Appendix 8. Endurance Test for Engine Certification and Showing of Compliance for
Engineering Changes
1. The endurance test is an essential part of all engine type certifications. The test may also be used to show compliance for engineering changes introduced by repairs and PMA applications.
We discuss acceptable means of compliance with § 33.87 for engine type certification and major design changes in the following paragraphs.
a. New type certificate. Run the endurance test as prescribed in §§ 33.87(a) and 33.87(b) through (g), as applicable, i.e., standard 150-hour endurance test, with no deviations.
b. Amended type certificate. Run the endurance test as prescribed in § 33.87(a) and the applicable endurance test cycle in §§ 33.87(b) through (g), with no deviations, unless one of the following conditions applies:
(1) The engine is a derivative model with minor or no design changes, and with the same or lower ratings and same or lower operating limitations, such that the original endurance test is still applicable.
(2) The engine is a derivative model with major design changes, and the same or lower ratings and operating limitations. These changes, if viewed individually or in combination, would have no impact on engine operability or durability within the approved ratings and limitations. Therefore, the data from the original endurance test would show compliance for the proposed hardware changes.
(3) The engine is a derivative model with major design changes, and the same or lower ratings and operating limitations, and for which we have determined that a repeat demonstration of the endurance test under §§ 33.87(b) through (g), as applicable, is needed to show compliance for the design change. Applicants must comply with all subparagraphs of § 33.87(a). We will determine:
1 Which subparagraphs of § 33.87(a) are affected by the proposed design changes, and require reevaluation by test, and 2 Which subparagraphs can be complied with by similarity using existing data from a prior applicable test under § 33.87.
(4) The above exceptions are based on the assumption that the applicant has conducted a standard endurance test on the original model or subsequent derivative model, in compliance with the requirements of § 33.87, such that the approved data from that previous test would apply.
c. Supplemental type certificate (STC) and amended STC. The applicant should identify the effects of the major design change on the validity of the endurance test run during the engine type certification. Compliance with all subparagraphs of § 33.87(a) is required. Our certifying office will determine which subparagraphs of § 33.87(a) are affected by the proposed design A8-1
changes and require reevaluation by test. The certifying office will also determine which subparagraphs have existing data from a test completed under § 33.87 for which compliance findings by similarity can be made. When the major design change requires a repeat demonstration of § 33.87, the applicant should run the applicable endurance test cycle prescribed in §§ 33.87(b) through (g), with no deviations.
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Appendix 9. Rotorcraft Operation with OEI Ratings and Endurance Test
1. Endurance Test for 30-second OEI and 2-minute OEI Ratings.
a. The maximum gross weight of a multi-engine rotorcraft is limited by the power available from the remaining operating engine(s) when one engine fails or is shutdown during flight.
b. In the event of an engine failure at the critical decision point (CDP) of an aircraft during takeoff or landing, a short burst of very high power, referred to as the 30-second OEI power, is required to complete the takeoff, or complete a rejected takeoff, or complete a balked landing.
This power level should enable the aircraft, at any point at or above CDP, to achieve continued flight, meet obstacle clearance requirements, and gain forward speed for taking off. At any point at an airspeed/altitude up to CDP, this power level should also enable the aircraft to safely complete a rejected takeoff. If the OEI condition occurs during landing, this high power level should be sufficient to lift the aircraft to a safe altitude, clear obstructions in the flight path, and initiate a climb out (balked landing).
c. A somewhat longer period of 2 minutes and at a lower power level, but still higher than takeoff power, referred to as 2-minute OEI power, is required to complete the climb out to a safe altitude and obtain the desired forward airspeed. Once the aircraft has reached the desired safe altitude and airspeed, a longer period at a lower power (still equal to or higher than maximum continuous power) is required to continue flight until a suitable landing site is reached. This power level will either be rated maximum continuous, 30-minute OEI, or continuous OEI power depending on the rating structure of the engine. For these reasons, the engine rating structure for a rotorcraft having 30-second OEI and 2-minute OEI is typically selected from one of the following three combinations of ratings:
(1) Maximum continuous, rated takeoff, and 30-second and 2-minute OEI.
(2) Maximum continuous, rated takeoff, 30-second and 2-minute OEI, and 30-minute OEI.
(3) Maximum continuous, rated takeoff, 30-second and 2-minute OEI, and continuous OEI.
d. For the endurance test, the test schedule is prescribed in § 33.87(f) for rotorcraft engines for which 30-second OEI and 2-minute OEI ratings are desired. The intent of this 2-hour test run is to assure that the engine is capable of producing these OEI rating powers in a deteriorated state. Therefore, this is an add-on test conducted separately on the same engine hardware after completion of a 150-hour endurance test under §§ 33.87(b), (c), (d), or (e).
2. Endurance Test for 2½-minute OEI Rating.
a. The 2½-minute OEI rating power is intended for the engine to perform similar functions as 30-second OEI and 2-minute OEI ratings in an OEI flight condition for a rotorcraft, but at a A9-1
generally lower power level relative to these two ratings. Similarly, the engine rating structure for a rotorcraft having 2½-minute OEI is typically selected from one of the following combinations of ratings:
(1) Maximum continuous, rated takeoff, and 2½-minute OEI.
(2) Maximum continuous, rated takeoff, 2½-minute OEI, and 30-minute OEI.
(3) Maximum continuous, rated takeoff, 2½-minute OEI, and continuous OEI.
b. The endurance test prescribed in § 33.87(e) applies to the above rating structures.
3. The Difference Between the OEI Ratings.
a. In rotorcraft operation, 30-second OEI and 2-minute OEI ratings perform very similar functions as 2½-minute OEI ratings. The significant difference between them is the limited use in service with mandatory inspection and maintenance actions for 30-second OEI and 2-minute OEI power ratings use. These two higher power ratings are intended to safely use available engine design margin for brief periods of exposure that may result in engine part or component deterioration beyond serviceable limits and, therefore, possibly be unavailable for further use.
b. The purpose of mandatory maintenance action is to restore the safety margin of the engine back to the level required in the airworthiness standards of part 33. Additionally, the teardown inspection standards differ as follows:
(1) The requirement for teardown inspection for a 2½-minute OEI rating after an endurance test under § 33.93(a) is that each engine part must be eligible for incorporation into an engine for continued operation in accordance with information submitted in compliance with § 33.4.
(2) The requirements for teardown inspection for 30-second and 2-minute OEI ratings under § 33.93(b) are that each engine part may exhibit deterioration in excess of that permitted under § 33.4, including some engine parts or components that may be unsuitable for further use, provided the structural integrity of the engine is maintained.
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Appendix 10. Accumulated Endurance Testing Times for § 33.87(e)
1. The endurance tests under § 33.87(e) for a 2½-minute OEI rating should be run in one of the following three combinations of ratings, as described in the “Guidance” section of § 33.87(e).
(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.
2. The accumulated testing times for 150 hours of testing of the three schedules are found in table 3, which represents the rating combinations in paragraph 1 above.
Table 3. Accumulated Test Times for Each Rating Combination Under 33.87(e) Time (in hours) at Indicated Power Level Schedule 2½ Min OEI 30 Min OEI Cont.
OEI Takeoff Max.
Cont.
Rotor Speed Steps Idle
(a) Rated takeoff, maximum continuous, and 2½-minute OEI
2.17* 16.58 45 62.50 23.75
(b) Rated takeoff, maximum continuous, 2½ minute OEI, and 30-minute OEI
2.17* 12.41** 11.67 37.5 62.50 23.75
(c) Rated takeoff, maximum continuous, 2½ minute OEI, and continuous OEI
2.17* 24.91*** 16.67 20 62.50 23.75 * 2.17 hours = 2 hours and 10 minutes.
** 12.41 hours = 12 hours and 25 minutes.
*** 24.91 hours = 24 hours and 55 minutes.
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Appendix 11. Teardown Inspection Requirements—Dirty Inspection and Clean Inspection
1. The FAA’s manufacturing inspector, project engineer, or authorized designees should witness the teardown inspection. Refer to Order 8110.4C paragraph 5-14 for the teardown inspection requirements. The teardown inspection consists of two parts—a dirty and a clean inspection. Applicants should conduct the dirty inspection at part level, partial assembly, subassembly, or assembly, without prior cleaning. The degree of disassembly largely depends on the findings of the engine and components before, during, or after disassembly.
2. Tear down inspection requirements.
a. Dirty inspection. At a minimum, the applicant should note the items listed below, and our manufacturing inspector should verify that the appearance of subassemblies and parts during the teardown is carefully noted.
(1) Component’s adjustment setting and functioning characteristics not within the limits established and recorded at the beginning of the endurance test;
(2) Any abnormal indication of leaks in valves, seals, or fittings;
(3) Indications of excessive or lack of lubrication;
(4) Excessive coking;
(5) Metal or foreign particles in the oil screens or passages;
(6) Sticking or broken parts;
(7) Lack of freedom of moving parts;
(8) Excessive breakaway torques; and
(9) Any other condition that may not be noticeable after complete disassembly and cleaning.
b. Clean inspection.
(1) Visual inspection. Thoroughly clean all engine parts and subassemblies.
(a) Visually inspect all engine parts for indications of galling, metallic pickup, corrosion, distortion, interference between moving parts, and cracks;
(b) Check parts for discoloration that may be due to excessive heat or lack of lubrication;
(c) Give special attention to bearings, gears, seals, or other rotating parts; and A11-1
(d) Inspect hot section parts for indications of cracking, overheating, or burning.
(2) Nondestructive inspection. Inspect highly stressed engine parts for cracks or incipient failures using suitable nondestructive testing methods, such as magnetic particle inspection, x-ray, penetrant, ultrasonic, or eddy current.
(3) Dimensionally inspect all critical engine parts that are subject to wear, growth, rubbing, or distortion to determine the extent of any changes that occurred during the test.
Dimensional changes may be determined by comparison of pre- and post-test dimensional measurements.
3. The applicant should submit the inspection report, as verified by our manufacturing inspector, as an attachment to the Conformity Inspection Record, FAA Form 8100.1. The applicant should include the inspection results in this report, giving a comprehensive description of all defects, failures, wear, or other unsatisfactory conditions, including photographs, as required. The manufacturing inspector should ensure that the applicant identified and retained all questionable parts in safe storage for review, if requested.
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Appendix 12. Historical Background for the Calibration Tests, Endurance Tests, and Teardown Inspection Requirements
1. General.
a. The basic requirements under § 33.87 regarding test cycles and testing at maximum thrust, power, torque, redline rotor speeds, and engine turbine gas temperatures have remained substantially the same since their introduction in 1958 and through amendment 2 of part 13 of the Civil Air Regulation (CAR), despite several regulatory reviews. These endurance test requirements constitute an inseparable part of the engine durability and operability certification process in part 33 that has provided an acceptable level of safety for aircraft gas turbine engines for more than five decades.
b. The endurance test cycle was not designed to represent a typical in-service flight cycle.
Instead, it represents an accelerated demonstration of durability, operability, and reliability with the engine operating at the power, thrust or torque, rotor speeds, and gas temperature limits for which it will be certificated.
2. Civil Air Regulation (CAR) 14 CFR part 13—Aircraft Engine Airworthiness.
a. CAR part 13, effective March 5, 1952. The Civil Aeronautics Board adopted a revised CAR part 13, on January 28, 1952. The Board promulgated the previously effective CAR part 13 in 1941. Part 13 remained substantially unchanged until 1952, and represented the first time turbine-type engine certification rules were included in the regulations. The Board designated general block tests, calibration tests, and a 150-hour endurance test for turbine engines as §§ 13.250, 252 and 254, respectively, in part 13. The technical content of the calibration test was similar to Federal Aviation Regulation (FAR) 14 CFR part 33.85(a), except that the CAR part 13 did not address compressor air bleed for the test. The CAR part 13 endurance test consisted of 30 run periods of 5 hours each, as specified by the test schedule. This endurance test resembled today’s FAR part 33.87(b)(1), but it used 91 percent takeoff, and 90 and 75 percent maximum continuous power or thrust test runs instead of §§ 33.87(b)(2) through 33.87(b)(4). Sections
33.87(b)(2) through 33.87(b)(4) today specify the testing at maximum continuous, takeoff, and incremental cruise power or thrust. The CAR part 13 teardown inspection under § 13.256 required a complete engine disassembly and detailed inspection of the tested engine to check for fatigue and wear.
b. CAR part 13, amendment 13-1, effective August 12, 1957.
(1) This amendment revised two subparagraphs in the endurance test schedule from “91 percent takeoff” and “90 percent and 75 percent maximum continuous power/thrust” to “Takeoff and idling” and “Incremental cruise power and/or thrust,” respectively, which made these two essentially the same as the FAR §§ 33.87(b)(2) and 33.87(b)(4).
(2) Other revisions were:
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(a) The addition of a requirement that during the endurance test, the engine power/thrust and rotational speeds be controlled within +/- 3 percent of the specified value.
(b) The addition of test requirements for engines with an augmented power or thrust rating.
(c) The addition of maximum gas temperature and oil temperature requirements at all engine rating runs during the test, which is same as the equivalent requirements in § 33.87(a)(7).
c. CAR part 13, amendment 13-2, effective May 17, 1958. This amendment revised the specification of power and/or thrust and of engine rotational speed of a tolerance in the endurance test from “+/- 3 percent of the specified values” to “at not less than 100 percent of the specified values.”
d. CAR part 13, amendment 13-5, effective February 12, 1963. This amendment revised part 13 by defining and adding a new rating, “30-minute power for helicopter turbine engines,” and adding a new test schedule to § 13.254, Endurance test, for substantiating this rated power for turbine engines used in rotorcraft. The technical content of this new schedule is essentially the same as the FAR part 33.87(c), “Rotorcraft engines for which a 30-minute OEI power rating is desired.”
e. CAR part 13, amendment 13-6, effective April 22, 1964. This amendment established a 2½-minute power rating for a one engine inoperative (OEI) event at any instant after the start of takeoff for multi-engine rotorcraft. This was accomplished by defining and adding a new rating, “2½-minute power for helicopter turbine engines,” and adding a new test schedule to § 13.254(c) for an engine with a “2½-minute power” and a “30-minute power” to substantiate these rated powers. The test requirements for these ratings are essentially the same as § 33.87(e), except:
(1) Section 33.87(e)(2) allows three OEI rating combinations for rotorcraft performance needs, while the CAR § 13.254(c) allowed only the following option (b). The options allowed by FAR § 33.87(e)(2) are:
(a) 2½-minute OEI/maximum continuous,
(b) 2½-minute OEI/30-minute OEI, and
(c) 2½-minute OEI/continuous OEI ratings.
(2) Section 33.87(e)(2) requires that the last 5 minutes of the 30-minute OEI in one of the endurance test cycles be run to 2½-minute OEI power, while the CAR § 13.254(c) does not.
Note: Section 13.250, General block tests, § 13.252, Calibration test, and § 13.256, Teardown inspections remained unchanged through amendment 13-6.
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3. 14 CFR, part 33 – Airworthiness Standards: Aircraft Engines.
a. Part 33, effective February 1, 1965.
(1) The new 14 CFR part 33 of the FAA recodification program was issued to replace the airworthiness requirements contained in part 13 of the CAR.
(2) Section 33.85, Calibration tests, is the same as its equivalent paragraph in the CAR part 13, except for an additional requirement that the engine power control be adjusted to produce the maximum allowable gas temperatures and rotor speeds at takeoff operating conditions before the endurance test, and may not be changed during calibration tests and the endurance test.
(3) The contents of §§ 33.87, Endurance test and 33.93, Teardown inspections are the same as their equivalent paragraphs in the CAR part 13, except for minor editorial differences.
(4) Section 33.99, General conduct of block tests was completely revised from its equivalent paragraph in the CAR part 13.
b. Part 33, amendment 6, effective October 31, 1974.
(1) The new § 33.82, General, adopted by this amendment, requires the applicant to establish and record certain adjustment settings and functioning characteristics of engine components before starting the endurance test. This paragraph is the same in the current regulations.
(2) Section 33.85(a) of the calibration test was revised to include only the compressor air bleed essential for engine functioning during the test. Section 33.85(b) was revised to the current regulation. Both paragraphs now agree with the current regulation.
(3) Section 33.87, Endurance test, was revised in response to the increasing complexity of airframes, engines, and their interfaces. This amendment added § 33.87(a)(3) to address the allowance for multiple engine tests when all engine parameters could not be held simultaneously at the 100 percent level that was to be certified. We made this change based on our certification experience with high-bypass large turbofan engine certifications. Section 33.87 was revised by:
(a) Expanding § 33.87(a) to the current requirements, except:
1 The test may be run at reduced power or thrust or rotor speeds below the 100 percent value specified in § 33.87(a)(3) during maximum air bleed runs, and 2 The testing of accessory drives and mounting attachments may be accomplished on a rig in § 33.87(a)(6).
(b) Including the endurance test schedule for supersonic aircraft engine.
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(4) Section 33.93(a), Teardown inspection, was revised to the current requirements.
