Chapter 1. General
1-1. Definitions. The following definitions apply to the terms used in this AC.
a. Engine component. Systems and accessories that are part of the engine type design.
b. Endurance test cycle. The series of runs or sequences prescribed in each §§ 33.87(b) through (g).
c. Germane or primary hardware. The hardware used in the certification test that is affected by, or directly affects the outcome of the certification test and the pass or fail criteria for the given test objectives.
d. Redline conditions. The highest physical rotor speed, gas temperature, or output shaft torque, including transients, which may not be exceeded during operation at any applicable rating. The type certificate data sheet (TCDS) prescribes the limitations for each approved engine rating.
e. Test hardware. The parts and components assembled on the engine and to be tested, or, if not mounted on the engine, are part of the engine type design. The test hardware may deviate from the type design if it remains representative of the type design and the applicant justifies the differences in the test plan. This justification is frequently referred to as “reconciliation.”
f. Triple redline test condition. The engine operation at simultaneous redline rotor speeds, redline gas temperature, and full rated thrust or torque, as required by § 33.87(a)(3). For example, for a two-shaft system engine, the engine operation at simultaneous redlines of the two rotor speeds (e.g. low pressure and high pressure rotor systems) and redline gas temperature, in addition to full rated thrust or torque.
Chapter 2. Calibration Tests
2-1. Power Characteristics.
a. The power characteristics should be established based on the endurance test of the engine that represents the type design and is in the following configuration:
(1) No air bleed for aircraft services, and
(2) With only those accessories installed that are essential for engine functioning, except when the TCDS defines other conditions that are used to determine the engine ratings.
b. The test data is then referenced to the standard atmosphere conditions for which the engine rating is defined.
2-2. General Requirements in §§ 33.85(a), 33.85(b), and 33.85(c).
a. The pre- and post-endurance calibration tests required by § 33.85, establish the engine’s power characteristics in the type design configuration over its entire operating range of speeds, pressures, and temperatures. These tests provide the means for measuring the engine deterioration caused by the endurance test.
b. To identify the changes in engine power characteristics that may occur during the engine endurance test of § 33.87, applicants must compare the engine’s power characteristics established before and after the endurance test, except as provided by § 33.85(d) for the 30 second and 2-minute OEI ratings. This exception is further discussed in paragraph 2-3 of this AC.
c. Applicants should run the calibration tests with a clean inlet and exhaust, and without special test equipment, such as inlet screen, pre-swirler, mixer screen, or non-type design exhaust nozzles. In addition, the facility hardware must be similar to a production test cell (e.g., engine cowling, bell mouth, and normal engine control schedules). When special test equipment is necessary to obtain simultaneous redline conditions during the endurance test, applicants must address the engine configuration changes. See paragraph 3-1.c. and appendix 4 of this AC for further guidance. The power settings should include the highest power or thrust rating capability of the engine during the initial and final power calibrations. 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.
d. The calibration test data should show that following completion of the endurance test, the engine is capable of producing its rated power or thrust without exceeding any speed, gas temperature, or operating limit(s) specified in the TCDS.
e. The applicant should record the calibration test measurements only after the engine operating condition stabilizes at each rating condition. The only exception is the recording of data at the 30-second and 2-minute OEI rating conditions described in § 33.85(d), during which the gas temperature need not be stabilized at the end of the rating time limit.
2-3. Provisions for 30-second and 2-minute OEI Ratings, § 33.85(d). The extended engine operation at the 30-second OEI and 2-minute OEI ratings during calibration testing could increase hardware deterioration beyond what the endurance test would demonstrate for the ratings alone. Therefore, the data recorded during the OEI power segments in § 33.87(f) may be used toward compliance with the calibration test requirements of these two ratings. However, the applicant should record this data during the first and last test cycle to properly represent the pretest and post-test performance capability of the engine. The following paragraphs in this section provide guidance on how the data from § 33.87 may be used to support compliance with § 33.85.
a. The applicant should determine the engine’s power characteristics at the 30-second and 2-minute OEI rating conditions. These power characteristics should include deterioration determined from the pretest calibration prior to the endurance test of § 33.87(f), up to and including the third test sequence of the 30-second rated power. The power deterioration through the third test sequence should be the best indicator of the worst-case power deterioration that could occur during actual usage at the rating. This data must be included in the installation manual to define performance characteristics of the engine.
b. If power deterioration exceeds 10 percent at the 30-second rating over the course of the 2-hour test in § 33.87(f), and even if the engine still produces rated power at the end of the test, the applicant should evaluate the mode of deterioration to ensure that the availability of 30 second rated power in service will not be compromised by deterioration variability.
c. In addition to the calibration data from § 33.85(d), any available information from tests in §§ 33.88 and 33.90 should also be used to establish the engine characteristics throughout the engine operating envelope.
