8.9.2 Control surfaces such as elevators, ailerons, flaps, and spoilers, especially those constructed of thin metallic, nonmetallic, or composite materials, are also subject to damage. Currently available trajectory and impingement analyses may not adequately predict such damage. Unpredictable ice shedding paths from forward areas such as radomes and forward wings (canards) have been found to negate the results of these analyses. For this reason, a damage analysis should consider that the most critical ice shapes will shed and impact the areas of concern. For analysis of the radome as a potential airframe ice source, the applicant may use qualitative analysis of the design supported by similarity to previous design that has shown successful service history to have confidence that the historical methodology for certification represented by Table 1—Minimum Ice Slab Dimensions Based on Engine Inlet Size of § 33.77 is appropriate.
8.10 Propeller Deicing Analysis.
The applicant should perform a propeller deicing analysis that—
8.10.1 Substantiates ice protection coverage in relation to chord length and span,
8.10.2 Substantiates the IPS power density, and
8.10.3 Considers the effect of inter-cycle ice accretions and potential for propeller efficiency degradation. Qualitative analysis of the design supported by similarity to prior designs with successful service histories in icing may be used to show compliance with this aspect.
8.11 Airspeed Indication and Static Pressure System Ice Protection.
8.11.1 Compliance with the minimum performance standard in Technical Standard Order (TSO) TSO-C16a, Electrically Heated Pitot and Pitot-Static Tubes, is not sufficient by itself to show compliance with § 25.1323. Section 25.1323 requires a heated pitot probe or an equivalent means of preventing malfunction due to icing conditions specified in Appendix C, certain Appendix O icing conditions, and in specific mixed phase icing and ice crystal conditions. It is unlikely that the icing conditions critical to the equipment will be encountered during flight tests. Consequently, certification programs should supplement the icing flight tests with icing tunnel test and/or tanker test data, as necessary. In-service experience during severe atmospheric conditions has shown that use of pitot probes qualified to the older standards has resulted in airspeed fluctuations and even loss of indicated airspeed. However, similarity to a previous pitot probe installation with positive in-service history may be a consideration when showing compliance. Section 25.1326 requires a system to advise the flightcrew when a pitot heating system is not operating. An acceptable indication system may consist of separate lights or a flightcrew alert indication on an electronic display for each pitot source. Additional guidance on an acceptable means of compliance with § 25.1326 is provided in AC 25-11B, Electronic Flight Deck Displays.
8.11.2 TSO-C16a references Society of Automotive Engineers (SAE) Aeronautical Standard AS8006, Minimum Performance Standard for Pitot and Pitot-Static Tubes, and
21 supplements the icing requirements with specific part 25, Appendix C icing conditions and specific liquid water content tests from British Specification 2G 135, Specification for Electrically-Heated Pitot and Pitot-Static Pressure Heads. However, it does not contain tests for Appendix O icing conditions, mixed phase, or ice crystal conditions.
The applicant is responsible for showing that the pitot heat is adequate for the Appendix C and applicable Appendix O icing conditions. If this proof of compliance is not obtained in flight tests, analysis or icing tunnel test data should be submitted.
8.11.3 Although Appendices C and O of part 25 only consider the liquid water content of icing conditions, recent cloud characterization research has indicated that approximately 40 percent of icing condition events consist of liquid water drops and ice crystals (mixed phase icing conditions). Also, glaciated atmospheric conditions, which consist solely of ice crystals, are encountered during airplane operations. The ice crystal environment may be more critical than liquid water for thermal systems because evaporating ice crystals requires more energy. Section 25.1323(i) requires a means to prevent malfunctions in the mixed phase and ice crystal requirements described in part 33, Appendix D. Appendix D of part 33 provides an environmental definition of conditions that may be encountered. These environmental conditions include parameters for total water content as a function of temperature, altitude, and horizontal extent.
Altitude is a parameter identified in Appendix D of part 33 and must be considered when developing the test conditions and supporting analysis necessary to show compliance. In addition, ice crystal concentrations at an exterior mounted probe could be higher than the free stream conditions defined in Appendix D of part 33. Therefore, installation effects should be evaluated. A combination of CFD codes and icing tunnel testing would be an acceptable method of compliance. If not enough heat is provided to melt the ice crystals in mixed phase or ice crystal conditions, blocked pitot probes can result. Blocked pitot probes can prevent accurate airspeed indications. Icing tunnel tests with simulated mixed phase and glaciated icing combined with analysis is an acceptable way to show compliance with § 25.1323.
8.11.4 To ensure proper drainage, airspeed indication probes should be evaluated in the heavy rain conditions described in § 25.1323(i) as installed on the airplane. It is foreseeable that flight into freezing conditions could occur after encountering heavy ground level rain. If not enough heat is provided under these conditions, blocked pitot probes and inaccurate airspeed indications can result. Pitot probes should continue to operate after such an encounter.
8.12 Stall Warning System and Angle-of-Attack Ice Protection.
8.12.1 Compliance with the minimum performance standard for stall warning instruments in TSO-C54, Stall Warning Instruments, is not sufficient by itself to show compliance with § 25.1324. Section 25.1324 requires a heated angle-of-attack system sensor or equivalent means to prevent malfunction due to icing conditions specified in Appendix C, certain Appendix O icing conditions, and specific mixed phase icing and ice crystal conditions. AC 25-25A contains additional guidance on stall warning requirements.
8.12.2 With certain exceptions and additions, TSO-C54 requires compliance with performance specifications of SAE Aerospace Standard AS403A, Stall Warning Instrument. As in AS393B, Airspeed Tubes Electrically Heated, these requirements include a test to demonstrate deicing and anti-icing capability, but only temperature and airspeed are specified. The precipitation test conditions of SAE AS403A include moderate icing conditions for Type II instruments. “Moderate,” however, is not defined. Although functioning of angle-of-attack sensors is evaluated in natural icing conditions during certification test programs, there is no requirement to flight test at the Appendix C or Appendix O icing limits because the low probability of finding those conditions imposes a burden. The applicant is responsible for showing that stall warning heat is adequate for the Appendix C and applicable Appendix O icing conditions. If proof of this is not obtained in flight tests, analysis or icing tunnel test data should be submitted.
8.12.3 Section 25.1324 also requires consideration of the mixed phase and glaciated icing conditions defined in part 33, Appendix D, for angle-of-attack sensors. Conventional trailing vane designs are such that ice crystals are not likely to be captured. A qualitative analysis based on the service history on trailing vane angle-of-attack sensors is an acceptable means of compliance.
8.12.4 In addition, § 25.1324 requires consideration of heavy rain conditions. To ensure proper drainage, angle-of-attack sensors should be evaluated in the heavy rain conditions as installed on the airplane. It is foreseeable that flight into freezing conditions could occur after encountering heavy ground level rain. Angle-of-attack sensors should continue to operate after such an encounter.
8.13 Runback Ice.
Water not evaporated by thermal IPSs and unfrozen water in near-freezing conditions (or in conditions when the freezing fraction is less than one) may run aft and form runback ice. This runback ice can then accumulate additional mass from direct impingement. The determination of runback ice is described in further detail in AC 20-73A. Runback ice should be considered when determining critical ice shapes.
Simulated runback ice shapes may be used when evaluating effects of critical ice shapes. The applicant should consider potential hazards resulting from the shedding of runback ice.
9 SIMULATED ICE SHAPES AND ROUGHNESS.
AC 25-25A contains guidance on icing exposure during various phases of flight that should be considered when determining simulated ice shapes and surface roughness.
AC 20-73A also contains additional guidance related to simulated ice shapes. The shape and surface roughness of the ice should be developed and substantiated with acceptable methods. Common practices include—
9.1 Use of computer codes for modeling drop impingement limits and ice shape predictions (see SAE ARP5903, Droplet Impingement and Ice Accretion Computer Codes),
9.2 Flight in measured natural icing conditions,
9.3 Icing wind tunnel tests (see SAE ARP5905, Calibration and Acceptance of Icing Wind Tunnels), and
9.4 Flight in a measured controlled simulated icing cloud (e.g., behind an icing tanker, see SAE ARP5904, Airborne Icing Tankers).
10 CRITICAL ICE SHAPES.
When developing critical ice shapes, the applicant should consider ice accretions that will form during all phases of flight and those that will occur before activation of the IPS. If applicable, runback, residual, and inter-cycle ice accretions should also be considered.
10.1 The applicant should assume that, during holding conditions, the airplane will remain in a rectangular “race track” pattern, with all turns being made within the 17.4 nautical mile icing cloud. Therefore, no horizontal extent correction should be used for this analysis.
10.2 The applicant should substantiate the drop diameter distribution (mean effective, median volume, spectra), liquid water content, and temperature that will cause formation of an ice shape critical to the airplane’s performance and handling qualities.
10.3 Ice roughness used should be based on icing tunnel, natural icing, or tanker testing.
10.4 See AC 20-73A for more information on critical ice shapes.
11 COMPLIANCE TESTS (FLIGHT/SIMULATION).
11.1 The following paragraphs address methods normally used to show compliance with §§ 25.1419 and 25.1420. These requirements state that the airplane must be able to operate safely in the continuous maximum and intermittent maximum icing conditions described in Appendix C and in the applicable icing conditions described in Appendix O. The airplane should be shown to comply with certification requirements when all IPS are installed and functioning. This can normally be accomplished by performing tests in those conditions found to be most critical to basic airplane aerodynamics, IPS design, and powerplant functions. All IPS equipment should perform its intended function throughout the entire operating envelope.
11.2 The primary purposes of flight testing are to—
11.2.1 Determine that the IPS is effective and performs its intended functions during flight as predicted by analysis or ground testing.
11.2.2 Evaluate any degradation in performance and flying qualities.
11.2.3 Show compliance with the performance and handling qualities requirements identified in § 25.21(g). Flight tests to show compliance with these requirements are addressed in
24 AC 25-25A. Flight testing also verifies the adequacy of flightcrew procedures as well as limitations for the use of the IPS in normal, abnormal, and emergency conditions.
11.2.4 Confirm that the powerplant installation as a whole (engine, inlet, anti-ice system, etc.)
performs satisfactorily in icing conditions. If the auxiliary power unit performs essential functions, it should be evaluated for performance in icing conditions. In accordance with part 33, engine icing certification results should also be carefully reviewed before any part 25 engine or airframe icing flight tests to determine if there were any anomalous results requiring special airplane/engine operating procedures.
11.3 Both §§ 25.1419 and 25.1420 require two or more means of compliance for flight in icing approval. It is common to use a combination of methods in order to adequately represent the conditions and determine resulting degradation effects with sufficient confidence to show compliance.
11.3.1 Dry Air Ground Tests.
Depending on details of the IPS design, some preliminary ground tests of the equipment may be warranted to verify the basic function of each item. The applicant should collect quantitative data, as necessary, to verify the system designs. These data should include operating temperatures of thermal devices, deicing fluid flow rates, and operating pressures and rates of inflation of pneumatic components.
11.3.2 Dry Air Flight Tests with Ice Protection Equipment Operating.
The first flight tests conducted to evaluate the airplane with the IPS operating are usually dry air flight tests. The initial dry air tests are conducted to verify that the IPS does not affect flying qualities of the airplane in clear air, and to obtain a thermal profile of an operating thermal IPS.
11.3.3 Typical Dry Air Flight Test Practices for Commonly Used Ice Protection Systems.
Several commonly used IPSs and components are discussed below to illustrate typical dry air flight test practices. Other types of equipment should be evaluated as their specific design dictates.
11.3.3.1 Thermal Ice Protection Leading Edge Systems.
Dry air flight tests are conducted to verify the system design parameters and thermal performance analyses.
11.3.3.1.1 Normally, instruments are installed on system components to measure the anti-icing mass flow rate or energy input (for electrical systems), supply air temperature, and surface temperatures. The dry air test plan generally includes operating conditions such as the climb, holding, and descent phases of a normal flight profile. Since the presence of moisture can affect surface temperatures, tests should be conducted where no visible moisture is present.
11.3.3.1.2 Measurements of supply air mass flow rate, energy input, and air temperature allow determination of how much heat is available to the
25 system. The adequacy of the IPS can then be demonstrated by comparing the measured data to the theoretical analysis for both Appendix C and either Appendix O for compliance with § 25.1420(a)(3) or the portion of Appendix O proposed for compliance with § 25.1420(a)(2). Surface temperatures measured in the dry air, for example, can be useful in extrapolating the maximum possible leading edge surface temperature in flight, the heat transfer characteristics of the system, and the thermal energy available for the IPS. Supply air temperatures or energy input may also be used to verify that the IPS materials were appropriately chosen for the thermal environment.
11.3.3.2 Pneumatic Leading Edge Boots.
Tests should demonstrate a rise and decrease in operating pressures, which results in the effective removal of ice. This pressure rise time, as well as the maximum operating pressure for each boot, should be evaluated throughout the altitude range defined in Appendices C and O. Boots should be operated in flight at the minimum envelope temperature (-22 °F) of Appendix C continuous maximum icing conditions and Appendix O.
This is to demonstrate adequate performance throughout the entire flight envelope and to demonstrate that no damage occurs during inflation and deflation. The appropriate speed and temperature limitation (if any) on boot activation should be included in the AFM. Boot inflation should have no significant effect on airplane performance and handling qualities.
11.3.3.3 Electrically Heated Propeller Boots.
For compliance with provisions of §§ 25.901(c), 25.903(b), 25.929, and
25.1419(c):
11.3.3.3.1 When flight testing in dry air, surface temperature measurements should be made and monitored to confirm proper function. These measurements are useful for correlating analytically predicted dry air temperatures with actual temperatures, and as a general indicator that the system is functioning and that each deicer is heating. It is suggested that system current, brush block voltage (i.e., between each input brush and the ground brush), and system duty cycles be monitored to ensure that adequate power is applied to the deicers.
11.3.3.3.2 System operation should be checked throughout the full revolutions per minute and propeller cyclic pitch range expected during flight in icing. All significant vibrations should be investigated.
11.3.3.3.3 The applicant should consider the maximum temperatures a composite propeller blade may be subjected to when deicers are energized. It may be useful to monitor deicer bond-side temperatures. When performing this evaluation, the most critical conditions should be investigated (e.g., airplane on the ground or propellers not rotating) on a hot day with the system inadvertently energized.
11.3.3.4 Windshield Anti-Ice.
Section 25.773(b)(1)(ii) requires that the airplane have a means of maintaining a clear portion of windshield in the icing conditions defined in Appendix C and in certain Appendix O icing conditions. The applicant should conduct dry air flight tests to verify the thermal analysis.
Measurements of both the inner and outer surface temperature of the protected windshield area may be needed to verify the thermal analysis.
Thermal analysis should show that the windshield surface temperature is sufficient to maintain anti-icing capability without causing structural damage to the windshield. An evaluation of visibility, including distortion effects through the protected area, should be made for both day and night operations. In addition, the size and location of the protected area should be reviewed to confirm that it provides adequate visibility for the flightcrew, especially during the approach and landing phases of flight.
Analysis of exposures to Appendix C and Appendix O conditions should consider holding operations consistent with the applicable “Holding Ice” definition contained in Part II of those appendices.
11.3.3.5 Bleed Air Systems.
Effects of bleed air extraction on engine and airplane performance, if any, should be examined and included in the AFM performance data. The surface heat distribution analysis should be verified for varying flight conditions including climb, cruise, hold, and descent. Temperature measurements may be necessary to verify the thermal analyses. In accordance with provisions of § 25.939(a), the maximum bleed air for ice protection should have no detrimental effect on engine operation throughout the engine’s power range.
11.3.4 Dry Air Flight Tests with Predicted Simulated Ice Shapes and Roughness.
The primary function of dry air flight tests with simulated ice shapes is to demonstrate the ability of the airplane to operate safely with an accumulation of critical ice shapes based on exposure to Appendix C icing conditions and applicable Appendix O icing conditions. The specific flight tests used to evaluate airplane performance and handling qualities are addressed in AC 25-25A. In addition, effects of ice accretion on locations forward of pitot-static sensors, angle-of-attack sensors, and static pressure ports should be assessed. For failure conditions of the IPS that are not extremely improbable, validation testing may be required to demonstrate that the effect on safety of flight (as measured by degradation in flight characteristics) is commensurate with the failure probability. Such validation testing may consist of flight tests, flight simulator tests, wind tunnel tests, or validated CFD analyses.
11.3.5 Icing Flight Tests.
Flight tests in measured natural icing and tests performed with artificial icing tools, such as icing tankers, are normally used to demonstrate that the IPS performs during flight as predicted by analysis or other testing. Such tests are also used to confirm analyses used in developing the various components, such as ice detectors, and ice shapes.
27 Section 25.1419 requires measured natural icing flight tests within the icing conditions of Appendix C. If necessary, measured natural icing flight tests should be accomplished in Appendix O conditions. The natural icing flight tests are accomplished to corroborate the general nature of the effects on airplane handling characteristics and performance determined with simulated ice shapes (see AC 25-25A), as well as to qualitatively assess the analytically predicted location and general physical characteristics of the ice accretions. If necessary, there should be a means to measure and record ice accumulations and impingement limits. These can be approximated by various means, such as a rod mounted on the airfoil and black paint on the airfoil to increase the contrast between the ice accretion and the airfoil.
11.3.5.1 Instrumentation.
The applicant should plan sufficient instrumentation to allow documentation of important airplane, system, and component parameters, as well as icing conditions encountered. The following parameters should be considered:
11.3.5.1.1 Altitude.
11.3.5.1.2 Airspeed.
11.3.5.1.3 Engine power level or speed.
11.3.5.1.4 Propeller speed and pitch, if applicable.
11.3.5.1.5 Temperatures that could be affected by ice protection equipment or ice accumulation or that are necessary for validation of analyses, such as the temperatures of—
• Static air,
• Engine components,
• Electrical generation equipment,
• Surfaces, and
• Structural components.
11.3.5.1.6 Liquid water content. This should be measured over the complete water drop size distribution.
11.3.5.1.7 Median volume drop diameter and drop diameter spectra. When measurement of the icing environment drop diameter is necessary, instrumentation used for measuring drop sizes should be appropriate for the Appendix C and Appendix O icing conditions. For airplanes to be certified to a portion or all of Appendix O, measuring and recording drop diameter spectra should be accomplished.
11.3.5.1.8 Microscopic measurement of drop impact craters on a gelatin oil or soot slide exposed to an icing cloud may be an impractical way of measuring a median volume drop diameter because of the splashing and bouncing characteristics of large drops. