AMC E 780 Icing Conditions

ED Decision 2015/009/R

(1)     Introduction

This AMC provides Guidance Material and Acceptable Means of Compliance for showing compliance with CS-E 780.

Test evidence is normally required for Supercooled Liquid Water (SLW) icing conditions. For other applicable icing conditions, compliance may be demonstrated by a combination of test, analysis and service experience.

(1.1)   Definitions

Auto-Recovery Systems: Engine systems that ensure that Engines operate just before or immediately after an upset (that is, power loss or stall) without operator intervention. Auto-recovery systems include auto-relight systems, stall recovery systems, and other Engine systems intended to recover the operability of an Engine following a flameout, surge, stall, or a combination of these.

Freezing Fraction: The ratio of the mass of water that freezes at a point on a surface to the total mass of incoming water at that point.

Highlight Area: The area bounded by the leading edge of the nacelle inlet. This may be different for turboshaft installations where complex inlet schemes are utilised.

Ice Formations: Ice formations resulting from the impact of supercooled water droplets on propulsion system surfaces are classified as follows:

(a)     Glaze Ice: This is a transparent or translucent ice formed by liquid water droplets that do not freeze immediately on impact and has horns. Droplets impacting the surface do not freeze immediately, but run back along the surface until freezing occurs. Glaze ice typically has a non-aerodynamic shape and is more susceptible to aerodynamic forces that result in shedding. Glaze ice typically has both a lower freezing fraction and lower adhesive properties than rime ice. Glaze ice is often a concern for static hardware while rime ice is often a concern for rotating hardware.

(b)     Rime Ice: This is a milky and opaque ice formed by liquid cloud droplets that freeze immediately on impact. Rime ice typically forms in an aerodynamic shape, on both rotating and static Engine hardware. The freezing fraction is high for rime ice, typically approaching a value of 1.0. Rime ice typically has greater adhesion properties than glaze ice but often a lower density. Adhesion properties increase with lower temperature up to a test point where no additional adhesion is gained with additional lower temperature.

Ice Shed Cycle: The time period required to build up and then shed ice on a propulsion system surface for a given power and icing condition. A shed cycle can be identified visually  (for example, with high-speed cameras), and with Engine instrumentation (such as vibration pickups, temperature probes, pressure probes, speed pickups, etc.). The ice shed cycle for rotating surfaces, such as fan blades, is strongly influenced by rotor speed and the adhesive strength of the ice to the surface. In general, ice adhesive strength increases as surface temperature decreases.

Icing Conditions: The presence of supercooled liquid water drops and temperature conducive to aircraft icing. These conditions are defined by the following parameters:

—                  Liquid Water Content (LWC): The total mass of water contained in liquid drops within a volume or mass unit of cloud or precipitation, usually given in units of grams of water per cubic meter or kilogram of dry air (g/m3, g/kg);

—                  Median Volumetric Diameter (MVD): The drop diameter which divides the total liquid water content present in the drop distribution in half, i.e., half the water volume will be in larger drops and half the volume in smaller drops. (Also sometimes called Median Volume Diameter). Note the MVD used in Appendices O and P to CS-25 is equivalent to the MED used in Appendix C to CS-25 and CS-29, CS-Definitions for an assumed Langmuir type droplet distribution);

—                  Mean Effective Diameter (MED): A term used with the rotating multicylinder method for measuring LWC in clouds. A droplet diameter which, when assigned to the midpoint of one of the Langmuir distributions, gives the best agreement between the computed and measured differential ice mass accumulations on a set of rotating multicylinders. The MED is approximately equal to the MVD;

—                  Total Air Temperature (TAT): The ambient air temperature plus the ram air temperature rise. For icing testing in test cells, the total Engine inlet temperature includes the static air temperature of the cloud from the applicable icing environment, plus the assumed flight airspeed; and

—                  Static Air Temperature (SAT): The local measured air temperature minus the air temperature rise from velocity effects.