Chapter 3. Endurance Test
3-1. General Requirements in § 33.87(a).
a. The endurance test. The endurance test is not a simulation of expected in- service operation, but an accelerated severity test intended to demonstrate a minimum level of engine operability and durability within the approved engine ratings and operating limitations.
(1) The test requirements of §§ 33.87(b), (c), (d), (e), or (g) are to demonstrate that at the end of the test, the engine conforms to its type design and is safe for continued operation.
The requirement for § 33.87(f) is to demonstrate that at the end of the supplemental test, at a minimum, the structural integrity of the engine is maintained. This determination should be based on the inspection and maintenance requirements that are defined in the instructions for continued airworthiness (ICA) that will be submitted in compliance with § 33.4. To demonstrate this minimum level of durability and operability, the engine should:
(a) Continue to produce its rated power or thrust at the end of the test in §§ 33.87(b), (c), (d), (e), or (g) without exceeding any of the operating limitations defined by compliance with §§ 33.4, 33.5, and 33.7 and prescribed in the TCDS.
(b) Demonstrate acceptable operability from minimum to rated takeoff power or thrust without overtemperature, surge, stall, or other detrimental occurrences.
(c) Demonstrate durability while operating up to rated thrust, power, or torque;
redline and transient rotor speeds and gas temperatures; and when applying maximum rated loads on accessory drives and mounting attachments.
(d) Demonstrate the maximum compressor bleed air capability for engine and aircraft use. Paragraph 3-2.e. of this AC provides guidance for maximum compressor bleed air.
(e) Demonstrate acceptable operation at minimum and maximum fuel and hydraulic fluid pressure limit conditions. Advisory Circular 33.7-1 provides information and guidance on the fuel pressure limit locations.
(f) Demonstrate acceptable operation at the minimum and maximum lubrication oil pressure and temperature limit conditions.
(g) Demonstrate starting capability and drainage or expulsion of unwanted fluids or fuel that may accumulate after a false start.
(2) The test engine may also be used for other part 33 engine tests in conjunction with the endurance test runs. In addition, the data and results from the endurance test may also be used to show compliance to other part 33 regulations. These regulations may include:
(a) Engine ratings and operating limitations in § 33.7.
(b) Engine power and thrust settings in § 33.8(b).
(c) Suitability and durability of materials in § 33.15.
(d) Fluid drainage demonstration in § 33.17(f).
(e) Durability in § 33.19(a).
(f) Accessory attachments in § 33.25.
(g) Surge and stall characteristics in § 33.65.
(h) Bleed air systems in § 33.66 and bleed and power extraction in § 33.89.
(i) Ignition system function in § 33.69.
(j) Fuel system in § 33.67(a) and (b).
(k) Lubrication system, § 33.71(a) and (b).
(l) Hydraulic actuating systems, § 33.72.
(m) Power or thrust response, § 33.73, and acceleration time in § 33.89(a).
(n) Engine-propeller system tests for turbopropeller engines in § 33.95.
(o) Thrust reverser test in § 33.97.
(p) Fuel venting emissions in Subpart B of 14 CFR part 34.
(3) The general endurance test requirements specified in § 33.82, the calibration test in § 33.85, the endurance test in § 33.87, the teardown inspection in § 33.93, and the general conduct of block test requirements specified in § 33.99 form integral parts of the engine’s endurance testing. However, some systems or components may not be adequately substantiated by endurance testing alone under the requirements of § 33.87. For these systems or components, applicants should complete additional component level tests under the requirements defined in § 33.91 to establish that the systems, or components, can function reliably in all engine operating and atmospheric conditions that will be certificated. For example, to properly demonstrate its operability and durability characteristics, the main accessory gearbox must be tested at the maximum rated pad loads and power extraction loads that will be certified. If these extreme gearbox load conditions exceed the capability of the applicant’s endurance testing facilities, then the applicant may choose to complete an additional gearbox rig test at these maximum rating conditions.
