Figure 3. Normal Component of Kinetic Energy of Ice Slab Relative KE component normal to leading edge is critical Figure 4. Ice Slab Orientation Effects Dependent on orientation at fan face the “bite” can be increased substantially Load shared by 6-7 blades Load shared by 3 blades rotation slab 37
(3) The relative kinetic energy of the ice slab should be determined from an assessment of flight conditions that control engine rotor speed versus ice slab velocity. Engine test results from previous ice slab testing may be used to support the predicted ice slab velocity. The applicant’s analysis should assume the most critical orientation, unless they can show that an alternate ice slab orientation is more conservative for ice slab testing purposes.
e. Ice Slab Break Up. Typically, the ice slab breaks into smaller pieces during ingestion.
The applicant’s analysis should use the largest slab size consistent with a conservative assessment of slab “break up” that can occur within the air stream ahead of the fan. Data derived from a number of tests are included in Figure 5 below. The largest ice piece is typically 1/3 to 1/2 the original size. For compliance by analysis, the applicant should assume 1/2 of the original slab length unless evidence suggests that this is not conservative relative to ice slab testing.
Figure 5. Ice Slab Break Up Experience
½ slab dimension – minimum allowed 38
f. Test Results. Section 33.77(c) requires that under the conditions in § 33.77(e), ice ingestion must not cause a sustained power or thrust loss, or require the engine to be shutdown.
The following paragraphs cover these points in greater detail:
(1) Sustained Power Loss. Applicants should evaluate the impact of any first stage blade bending or damage on potential sustained engine power loss. Power loss associated with blade damage from the slab should be less than 1.5%. As soft body blade damage is common from medium bird ingestion, applicants may use the medium bird ingestion test results to show compliance with this requirement. If the medium bird ingestion test results is less than 1.5% permanent power loss, and no cracks, tears, or blade piece breakout occurs due to a bird introduced at the outer 1/3 of the fan blade span, then the § 33.77 requirement is met.
(a) If power loss exceeds 1.5% when utilizing the bird test, the manufacturer must provide a validated analysis that shows consistency with the bird test results. The manufacturer must also demonstrate the standard ice slab would produce less than the 1.5% power loss.
(b) Applicants must also demonstrate by test that any cracks, tears, or blade piece breakout will not result in “unacceptable sustained power or thrust loss” within 100 flight cycles.
This is considered sufficient to allow continued engine use until the next scheduled “A” check for fixed wing applications, or similar inspection periods for rotorcraft applications. Note that any damage resulting from this test must be documented in the engine installation manual.
(2) Engine Operability. Engine damage should not cause surge, flameout, or prevent transient operation.
(3) In-Service Capability. Engine damage should not result in a failure or a performance loss that would prevent continued safe operation during a conservative flight operations scenario.
For example, an engine should be able to operate within the time period for an “A” check or greater, if appropriate testing validates a continued period of in-service capability. The period of in-service capability to be demonstrated may vary with installation if the damage is not readily evident to the crew or visible on preflight inspection (for example, tail mounted engine location).
(4) Other Anomalies. Engine damage should not result in any other anomaly (for example, vibration) that may cause the engine to exceed operating or structural limitations.
(5) Auto-Recovery Systems. If during the ice slab ingestion testing under § 33.77, an engine incurs a momentary flameout and auto-relight, then the acceptance of that test is predicated on including the auto-relight system as a required part of the engine’s type design.
Furthermore, additional criteria would be required where the ignition system is fully operable before each dispatch. The reason for the additional dispatch criteria is to ensure the ignition system’s critical relight function is reliably available during the next flight. We allow use of the auto-recovery system during § 33.77 certification testing to account for ice accretion and shedding that results from an inadvertent delay in actuating the anti-icing system. We consider this delay to be an abnormal operational result where operability effects, like momentary flameout and relight, may be accepted.
g. Communication of Test Results.
(1) The installation and operation instructions required by § 33.5 should provide information on the size, thickness, and density of the ice slab ingested; any anomalous behavior such as high vibrations, and any affect on the engine’s ability to operate at the commanded power setting or rating. By including the information in the installation manual, you provide these test results to the installer.
(2) The icing certification report addressing § 33.77 compliance should include information regarding ice slab orientation and trajectories, slab breakup, impact locations, descriptions of any resultant damage, as well as all other pertinent data defining the engine's capability or response to the ice ingestion event. Additionally, if the auto-recovery system is required to comply with § 33.77, then the functional state of the recovery system (for example, one igniter inoperative) becomes a limitation that needs to be communicated to the installer via the engine installation manual to ensure compliance with the delayed activation requirements of §§ 23.1093, 25.1093, 27.1093, or 29.1093.
11. Induction System Icing Protection (§§ 23.1093, 25.1093, 27.1093, and 29.1093).
Applicants usually use the results of § 33.77 for compliance with §§ 23.1093, 25.1093, 27.1093, and 29.1093, instead of a test demonstration of the 2-minute delayed activation of inlet anti-ice (1-minute for turbo shaft engines). The § 33.77 results have also been used to show compliance with §§ 23.1093, 25.1093, 27.1093, and 29.1093 for other airframe ice sources. The engine manufacturer and the airframer should coordinate closely to ensure the § 33.77 test covers all potential ice sources. Similarly the icing compliance demonstration of § 33.68 should be coordinated with the installer to verify the engine icing compliance demonstration is applicable to the installation icing compliance requirements. The icing conditions depicted in appendix O of part 25 are only applicable to turbine engine induction systems on part 25 airplanes with a maximum takeoff weight less than 60,000 pounds. Turbine engine induction systems on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds need only comply with the icing conditions depicted in appendix C of part 25, appendix D of part 33 and in falling and blowing snow within the limitations established for the airplane for such operation.
a. Acceptable Means of Compliance (§§ 23.1093(b)(1), 25.1093(b)(1), 27.1093(b), and
29.1093(b)). As a general rule, engine induction systems should operate continuously in icing conditions without regard to time, as in a hold condition. For fixed wing aircraft, a 45-minute test duration in a standard continuous maximum cloud is an acceptable demonstration of continuous in-flight operation, so long as the test demonstration result isn’t indicating an imminent test criteria failure. An exception would be for low engine power conditions where a sustained level flight is not possible. Even then, a conservative approach must be used when a series of multiple horizontal and vertical cloud extent factors are assumed. The horizontal cloud extent factor is not intended to be used to limit the severity of exposure to those icing conditions where it is reasonable to assume the aircraft will be required to operate.
(1) Applicants must adequately analyze the engine inlet anti-ice system performance and address any potential ingestion hazards to the engine from predicted ice buildup on the engine inlet. This includes runback and lip ice. If an applicant can show that both the inlet anti-ice system performance and the first stage compressor blade (for example, fan blade for turbofan
engines) capability are equivalent to previous certification experience, then certification may be shown through similarity to previous designs.
(2) You must show by analysis, and verify by test, that the engine inlet anti-ice system provides adequate protection under all flight operations. Your analysis should include the conditions shown in Table C of this AC. Note that additional critical points may be needed depending on the specifics of the airplane and engine design. Bleed crossover points may also need to be analyzed, if applicable.
b. Inlet Design Point Selection. If the inlet is evaporative under the critical points in continuous maximum icing conditions, and if it is running wet under intermittent maximum icing conditions (refer to appendix C of parts 25 and 29), then the design is satisfactory. Turbofan and turboprop engine inlet anti-ice systems have historically demonstrated good service experience with inlet anti-ice systems that run wet, when runback ice has been evaluated at the conditions described in Table C of this AC. For turbofan and turboprop inlets that are certified for unlimited operation in icing, runback ice formation is allowed during hold, descent, and diversions under the conditions shown in Table C of this AC. Turbofan and turboprop installers should determine the critical accretion conditions for their inlets in each of the scenarios in Table C below. They should then compare each result to the amount of ice the engine was satisfactorily demonstrated to ingest during engine certification (refer to § 33.77).
Table C. Inlet Lip and Runback Ice (part 33 turbofan and turboprop engines only)
Flight Descent Hold Straight-Line Flight Condition Icing Design 6500 foot 45-minute hold in a 45-minute exposure in a Condition descent through continuous continuous maximum cloud, as a continuous maximum cloud, as depicted in appendix C of part 25, maximum depicted in with a cloud extent factor as cloud, as appendix C of part depicted in appendix C of part 25, depicted in 25, with a cloud followed by an intermittent appendix C of extent factor of 1. maximum cloud exposure, as part 25, with a depicted in appendix C of part 25.
cloud extent factor of 1, followed by an intermittent maximum cloud exposure, as depicted in appendix C of part 25.
c. Addressing Specific Operational Conditions.
(1) Descent. If the engine inlet anti-ice system is not fully evaporative during descent through a continuous maximum cloud as depicted in appendix C of part 25, then the total amount of ice accretion (including inlet lip and runback ice) must be calculated for both the continuous maximum and intermittent maximum icing conditions depicted in appendix C, and for the operating conditions shown in Table C above. Airspeed and scoop factor (if applicable for the inlet design) should be part of this assessment. Ingestion of these calculated quantities of ice buildup for a given condition must not result in more ice ingestion damage (that is, based on slab size or kinetic energy) than the amount of ice the engine was satisfactorily demonstrated to ingest during the engine certification (refer to § 33.77).
(2) Holding. The engine inlet anti-icing system must be capable of safe operation in the cloud conditions depicted in appendix C of parts 25 or 29 for extended operations. The extended holding condition is for 45 minutes (except, 30 minutes is used for parts 27 and 29) at the critical accretion ice conditions for the continuous maximum icing cloud, using a horizontal cloud extent factor of one.
(a) If the inlet is evaporative under continuous maximum icing conditions, and it is running wet under intermittent maximum icing conditions, then the design is satisfactory. This is because the descent condition provides less power to the anti-ice system and will always become the critical accretion condition in intermittent maximum icing conditions.
(b) If the inlet is running wet in a maximum continuous atmospheric condition, then the applicant should calculate the amount of ice that would accumulate during the holding conditions as described in Table C above. The total amount of calculated ice should include runback and lip ice for the condition being analyzed. Ingestion of this ice must not result in more damage than the amount of ice the engine was satisfactorily demonstrated to ingest during engine certification tests (refer to § 33.77). This damage assessment should be based on the kinetic energy at the leading edge of the first stage compressor blade (for example, the fan for turbofan engines), and should include considerations of ice slab size, velocity, density, and compressor (or fan) rotor speed.
(3) Straight-Line Flight. For straight-line flight (for example, cruise, and diversion to alternate airport) the engine inlet anti-icing system should be capable of operating sufficiently to assure safe operation for extended time in icing conditions. The straight-line flight evaluation must be evaluated and may include the use of the cloud extent factor depicted in appendix C of parts 25 or 29. If the inlet is evaporative under continuous maximum icing conditions, and if it is running wet under intermittent maximum icing conditions, then the design is satisfactory. If the inlet is running wet in a maximum continuous atmospheric condition, then the applicant should calculate the amount of ice that would accumulate during the straight-line flight conditions identified in Table C of this AC (that is, continuous maximum exposure followed by an intermittent maximum exposure). Ingestion of these calculated quantities of ice (that is, the total amount of ice including runback and lip ice for the condition) should not result in more ice
ingestion damage than the amount of ice the engine was satisfactorily demonstrated to ingest during engine certification tests (refer to § 33.77).
(4) Running Wet Inlet Anti-ice Systems. For running wet systems, an alternative method of compliance would be to demonstrate the normal (no crew action required) ice build and shed cycles during exposures representative of descent, holding, or straight-line flights (turboshaft engines can propose alternate representative exposures). This may be accomplished through tanker or ground testing, with conservative corrections for water contents consistent with exposures discussed in paragraphs (1) through (3) above. Ingestion of this ice must not result in more damage than the amount of ice the engine has satisfactorily demonstrated to ingest during engine certification tests (§ 33.77). The period of in-service capability to be demonstrated may vary with installation if the damage is not readily evident to the crew or visible on preflight inspection (for example, tail mounted positions). Damage should be reviewed with the FAA and documented in the certification report.
d. Two-minute Delayed Selection of Inlet Anti-Ice Accretion Analysis (part 33 turbofan and turboprop engines only). The § 33.77 testing criteria were developed to account for historical means of compliance regarding the 2 minute delayed selection of inlet anti-ice. For traditional pitot-style engine inlet designs, a part 23 or 25 applicant does not need to consider this scenario if the applicant shows compliance with § 33.77 testing criteria that utilizes a standard slab size.
(1) For inlet designs other than the traditional pitot-style inlets, a 2-minute delay calculation may be required, given that inlet lip ice can form during the delayed activation of the engine inlet anti-ice system. Applicants should calculate the amount of inlet lip ice that forms using a continuous maximum condition depicted in appendix C of part 25, with a cloud extent factor of one. Of the total lip ice, only the ice on the inner barrel side of the stagnation point would be ingested into the engine.
(2) For the 2-minute delay approach, applicants may assume that 1/3 of the ice on the inlet perimeter is ingested as one piece. This assumption is consistent with the historical approach taken by the engine manufacturers. The maximum damage to the fan blade occurs at a high fan speed; the critical condition occurs with the maximum inlet nacelle heat. Therefore, 2 minutes is a reasonable time to include the pilot reaction to select the anti-ice system and, for the engine thereafter, to shed the ice from the inlet perimeter.
e. Extended Operations Performance Standards (ETOPS) (part 25 installations only). If ETOPS certification is desired, the applicant should consider the maximum ETOPS diversion scenarios. For example, if a lightweight two-engine diversion occurs at a 10,000 foot altitude followed by a cabin depressurization, then this puts the airplane into icing conditions that will promote ice buildup. Therefore, the inlet icing should be evaluated. Ingestion of the ice must not result in more ice ingestion damage than has been satisfactorily demonstrated during engine certification ice ingestion (refer to § 33.77).
f. Potential Airframe Ice Sources, Including Inlets. The engine manufacturer should consider the potential installation effects of the engine induction system and work with the installer to ensure that potential ice accretions formed at airframe ice accumulation sites are adequately
tested during the engine ice ingestion testing of § 33.77. Fuselage sources of ice accretion include the inboard section of the wing for an aft fuselage mounted engine, and a radome for any installation configuration, to name two. The induction system manufacturer or installer should assess ice accumulations in accordance with parts 23, 25, 27, or 29 (that is, §§ 23.901(d)(2),
23.1093, 25.1093, 27.1093, or 29.1093). They should also provide pertinent test variables to the engine manufacturer for incorporation into the § 33.77 test demonstration. If an application or product inlet has not been selected by the time of engine certification, the engine manufacturer should list all pertinent inlet assumptions, test data, and results in the engine installation manual for use by the future installer.
(1) It is normally sufficient to show that ice accretions from these sites are smaller in size, and therefore have less or equal kinetic energy potential than the ice slab used in § 33.77 testing.
As described in paragraph 10.e of this AC, the slab typically breaks into pieces when entering the inlet. For the comparison of airframe ice to the results of the § 33.77 test, it may be assumed that airframe ice sources break in half when entering the inlet.
(2) The applicant may elect to compare the ice based on the kinetic energy of the ice slab.
Kinetic energy may be used as an acceptable method for comparing the airframe ice source to the results of the § 33.77 test. Any kinetic energy method must be agreed to by the applicant’s ACO.
(3) Demonstration of ice shedding with natural or icing tanker testing can be an acceptable means of compliance for airframe ice that accumulates in front of the engine, while in the conditions depicted in appendix C of part 25 or 29. However, the applicant must demonstrate conservatism relative to long durations in these icing conditions (such as for several successive cloud extent exposures). Ingestion of shed ice during natural or icing tanker testing must not result in more damage than the amount of ice the engine was satisfactorily demonstrated to ingest during engine certification tests (refer to § 33.77). The period of in-service capability to be demonstrated may vary with the installation if the damage is not readily evident to the crew or visible on preflight inspection (for example, tail mounted positions).