(5) Section 33.99, General conduct of block tests, was revised to the current regulation by adding requirements for engine service, malfunction, and stoppage during endurance test.
c. Part 33, amendment 10, effective March 26, 1984.
(1) Section 33.87, Endurance test, was revised as follows:
(a) Sections 33.87(a)(3) and (a)(5) were revised to allow the applicant to reduce power or thrust or rotor speeds below the 100 percent value specified in § 33.87(a)(3) during maximum compressor air bleed runs, as it is not always possible to reach redline speeds at takeoff and maximum continuous thrust/power without exceeding gas temperature limits.
(b) Section 33.87(a)(6) was revised to allow separate rig testing of accessory drives and mounting attachments.
(c) A new requirement was added to specify that the rated 2½-minute OEI power must be applied during the last 25th cycle of the endurance test. Specifically, the 2½- minute OEI power must be applied during the last 5 minutes of the 30-minute run for the rated 30-minute OEI.
(d) The above changes updated §§ 33.87(a)(3), (a)(5), and (a)(6) to the current requirements.
d. Part 33, amendment 12, effective October 3, 1988.
(1) In § 1.1, the existing definitions of “2½-minute power” and “30-minute power” were revised to read: “2½-minute OEI power” and “30-minute OEI power” to make these definitions relate more closely to the name of these power ratings with their intended use. A new definition of “continuous OEI power” was introduced.
(2) In § 33.87, the new test schedule for an engine with a “continuous OEI power” rating was added to the endurance test as § 33.87(d).
e. Part 33, amendment 18, effective August 19, 1996.
(1) This amendment adopted new 30-second OEI and 2-minute OEI ratings applicable to rotorcraft turbine engines.
(2) New definitions of, “30-second OEI power” and “2-minute OEI power” ratings were introduced in § 1.1.
(3) Section 33.85 was revised by adding new paragraphs (c) and (d) to cover specific calibration test requirements for these two OEI ratings.
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(4) Section 33.87 was revised by adding a new 2-hour test schedule to paragraph (f) for the ratings.
(5) Section 33.93 was revised by adding new paragraphs (b) and (c) that define teardown inspection requirements after completion of the 2-hour endurance test in § 33.87(f).
f. Part 33, amendment 25, effective October 17, 2008. This change amended the endurance test and tear down inspection requirements for certain OEI ratings. This amendment was the result of a harmonization effort of a working group comprised of representatives of the FAA, the Joint Aviation Authorities (JAA), Transport Canada, and industry. The European Aviation Safety Agency (EASA) subsequently adopted these new requirements as part of its Certification Specifications for Engines.
(1) Revised § 1.1, General definitions, for all rated OEI powers by adding a second condition, an engine shutdown, to the failure condition that defines an OEI. Also revised the use for the 2½-minute OEI rating from “a period of use” to “periods of use” to be consistent with the usage definitions of 30-second OEI and 2-minute OEI ratings.
(2) Revised § 33.87(a)(6) to allow the applicant to run the tests under §§ 33.87(f)(1) through (f)(8) without loading the accessory drives and mounting attachments if the applicant can substantiate that the durability of any accessory drive or engine component is not significantly affected and if the equivalent power is added to the output drive shaft.
(3) Revised § 33.87(a)(5) to exempt engines with 30-second and 2-minute OEI ratings from testing with maximum air bleed if the applicant can show by testing, or analysis based on testing, that the validity of the endurance test is preserved.
(4) Added a requirement that the test schedule for 30-second and 2-minute OEI ratings in § 33.87(f) be run continuously without stopping.
(5) Clarified the test schedule in § 33.87(f)(4) related to 30-second and 2-minute OEI ratings.
(6) Revised the test schedule in § 33.87(c) related to 30-minute OEI rating to agree with the schedule in EASA’s certification specifications for engines. The result harmonizes the endurance test schedule for engines with a 30-minute OEI rating.
(7) Clarified the idle condition of § 33.87(f)(8) as flight idle.
(8) Revised § 33.93(b)(2) by replacing the parts or components identified as, “mounts, cases, bearing supports, shafts, and rotors,” with, “the engine,” to emphasize that after the test, the applicant needs to inspect and consider deterioration of any engine component that could affect the structural integrity of the engine, not just those listed above.
g. Part 33, amendment 30, effective November 2, 2009. This amendment added the transient engine overtorque and maximum engine overtorque. This amendment harmonized A12-5