AMC 25.1093(b) Power plant icing

• ED Decision 2016/010/R found in: CS-25 Amdt 27 - Large Aeroplanes (Jan 2023)

AMC 25.1093(b) Power plant icing ED Decision 2016/010/R Compliance with [CS 25.1093(b)](#_DxCrossRefBm1709821509) is required even if certification for flight in icing conditions is not sought. Applicants must, therefore, propose acceptable means of compliance which may include flight tests in natural icing conditions. The results of tests and analysis used for compliance with CS-E 780 may be used to support compliance with CS 25.1093(b). This requires close coordination between the engine manufacturer and the aeroplane manufacturer to make sure that CS-E 780 tests cover all potential ice sources. If an applicant can show that the ice protection and the ice ingestion capability of a powerplant is equivalent to a previously certified powerplant installation which has demonstrated a safe in-service experience, then certification may be shown by similarity to previous designs. Other airframe ice shedding sources should also be reviewed if necessary. (a) Compliance with [CS 25.1093(b)(1)](#_DxCrossRefBm1709821509) Compliance with CS 25.1093(b)(1) can be shown by analysis, laboratory testing, ground testing, dry air flight testing, similarity, and/or natural icing flight testing as necessary. As a general rule, engine air intake systems, including auxiliary components (e.g. scoops, oil coolers, struts, fairings…), should be shown to operate continuously in icing conditions without regard to time, as in a hold condition. An exception would be for low engine power/thrust 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. Applicants are reminded that the cloud horizontal extent factor is not intended to be used to limit the severity of exposure to icing conditions where it is reasonable to assume that the aircraft will be required to operate in that condition. The applicant will show by analysis, and verify by test, that the engine air intake Ice Protection System (IPS) provides adequate protection under all flight operations. If there is a minimum power/thrust required for descent to ensure satisfactory operation in icing conditions, the increase to that minimum power/thrust in icing conditions should be automatic when the IPS is switched on. The engine may revert back to normal flight idle for short term operation, such as on final approach to landing; in such a case, this reversion to normal flight idle should be assessed in term of engine ice ingestion, and any required operational time limitation or pilot action should be included in the AFM. 1. Analysis & Test Point Selection. Applicants will adequately analyse the engine air intake IPS performance and address potential ingestion hazards to the engine from any predicted ice build-up on the engine air intake, including any runback or lip ice. In establishing compliance with the requirements of CS 25.1093(b)(1), reference should be made to [AMC 25.1419](#_DxCrossRefBm1709821102) paragraph (a) for the assessment of the CS-25 [Appendix C](#_DxCrossRefBm1709820681) icing environment. In particular for the following aspects: — Analytical Simulation Methods; — Analysis of areas and components to be protected; — Impingement Limit Analysis; — Ice Shedding Analysis; — Thermal Analysis and Runback Ice; and — Similarity Analysis. In establishing compliance with the requirements of CS 25.1093(b)(1), reference should be made to [AMC 25.1420](#_DxCrossRefBm1709820703) paragraph (d) for the assessment of the [Appendix O](#_DxCrossRefBm1709820665) icing environment in particular for the following aspects: — Analysis of areas and components to be protected; — Failure analysis, and — Similarity analysis. In addition, the following specific analysis should be conducted: 1.1 Critical Points Analysis (CPA) A Critical Points Analysis (CPA) is one analytical approach to identify the most critical operational icing conditions to show that an engine air intake system, including auxiliary components (e.g. scoops, oil-coolers, struts, fairings…), complies with CS 25.1093(b)(1). For [Appendix C](#_DxCrossRefBm1709820681) icing conditions, in lieu of a detailed CPA, the conditions specified in paragraph 2.1, “Icing wind tunnel tests”, are acceptable and can be used for testing without further justification. The CPA provides a means to predict critical conditions to be assessed and allows for a selection of conditions which will ensure that the ice protection system will be adequate throughout the combined aircraft operation/icing envelope. The CPA should include ice accretion calculations that account for freezing fraction and aerodynamic effects of the ice as it moves into the air intake, forward aircraft airspeed effects, engine configuration effects and altitude effects such as bypass ratio effects. It should also include prolonged flight operation in icing (for example, in-flight hold pattern), or repeated icing encounters. The CPA should consider: 1. the aircraft/engine operating envelope. This should consider climb, cruise, hold and flight idle descent conditions in the icing envelopes. 2. the environmental icing envelopes defined in CS-25 Appendices C, O and P. The Intermittent Maximum Icing Conditions of Appendix C envelope extension down to −40°C should also be considered. 3. thermal behavior of the ice protection system in icing conditions. For each icing condition a heat balance can be made to assess the material temperature and runback water/ice accretion in icing conditions. This balance considers the heat available from the de-icing/anti-icing system and the heat lost to the impinging liquid water and external convection. The result determines the need to undertake an icing test at that point. Applicants should determine the critical ice accretion conditions and compare each of them individually with the amount of ice the engine has satisfactorily demonstrated to ingest during engine certification (CS-E 780). Applicants may assume that 1/3 of the ice on the air intake perimeter is ingested as one piece. This assumption is consistent with the historical approach taken by the engine manufacturers. The critical ice accretion including runback ice (if any) may be different for each flight phases. If this is the case, the engine manufacturer should provide the relevant information. A particular attention should be made to: — ice accretion occurring during the holding phase, which may be ingested during descent at Idle power/thrust (potentially critical for engine performance and handling characteristics) or — ice accretion occurring during the descent at Idle power/thrust (with potentially reduced ice protection availability), which may be ingested during a Go Around at Take-Off power/thrust (potentially critical for mechanical damage). Airspeed and scoop factor should be part of this assessment. Applicants should demonstrate that the full flight envelope and the full range of atmospheric icing conditions specified in Appendices C, O and P to CS-25 have been considered, including the mean effective drop / particle diameter, liquid / total water content, and temperature appropriate to the flight conditions (for example, configuration, speed, angle-of-attack, and altitude). To demonstrate unlimited operation of an air intake system in icing conditions, the system should: — either operate fully evaporative, or — any ice accretion, including runback ice, which forms should result in less ice than the engine has been demonstrated to ingest per CS-E 780. The test duration may be reduced if a repeatable build and shed cycle is demonstrated. It has been historically shown that an air intake thermal IPS designed to be evaporative for the critical points in [Appendix C](#_DxCrossRefBm1709820681) continuous maximum icing conditions, and running wet in [Appendix C](#_DxCrossRefBm1709820681) intermittent maximum icing conditions, provides satisfactory performance. If the air intake is running wet in continuous maximum icing conditions, then the applicant should calculate the amount of runback ice that would accumulate during any relevant flight phase and compare that to the maximum certified ingestion capability of the engine per CS-E 780. Scenario to be considered: The applicant should justify the icing scenarios to be considered when determining the critical ice accretion conditions. The flight phases as defined in [Part II of Appendix C](#_DxCrossRefBm1709820680) and [Part II of Appendix O](#_DxCrossRefBm1709820664) could be used to support the justification. For holding ice accretion, the applicant should determine the effect of a 45-minute holding in continuous maximum icing conditions of [Appendix C](#_DxCrossRefBm1709820681). The analysis should assume that the aeroplane remains in a rectangular “race track” pattern, with all turns being made within the icing cloud. Therefore, no horizontal extent correction should be used for this analysis. If ETOPS certification is desired, the applicant should consider the maximum ETOPS diversion scenarios. 