16. CRITICAL POINT ANALYSIS PROCEDURE. The critical point analysis is an assessment of the engine's capability throughout its operating envelope, given the range of event variables described above and any engine operability condition which is affected by ingested rain or hail.
Typical operability conditions to consider include; surge and stall margin, fuel control run-down margin, combustor flameout margin, and instrumentation sensing errors. The critical point analysis should also address case contraction.
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SECTION 4. COMPLIANCE METHODS
17. GENERAL. An engine compliance test method consistent with the critical point analysis may include the use of a ground-level static facility. This method should include appropriate means to conduct engine tests during simulated rain and hail ingestion at the increased concentrations. This would simulate in-flight rain and hail scoop factor concentration amplification effects, and compensate for the differences between the critical point conditions and the ground level test conditions. Other possibilities for demonstrating compliance include;
wind tunnel testing, direct core water-injection tests, component rig tests, scale model tests, and analyses.
18. TEST POINT SELECTION. The critical hail point(s) and rain point(s) that yield the least operability margin should be demonstrated by engine ingestion testing. Additional test points should be considered if any of the operability margins are determined to be minimal (e.g., compressor surge and stall, combustor blow out, fuel control run-down, instrumentation sensing errors, etc.).
19. CRITICAL POINT TESTING AT GROUND LEVEL. The applicant may test the engine at ground level conditions, provided the relevant engine operational factors of the critical points are reproduced in a relevant manner.
a. Test compensation. The applicant should compensate for differences between the critical point conditions and the test facility conditions. These differences may include:
(1) Air density. The critical point percentage of rain and hail concentration by weight should be reproduced during the test. For example, 20 g/m3 of rain at 20,000 feet is approximately three-percent water by weight. At sea level, this percentage of water requires nearly 40 g/m3 to compensate for the higher air density (Refer to Appendix B, FIGURE B1).
(2) Scoop factor. The appropriate rain and hail concentration amplification due to the scoop factor effect should be applied to further increase the quantities of rain and hail for the ground level tests. Determining this effect necessitates having knowledge of the inlet diffusing flow field throughout the engine power or thrust range and flight envelope.
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(3) Engine rotational speeds. The low rotor speed for the ground level test should be no greater than the altitude critical point condition. This is particularly important for turbofan engines since rotational speed determines the rain and hail centrifuging effects which prevent some of the rain and hail from reaching the engine core. The rain and hail concentrations may be adjusted to compensate for any necessary deviation from critical point rotational speeds.
(4) Variable systems. All variable systems, such as engine bleeds, whose position can affect engine operation in rain and hail, should be set in the position associated with the critical point.
(5) Engine power extraction. It should be shown by analysis or test that sufficient margin exists for extraction of the representative electrical or shaft power loads and service air bleeds.
(6) Thermodynamic cycle differences. There may be thermodynamic cycle differences between the test point and the critical point, which affect the operability of the engine. There should be compensation for these cycle differences, or it should be shown that these differences provide additional conservatism.
(7) Enthalpy of water. Rain and hail concentrations may be adjusted to ensure that the heat extraction resulting from their ingestion are the same as the critical point. If the ingestion of liquid water droplets is accepted (see paragraph 20 of this AC for compliance alternatives) for critical hail point testing, then the water concentration should, as a minimum, be increased to compensate for the heat of fusion of ice.
(8) Rain droplet breakup. In the ground level test environment, forces applied to accelerate the simulated rain droplets to flight speed, as well as shear forces between the droplets and the engine airflow, are likely to break up the droplets. This breakup can result in reduced conservatism due to additional centrifuging by the fan or propeller and spinner. The concentration of the rain may need to be increased to compensate for the added centrifuging resulting from ground level testing.
b. Engine test facility. The engine test facility should provide a uniform water droplet or hail spatial distribution within the critical area of a geometric plane within the engine inlet.
These uniform spatial distributions need to be agreed to by the appropriate FAA ACO. The facility should also provide proper droplet or particle sizes, and proper velocity distributions, unless otherwise justified, in accordance with Appendix B.
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c. Instrumentation. Instrumentation and data sampling rates should be sufficient to establish; rain and hail temperature and concentrations, particle velocities and size distributions, and engine response. Primary exhaust water to air ratio measurements, by way of gas sampling should be considered. Instrumentation accuracy and repeatability should be demonstrated by suitable means.
d. Test procedure. The test procedure should consider the following for operability critical point tests, and for the thermal shock (rain only) critical point test:
(1) Stabilize the engine at the critical point conditions.
(2) Take steady-state data readings before introducing rain or hail.
(3) Start continuous transient data recording prior to initiation of rain or hail flow.
(4) Establish altitude equivalent rain or hail flow at proper inlet velocity and size distribution.
(5) Conduct operability critical point tests at the following steady-state conditions:
(a) Deliver rain for a minimum of three minutes, at the altitude equivalent concentration defined in Appendix B, FIGURE B1 and Table B1.
(b) Deliver hail for a minimum of 30 seconds, at the altitude equivalent concentration defined in Appendix B, FIGURE B1 and Table B2.
(6) When testing low power critical points (i.e., minimum flameout and/or run-down margin), conduct tests with ingestion at the following transient conditions:
(a) Accelerate the engine with one-second throttle movement to rated takeoff power or thrust from the minimum rotor speed defined by the critical point analysis.