(4) The endurance test must include at least 150 hours of testing in accordance with the series of test schedules specified in §§ 33.87(b), (c), (d), (e), and (g), depending upon the type 5
and contemplated use of the engine. The applicant must conduct the prescribed 6-hour endurance test cycle 25 times to complete the required 150 hours of operation, unless we approve a different test sequence. For engines seeking the 30-second OEI and 2-minute OEI ratings, the applicant must conduct an additional 2-hour test in accordance with the test schedule in § 33.87(f), for a total test time of at least 152 hours. Appendix 1 of this AC presents the graphical profiles of the endurance test schedules specified in §§ 33.87(b), (c), (d), (e), and (f).
(5) The limits of rotor speed, power or thrust, output torque, and gas temperature that are included in the engine TCDS, as required by § 33.7, must be less than or equal to those values that were demonstrated during the endurance test for each engine rating condition.
b. Endurance test plan requirements.
(1) Applicants should prepare a test plan for the endurance test defining the manner in which the test will be conducted. Applicants should submit this test plan early enough to allow us the time to review and approve the plan prior to the start of the test. At a minimum, the plan should contain:
(a) The applicable part 33 section(s),
(b) A description of the germane or primary hardware to be tested and certified,
(c) A description of properly calibrated test equipment necessary to conduct the test,
(d) Conformity requirements of the test item(s) and test setup,
(e) The test procedure with sufficient detail to fully describe the method of compliance, and
(f) The pass/fail criteria.
(2) See appendix 2 of this AC for additional information about test plan requirements.
c. Test engine configuration.
(1) The test engine configuration must substantially conform to the final type design.
This means that all engine hardware, software, and systems to be certified should have, at most, only minor deviations from the type design. The applicant should disclose, in the test plan, any deviations (referred to as “nonconformances”) of the hardware, software, and components from the type design. The applicant should provide acceptable evidence that these non-type design parts are representative of the capabilities, durability, and operating characteristics of the type design parts, and they will not adversely affect the outcome or the integrity of the test. Further, the non-type design part should not adversely affect the functioning of any other part or system.
Justification of nonconformance acceptability is frequently referred to as reconciliation.
Applicants should submit all such non-type design configuration deviations and associated reconciliations to us for approval.
(a) The standard type design configuration and operating characteristics of some turbine engines may preclude achieving a triple redline test for compliance with § 33.87(a)(3).
Modification of certain test equipment, engine configurations, and test sequences may be necessary to run the test at triple redline conditions. The applicant should show through analysis that the test engine, with any modifications, still represents the durability and operating characteristics of a typical type design engine, and that the engine is capable of complying with § 33.87 requirements.
(b) The applicant may run more than one test on the same set of hardware to be presented for certification if all operating parameters cannot be held simultaneously at 100 percent of the values to be certificated. See appendix 3 of this AC for additional information on an acceptable endurance test cycle for multiple engine tests.
(2) Appendix 4 of this AC shows methods that may be used for matching engine speeds and temperatures for the desired test conditions.
(3) Appendix 5 of this AC provides guidance for the endurance test engine conformity inspections.
(4) The applicant should provide information on aircraft components that will be mounted on or driven by the engine, and that are not part of engine type design. These components may affect the operation of the engine and thus the outcome of the endurance test.
An example of this type of information is the weight, overhung moment, or power extraction loads imposed on the engine by each accessory. The applicant may install the component or simulating loading device on the test engine when approved by us.
(5) Some engine components, such as large integrated starter generators, may require high levels of horsepower extraction from the engine core rotor through the gearbox. High levels of power extraction from the engine core could stall the engine at flight idle or below, or during transient operations. To determine the level of horsepower extraction appropriate for the various segments of the endurance test, the applicant should consider:
(a) The effects of high levels of horsepower extraction on engine stability, operating characteristics, and durability of the power extraction hardware, and
(b) The effects of electrical load transients at critical engine operating conditions, including idle.
d. Engine component and test equipment calibrations.
(1) Before and after the endurance test, the applicant should establish and record the adjustment setting and functioning characteristic of each engine component that can be adjusted independently of installation on the engine, as required by §§ 33.82 and 33.93(a)(1), 7
respectively. These components may include, but are not limited to, the control system, pumps, actuators, heat exchangers, and valves. During the endurance test, all components should operate in a manner consistent with both the type design and the engine operating instructions.
(2) The fuel, lubricating oil, and hydraulic fluid used for the endurance test must conform to the specifications prescribed in the TCDS and ICA. The applicant should provide a statement in the test report identifying the specifications of all engine fluids used during the test.
Verification of fluid properties is the responsibility of the applicant, not the fluid suppliers.
(3) Prior to starting the endurance test, applicants should calibrate:
(a) All test equipment and measuring instrumentation necessary to operate and monitor the engine and the test facility to FAA-approved standards,
(b) All engine components according to their component calibration schedules.
(4) Applicants should maintain instrumentation calibration and measurement system accuracy throughout the duration of the test. This may be accomplished by verifying the posttest calibrations or using recognized and documented applicant procedures available to the FAA if requested.
e. Engine vibration signature. Applicants should establish the engine vibration signature during both pre-endurance and post-endurance tests by recording the vibration amplitudes as a function of engine rotor speed. Engine accelerometers mounted on the engine cases may be used to record the vibration data. Changes to the engine’s vibration signature over its operating range due to any part deterioration caused by the endurance test can be conveniently measured by comparing vibration surveys conducted before and after the test. Applicants should conduct these vibration surveys by applying slow accelerations and decelerations of not less than 2 minute durations that cover the range from minimum to maximum permissible rotor speeds, including any transient operations. Post-endurance test vibration signatures should not show a significant change from the data recorded during the pre-endurance vibration survey, and should still be at or below the allowable limits established for compliance with part 33 requirements.
The only purpose of the surveys conducted during endurance testing is to determine potential vibration signature changes due to engine deterioration. Applicants should address significant changes in vibrations to determine their causes and, if necessary, make applicable design changes; the test report should document all findings and actions taken.
f. Servicing and repairs during the endurance test.
(1) Applicants should monitor oil consumption during the test and ensure it remains within the allowable limits prescribed in the engine operating documents. If the oil consumption exceeds the allowable limits, the applicant should determine the cause and take corrective action during the test, or otherwise revise the engine manuals to reflect the consumption observed during the test. We also recommend that applicants periodically check oil samples for the presence of metallic particles or other debris that may indicate an impending failure of one or more parts.
(2) During the endurance tests, only servicing and minor repairs may be permitted by the ICA service and maintenance instructions, refer to § 33.99(b). 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 may be considered minor repairs.
(3) Section 33.99(b) requires that the applicant must subject the engine or its parts to additional test(s) or penalty test run(s) that we find necessary if during the endurance test, the frequency of service or the number of stops due to engine malfunction is excessive; or if a major repair or replacement of a part is found necessary; or because of findings from the teardown inspection.
(4) If an engine part fails during the test, the applicant should determine the cause of failure and assess the effect on the durability and operability of the engine. The applicant should determine and justify any corrective actions prior to resuming the test.
g. Endurance test report requirements.
(1) The certification test report(s) should contain sufficient data (for example, plots and tabulations) and discussion showing that the engine has successfully demonstrated compliance to all requirements of the calibration testing, endurance testing, and the teardown inspection regulations. In addition to test results and data analyses, the report should contain a list of engine faults and a complete description of all hardware distress, including any corrective actions that were implemented or are planned for implementation after the test.
(2) The report should also provide comparative tabulations (test results as related to § 33.87 requirements) of:
(a) The total time spent at each sequence of the endurance test cycle and associated power condition, and
(b) The minimum certificated parameter values obtained during each of those test cycle sequences.
(3) To show engine stabilization at or above the required operating parameter values, such as the rotor speed and gas temperature to be certificated, applicants should include a trace of one typical takeoff and idling cycle segment performed without any air bleed required by § 33.87(b)(1), and another for one acceleration and deceleration segment required by § 33.87(b)(5). These traces are necessary to prove that a parameter may not stabilize at its 100 percent value within §§ 33.87(b)(1) or § 33.87(b)(5) test segment. Applicants should provide supporting data showing that any deviations or exceptions to the test requirements are acceptable. This requirement also applies to the corresponding test runs of §§ 33.87(c), (d), (e),
(f), or (g).
(4) Appendix 6 of this AC provides additional information on test report requirements.
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.