3-2. Specific Requirements in §§ 33.87(a)(1) through (a)(9). Unless otherwise specified, the guidance in the following paragraphs regarding § 33.87(b) requirements also applies to the corresponding test run requirements of §§ 33.87(c), (d), (e), (f), or (g).
a. Test sequence, § 33.87(a)(1). Applicants should follow the 6-hour endurance test cycle sequence prescribed in §§ 33.87(b) through (e) and the 2-hour test sequence in § 33.87(f) when applicable. We may allow test segments to be run out of their normal sequence when applicants have difficulty with readjusting the engine and test facility configuration to run all test segments sequentially, as prescribed. Applicants intending to run the tests with sequence changes should show that changes to the published endurance test cycle sequence(s) do not lessen the severity of the endurance test cycle. The applicant must also show that all test cycle requirements, including the required number of accelerations to rated power and decelerations back to idle power, were properly completed.
b. Automatic control of the engine, § 33.87(a)(2). Any automatic engine control that is part of the engine type design must control the engine operation during the endurance test, except for operations where manual, instead of automatic control is permitted, or where manual control is otherwise specified for a particular test run. Software used for engine control should be representative of the type design version. See paragraph 3-1.c of this AC for additional guidance related to test engine configuration.
c. Endurance test at redline conditions, § 33.87(a)(3).
(1) During the endurance test, the engine thrust or power, torque, gas temperature, and rotor shaft rotational speed(s) must be maintained simultaneously at least at 100 percent of the values associated with the particular engine operation being tested with exceptions under §§ 33.87(a)(5), (a)(6), and (a)(7). This means that applicants must run the endurance test to the maximum permissible (or redline) rotor speeds, gas temperature, and rated power, torque, or thrust values proposed for certification and prescribed in the engine TCDS. These parameters should achieve steady-state operating values with minimal fluctuations or cyclic type variations, and with any minimum variations remaining at or above the 100 percent values to be certificated.
(2) At each rating condition during the endurance test, the test engine must maintain rated power, thrust, or torque, which are the minimum physical power, thrust, or torque values meeting the definition in 14 CFR 1.1 and the requirements of §§ 33.7(a), (c), and 33.8(b). The rated power, thrust, or torque value that is prescribed in the TCDS is normally referenced to sea level, static, standard day pressure (14.696 psia or 101.3254 kilopascals) and the rated air temperature, with no customer air bleed or horsepower extraction allowed, an ideal inlet (100 percent ram pressure recovery), and engine inlet and exhaust test hardware as specified in the TCDS. See paragraphs 3-4.a.(2)(a) and (2)(b) of this AC for power or thrust setting adjustments and permitted power or thrust shortfalls.
(3) If the engine cannot achieve the rated minimum physical thrust or power requirement, then the applicant should present data or analysis that shows that any difference is inconsequential to the durability of those engine parts that are subject to thrust or power loads.
We will determine whether the showing is adequate or whether it requires an equivalent level of safety in accordance with § 21.21(b)(1).
(4) The ambient test conditions or the test engine configuration may not allow the engine to reach the triple redline test condition. In this case, the applicant may conduct more than one test, if necessary, so that all of the required testing at redline conditions is completed.
However, all endurance testing should be conducted on a single set of engine hardware.
(a) When the applicant proposes more than one endurance test, the second test should be run at the rated thrust with at least two redline conditions maintained simultaneously.
For turbopropeller and turboshaft engines, we will base the approval of torque and output shaft speed limits on those limits demonstrated simultaneously during the endurance test.
(b) Appendix 3 of this AC presents an example of an acceptable two-part test.
(5) If a parameter limit, such as rotor speed, for a particular engine rating, such as maximum continuous, is not defined, then the applicant should run the test segments associated with that rating condition to the maximum engine redline condition, as defined in the TCDS.
(6) For a transient overtemperature having the time limit greater than 30 seconds and less than or equal to 2 minutes, the applicant should apply the 100 percent gas temperature limit required by 33.87(a)(3) to the value of the transient overtemperature for the duration proposed.
The applicant should apply this transient overtemperature for each test period at takeoff power or thrust conditions, and for the entire time of all the 30-second periods at takeoff power or thrust, unless § 33.87(a)(7) applies. See appendix 7 of this AC for guidance on two-minute transient overtemperature limit approval.
d. Fuel and lubricating oil for endurance test, § 33.87(a)(4).
(1) The fuel, lubricating oil, and hydraulic fluid used for the endurance test must conform to the fuel, lubricating oil, and hydraulic fluid requirements listed in the TCDS and installation instructions.
(2) To verify that the lubrication system functions within design intent, the applicant should monitor oil consumption during the endurance test and summarize the results in the certification report. To verify oil type and quality, the applicant should take oil samples before and after the test, and analyze these samples to evaluate any oil property changes. The applicant should analyze the post-test sample for the presence of any metallic particles or other contamination that may indicate the deterioration of one or more engine parts. The applicant should disclose any evidence of oil property degradation or particle contamination in the test report. Additional oil sampling should be done during the test. The additional sampling will assist oil monitoring for particle contamination, which in turn will help to identify the beginning of significant part deterioration. The oil consumption rate, the system pressure, and temperature operating limits that were demonstrated during the endurance test should be consistent with any limits prescribed in the engine installation or operating instructions.
(3) The fuel characteristics should be consistent throughout the test and conform in all respects to the specifications prescribed in the TCDS. Applicants should disclose and justify any fluid property differences from these specifications, including the use of any additives. To establish a baseline, the applicant should take a fuel sample from the engine fuel supply line prior to the initial power or thrust calibration check. We may require additional sampling during the endurance test if fuel is used from sources other than the one from which the initial sample was taken.
e. Maximum air bleed testing, § 33.87(a)(5).
(1) The applicant should ensure that the maximum permissible total air bleed be extracted from the engine during at least five of the 25 endurance test cycles, except for the 30 second and 2-minute OEI test run under 33.87(f) (see paragraph e.(6) below). For these five test runs with the maximum air bleed, the regulation permits the power, thrust, or rotor speeds to be less than their 100 percent redline values; however, the gas temperature must be maintained to at least the 100 percent redline value. Any power, thrust, or rotor shaft rotational speed reduction should not exceed the reductions due to bleed extraction if the engine were in the type design configuration.
Note: We recommend that only five of the 25 endurance test cycles be run with maximum permissible air bleed flow to comply with the intent of § 33.87(a)(5). Running more than five of the endurance test cycles with maximum permissible air bleed flow may not allow the engine to demonstrate compliance to the intent of § 33.87(a)(3) and, therefore, should be avoided.
(2) In the test plan, the applicant should specify the total compressor air bleed flow rate limit and the proportional split from each compressor air bleed port on engines having multiple bleed ports. Examples of aircraft service air bleed uses are wing and cowling anti-ice, aircraft environmental control systems (for example, air conditioning), and engine cross-bleed starting.
During the endurance test, the applicant must demonstrate the maximum allowable air bleed flow rate and the individual air bleed port flow limits specified in the TCDS and in the § 33.5 manual.
(3) On engines with multiple air bleed ports, the air bled from each port should be extracted simultaneously at rates consistent with normal engine operation and design intent. The combined extraction should equal the total air bleed flow limit consistent with the rating condition being tested.
(4) During the test runs with maximum air bleed, the power or thrust or the rotor speed(s), or both, may be less than 100 percent of the value associated with the test condition, provided the following conditions are met:
(a) Any power or thrust or rotor shaft rotational speed reduction should not exceed reductions that would be due to air bleed extraction if the engine were in the type design configuration.
(b) The gas temperature must be maintained at least at 100 percent of the redline value at each power rating to be certified.
(5) The secondary airflow system, including the amount of air bleed for internal engine hardware cooling, should function as intended by the type design.
(6) For the test in § 33.87(f), an exception is permitted by the regulation to reduce test complexity and improve the flexibility necessary to attain key parameters, such as speed, temperature, and torque. For this test, applicants do not need to use maximum air bleed for engine and aircraft services if they show by test, or analysis based on test, that the engine’s ability to meet the teardown inspection requirements under § 33.93 is not enhanced. See paragraph 3-8.f. of this AC for guidance regarding what the analysis should include.
f. Accessory drive and mounting attachment loads, § 33.87(a)(6).
(1) Each accessory drive and mounting attachment must be loaded throughout the tests, either with the equipment listed in the applicant's assembly drawing or with facility units of a similar type. An exception is permitted by 33.87(a)(6)(iii) for the 30-second and 2-minute OEI test run under § 33.87(f) (see paragraph f.(5) below). The intent of this requirement is to simulate power extractions, weights, and overhung moments of the type design hardware for all operating power or thrust levels throughout the duration of the endurance test. However, the load imposed by each accessory used for aircraft service only should be the limit load consistent with rated power or thrust levels to be certified. The required loads for all other power or thrust levels are the normal operating loads.
(2) The accessory drives referred to above are typically for aircraft supplied equipment, such as starter, generator, gearbox spare drive, and engine components supplied by an airframe manufacturer. The mounting attachment points, including the mounting pads for accessory drive(s) and static structures, must also be loaded. Engine accessories, which are part of the engine type design, must be loaded for the conditions being tested.
(3) For any accessory drive and mounting attachment that cannot be adequately loaded or substantiated by the endurance test, the applicant should conduct additional component tests on a rig in accordance with §§ 33.87(a)(6) and 33.91, Component test, to establish that the components can function reliably in all anticipated engine operating conditions. For example, the main accessory gearbox may require both endurance rig testing and engine endurance testing to show compliance with § 33.87.
(4) When a gearbox rig test is necessary, the rig test is generally run at maximum drive-pad loading, maximum power extraction loading, and at extreme operating and environmental conditions to be approved. These extreme conditions may be outside of the engine endurance test set-up capability. In this example and during the engine endurance test, the gearbox drive position may have facility hardware, either actual or nonfunctioning, mounted to represent its weight and overhung moment.
(5) The applicant does not need to load the accessory drives and mounting attachments when running the test required for the 30-second and 2-minute OEI ratings under 33.87(f). In this case, the applicant should show that there is no effect on the durability of any accessory drive or engine component. If the accessory drives and mounting attachments are not loaded, the equivalent power should be added to the required power at the output drive. This will ensure 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.
g. Redline gas temperature or oil temperature exclusion, and fuel, oil, and hydraulic fluid tests, § 33.87(a)(7).
(1) The regulation allows for an exception from testing at the 100 percent oil or exhaust gas temperature “where test periods are not longer than 5 minutes and do not allow stabilization.” This exception applies if at the end of any test period of 5 minutes or less, the type design characteristics of the engine are such that either the oil or exhaust gas temperature are still increasing and have not yet achieved their limiting value. To satisfy the requirements of this exception, the applicant should provide supporting data showing that these are the engine’s normal engine operating characteristics.
(2) The term “stabilization” of this requirement means that engine operating parameters, such as oil temperature or gas temperature, have achieved steady-state operating values with minimal fluctuations or cyclic type variations and any minimum variations remain at or above the 100 percent value to be certificated.
(3) To establish the time duration it takes for parameters to stabilize, the applicant should start measuring time from when the throttle is first moved from the minimum idle position toward the takeoff power position. Move the throttle from the minimum idle position to the takeoff power position within one second.
(4) The exception to the § 33.87(a)(7) requirement to maintain the gas temperature at the 100 percent or redline value does not apply to the 5-minute or longer runs at takeoff power or thrust during the 5-minute test periods in §§ 33.87(b)(1), (c)(1), (d)(1), and (e)(1). Nor does it apply to the 30-minute test periods in §§ 33.87(b)(2)(ii), (c)(2)(ii), (d)(2)(ii), and (e)(2). This means that the gas temperature must be stabilized at or above the 100 percent redline value before those 5-minute and 30-minute test periods, respectively, can begin. For most engines, the gas temperature does not generally require a 5-minute run period to stabilize at the redline value.
(5) For rotorcraft engines, the exception to the § 33.87(a)(7) requirement to maintain the gas temperature at the 100 percent or redline value to be certificated does not normally apply to the test periods rated continuous OEI, rated 30-minute OEI, 2 ½-minute OEI, 2-minute OEI, or even 30 second OEI runs.
(6) To request the exception to the gas temperature stabilization for the 30-second test run segments in § 33.87(b)(5), the applicant should follow the conditions specified in paragraph
3-4.b.(4)(b) of this AC. Similarly, for exceptions to §§ 33.87(c)(6), (d)(6), and (e)(2), follow the guidance as provided in the corresponding paragraphs 3-5.f, 3-6.e, and 3-7 of this AC.