(b) Decelerate the engine with one-second throttle movement from an appropriate power or thrust setting (e.g., 50-percent rated takeoff power or thrust) to the minimum rotor speed defined by the critical point analysis.
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(c) If test conditions or test facility limitations prevent transient testing as defined above, the applicant may propose alternative test criteria. This alternative test criteria must provide test criteria (and any complementary substantiation) which validates that the engine has sufficient operability margins to account for likely flight operations such as missed approaches (i.e., go-around) and likely throttle movements during descent.
(7) Conduct the thermal shock critical point test by delivering rain for three minutes at the critical power or thrust condition, following a normal stabilization period without water ingestion. The maximum rain ingestion rate should occur within five seconds from onset.
e. Probable factors. It should be demonstrated by test or analysis, that the engine tested in accordance with paragraph 19.d. of this AC would have operated acceptably if exposed to other probable factors associated with a rain or hail encounters. These other probable factors would include, but are not be limited to; typical engine performance losses, installation effects, and typical auto-throttle power excursions.
f. Acceptance criteria. Acceptable engine operation excludes flameout, run-down, continued or non-recoverable surge or stall, or loss of acceleration and deceleration capability.
A momentary surge or stall that arrests itself without operational intervention (e.g., without throttle manipulation) is usually acceptable (see paragraph 19.f.(1), below). If, after the test, it is found that damage has occurred, further running or other evidence may be required to show that subsequent failures resulting from the damage are unlikely to occur before the damage is rectified. Engine performance should be measured before and after the rain and hail ingestion tests to assess steady state performance changes. Data should be normalized according to the applicant's standard practices, and the evaluation of sustained loss or degradation of power or thrust should encompass the full range of engine power or thrust.
(1) Use of auto-recovery systems during certification testing. Auto-recovery systems typically include auto-relight systems, stall recovery systems, or any other engine system intended to restore operability of an engine following a flameout, surge or stall.
Although auto-recovery systems do not need to be disabled during rain and hail certification testing, a test result that indicates that the auto-recovery system interceded to
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recover from a flameout, even a momentary flameout, would be an unacceptable result. This is also true for unrecoverable surges, stalls or run-downs. Engine systems that preemptively prepare the engine for operation in inclement weather via control system or variable geometry system reconfigurations, may be enabled during rain and hail certification testing.
(a) A momentary flameout of only one to two seconds duration may be considered acceptable if it can be shown that it is unnoticeable to the flight crew and does not affect aircraft operation. Additionally, if a momentary flameout does occur, an analysis should be performed to account for the worst case combination of engine bleeds, component performance, and expected transients, to show an adequate relight margin in all configurations and expected conditions. If compliance to these criteria is dependent upon the functioning of an auto-recovery system, then availability of this system is considered critical for dispatch and should therefore have a higher standard of reliability. Since the rule precludes flameout, then this momentary flameout would necessarily need to be analyzed and potentially reviewed against, and potentially accepted to part 21, paragraph 21.21(b)(1), as an equivalent level of safety, if it is judged to meet that criteria, and should be so notated on the Type Certificate Data Sheet.
(2) Sustained power or thrust loss. As a result of the test, any shifts or errors in measured thrust or power, which occur against the primary thrust or power set parameter(s) (i.e., fan speed, engine pressure ratio, torque, shaft horsepower), should be limited to three-percent for aircraft safety.
(3) Power or thrust degradation. A change of engine corrected thrust or power of up to 10-percent from rated or pretest levels, excluding the primary thrust or power setting parameter, is acceptable provided the criteria for sustained power or thrust loss is met. This change in performance is based on the applicant’s standard performance parameters (e.g., exhaust gas temperature, rotor speed, etc.).
20. OTHER COMPLIANCE ALTERNATIVES. Analysis may be used in place of, or in combination with, engine testing to demonstrate compliance with the requirements. The analytical methods used should have a sufficient validation basis to justify the accuracy of the predictions, or be shown to yield conservative results. The amount of validation (e.g., engine test, rig test, experimental test, etc.) should be proportional to the complexity of the analytical methods used, and to the criticality of the particular calculation to the prediction of engine operability.
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SECTION 5. GUIDANCE ON ENGINES FOR SUPERSONIC AIRPLANES
21. GENERAL. This Section provides additional guidance to the requirements of § 33.78(c), titled, Engines for supersonic airplanes. This additional guidance is intended to outline an acceptable harmonized compliance plan which is consistent with the intent of JAR-E 790(c), while not altering the requirement or intent of § 33.78(c).
(1) Compliance requirements. In addition to complying with paragraphs (a)(1) and (a)(2) of § 33.78, a separate supersonic airplane engine test is required to be conducted with three hailstones ingested at supersonic cruise velocity, except as otherwise noted in § 33.78(c).
(2) Operating conditions. The engine’s rotor speeds, component loadings, and component temperatures for this test should be representative of supersonic cruise flight operation.
(3) Hailstone ingestion sequence. The hailstones should be ingested in a rapid sequence to simulate an in-flight hailstone encounter.
(4) Alternate large hailstone option. Section 33.78(c) allows an alternate hailstone test where three large hailstones may be ingested in a rapid sequence at subsonic velocities. It should be shown that for this alternate subsonic test, the ingestion is equivalent to the applicable supersonic ingestion with respect to engine component loadings and strengths, the kinetic energy of hailstones and their depth of penetration into the engine.
/s/ Thomas A. Boudreau Acting Manager, Engine and Propeller Directorate Aircraft Certification Service
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