g. Wing Sourced Ice for Aft Mounted Engines (part 33 turbofan and turboprop engines, only). Clear ice may occur on the upper surfaces of the wing when cold-soaked fuel (due to aircraft prolonged operation at high altitude) remains in contact with the fuel tanks’ upper surfaces after landing, and during time on the ground when the airplane is exposed to conditions of atmospheric moisture (for example, fog, precipitation, and condensation of humid air) at ambient temperatures above freezing. Atmospheric moisture, when in contact with cold wing surfaces, may freeze. If undetected and still present during takeoff, the ice is most likely to shed when the wings flex at takeoff rotation. Simultaneous ice shedding from both wings of an airplane with aft mounted engines has resulted in ice ingestion damage and power loss in both engines during takeoff.
(1) When fuel management procedures allow wing fuel tanks to remain mostly full after landing, cold fuel is in contact with the wing upper skin. As a result, clear ice may form on cold wing surfaces during the time on the ground; this holds true even if conditions remain above
freezing and are not expected to be, or recognized as, icing conditions. Fuel temperature may need to be analyzed, including the effect of fuel schedules and refueling procedures.
h. Aft Mounted Engine Installations. For aircraft with aft mounted engines, applicants should demonstrate that any undetected ice in front of engine inlets is not greater than what was shown during engine and aircraft certification. Applicants should also demonstrate that shedding the ice accumulated due to cold-soaked fuel will not result in hazardous engine operation.
(1) For fixed wing applications, a wing mounted ice detection system may allow minimal accretion prior to alerting the flight crew as long as the accumulated ice thickness is limited to a defined value. This defined value should demonstrate a sufficient margin relative to both engine ingestion (refer to § 33.77) and performance and handling qualities as demonstrated under § 25.1419. The largest clear ice slab area that could be ingested into an engine is the same area as the engine’s inlet highlight area. In cases where this large slab is very thin, it may not be possible to reach the engine in one piece. Therefore, consistent with a conservative assessment of slab “break up” that can occur within the air stream ahead of the fan, the applicant may consider the ice slab as multiple pieces. You may evaluate any potential resulting damage to individual engine components, such as fan blades, by comparison to other ice ingestion tests such as the ice slab discussed in paragraph 10 of this AC. Additionally, you may compare the ingestion results to other soft body ingestion test results, such as the 2-inch hail ingestion tests, as long as the mass and kinetic energy effects are accounted for. Finally it may be appropriate to address the potential effect on the engine fan shaft, or other structure, if the combined impact of a large sheet of ice (or multiple pieces) striking many fan blades simultaneously could generate unacceptable loads on the engine components. This might in some instances be addressed by analysis.
i. Compliance with Mixed Phase and Glaciated Atmospheric Conditions Under § 25.1093(b)(1) (part 33 turbofan and turboprop engines only). The results of previous airfoil testing in a mixed phase icing environment indicates these icing conditions do not appreciably accrete on unheated aircraft wings. Furthermore the testing showed mixed phase environment results in the same or less ice accretion than an equivalent amount of supercooled liquid water.
The overall power required by the running-wet ice protection system was essentially unchanged between all-liquid and mixed phase conditions.
(1) However, in the running-wet mode, the local power density was much higher around the stagnation area in the mixed phase conditions, compared to the purely liquid conditions. This is due to the power required to offset the thermodynamic heat-of-fusion necessary to melt the impacting ice particles that either fully or partially stick to the surface.
(2) This may also explain why pitot-style inlets have not proved to be susceptible to mixed phase ice accretion within the inlet, and why the compliance methods to § 25.1093 for the icing conditions depicted in appendix C of part 25, have historically adequately addressed those inlets.
Engines designed with reverse flow intakes, or with intakes involving considerable changes in airflow direction should demonstrate acceptable operation in the icing conditions depicted in appendix D of part 33. Compliance for pitot-style inlets, without considerable changes
in airflow direction, may be shown through qualitative analysis of the design, and supported by similarity to previous designs that have shown successful service histories.
j. Compliance with the Supercooled Large Drop (SLD) Conditions Under § 25.1093(b)(1) (part 33 turbofan and turboprop engines only). The icing conditions depicted in appendix O of part 25 are not applicable to turbine engine induction systems on airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds.
(1) In-Flight Exposure to Appendix O of Part 25 Conditions.
(a) For airframe ice sources, the installer may use either quantitative or qualitative analysis of the design. The qualitative analysis must be supported by similarity to previous designs that have shown a successful service history in order to have confidence that the historical methodology for certification as represented by § 33.77, Table 1 – “Minimum Ice Slab Dimensions Based on Engine Inlet Size”, is appropriate.
1. For engine inlets, compliance may be shown through quantitative or qualitative analysis of the design. Qualitative analysis must be supported by similarity to previous designs that have shown a successful service history. Compliance to the “Large Droplet Condition” of Table 1 of § 25.1093(b)(2), titled “Table 1 - Icing Conditions for Ground Tests”, would still be required for ground idle taxi conditions. If similarity is not shown, then an assessment of inlet impingement limits, differing catch efficiency, distribution effects, and water contents depicted in appendix O of part 25 should be accomplished.
2. To demonstrate compliance with § 25.1093, the exposures should be consistent with, but conservative to the aircraft operational approvals, as well as the applicable definitions in Part II of appendix O of part 25, for both the ice protection system and the potential airframe accumulations with respect to shedding.
(2) Ground Taxi Exposure to Appendix O of Part 25 Conditions. The service experience considered for this advisory material indicates that engine fan damage events exist which resulted from exposure to SLD during ground taxi operations. For this reason, an additional compliance condition was added to § 25.1093(b)(2) for a 30-minute, idle power exposure to SLD on the ground. Applicants should include the terminal falling velocity of SLD (for example, freezing rain, freezing drizzle) in their trajectory assessment, relative to the protected sections of the inlet. The 100 micron minimum mean effective diameter (MED) was selected as a reasonable achievable test condition, given current technology. We recommend however, that applicants who choose to certify by analysis, should evaluate engine operation in the icing conditions depicted in appendix O of part 25. This evaluation should include various drop sizes, with the appropriate drop size distribution, to find a critical condition.
k. Natural Icing Flight Tests. Natural icing flight tests are intended to demonstrate that each turbine engine is capable of operating throughout the flight power range of the engine (including idling), without an adverse effect. This includes the accumulation of ice on the engine, inlet system components, or airframe components that would have an adverse effect on the engine operation or cause a serious loss of power or thrust. The icing conditions depicted in appendix O
of part 25 are only applicable to turbine engine induction systems on part 25 airplanes with a maximum takeoff weight less than 60,000 pounds.
(1) Based on multiple natural ice engine damage and operability events experienced on natural icing flight tests and in-service airplanes, natural ice encounters must be used to show compliance with §§ 23.1093(b)(1) or 25.1093(b)(1), and in some cases for §§ 27.1093(b)(1) and
29.1093(b)(1). For airplanes that are not intended to be certificated for flight in icing conditions, other methods may be used to replace the natural icing flight tests. When replacing the natural icing flight tests on these airplanes, you can show compliance to § 23.1093(b)(1) by analysis such as ground testing, dry air flight testing, and similarity.
(2) In addition to showing your engine inlet icing analysis model is accurate, several other key issues exist that the natural ice encounter addresses. These include:
• The adequacy of flight crew procedures when operation in icing conditions;
• The acceptability of control indications to the flight crew as the airplane responds to engine fan blade ice shedding during various conditions;
• The performance of the engine vibration indication system, as well as other engine indication systems; and
• The confirmation that the powerplant installation performs satisfactorily while in icing conditions. This powerplant installation is comprised of the engine, inlet, and the anti-ice system.
l. Identification of Ice Source. Applicants should provide a means to identify the source of any ice that may be ingested by the engine during the natural icing certification testing. Special attention should be given to non-representative ice accretions on flight test instrumentation probes or other surfaces forward of the engine during prolonged operation in icing conditions.