1.2 Two Minutes Delayed Selection of Air intake IPS Accretion Analysis It should be demonstrated that the ice accretion is acceptable after a representative delay in the selection of the ice protection systems, such as might occur during inadvertent entry into the conditions. In lack of other evidence, a delay of two minutes to switch on the IPS should be assumed. For thermal IPS, the time for the IPS to warm up should be added. Applicants should calculate the amount of air intake lip ice that forms using a continuous maximum condition from [Appendix C](#_DxCrossRefBm1709820681) to CS-25, with a liquid water content 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. Applicants may assume that 1/3 of the ice on the air intake perimeter is ingested as one piece. 1.3 Ice accretion sources Examples of airframe sources of ice accretion include the radome, the spinner, the antenna and the inboard section of the wing for aft fuselage mounted engines. Clear ice may also occur on the wing upper surfaces when cold-soaked fuel (due to aircraft prolonged operation at high altitude) is in contact with the fuel tanks’ upper surfaces, or cold soaked structural part is in contact with upper surfaces, and the aeroplane is exposed to conditions of atmospheric moisture (for example, fog, precipitation, and condensation of humid air) at ambient temperatures above freezing. This atmospheric moisture, when in contact with cold wing surfaces, may freeze. Simultaneous ice shedding from both wings of an aeroplane may damage surrounding components or structure parts and result in ice ingestion damage and power/thrust loss in all engines during take-off of flight for aeroplanes with aft fuselage mounted engines. Identification of Engine Air intake ice accretion sources includes, for Appendix O to CS-25 icing environment, an assessment of air intake differing impingement limits, catch efficiency, distribution effects, and water contents. The applicant should evaluate the potential ice accumulation aft of the engine air intake protected surfaces for the possibility of ice ingestion by the engine. The applicant should assess the ice accumulations and compare them on the basis of the size or the kinetic energy of the ice slab. It is possible to show that ice accumulations are smaller in size and therefore have equal or less kinetic energy than the CS-E 780 ice ingestion demonstration. Alternatively, kinetic energy may be used as an acceptable method for comparing the airframe ice source to the results of the CS-E 780 ice ingestion demonstration. Any kinetic energy method must be agreed to by the Agency. 1.4 Ice Detection 1.4.1 Upper wing mounted ice detection systems For aircraft with aft fuselage mounted engines equipped with upper wing mounted ice detection systems to warn the flight crew of clear ice build-up on the upper surface of the wings, applicants should demonstrate that any undetected ice, including ice formed from cold-soaked fuel, is not greater than the ice ingestion demonstrated for CS-E 780 compliance. 1.4.2 Primary Ice Detection System (PIDS). The relevant provisions of the [AMC 25.1419](#_DxCrossRefBm1709821512) paragraph (d) apply. In addition, if a detection threshold exists in the PIDS (in terms of Liquid Water Content (LWC), amount of ice accretion, etc…) it must be demonstrated that the ice accretion that will occur before the actual detection threshold is reached is consistent with CS-E 780 ice ingestion demonstration. Prolonged exposure (up to a 45-minute holding configuration in continuous maximum condition from Appendix C to CS-25) shall be considered at the limit of the detection threshold to evaluate a conservative amount of ice accretion. For aft fuselage mounted engines, both the engine air intake and the part of the wing in front of the engines should be considered. A conservative assumption is that the ice accretion may detach from both sites simultaneously and be ingested by the engines when the IPS is switched on. 1.5 Appendix P Icing Environment and Pitot-style air intakes design The results of FAA aerofoil testing in a mixed phase icing environment indicate that these icing conditions do not appreciably accrete on unheated aircraft wings. Furthermore the testing showed that exposure to mixed phase environment results in the same or less ice accretion than exposure to supercooled liquid water environment with the same Total Water Content (TWC). The overall power required by the running-wet ice protection system was essentially unchanged between all-liquid and mixed-phase conditions. 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. This may also explain why Pitot-style air intakes have not proved to be susceptible to mixed phase ice accretion within the air intake, and why [Appendix C](#_DxCrossRefBm1709820681) to CS-25 compliance methods adequately address those air intakes. Engines designed with reverse flow air intakes, or with air intakes involving considerable changes in airflow direction should be shown to comply with [Appendix P](#_DxCrossRefBm1709821510) to CS 25. Compliance for Pitot-style air intakes, 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. 1.6 Falling and Blowing Snow 1.6.1 CS 25.1093(b)(1) requires that each engine, with all icing protection systems operating, operate satisfactorily in falling and blowing snow throughout the flight power/thrust range, and ground idle. Falling and blowing snow is a weather condition which needs to be considered for the powerplants and essential Auxiliary Power Units (APUs) of transport category aeroplanes. 1.6.2 All engine air intakes, including those with plenum chambers, screens, particle-separators, variable geometry, or any other feature, such as an oil-cooler, struts or fairings, which may provide a potential accumulation site for snow, should be evaluated. 1.6.3 Although snow conditions can be encountered on the ground or in flight, there is little evidence that snow can cause adverse effects in flight on turbojet and turbofan engines with traditional Pitot style air intakes where protection against icing conditions is provided. However, service history has shown that inflight snow (and mixed phase) conditions have caused power interruptions on some turbine engines and APUs with air intakes that incorporate plenum chambers, reverse flow, or particle separating design features. 1.6.4 For turbojet and turbofan engines with traditional Pitot (straight duct) type air intakes, icing conditions are generally regarded as a more critical case than falling and blowing snow. For these types of air intake, compliance with the icing specifications (at least including the icing environment of Appendix C to CS-25) will be accepted in lieu of any specific snow testing or analysis. 1.6.5 For non-Pitot type air intakes, demonstration of compliance with the falling and blowing snow specification on ground should be conducted by tests and/or analysis. If acceptable powerplant operation can be shown in the following conditions, no take-off restriction on the operation of the aeroplane in snow will be necessary. a. Visibility: 0.4 Km or less as limited by snow, provided this low visibility is only due to falling snow (i.e. no fog). This condition corresponds approximately to 1 g/m3. b. Temperatures: − 3 °C to + 2 °C for wet (sticky) snow and – 9 °C to – 2 °C for dry snow, unless other temperatures are found to be critical (e.g. where dry snow at a lower temperature could cause runback ice where it contacts a heated surface). c. Blowing snow: Where tests are conducted, the effects of blowing snow may be simulated by taxiing the aircraft at 15 to 25 kts, or by using another aircraft to blow snow over the test powerplant. This condition corresponds approximately to 3 g/m3. d. Duration: It must be shown that there is no accumulation of snow or slush in the engine, air intake system or on airframe components, which would adversely affect engine operation during any intended ground operation. Compliance evidence should consider a duration which corresponds to the achievement of a steady state condition of accretion and (possible) shedding. Any snow shedding should be acceptable to the engine. e. Operation: The methods for evaluating the effects of snow on the powerplant should be agreed by the Agency. All types of operation likely to be used on the ground should be considered for the test (or analysis). This should include prolonged idling and power transients consistent with taxiing and other ground manoeuvring conditions. Where any accumulation does occur, the engine should be run up to full power, to simulate take-off conditions and demonstrate that no hazardous shedding of snow or slush occurs. Adequate means should be used to determine the presence of any hazardous snow accumulation. f. Snow concentration corresponding to the visibility prescribed is often extremely difficult to locate naturally and it is often difficult to maintain the desired concentrations for the duration of testing. Because of this, it is likely that exact target test conditions will not be achieved for all possible test conditions. Reasonable engineering judgment should be used in accepting critical test conditions and alternate approaches, with early coordination between the applicant and the Agency addressing these realities. 1.6.6 For in-flight snow (and mixed phase) conditions, some non-Pitot type air intakes with reverse flow particle separators have been found to accumulate snow/ice in the pocket lip (sometimes referred to as the “bird catcher” section) just below the splitter which divides the engine compressor from the air intake bypass duct. Eventually, the build-up of snow in the pocket (which can melt and refreeze into ice) either spans across to the compressor air intake side of the splitter lip or, the snow/ice build-up is released from the pocket and breaks up whereupon some of the ice pieces can be re-ingested into the compressor side of the inlet. The ingestion of this snow/ice has caused momentary or permanent flameouts and in some cases, foreign object damage to the compressor. Some aeroplane manufacturers have tried to correct this condition by increasing the amount and/or frequency of applied thermal heat used around the pocket, splitter, and bypass sections of the air intake. However, short of modifying the engine ice protection systems to the point of operating fully evaporative, these fixes have mostly failed to achieve acceptable results. 1.6.7 Aeroplanes with turbine engine or essential APU air intakes which have plenum chambers, screens, particle separators, variable geometry, or any other feature (such as an oil cooler) which may provide a hazardous accumulation site for snow should be qualitatively evaluated for in-flight snow conditions. The qualitative assessment should include: 1) A visual review of the installed engine and air intake (or drawings) to identify potential snow accumulation sites, 2) A review of the engine and engine air intake ice protection systems to determine if the systems were designed to run wet, fully evaporative, or to de-ice during icing conditions, and 3) Unless the air intake ice protection means (e.g. thermal blanket, compressor bleed air, hot oil) operates in a fully evaporative state in and around potential air intake accumulation sites, inlet designs with reverse flow pockets exposed directly to in-flight snow ingestion should be avoided. Flight testing may be necessary to validate the qualitative assessment. 2. Testing The engine air intakes may be tested with the engine and propeller where appropriate in accordance with the specifications of CS-E 780 and AMC E 780. Where the air intake is assessed separately (e.g. icing wind tunnel evaluation of IPS performance, lack of suitable test facilities for engine and air intake, change in the design of the air intake, air intake different from one tested with the engine), it should be shown that the effects of air intake icing would not invalidate the engine tests of CS-E. Factors to be considered in such evaluations are: — distortion of the airflow and partial blockage of the air intakes, — the shedding into the engine of air intakes ice of a size greater than the engine has been shown to ingest per CS-E 780, — the icing of any engine sensing devices, other subsidiary air intakes or equipment contained within the air intake, and — the time required to bring the protective system into full operation. In establishing compliance with the requirements of CS 25.1093(b)(1), reference should be made to [AMC 25.1419](#_DxCrossRefBm1709821512), paragraph (b), for the assessment of the Appendix C icing environment. In conjunction with the CPA, a thorough validation of the IPS may include in particular the following aspects: — flight tests in dry air with ice protection equipment operating, — flight tests in icing conditions, natural or artificial, and — ground tests in icing wind tunnel. In establishing compliance with the requirements of CS 25.1093(b)(1), reference should be made to AMC 25.1420, paragraph (d), for the assessment of the Appendix O icing environment. 2.1 Icing wind tunnel tests Icing wind tunnels provide the ability to simulate natural icing conditions in a controlled environment and they have also been used in particular to evaluate performance of ice protection systems (IPS), such as pneumatic and thermal systems. When the tests are conducted in non-altitude conditions, the system power supply and the external aerodynamic and atmospheric conditions should be so modified as to represent the required altitude condition as closely as possible. Where an altitude facility is available, the altitudes to be represented should be consistent with the icing scenario considered. The appropriate inlet incidences or the most critical incidence should be simulated. Icing tests may be performed in sea level facilities. In order to compensate for the altitude effects, consideration is given to the necessary amendments to the test parameters in order to achieve an adequate evaluation. Flight conditions may need to be corrected to allow simulation in a wind tunnel. To achieve this, the location of the stagnation point on the inlet lip and the amount of water runback at the throat should be maintained between flight and wind tunnel conditions. Other test parameters, such as static or total air temperature, may require similitude adjustments to achieve the best match of icing condition parameters, such as those described in FAA AC 20-73A. For each test, the ice protection supply should be representative of the minimum engine power/thrust for which satisfactory operation in icing conditions is claimed. At the conclusion of each test, the applicants should assess the ice accumulations and compare them with the amount of ice the engine has satisfactorily demonstrated to ingest during engine certification (CS-E 780). Test results may be used to validate the CPA in term of ice accretion prediction. For the evaluation of the performance of the IPS, either the critical points determined by a CPA or the conditions defined in Table 1 below may be used to simulate CS-25 [Appendix C](#_DxCrossRefBm1709820681) conditions: Table 1 – Appendix C test conditions <table border="1" cellpadding="0" cellspacing="0" width="529"><thead><tr><td rowspan="2" width="87"><p align="center"><b>Ambient Air Temperature ° C</b></p></td><td colspan="2" width="120"><p align="center"><b>Altitude</b></p></td><td colspan="2" width="209"><p align="center"><b>Liquid Water Content </b><b>g/m³</b></p></td><td rowspan="2" width="113"><p align="center"><b>Mean Effective Droplet Diameter</b></p><p align="center"><b>µm</b></p></td></tr><tr><td width="60"><p align="center"><b>Ft</b></p></td><td width="59"><p align="center"><b>m</b></p></td><td width="105"><p align="center"><b>(a) Continuous Max</b></p></td><td width="104"><p align="center"><b>(b) Intermittent Max</b></p></td></tr></thead><tr><td width="87"><p align="center">− 10</p><p align="center">− 20</p><p align="center">− 30</p></td><td width="60"><p align="center">17 000</p><p align="center">20 000</p><p align="center">25 000</p></td><td width="59"><p align="center">5 182</p><p align="center">6 096</p><p align="center">7 620</p></td><td width="105"><p align="center">0.6</p><p align="center">0.3</p><p align="center">0.2</p></td><td width="104"><p align="center">2.2</p><p align="center">1.7</p><p align="center">1.0</p></td><td width="113"><p align="center">20</p></td></tr></table> Note: The conditions of water concentration required by these tests are somewhat more severe than those implied by the Appendix C to CS-25 so as to provide margins. A separate test should be conducted at each temperature condition of Table 1 above, the test being made up of repetitions of one of the following cycles: 1) 28 km (15.1 NM) in the conditions of Table 1, column (a), appropriate to the temperature, followed by 5 km (2.7 NM) in the conditions of Table 1, column (b), appropriate to the temperature, for a total duration of 30 minutes, or 2) 6 km (3.2 NM) in the conditions of Table 1, column (a), appropriate to the temperature, followed by 5 km (2.7 NM) in the conditions of Table 1, column (b), appropriate to the temperature, for a total duration of 10 minutes. Each test should be run at, or should simulate, different engine power/thrust conditions, including the minimum power/thrust for which satisfactory operation in icing conditions is claimed. Flight Idle power/thrust should be assessed against the conditions defined in Table 1 both for Column (a) and Column (b). If there is a minimum power/thrust required for descent to ensure satisfactory operation in icing conditions, the increase to that minimum power/thrust in icing conditions should be automatic when the IPS is switched on, and this minimum power/thrust associated with descent in icing conditions should be assessed against the conditions in Table 1 above. The test duration expressed above assume that steady state conditions (ice shedding cycles) are established. If this is not the case, the test should continue until a maximum duration of 45 minutes when using test 1) above or 15 minutes when using test 2) above, except for descent where the test duration may be limited to the time needed to cover an anticipated descent of 3 000 m. Where an altitude facility is available, the altitudes to be represented should be as indicated in Table 1. 2.2 Delayed activation of the air intake IPS When the ingestion tests under CS-E 780 do not adequately represent the particular airframe installation, then the delayed IPS activation test should be considered, even for aircraft equipped with PIDS to consider possible manual IPS activation in “degraded” mode. Either by separate tests, or in combination with those of paragraph 2.1 above, it should be demonstrated that the ice accretion is acceptable after a representative delay in the selection of the IPS, such as might occur during inadvertent entry into the conditions. In lack of other evidence, a delay of two minutes to switch on the IPS should be assumed when exposed to Continuous Maximum exposure of Appendix C to CS-25. For thermal IPS, the time for the IPS to warm up should be added. Similar to the accepted compliance with CS-E 780 ice ingestion tests, the use of engine auto-ignition and recovery systems are allowed to show compliance with the delayed activation tests of CS-25, as long as these automatic systems cannot be easily turned off by the flight crew. In the case of De-iced air intakes (designed for a cyclic shedding of ice from the engine air intake into the engine) which incorporate, as part of their design, an air intake particle-separator that stops the ingestion of ice into the core of the engine, engine auto-recovery systems should not be a compensating design feature utilized to minimize the negative effects of an inadequate particle-separating air intake that is not in full compliance with CS 25.1093. 2.3 Natural Icing Flight Tests Natural icing flight tests may also be used to show compliance with CS 25.1093(b)(1). In this context, natural icing flight tests are intended to demonstrate that the engine is capable of operating throughout its flight power/thrust range (including idling), without an adverse effect. This includes the accumulation of ice on the engine, air intake system components, or airframe components that would have an adverse effect on the engine operation or cause a serious loss of power or thrust. In addition to proving that the engine air intake icing analysis model is accurate, several other key issues exist, which the natural ice encounter may address. These include: — the adequacy of flight crew procedures when operation in icing conditions, — the acceptability of control indications to the flight crew as the aeroplane 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 whole powerplant installation includes the engine, air intake, and the IPS system. 2.4 Testing in Non-Representative Conditions When damage results from icing test conditions that fall significantly outside Appendices C, O and P to CS-25 icing envelopes, or when the aeroplane flight test is conducted in an abnormal manner and results in excessive ice shed damage, this may result in a test failure relative to the pre-test pass or fail criteria. Any abnormal conditions should be discussed with the Agency to determine if the test can be deemed “passed.” An example of an abnormal operation could be flying with one engine at idle while the aircraft is operated in level flight. 3. Comparative Analysis. For showing compliance with the CS-25 certification specifications relative to SLD icing conditions represented by Appendix O, the applicant may use a comparative analysis. AMC 25.1420(f) provides guidance for comparative analysis. (b) Compliance with [CS 25.1093(b)(2)](#_DxCrossRefBm1709821509) Ground taxi exposure to Appendices C and O to CS-25 1. Critical Points Analysis (CPA). The temperatures should result from a CPA, considering the full range of temperatures specified in CS 25.1093(b)(2), conducted to determine the critical ice accretion conditions for the air intake. 2. Ground taxi exposure to [Appendix O](#_DxCrossRefBm1709820665) conditions. The service experience indicates that engine fan damage events exist from exposure to SLD during ground taxi operations. For this reason, an additional condition of a 30-minute, idle power/thrust exposure to SLD on the ground must be addressed. 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 air intake. The 100 micron minimum mean effective diameter (MED) is selected as a reasonably achievable condition, given current technology. To certify by analysis the applicant should evaluate the Appendix O drop sizes up to a maximum of 3 000 microns particle size to find a critical condition. For showing compliance with the CS-25 certification specifications relative to SLD icing conditions represented by Appendix O, the applicant may use a comparative analysis. AMC 25.1420(f) provides guidance for comparative analysis. 3. Operating limitation. The conditions defined in CS 25.1093(b)(2), in terms of time and temperature, should be considered as limitations necessary for the safe operation in freezing fog, and made available to the crew in the Aeroplane Flight Manual (refer to [CS 25.1581](#_DxCrossRefBm1709820707)). Nevertheless, the applicant may use an analysis to substantiate safe operation of the engine at temperatures below the demonstrated minimum temperature. No limitation would then be required in the Aeroplane Flight Manual. [Amdt 25/16] [Amdt 25/18]