(7) If the applicant demonstrates that the gas temperature stabilization exception is permitted during the 30-second test run segments in §§ 33.87(b)(5), (c)(6), (d)(6), and (e)(2), then the total run time that can be claimed at takeoff redline gas temperature should be reduced by the 1.25 hour duration of these segments. For example, the total run time that may be claimed at the takeoff redline gas temperature for compliance with § 33.87(b)(5) is reduced from 18.75 hours (accounting for the §§ 33.87(b)(1) and (2)(ii) segments and § 33.87(b)(5) segments) to
17.5 hours (which accounts for the §§ 33.87(b)(1) and (2)(ii) segments only). We will, however, still accept this reduced takeoff power redline run time to meet the redline gas temperature run time requirements of §§ 33.87(a)(3) and (a)(8).
(8) During the test segments run at gas temperature below the 100 percent redline, because the gas temperature stabilization exception is permitted, the applicant should maintain the rated thrust or power with rotor speeds (i.e., N1 and N2) at or above redline values.
(9) The applicant should qualify the engine with the same exhaust gas temperature stabilization characteristics that it will display in service.
(10) The applicant should control the engine using the same exhaust gas temperature measurement and control system that it will use in service.
(11) At least one endurance test cycle must be run with fuel, oil, and hydraulic fluid at the minimum pressure limit, and at least one endurance test cycle must be run with fuel, oil, and hydraulic fluid at the maximum pressure limit. During each of these two cycles, the fuel, oil, and hydraulic fluid pressures must be maintained at their maximum or minimum values simultaneously. The applicant may artificially adjust the fluid temperature with a test facility heat exchanger to achieve the desired value, or may use other means if it is shown not to affect the test outcome.
h. Transient overspeed, overtemperature, and overtorque demonstrations for a maximum period of 30 seconds, § 33.87(a)(8).
(1) Requested transient conditions that are being certified as part of an engine rating limitation must be demonstrated during the acceleration cycles required by §§ 33.87(b) through
(f), as applicable.
(2) A transient condition, referred to in § 33.87(a)(8), is rotor speed, gas temperature, or shaft torque value that exceeds the approved limit for a period of 30 seconds or less for transients associated with the takeoff, continuous OEI, and 30-minute OEI ratings. Applicants should limit rotor speed, gas temperature, or shaft torque transients associated with the 2 ½- minute, 2-minute, and 30-second OEI ratings to very brief periods of 10 seconds maximum.
Note: The FAA has approved transient overtemperatures that exceeded 30 seconds and up to 2 minutes. However, § 33.87(a)(8) is intended and remains suitable to evaluate rotor speed and gas temperature transient overtemperature for periods of 30 seconds or 15
less. See appendix 7 of this AC for guidance on two-minute transient overtemperature limit approval.
(3) Applicants may not use transient limits as supplementary limitations, regardless of their duration, for engine power setting purposes. The transient limits of shaft overspeed, gas overtemperature, and shaft overtorque prescribed in §§ 33.7(c)(14), (c)(15), and (c)(16), respectively, are intended for a transitory overshoot before reaching steady-state limit values following an engine acceleration in normal operation.
(a) If the number of occurrences of transient conditions up to 30 seconds is limited in normal engine operation, then the same number of accelerations required by §§ 33.87(b) through (g) must be made to the limiting transient conditions during the endurance test.
(b) If the number of occurrences of the transient conditions is not limited in normal engine operation, then at least 50 percent of the required endurance test accelerations must be made to the limiting transient conditions. For example, §§ 33.87(b)(1), (b)(2), and
(b)(5) require a total of 310 accelerations (150, 10, and 150, respectively) to takeoff power or thrust. Therefore, a minimum of 155 (50 percent of 310) accelerations must be made to the limiting takeoff power transient conditions during the endurance test.
(c) Section 33.87(a)(7) defines the basis for a possible exception to the requirement under § 33.87(a)(3) to maintain gas temperature at the 100 percent, or redline value, to be certificated during all takeoff test segments. Although an applicant may present data showing that an engine cannot achieve a stabilized takeoff redline gas temperature during the 30-second takeoff power segments under § 33.87(b)(5), these accelerations to takeoff power still count toward the total 310 accelerations required during the endurance test.
(4) We recommend conducting the transient condition test runs during the 150 accelerations to the 5-minute takeoff test segments of §§ 33.87(b)(1), (c)(1), (d)(1), and (e)(1) and during the 10 accelerations to the 30-minute takeoff test segments of §§ 33.87(b) (2)(ii), (c)
(2)(ii), (d) (2)(ii), and (e) (2). Some turbine engines, particularly large turbofan models, may not reach a stabilized gas temperature during the 30-second takeoff power segments of the test sequence under § 33.87(b)(5). We do not recommend attempting to run the transient tests during these segments, unless the transient duration to be certified is short enough to allow gas temperature stabilization at the required transient condition.
i. Additional test requirements for supersonic aircraft engines, § 33.87(a)(9). We may address the test requirements for supersonic aircraft engines in a future AC.
3-3. Special Topics.
a. Endurance test cycles for major design changes, parts manufacturer approvals (PMAs), repairs, and alterations.
(1) Applicants should consider the endurance test as an essential part of all major design change approvals, i.e., amended type certificates (TCs), supplemental type certificates 16
(STCs), and amended STCs. See appendix 8 of this AC for guidance on compliance with § 33.87 for engine TCs and major design changes, i.e., amended TCs, STCs, and amended STCs.
(2) Parts manufacturer approvals and major repairs and alterations may require running certain endurance testing for compliance with § 33.87. Advisory Circular 33.87-2, provides guidance on showing compliance with § 33.87 for PMAs by the means of comparative test methods.
b. Endurance test for multiple engine testing.
(1) When all engine parameters (thrust, power, torque, speed, and gas temperature) cannot be held at redline conditions simultaneously during the endurance test, then the applicant may conduct more than one test, so that all required testing is completed at redline conditions.
However, applicants should conduct all endurance testing on the same set of engine hardware to be presented for certification.
(2) Appendix 3 of this AC presents additional information on an acceptable endurance test cycle for multiple engine tests.
3-4. Endurance Test Schedule, § 33.87(b), for Engines Other Than Certain Rotorcraft Engines.
a. Explanation.
(1) The applicant must perform this endurance test cycle 25 times, for a total of 150 hours of testing on all turbojet, turbofan, and turbopropeller engines. The endurance test cycle is also required for rotorcraft turbine engines, except those rotorcraft engines for which the 30-minute OEI rating, continuous OEI rating, or 2½-minute OEI rating are desired. The 150 hours of testing time will accumulate 18.75 hours at takeoff power rating, 45 hours at maximum continuous power rating, 62.5 hours at incremental power step time period, and 23.75 hours at minimum idle condition. This endurance test cycle for § 33.87(b) is presented graphically in appendix 1, figures 1 and 2 of this AC.
(2) The test engine must maintain the rated power or thrust. Advisory Circular 33.7-1 provides information and guidance on the engine ratings and associated limitations. Specifically, the methods, ambient conditions, and engine configuration the applicant should use to establish and demonstrate the ratings that are then applied during the endurance test.
(a) The minimum physical power or thrust value required for testing under § 33.87(b) may be adjusted from the actual testing conditions to the conditions specified in the TCDS under which the rated power or thrust is determined. Applicants are not allowed to adjust the physically measured power or thrust to account for losses due to special test hardware that is incidental only to the conduct of the test. Special test hardware, such as fan inlet or exit guide vanes, an adjustable fan exhaust nozzle, or an altitude test cell, may be required to achieve simultaneous rotor shaft rotational speed and gas temperature redline conditions; however, the engine may not be able to produce physical rated power or thrust under these operating 17
conditions. The shortfall in physical power or thrust may result in insufficient loading on loadcarrying members of the engine, such as thrust bearings, struts, mounts, and cases to comply with the requirements under § 33.87(a)(3).
(b) The applicant may propose data to justify the power or thrust shortfall when the minimum physical power or thrust target cannot be achieved. The data should address the engine parts that are affected by the power or thrust magnitude and should show that their durability is not affected. We will review the proposal to see if it needs to be evaluated as an equivalent level of safety under § 21.21(b)(1).
(c) For all test conditions, the applicant should identify in the test plan the parameters relevant to the purpose of the test together with the appropriate times for their recording during the test. Except during transient conditions, the applicant should allow the engine to stabilize before recording the data.
b. Guidance. Unless we approve an alternate sequence, the test must include the following runs defined in §§ 33.87(b)(1) through (b)(6). All accelerations to rated power or thrust must start from minimum idle power condition and all decelerations must return to minimum idle power condition.
(1) Section 33.87(b)(1), Takeoff and idling. The intent of this requirement is to expose the engine hardware to the maximum cyclic thermal and mechanical stresses associated with accelerations and decelerations between the lowest operating power or thrust condition (minimum idle) and the maximum operating power or thrust condition (takeoff). All certifying parameters, such as rated thrust or power or rated torque for rotorcraft and turbo-propeller engines, maximum permissible speeds and gas temperature, should stabilize for 5 minutes at the takeoff power condition redline values. Similarly, power or thrust should be maintained for 5 minutes at the minimum idle condition following stabilization.
(a) Applicants should conduct one hour of alternate 5-minute periods at rated takeoff power or thrust and at minimum idle power or thrust with the following requirements:
1 Prior to beginning any of the 5-minute takeoff segments, the physical rotor speeds, indicated gas temperature, and physical thrust or power should be stabilized at or above the takeoff power values to be certified per § 33.87(a)(3), unless the applicant elects to run more than one 150-hour endurance test. See appendix 3 of this AC for multiple endurance test requirements.
2 Prior to beginning any of the 5-minute minimum idle segments of the test run, the physical rotor speeds and physical power or thrust should be at or below the minimum idle power operating values to be certified. While the indicated gas temperature may continue to cool after the 5-minute minimum idle part has begun, it should stabilize at the nominal minimum idle power operating temperature consistent with the ambient test conditions prior to beginning the next acceleration to the takeoff thrust or power level.
(b) For engines with augmented takeoff thrust ratings that involve an increase in turbine inlet temperature, rotor speed, or shaft power, all test runs at takeoff power or thrust must be at the augmented takeoff rating. For engi nes with augmented takeoff thrust ratings that do not materially increase operating severity:
1 If use of the rating is not limited, all takeoff runs must be made at the augmented rating, and 2 If use of the rating is limited, the limited number of runs conducted at the augmented rating must be made at the augmented rated thrust.
(c) In complying with § 33.87(b)(1), the power-control lever should be moved from one extreme position to the other in one second or less. If the applicant incorporates different regimes of control operation requiring control scheduling, then a longer period of time is acceptable, but for not more than two seconds. Examples of different modes of control operations requiring control scheduling are separate stops or detents in the power-control level lever for another rating, which is incorporated between idle and takeoff thrust or power, and which requires scheduling of the power-control lever motion in going from one extreme position to the other.
(d) All accelerations should start from the minimum idle power or thrust, and all decelerations should return to the minimum idle power or thrust.
(2) Sections 33.87(b)(2) and (b)(3), Rated maximum continuous and takeoff thrust or power.
(a) In complying with these two requirements, the engine must run 15 of the 25 endurance test cycles at the rated maximum continuous thrust or power for two hours during each cycle. The engine must run the remaining 10 of the 25 endurance test cycles at the rated takeoff power or thrust for 30 minutes followed immediately by running at maximum continuous power or thrust for 1.5 hours during each endurance test cycle.
(b) Prior to beginning the test run at either the maximum continuous rating or the takeoff rating, the physical rotor speeds, indicated gas temperature, and physical thrust or power must be at or above the redline values for each rating to be certificated.
(c) If the applicant does not plan to define redline rotor speeds for the maximum continuous rating, then the takeoff redline rotor speeds will also be the required redline rotor speeds for the maximum continuous rating segments.
(d) The cycle profile shown in figure 1 in appendix 1 of this AC depicts an apparent acceleration from idle to the maximum continuous power or thrust to begin the 30-minute segments that are required for 15 of the 25 cycles under § 33.87(b)(2)(i); however, that depiction is only used to simplify the presentation of the cycle profile. These 30-minute segments may also be started after a deceleration from the last 5-minute takeoff power or thrust segment under § 33.87(b)(1). A rapid acceleration using a one-second throttle movement from 19
idle to maximum rated power or thrust is not required to start these 15 cycles, unless the applicant requested to use those accelerations for additional transient accelerations.
(e) The cycle profile shown in figure 2 in appendix 1 of this AC depicts an acceleration from idle to takeoff rated power or thrust prior to starting the ten 30-minute segments prescribed in § 33.87(b)(2)(ii). These 10 accelerations are an integral part of the requirement to complete 310 accelerations to takeoff power or thrust in the test under § 33.87(b).
(3) Section 33.87(b)(4), Incremental cruise power and thrust.
(a) In complying with this requirement, the applicant must run all 25 of these segments for 2 hours and 30 minutes at equal speed and time increments by moving the power lever. Specifically, the successive power lever positions must correspond to at least 15 approximately equal speed and time increments between maximum continuous engine rotational speed and minimum idle rotational speed. However, if the engine exhibits significant peak vibration anywhere between the minimum idle and maximum continuous rating conditions, the applicant should increase the number of increments to increase the amount of running time made while subject to the peak vibrations, up to no more than 50 percent of the total time spent in incremental running. Applicants may vary thrust and power in place of rotor speed for engines operating at constant rotor speed.
1 If the applicant finds any significant vibration within the operating range of the engine, then the relevant parts or components should be subject to sufficient vibration running time at, or close to, the resonance peak speed. The vibration characteristic data for each rotor system may be obtained from the vibration test conducted under § 33.83. The running time allocated for any peak vibration should be sufficient to accumulate at least the minimum vibration cycles for endurance limit. Based on industry practices, the recommended minimum number of dwell cycles at each step is 10 million. The required dwell time spent at each selected speed depends on the frequency of the vibration resonance. The applicant may calculate the test time by dividing the number of cycles by the frequency.
2 Any engine vibration that occurs within the declared operating speed ranges, including transients, is considered significant when it occurs at or near a known resonance point, or within a steady-state speed operation and presents any of the following characteristics:
(aa) Amplitudes that indicate stresses near the endurance limits, or (bb) Amplitudes or frequencies that may affect the functionality of engine rotors, systems, or components.
3 The relevant engine parts or components for vibration dwell consideration under § 33.87(b)(4) should include internal and external components of the engine. For example, consider blades, stators, turbine or compressor assemblies, pumps, and the oil tank.
(4) Section 33.87(b)(5), 30 minutes of accelerations and deceleration runs.
(a) In complying with this requirement, all 25 endurance test cycles must contain one 30-minute run per cycle consisting of alternating 30-second periods at rated takeoff thrust or power immediately followed by 4.5 minutes at idling thrust or power.
1 All accelerations should start from minimum idle thrust or power and all decelerations should return to minimum idle thrust or power. During runs at rated thrust or power, if the redline gas temperature is expected to stabilize within a 30-second period, then the physical speeds, indicated gas temperature, and physical thrust or power must be at or above the redline values and must be maintained for at least 30 seconds as required under § 33.87(a)(3). If the redline gas temperature does not stabilize within 30 seconds, then only the rated takeoff thrust or power and rotor speeds need to be maintained at or above their 100 percent values to be certified during each 30-second period. The power lever control movement requirements specified in paragraph 3-4.b.(1)(c) of this AC should be followed for all test runs.
2 Prior to beginning any of the 4.5-minute minimum idle parts of the test run, the physical speeds and physical power or thrust should be at or below the proposed minimum idle power operating level. While the indicated gas temperature may continue to cool after the
4.5-minute minimum idle run part has begun, it should be stabilized at the nominal idle power operating temperature prior to beginning the next acceleration to the takeoff thrust or power level.
(b) Sections 33.87(a)(3) and 33.87(a)(7) require that the gas temperature and oil inlet temperature be at least at 100 percent of the value for each rating condition being tested to be certificated. Section 33.87(a)(7), however, permits an exception to this requirement for test segments that are not longer than five minutes and do not allow these parameters to stabilize within those segments at their 100 percent or redline value. Experience has shown that the gas temperatures of many turbine engines do not stabilize during these 30-second takeoff runs under § 33.87(b)(5). For these engines, the applicant may elect to use this option. If the applicant does not run this part of the test to redline gas temperature, then the total steady-state run time that can be claimed at takeoff redline gas temperature will be reduced from 18.75 hours to 17.5 hours.
Before the applicant can use the gas temperature stabilization exception, the following conditions apply:
1 The applicant should supply data in the test plan showing that the time required to achieve gas temperature stabilization at or above the 100 percent value exceeds a 30 second duration in the type design configuration, e.g., a plot of gas temperature vs. time for a typical non-bleed cycle acceleration from idle to takeoff power or thrust for a type design engine.
2 If this data will not be available until the engine is installed in the endurance test facilities, then the test plan should at least contain a prediction of the engine acceleration characteristics and a statement that the exclusion under § 33.87(a)(7) will be sought for the 30-second test run segments under § 33.87(b)(5). The actual acceleration vs. time plots should then be included in the test report.