m. Icing Test Point Monitoring. The applicant should provide acceptable monitoring of the icing test point condition (that is, LWC, droplet diameter, temperature) for sufficient time exposure to ensure the icing encounter is representative of the environmental icing conditions depicted in appendices C and O of part 25 certification icing conditions, appendix C of part 29, or appendix D of part 33 conditions, as applicable.
n. Compliance. Compliance with §§ 23.1093 or 25.1093 is required even if flight into icing conditions is not intended or approved (§§ 23.1419 or 25.1419 compliance). Applicants must therefore, propose an acceptable icing engine-installation test. The proposed test should include the natural ice encounter criteria. The icing conditions depicted in appendix O of part 25 are only applicable to turbine engine induction systems on part 25 airplanes with a maximum takeoff weight less than 60,000 pounds.
(1) Typically, an adequate test sequence includes three natural fan ice shed cycles at each of the following conditions (inlet anti-ice turned "on"): descent (flight-idle, or anti-ice idle), holding (power set to maintain level flight through a range of anticipated airplane gross weight), and maximum climb, unless a more critical engine power setting exists. These fan shed cycles should result from natural ice accumulation and shedding, not induced or forced shedding by throttle or power lever excursions or manipulations during each condition.
(2) This test should be conducted at a steady-state engine thrust level. The test may also involve flying through the same icing cloud multiple times (lapping) for the fan to accumulate enough ice for a shed cycle to occur. The airplane may exit the icing conditions between each fan shed cycle as needed to clear any other unprotected airplane surfaces from ice. Applicants should ensure the test engine ignition system is off during the icing conditions. This will avoid masking any adverse engine operating conditions during the natural icing encounter. This may require pulling several airplane circuit breakers to disable the test engine’s auto-ignition or recovery system, or both. We recommend that applicants establish engine damage criteria and obtain FAA approval of these criteria before conducting the natural ice encounter test. Our experiences have shown that pass or fail criteria, such as allowable limits of engine damage, are best agreed on before certification testing.
o. Falling and Blowing Snow. Sections 23.1093(b)(1)(ii), 25.1093(b)(1), 27.1093(b)(ii), and
29.1093(b)(ii) require engines to operate satisfactorily in falling and blowing snow throughout the flight power range, and ground idle. Ground idle would include aircraft operations on or around the airport environment, both before and after flight. Accordingly, the effect of ingesting snow during ground operations should be evaluated.
(1) Consistent with service experience, airports continue operations with falling snow concentrations that result in a 0.25 mile or less visibility (about 0.9 gm/m3, see paragraph 9.g.(2) of this AC). For turbine engines operating in these conditions, in-flight service experience shows that snow sheds from engine or aircraft accumulation sites can cause severe operability affects when ingested by these engines. Typical turbofan or turbojet installations, using simple pitot (straight duct) inlets, have minimal areas for snow accumulation. For these inlets, in-flight icing tests have generally been found to be more critical than snow tests. The applicant should evaluate engine inlets that incorporate plenum chambers, screens, particle-separators, variable geometry, inlet barrier filters (rotorcraft), or any other feature, such as an oil-cooler, struts or fairings, which may provide a potential accumulation site for snow. Further, any airframe accumulation sites upstream of the engine inlet should also be considered by the applicant.
(2) A temperature range of 250 to 320 Fahrenheit (-40 Celsius to +00 Celsius) is common in a heavy snow environment, and can result in “wet sticky snow” which may accumulate on unheated surfaces (airframe and engine) that are subject to impingement. Engine service events have demonstrated that a snow environment is conducive to ice accretion behind the fan in front compressor stages of the engine, at low engine power. Service experience has demonstrated compressor damage as a result of exposure to prolonged periods of falling snow during ground operation. The new engine test for snow conditions is developed to represent the engine conditions where snow can form accretions aft of the first stage compressor (for example, the fan on turbofan engines) on the core inlet and first stages of the core compressor. The applicant should choose a temperature within the range provided above to achieve icing behind the first stage compressor (for example, the fan for turbofan engines).
p. Test Results. The applicant should carefully consider all evidence of ingestion and damage to the engines, and their potential sources. If damage is incurred, the possible test outcomes include: