant must show that, after the test, the condition of engine parts and components comply with § 33.93(a) instead of § 33.93(b).
c. Test conduct. All requirements under § 33.87(a) apply to the conduct of the 2-hour test, except as permitted by § 33.87(f) and as discussed further in this section. The required minimum physical power and general endurance test requirements described in section 3-4.a, “Explanation,” of this AC for § 33.87(b) apply to this test schedule. In changing the power setting at the beginning and the end of each test sequence, the applicant should move the power control lever from one position to the other in no more than one second.
d. Test sequences.
(1) The applicant must conduct the prescribed 2-hour test sequence specified in §§ 33.87(f)(1) through (f)(8) a total of four times as follows:
(a) Three minutes at rated takeoff power;
(b) Thirty seconds at rated 30-second OEI power;
(c) Two minutes at rated 2-minute OEI power;
(d) Five minutes at whichever is the greatest of rated 30-minute OEI power, rated continuous OEI power, or rated maximum continuous power, except that this period must be 65 minutes during the first test sequence. However, where the greatest rated power is 30-minute OEI power, that 65-minute period must consist of 30 minutes at 30-minute OEI power followed by 35 minutes at continuous OEI power or maximum continuous power, whichever is greater;
(e) One minute at 50 percent takeoff power;
(f) Thirty seconds at rated 30-second OEI power;
(g) Two minutes at rated 2-minute OEI power; and
(h) One minute at minimum flight idle condition.
(2) The applicant must run the test sequences in §§ 33.87(f)(1) through (f)(8) four times, continuously and without stopping. In the event of a stoppage, the applicant should repeat the interrupted sequence in full, or the sequence may be restarted from the interruption point if there are technical justifications acceptable to us. If we determine that the sequence does not need to be repeated in its entirety, the applicant should restart the test from a point where the engine thermal condition would be the same as at the time of interruption. If we determine that the number of interruptions is excessive, the applicant should repeat the entire test under § 33.87(f).
e. Test time. The accumulated time of two hours of testing is split among four test sequences. The total duration of the first sequence is 75 minutes, while the duration for sequences 2, 3, and 4 is 15 minutes each. The time difference between these sequences is a result of the requirements under 33.87(f)(4). These requirements also dictate different test schedules for the first sequence depending on the other ratings that the certification engine has.
The intent of the power level of test condition under § 33.87(f)(4) is to demonstrate the highest en route OEI or non-OEI power. The following are test time breakdowns among the four sequences accounting for different engine ratings scenarios (see tables 1 and 2 below).
(1) Test time breakdown when (a) the engine does not have a 30-minute OEI rating, or
(b) the 30-minute OEI rating is less than either the continuous OEI, or the maximum continuous ratings, or both (see table 1).
Table 1.
Sequence 1
33.87 section Takeoff (minutes) 30-sec OEI (minutes) 2-min OEI (minutes) Flight idle (minutes) Greatest of continuous OEI or maximum continuous (minutes) 50% takeoff (minutes) f(1) 3 f(2) 0.5 f(3) 2 f(4) 65 f(5) 1 f(6) 0.5 f(7) 2 f(8) 1 Sequences 2, 3, and 4 f(1) 3 f(2) 0.5 f(3) 2 f(4) 5 f(5) 1 f(6) 0.5 f(7) 2 f(8) 1 Total test time (min) 12 4 16 4 80 4
(2) Test time breakdown when the 30-minute OEI r ating is greater than both the co ntinuous OEI and the maximum continuous ratings (see tab le 2).
Table 2.
Sequence 1
33.87 section Takeoff (minutes) 30-sec OEI (minutes) 2-min OEI (minutes) Flight idle (minutes) 30-min OEI (minutes) Greatest of Continuous OEI or maximum continuous (minutes) 50% takeoff (minutes) f(1) 3 f(2) 0.5 f(3) 2 f(4) 30 35 f(5) 1 f(6) 0.5 f(7) 2 f(8) 1 Sequences 2, 3, and 4 f(1) 3 f(2) 0.5 f(3) 2 f(4) 5 f(5) 1 f(6) 0.5 f(7) 2 f(8) 1 Total test time (min) 12 4 16 4 30 50 4
f. Air bleed during test. During the test in § 33.87(f), maximum air bleed for engine and aircraft services under § 33.87(a)(5) do not need to 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. We permit this exception to reduce test complexity and improve the flexibility needed to attain key parameters during the test (rotational speed, temperature and torque). The analysis should include:
(1) The effect of the bleed air extraction on 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).
g. Accessory drives and mounting attachments. If, during testing the accessory drives and mounting attachments are not loaded, the applicant must add the equivalent power extraction to 31
the engine shaft output drive, as required under § 33.87(a)(6)(iii). This is so that the power turbine rotor assembly is operated at or above the same levels as if the accessory drives and mounting attachments were loaded.
h. Engine operating limitations. The engine operating limitations of the 30-second and 2-minute OEI ratings should be based on the minimum values obtained during the application of the 2-hour test under § 33.87(f). Further, the applicant should identify the limitations in the TCDS. The applicant should also account for stabilization time, instrumentation range of accuracy, and automatic controls features declared under §§ 33.29(c) and 33.28(k).
3-9. Endurance Test Schedule, § 33.87(g), Supersonic Aircraft Engines. We may address the test requirements of supersonic aircraft engines in a future AC.
3-10. Endurance Test Schedule, § 33.84, Engine Overtorque Test.
a. Maximum engine overtorque test, § 33.84. These requirements apply to turbopropeller and turboshaft engines that incorporate a free power turbine. The test requirements may be addressed as part of the endurance test of §§ 33.87(b), (c), (d), or (e). This guidance only applies when maximum engine overtorque testing is incorporated in the endurance test schedule of § 33.87. This guidance does not address the alternative tests available under § 33.84(a)(1).
b. Maximum engine overtorque. The maximum engine overtorque is permitted for all engine ratings, except for the 30-second and 2-minute OEI ratings, as stated by § 1.1, General definitions.
(1) The maximum engine overtorque is an "over the limit'' condition that could last up to 20 seconds, and which occurs inadvertently. After this condition occurs, the engine must remain operational without any maintenance action other than to correct the cause. The applicant should identify the failure modes that could produce engine overtorque conditions. For example, sudden changes in the rotorcraft or aircraft blade pitch or power demand, such as an engine failure on a twin engine rotorcraft, can cause a significant decrease in the rotor or propeller speed.
(2) When overtorque conditions may occur in association with more than a single rating, the applicant should declare the highest torque value as the maximum engine overtorque.
The applicant should apply this value during the maximum overtorque test.
c. Test conditions. The required test conditions specified in § 33.84(b) apply to the declared maximum engine overtorque. The applicant should incorporate the test into the endurance sequence for the rated power where the overtorque occurs. For example, when the maximum overtorque is associated with the 30-minute OEI rating, the 15 minute test time required in § 33.84(b)(1) may be incorporated into § 33.87(c)(4), either by running the entire 15 minutes during one of the 25 endurance cycles, or splitting the 15 minutes among the 25 endurance cycles into periods of not less than 2½ minutes each. The following test parameters apply:
(1) The power turbine rotational speed must be equal to the highest speed at which the maximum overtorque can occur in service. It should not exceed the limit speed associated with the rating for which the maximum overtorque is considered.
(2) The turbine-entry gas temperature must be equal to the maximum steady-state temperature approved for use during periods longer than 20 seconds when operating at conditions not associated with 30-second or 2-minute OEI ratings. We may waive the requirement to run the test at the maximum approved steady-state temperature if the applicant shows that other testing provides the desired temperature effects when considered in combination with the other parameters identified in §§ 33.84(b)(1), (b)(2), and (b)(3).
(3) The engine oil temperature should be within the normal operating range. For engines incorporating a reduction gearbox, the gearbox oil temperature must be equal to the maximum temperature when the maximum engine overtorque could occur in service.
d Post-test engine inspection. The engine subjected to the maximum overtorque during the endurance test under § 33.87 must meet the teardown inspection requirements, §§ 33.93(a)(1) and (a)(2), which we discuss further in chapter 4 of this AC.
Chapter 4. Teardown Inspection
4-1. General Requirements for All Endurance Tests (Except for Engines with 30-second OEI and 2-minute OEI Ratings), § 33.93(a). The applicant must completely disassemble each engine after the 150-hour endurance test under §§ 33.87(b), (c), (d), (e), or (g). For rotorcraft engines with 30-second OEI and 2-minute OEI ratings, the applicant may complete the endurance testing under §§ 33.87(b), (c), (d), or (e), and then start the testing under § 33.87(f) without intervening disassembly and inspection. For this option, the applicable teardown inspection requirements are those of § 33.93(a) instead of § 33.93(b). The FAA’s Manufacturing Inspection District Office inspector may need to witness the teardown inspection activities.
a. Section 33.93(a)(1). The applicant must verify that the adjustment setting and functioning characteristics of each engine component are within the limits established and recorded in compliance with § 33.82 before the endurance test. These components include, at the minimum, engine control system components, pumps, actuators, heat exchanger, and valves.
b. Section 33.93(a)(2).
(1) Explanation.
(a) The engine and engine parts or components must conform to their type design consisting of the data and information prescribed under § 21.31. In addition, allowances are permitted for used part conditions provided by the information submitted in compliance with § 33.4 or the TCDS, which includes the engine manual or overhaul manual limits, as applicable.
(b) The phrase, “eligible for incorporation into an engine for continued operation,” means that the installation of the part will continue to keep the engine in an airworthy condition.
An aircraft engine is “airworthy” when it conforms to its approved design and is in a condition for safe operation, see 14 CFR part 21.1(3)(b)(1).
(2) Guidance.
(a) The applicant should inspect all engine parts and components in both “dirty” and “cleaned” conditions using the ICA inspection instructions. The applicant should record the findings in the certification report. The engine parts and components must conform to their type design after accounting for allowances for used parts condition. Appendix 11 of this AC provides more detailed guidance for teardown inspections.
(b) The dirty inspection after the endurance test is necessary to preserve evidence that may be lost after cleaning, such as oil-coked parts, temperature affected parts, metal particles, and soot deposits. The applicant may conduct the dirty inspection at the individual part level, partial assembly, or complete assembly level. The degree of disassembly and dirty layout largely depends on observations made before and during disassembly of the engine or components. The applicant shouldn’t clean any engine part or fully disassemble the test engine without completing the dirty inspection at each step, or otherwise the applicant may request deviation from this guidance.
(c) The clean inspection should include the following:
1 A visual inspection for unacceptable wear, e.g., galling, distortion, or cracks.
2 A nondestructive inspection for cracks or incipient failures.
3 A dimensional inspection for wear, growth, rub, or distortion.
4 A visual, bench, or teardown inspection, or all, of controls, component, and accessory hardware.
(d) The applicant should use the results from the “dirty” and “clean” inspections to determine if the parts conform to the type design and are eligible to be re-installed in the engine for continued safe operation. The manufacturing inspector or designee should validate the applicant’s conformity inspection. The applicant should use the inspection limits in the engine manual or the overhaul manual (overhaul, heavy maintenance, shop manual, off-wing manual), as applicable. The applicant should document any part that no longer conforms to its type design in the test report.
(e) When an engine part exceeds the manual inspection limit(s), the applicant may:
1 Change the manual inspection limit to agree with the results of the endurance test. In this case, the applicant should show that the part conforming to the new manual inspection limits remains in airworthy condition between maintenance periods (e.g., inspections, shop visits, and overhaul) of the engine; or 2 Redesign the part to meet the manual inspection limit; or 3 Maintain the manual inspection limit, and (aa) Modify the frequency and procedures of the engine inspection program in the ICA to ensure that the part is inspected and removed from service before reaching the prescribed limit condition, and (bb) Identify in the test report any part condition that exceeds manual return-to-service limits and justifies part acceptability. The justification may be based on an engineering analysis and/or service experience from a part of similar type design and must show that the part in the existing condition is:
1) Still airworthy;
2) Capable to perform its design functions; and 35
3) Safe for continued engine operation between engine maintenance periods (inspection, shop visit, and overhaul) as scheduled in the ICA.
(f) In the ICA, the applicant may specify inspection and maintenance limits or criteria that are more conservative than those established for compliance with § 33.93.
(g) All engine components must pass post-test inspection and functional tests, such as a calibration test or a component acceptance test, when applicable, to ensure that the components are in a condition for continued safe operation and will perform satisfactorily until the next overhaul. Any component that fails the inspection or functional test, or is a new component using the test under § 33.87 as its sole endurance test, should be torn down for inspection. Newly designed components, components with major modifications, or components existing on other engine models with significantly different operating conditions would generally require a separate component test under § 33.91(a). For the component requiring a separate component test, we may waive the post-test teardown inspection after the tests under § 33.87 if the applicant shows that the component test condition is more severe than the endurance test, and if the teardown inspection will be performed after completing the component test.
(h) Certain repairs may be allowed for the endurance test. These are minor repairs that are normally required to aid in the engine reassembly after overhaul, such as drive shaft or stub shaft mating surface plating for assembly.
4-2. Teardown Inspection Requirements for Engines with 30-second OEI and 2-minute OEI Ratings, § 33.93(b) and (c).
a. After completing the test under § 33.87(f), the applicant should record the components’ adjustment settings and functional characteristics that can be established independent of installation on the engine. The applicant must show that these adjustment settings and functional characteristics remain within the limits that were established and recorded at the beginning of the test.
b. The test under § 33.87(f) for 30-second OEI and 2-minute OEI ratings is run following a 150-hour endurance test under §§ 33.87(b), (c), (d), or (e). Before starting the endurance test of § 33.87(f), the applicant may completely disassemble the test engine to show compliance to § 33.93(a).
c. Following completion of the test under § 33.87(f), the applicant must perform a teardown inspection based on the pass or fail criteria prescribed in § 33.93(b). The applicant must show that no failure of any significant engine component has occurred during the test or during the subsequent teardown inspection. For the purpose of this paragraph, engine components considered significant are those that can affect structural integrity, including, but not limited to, engine mounts, cases, bearing supports, shafts, and rotors. If any failure is seen, the failure should be analyzed. Further, the applicant should establish by test, analysis, or both, that the cause of the failure is corrected or that certain limitations are imposed on the engine.
d. The applicant may choose to not disassemble and inspect the engine after completion of the 150-hour endurance test and prior to starting the supplemental test under § 33.87(f). In this case, the applicant must perform the teardown inspection under § 33.93(a) at the end of the 2-hour supplemental test.
e. The applicant should document in the test report the component and engine part deterioration during the test under § 33.87(f). Such deterioration should not indicate a potentially hazardous condition for the engine. In addition to the damage that is readily visible, the applicant should detect and assess damage that is not visible. Such non-visible damage may include the effects of creep, stress rupture, metallurgical effects, or life usage. The applicant should consider this overall evaluation when defining and justifying the inspections and mandatory maintenance actions in the ICA. The ICA should include means to proper ly identify these component conditions, and should define appropriate maintenance actions for maintaining the continued airworthiness of the engine.