Power/Thrust Loss Instabilities: Engine operating anomalies that cause Engine instabilities. These types of anomalies could include non-recoverable or repeating surge, stall, rollback, or flameout, which can result in Engine power or thrust cycling.

Scoop Factor (concentration factor): The ratio of nacelle inlet highlight area (AH) to the area of the captured air stream tube (AC) [scoop factor = AH/AC]. Scoop factor compares the liquid water available for ice formation in the Engine inlet to that available in the low-pressure compressor or Engine core, as a function of aircraft forward airspeed and Engine power condition. The scoop factor effect depends on the droplet diameter, the simulated airspeed and the Engine power level as well as the geometry and size of the Engine. This may be different for turboshaft installations where complex inlet schemes are utilised.

Sustained Power/Thrust Loss: This is a permanent loss in Engine power or thrust. Typically, sustained power loss is calculated at rated take-off power.

Water Impingement Rate: The rate of water collection on an Engine surface during a specific period of time. The units of the water impingement rate are g/m2/min.

Note: Additional definitions can be found in Reference 2. — SAE ARP 5624.

(1.2)   References

1.       Mixed-Phase Icing Condition: A review (DOT/FAA/AR-98/76), Dr. Riley, James T, Office of Aviation Research, Washington D.C. 20591, U.S. Department of Transportation, Federal Aviation Administration, December 1998.

2.       SAE Aerospace Recommended Practice (ARP) 5624 — ‘Aircraft Inflight Icing Terminology’, issued on 3.3.2008, reaffirmed on 23.4.2013.

(1.3)   Test Configuration — Engine

Because the Engine behaviour cannot easily be divorced from the effects of the Engine air intake and Propeller, where possible, it is recommended that the tests be conducted on an Engine complete with representative air intake, Propeller (or those parts of the Propeller which affect the Engine air intake), and Engine air data probes. Separate assessment and/or testing of the air intake, Propeller and air data probes are not excluded but in such circumstances, the details of the assumed Engine installation will be defined in the manuals containing instructions for installing and operating the Engine (under CS-E 20(d)). It would then finally be the responsibility of the aircraft manufacturer to show that the Engine tests would still be valid for the particular air intake and Propeller, taking into account:

—                  distortion of the airflow and partial blockage of the air intake as a result of, for example, incidence or ice formation on the air intake and Propeller;

—                  the shedding into the Engine of air intake and Propeller ice of a size greater than the Engine is able to ingest;

—                  the icing of any Engine sensing devices or other subsidiary air intakes or equipment contained within the Engine air intake; and

—                  the time required to bring the protective system into full operation.

Apart from tests carried out under paragraph (6) of this AMC, the icing tests should be carried out with all ice protection systems (IPS) operating. When dispatch is to be permitted with some ice protection systems inoperative, then the tests should address all configurations approved for aircraft dispatch.

CS-E 780(b) requires that Engine bleeds and mechanical power offtakes permitted during icing conditions be set at the level assumed to be the most critical, or their effect must be simulated by other acceptable means. If it is not possible to establish clearly which test configuration is most critical, the test should be repeated, if necessary, in order to ensure satisfactory operation in all permitted configurations.

(1.4)   Test Configuration — Facility

The tests may be completed with adequately simulated icing conditions either in an altitude test facility capable of representing flight conditions, or in flight, or under non-altitude test conditions.

Where non-altitude testing is used to simulate altitude conditions, appropriate justification should be presented to demonstrate that the test conditions are not less severe for both ice accretion and shedding than the equivalent altitude test points. The effects of density, hardness, and adhesion strength of the ice as it sheds should be assessed to realistic flight conditions. For example, in realistic flight conditions, the ice shed cycle for rotating surfaces, such as fan blades, is strongly influenced by the rotor speed and the adhesive strength of the ice to the surface. The adhesive strength of ice generally increases with decreasing surface temperature. The ice thickness, ice properties and rotor speed at the time of the shed define the impact threat.

(1.5)   Flight Testing

Flight testing is an acceptable method of demonstrating Engine operation in icing conditions. Under these conditions, two important flight testing considerations are the measurement of ambient meteorological data and the ability to correlate the measured Engine performance to the most critical icing point.

In practice, it may not be feasible to test the Engine in flight under natural icing conditions. However, testing in flight with simulated icing conditions could be possible and is not excluded. In this case, the applicant should define an acceptable means to establish and control the icing conditions.

(1.6)   Applicable Icing Environments

The applicable icing environments are those applicable to the aircraft on which the Engine is to be installed, defined in CS 23.1093(b), CS 25.1093(b), CS 27.1093(b) and CS 29.1093(b), as appropriate. This includes atmospheric icing conditions (including freezing fog on ground) and falling and blowing snow conditions. Falling and blowing snow conditions are defined in AMC 25.1093(b).

The test altitude need not exceed any limitations proposed for aircraft approval, provided that a suitable altitude margin is demonstrated, and the altitude limitation is reflected in the manuals containing instructions for installing and operating the Engine.

(1.7)   Compliance of Rotorcraft Engines with Icing Conditions

Specific provisions for rotorcraft Engines are currently not included in this AMC. Until guidance has been established, the necessary compliance method required for rotorcraft Engines should be agreed by the Agency.

(2)     Supercooled Liquid Water (SLW) Icing Conditions

(2.1)   Critical Points Analysis (CPA)

(a)     General Principle

A Critical Points Analysis (CPA) is one analytical approach to determine suitable Engine test conditions in view of showing compliance of the Engine with Certification Specifications in Supercooled Liquid Water (SLW) icing conditions (including Supercooled Large Drops, if applicable).

Compliance evidence should include a description of the methodology and tools used as part of the CPA. The validation of tools should also be addressed.

Whilst the CPA is primarily intended to identify whether test points should be added to those defined in paragraph (2.2), the principles outlined below may also be used for justifying the testing necessary for approval of ground operation in SLW icing conditions.

Where a CPA test point is in a similar condition to a Table 1 test point, the more severe of the two should be demonstrated.

The applicant should consider pertinent service experience as well as the anticipated use of the aircraft when selecting critical icing test points.

Compliance with the requirements of CS-E 780 includes identifying, through analysis, the critical operating test points for icing within the declared operating envelope of the Engine. The CPA should relate icing conditions to the aircraft speed range and Engine powers/thrusts as defined by the applicant. It should also include prolonged flight operation in icing conditions (for example, in-flight hold pattern), or a repetition of icing encounters. These combined elements within the CPA should identify the most critical operational icing conditions:

(i)      Applicants should ensure that their analysis is supported by test data. It should also include environmental and Engine operational effects on accumulation, accretion locations, and the most critical Engine operating conditions for ice shed and ingestion. The CPA may also be supplemented with development test data (for example wet and dry testing with thermocouple components).

(ii)     The CPA should include ice accretion calculations that account for freezing fraction and aerodynamic effects of the ice as it moves into the air inlet. For example, water ingestion into fan module and core inlets, water impingement rates for critical surfaces, forward aircraft airspeed effects, Engine configuration effects such as inter-compressor bleed, and altitude effects such as by-pass ratio effects. The CPA should also include an energy balance of critical Engine surfaces (for example, latent heat and heat of fusion effects, metal-to-ice heat transfer effects, and ice-insulating effects).

(iii)     For anti-iced parts, the CPA should identify a critical test point determined from energy balance calculations of required heat loads encompassing the range of possible combinations of icing conditions and Engine power/thrust. The effects of non-aerodynamic ice formations and their shedding as well as runback ice shedding should be assessed.

(iv)     As part of the analysis, the CPA should also contain an assessment of the assumptions and any limitations of the models used as well as their validation.

(b)     Elements of the CPA

The CPA should address, at a minimum, the following icing issues:

(i)      Ice Shed Damage.

Shed ice can cause Engine damage if it impacts an Engine surface with sufficient mass and velocity. The following types of damage are common, and applicants should include them in their CPA with an assessment of each:

(A)     Fan Module

Various parts of the fan module, both non-rotating and rotating, are susceptible to ice shed damage. For example, acoustic panels, fan rub strips, and fan blade tips are susceptible to ice shed from air intake probe(s), spinner, and fan blade roots.

In determining the critical conditions for fan module damage, the surface temperature, exposure time and rotor speed are important considerations as well as the atmospheric icing conditions and scoop factor. In particular, extended operation in a holding condition in very cold continuous maximum icing conditions will maximise the adhesion of ice on rotating fan components. This can result in large ice accretions and resulting sheds which can damage the Engine or cause power/thrust loss.

(B)     Compressor Damage

When ice formations on static components shed, they often result in damage to the next downstream rotor stage. For instance, this type of damage has occurred on the first blade set in the high pressure compressor (intermediate pressure compressor for three spool Engines). Establishing the critical conditions for these ice accretions therefore requires careful consideration as the critical condition may occur at specific limited conditions of low freezing fractions over a range of local Mach numbers and air densities. The critical conditions may not occur during any of the power settings discussed in this AMC (for example, flight-idle, 50 %, 75 % or 100 % of maximum continuous power/thrust), and so the power/thrust setting at the critical condition should be evaluated. Applicants should evaluate any Engine compressor damage that results from ice testing against the possibility of multiple occurrences, since icing is a common environmental condition.

(ii)     Engine Operability.

The applicant should consider Engine operability as part of their CPA. Engine accelerations and decelerations relative to operability challenges (for example, surge and stall) should also be considered. The most adverse Engine bleed settings for the condition being analysed should be assumed to minimise the operability margin. The establishment of CPA points should consider those conditions where the minimum operability margin is expected.

(iii)     Core and Booster Ice Blockage.

Ice accretion on internal Engine vanes from glaze ice accretions may affect airflow capacity and rematch of the Engine cycle. This should be considered in the CPA. At Engine powers/thrusts that can sustain flight, ice accretion should be reconciled through a demonstration of several ice build/shed cycles to demonstrate no adverse operating effects of either the ice builds or sheds.

(2.2)   Establishment of Test Points for In-Flight Operation

The test conditions outlined below are intended as a guide to establish the minimum testing necessary to comply with CS-E 780. These test points should be supplemented or, if applicable, replaced, by any test points identified by the CPA as applicable.

The conditions of horizontal and vertical extent and water concentration defined below are somewhat more severe than those implied by the SLW Icing Conditions in CS-Definitions, Appendix C to CS-25 and Appendix C to CS-29. Encounters with icing conditions more severe that those defined are considered possible, and it is, therefore, appropriate to ensure that a margin is maintained.

(a)     Tests points to demonstrate icing capability at a power/thrust at or above that required for sustained flight

One test point should be run to simulate each of the conditions of Table 1 at the Engine minimum power/thrust to maintain sustained flight in the intended installation. For turbofan Engines, a second point should be run at a higher power/thrust condition, if it is predicted to result in a higher energy of ice shed from the fan blades. If an acceptable means to predict the critical fan speed is not available, tests at 50 %, 75 % and 100 % of maximum continuous power/thrust should be run.

The minimum duration of each test point should be determined by repetitions of either the cycle:

(i)      28 km horizontal extent in the LWC conditions of Table 1, Column (a), appropriate to the temperature, followed by 5 km in the LWC conditions of Table 1, Column (b), appropriate to the temperature, for a total duration of 45 minutes, or 30 minutes if clear evidence of repeat build/shed cycles has been observed;

or the cycle:

(ii)     6 km horizontal extent in the LWC conditions of Table 1, Column (a), appropriate to the temperature, followed by 5 km in the LWC conditions of Table 1, Column (b), appropriate to the temperature, for a total duration of 20 minutes, or 10 minutes if clear evidence of repeat build/shed cycles has been observed.

At the conclusion of each test point, the Engine should be run up to the maximum power/thrust corresponding to the test conditions, using a one second thrust/power lever movement, to demonstrate any effect of ice shedding. If repeat build/shed cycles have been established, the acceleration should be delayed to maximise the impact energy of the ice shed.

                                                Table 1 — Standard test points

(b)     Tests points at power/thrust below that required for sustained flight

An additional test at the minimum power/thrust associated with descent in icing conditions should be conducted at an ambient temperature of -10°C or lower if necessary to ensure splitter/core inlet icing, consisting of repetitions of the following cycle:

A 28 km horizontal extent in the LWC condition of Table 1, Column (a), appropriate to the temperature, followed by 5 km in the LWC condition of Table 1, Column (b), appropriate to the temperature, for a sufficient duration to cover an anticipated descent of 3 000 m.

If the temperature required to ensure core icing is below an ambient temperature of -10°C, the LWC should be determined by interpolating between the conditions defined in Table 1.

At the conclusion of the test, the Engine should be subjected to an acceleration, using a one second power/thrust control lever movement, to maximum power/thrust conditions, so as to simulate a balked landing. The maximum power/thrust conditions should then be maintained for a sufficient period to ensure all ice is shed or, alternatively, it may be established by visual inspection that any remaining ice is insignificant.

Whenever a minimum power/thrust is required for safe operation of the Engine in icing conditions, the applicant should ensure that this minimum power/thrust will be selected when the aircraft is operating in icing conditions. If any action is required from the installer to fulfil this requirement, then the minimum power/thrust should be declared as a limitation in the manuals containing instructions for installing and operating the Engine.

(c)      Test Installation Considerations

Altitude and ram effect have a significant impact on the Engine operating conditions, ice accretion and ice shedding. Therefore, the use of an altitude test cell is the most direct method of compliance because this approach enables the test to be carried out in the most representative way, requiring the minimum of correction to correlate Engine and icing test conditions with the real operating environment. It also allows accurate control of the icing conditions. However, it is recognised that such facilities are not always available, and alternative test methods are also considered acceptable, providing that evidence demonstrates that such testing is at least as severe.

When a non-altitude test is used, any differences in Engine operating conditions, LWC and ice accretion between the altitude condition to be simulated and the test conditions, which could affect icing at the critical locations for accretion or shedding, should be taken into account when establishing the test conditions. This could involve modification of other test conditions of this paragraph in order to generate equivalent ice accretion. Effects which should be considered and corrected for include but are not limited to:

—                   Engine shaft speeds;

—                   ice concentration and dilution effects at Engine and core inlet (i.e. scoop factor);

—                   mass flow (total and core Engine); and

—                   temperature effects.

Justification should be provided to demonstrate that altitude conditions for ice accretion and shedding are adequately replicated under test conditions at all critical Engine locations. If there is more than one critical location for any given test condition, and it is not possible to adequately simulate the icing conditions at both locations, separate test points may need to be run.

(2.3)   Establishment of Test Points for Ground Operation

The Engine should demonstrate the ability to acceptably operate at minimum ground idle speed to be approved for use in icing conditions for a minimum of 30 minutes at each of the following icing conditions shown in Table 2, with the available air bleed for ice protection at its critical condition, without adverse effect. An acceleration to take-off power or thrust should be performed at the time when the maximum ice accretion is likely to have occurred. During the idle operation, the Engine may be run up periodically to a moderate power/thrust setting in a manner acceptable to the Agency.

Normally, the conditions established during the test in terms of time, temperature and run-up procedures will be deemed to be the limitations necessary for safe operation in the applicable environment provided that the acceptance criteria of CS-E 780(a) are met.

However, an analysis may be used to demonstrate that ambient temperatures below the tested temperature are less critical.

Moreover, the applicant may demonstrate unlimited time operation if complete ice shedding is shown to have occurred during the test, either through repeatable ice build/shed cycles or by using a run-up procedure.

In order to avoid any unsafe condition resulting from operation outside the demonstrated conditions, these limitations will be defined in the manuals containing instructions for installing and operating the Engine.

For rime and glaze ice conditions as defined in Table 2, approval for operation below -18°C may be substantiated by analysis. A reduced liquid water concentration may be acceptable subject to appropriate substantiation.

The applicant should demonstrate, taking into consideration expected airport elevations, the following:

Table 2 — Demonstration Methods for Specific Icing Conditions

(3)     Mixed-phase/Ice Crystal Conditions

This paragraph is provided for certification of turbine Engines to be installed on aircraft which have mixed-phase and ice crystal icing conditions included in their Certification Specifications.

Until validated full-scale ground test facilities for mixed-phase and ice crystal icing conditions are available, compliance should be based on flight test and/or analysis (supported by Engine/component tests, as necessary).

(a)     Design Precautions. The applicant should show that design precautions have been taken to minimise the susceptibility of the Engine to mixed-phase/ice crystal accretions.

           The analysis should also identify remaining features or locations in which ice accretion could not be excluded. Design features which may increase the susceptibility include but are not limited to:

(i)      stagnation points which could provide an increased accretion potential;

(ii)     exposed core entrance (as opposed to hidden core);

(iii)     high turning rates in the inlet, booster and core flow path (particularly compound turning elements);

(iv)     protrusions into the core flow path (for example, bleed door edges and measurement probes);

(v)     unheated surfaces on booster and front core stages;

(vi)     narrow vane-to-vane circumferential stator spacing leading to a small stator passage hydraulic diameter;

(vii)    variable stator vanes can accrete ice and shed it when rotated;

(viii)   extraction capability of bleeds; and

(ix)     runback ice formed downstream of internal Engine heated surfaces.

(b)     Comparative Analysis. If service experience of comparable Engine design(s) is available, the applicant should perform a comparative analysis between previous designs and the new design in mixed phase/ice crystal icing conditions. The analysis should compare both design features and operational factors.

Where the analysis under paragraph (a) above identifies potential for ice accretion due to design features, the applicant should conduct an analysis to review the service experience of the comparable Engine design(s) in order to identify any evidence indicating susceptibility to ice crystal/mixed phase accretion.

The applicant may demonstrate that the identified potential susceptibility to ice accretion is acceptable based on the good service experience demonstrated on comparable Engine design. Good service experience means the absence of any event involving Engine malfunction or unacceptable damage caused by ice crystal or mixed-phase conditions. To validate the credit from the comparable Engine design, the applicant should demonstrate that the design feature on the new design is similar in all pertinent aspects.

When the comparable Engine design has experienced field events determined to have been caused by mixed-phase or ice crystal icing conditions, the analysis should show that measures have been taken on the new design to address these field events and result in acceptable Engine operation. Acceptable operation includes the absence of rollback, rundown, stall, flameout, and unacceptable compressor blade damage.

(c)      Novel Design Features. Where the analysis under paragraph (a) above identifies potential for ice accretion due to novel design features for which a comparative analysis cannot be performed, additional tests should be made to establish satisfactory operation.

(4)     Ice Ingestion

(a)      Intent of Ice Slab Ingestion Test

The intent of the ice slab ingestion test required by CS-E 780(f) is to demonstrate tolerance to ice ingestion from ice shedding from nacelle surfaces. In addition, it also establishes limits for ice released from other aircraft surfaces in the frame of CS-23 or CS-25 certification.

The minimum ice slab dimensions for the ice slab ingestion test are provided in Table 3 below. The dimensions are related to Engine size (defined by inlet highlight area), based on service experience. The applicant should determine the ice slab dimensions by linear interpolation between the values of Table 3, based on the actual Engine’s inlet highlight area.

Table 3 — Minimum ice slab dimensions based on Engine inlet size

Note: Applicants should use a minimum ice slab density equivalent to a 0.9 specific gravity unless a different value is considered more appropriate.

The applicant should also include in its compliance plan an analysis of the potential installation effects of the Engine induction system.

The applicant and the installer should closely coordinate the ice slab sizing. This coordination ensures that potential airframe ice accumulation that can be ingested by the Engine are addressed under CS-E 780(f).

(b)     Compliance Considerations

Compliance may be demonstrated through the standard Engine ice slab ingestion test or by means of a validated analysis procedure that uses an equivalent soft body testing.

The test demonstration should use ice slab trajectories aimed at critical Engine locations. Applicants should pick locations based on the ice accretion and shed characteristics of the induction system likely to be installed on the Engine. The most critical impact location should be tested.

Engine operation will be at the maximum cruise power or thrust unless lower power or thrust is shown to be more critical.

(c)      Elements of a Validated Analysis

This analytical model may be used alone or in conjunction with the results of a certification medium bird ingestion test. A validated analysis should contain sufficient elements to show compliance. These elements may include:

—                  full fan (fan Engines) or first stage compressor (non-fan Engines) blade model using the latest techniques such as finite element analysis;

—                  blade material properties for yield or failure, or both, as appropriate;

—                  dynamic and time variant capability;

—                  thrust or power variance prediction if required to account for blade damage; and

—                  appropriate Engine or component testing, or both, with impact at the outer 1/3 of the first stage blade span location. The fan is the first stage blade row for turbofan Engines.

(i)      The analysis of the ice slab impact on the fan should properly account for critical controlling parameters:

—                   relative kinetic energy normal to the leading edge chord,

—                   incidence angle — relative slab speed and blade speed,

—                   slab dimensions,

—                   sab orientation, and

—                   impact location.

(ii)     Any predicted power/thrust loss or blade damage (distortion, cracking, tearing) should be assessed against the criteria of this AMC.

(iii)     The relative kinetic energy of the ice slab should be determined from an assessment of the 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 it can be shown that an alternate ice slab orientation is more conservative for ice slab testing.

(iv)     Ice Slab Break-Up. Typically, the ice slab breaks up into smaller pieces during an ice slab ingestion. The applicant’s analysis should use the largest slab size consistent with a conservative assessment of a slab ‘break-up’ that can occur within the air stream ahead of the fan. Data derived from a number of tests shows that the largest ice piece is typically 1/3 to 1/2 of the original size. For analysis purposes, the applicant may assume 1/2 of the original slab greatest dimension unless evidence suggests that this is not conservative relative to the ice slab testing.

(d)     Test Results

CS-E 780(f)(2) requires that, following ice ingestion, the Engine must comply with CS-E 780(a). The below elements should be considered:

(i)      Engine Loss of Performance. Applicants should evaluate the impact of any first stage blade bending or damage on potential sustained Engine power/thrust loss. Sustained power/thrust loss associated with first stage damage from the slab should be less than 1.5 %. Ice and birds are ‘soft body’ objects in their impact behaviour, i.e. they are both highly deformable on impact and flow over the structure, spreading the impact load. They also have similar densities; thus they create similar strain footprints and, consequently, similar damage. As soft body fan 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 in less than 1.5 % permanent power/thrust loss, and no cracks, tears or blade piece breakout occur due to a bird ingested at the outer 33 % of the first stage blade span, then the CS-E 780(f)(2) requirement is met.

(A)     If power/thrust loss exceeds 1.5 % when utilising the bird test, the applicant should provide a validated analysis that shows consistency with the bird test results. The applicant should also demonstrate that the standard ice slab would produce less than the 1.5 % power/thrust loss.

(B)     Applicants should 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 (considered sufficient to allow Engine operation until the next scheduled ‘A’ check). Furthermore, any damage resulting from this test should be documented in the manuals containing instructions for installing and operating the Engine.

(ii)     Engine Operability/Handling Characteristics. Ice slab ingestion should not cause surge, flameout, or prevent transient operation.

(iii)     In-Service Capability. Engine damage resulting from ice slab ingestion should not result in a failure or a performance loss that would prevent continued safe operation for a conservative flight operations scenario (for example, 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 pre-flight inspection (for example, tail-mounted Engines).

(iv)     Other Anomalies. Ice slab ingestion should not result in any other anomaly (for example, vibration) that may cause the Engine to exceed operating or structural limitations.

(v)     Auto-Recovery Systems. If during ice slab ingestion testing, 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 type design. However, additional dispatch criteria would also be required where the ignition system is fully operable before each dispatch. The reason for the additional dispatch criteria is to ensure that the ignition system’s critical relight function is reliably available during the subsequent flight. The use of an auto-recovery system is allowed during ice slab ingestion certification testing, in order to account for ice accretion and shedding as a result of an inadvertent delay in actuating the ice protection system. This is considered as an abnormal operational result where operability effects, like momentary flameout and relight, may be accepted.

(e)     Communication to the Installer. The manuals containing instructions for installing and operating the Engine should provide information on the Engine ingestion capability such as size, thickness and density of the ice slab ingested.

(5)     Engine Air Data Probe Icing

In accordance with paragraph (1.3) of this AMC, the accretion and shedding of ice from the Engine air data probe(s) should be evaluated either as part of the Engine test, or by separate assessment and/or testing.

In addition, if data from an Engine air data probe is critical to ensure acceptable Engine operation, then the applicant should demonstrate that the Engine air data probe will operate normally without any malfunction under icing conditions. The icing conditions against which the Engine is tested may not cover the icing conditions that are critical for the Engine air data probe, in particular if high airflow conditions like Maximum Continuous Thrust/Power were not selected for the Engine tests in paragraph (2.2) above. The applicant should determine those critical probe icing conditions. In that respect, the guidance material of AMC 25.1324 of CS-25 should be used along with appropriate consideration of the installation effects and dependence on Engine airflow. In doing that, the substantiation may be limited to the icing environment applicable to the aircraft on which the Engine is to be installed.

In assessing whether data from an Engine air data probe is critical, the Engine system(s) response to erroneous in-range data and to data during transitions to/from icing conditions should be considered.

Note: If Engine air data probe signals are used by the aircraft system(s) on a CS-25 aeroplane, the aeroplane manufacturer will be responsible for showing that the involved Engine air data probe complies with CS 25.1324 (including rain conditions).

(6)     Inadvertent Entry into Icing Conditions or Delayed IPS Activation

The ice ingestion demonstration of paragraph (4) of this AMC addresses the threat of ice released from protected airframe surfaces, including the Engine air intake, following a delay in the selection of the ice protection system such as might occur during inadvertent entry into icing conditions.

However, if satisfactory operation in any icing conditions relies on manual activation of Engine ice protection system(s), such as a raised idle function and/or an internal ice protection system, it should be demonstrated that the Engine characteristics are not unacceptably affected by the introduction of a representative delay in the initiation of operation of the Engine ice protection system(s).

In assessing the representative delay, the applicant should consider all factors that contribute to a delay in activation of the ice protection system(s).

This assessment should include the time for ice condition detection, pilot response time, time for the system to become operational, time for the system to become effective.

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.

(7)     Instructions for installing and operating the Engine

The applicant should declare all identified limitations to the installer in the manuals containing instructions for installing and operating the Engine. These should include but are not limited to the following items (see background in the previous paragraphs of this AMC):

—                   the icing environment in which the engine has been certified;

—                   details of the assumed Engine installation, including protection device(s);

—                   operational altitude limitation;

—                   Engine ingestion capability such as size, thickness and density of the ice slab ingested;

—                   Engine ice ingestion protection device to be provided by the installer (when not part of the Engine configuration);

—                   effects that may be observed during or after the encounter of icing conditions, such as vibrations, temporary power/thrust loss, change in Engine power/thrust response;

—                   anomalous Engine behaviour that has been found acceptable following ice shed ingestion;

—                   minimum power/thrust required for safe operation of the Engine in icing conditions (if necessary); and

—                   for ground icing operation, the conditions established during the test in terms of time, temperature (if any limitation exists) and run-up procedures.

If the Engine is certified under the assumption that the protection device considered under CS-E 780(f)(3) is provided by the aircraft installation, and if (with respect to ice formed forward of the protection device) the compliance with CS-E 780(f)(1) to (f)(2) is waived, then the Engine approval would be endorsed accordingly and the Engine instructions for installation would need to impose the conditions of CS-E 780(f)(3)(i) to (iii) to the installation.

[Amdt. No.: E/1]

[Amdt. No. E/4]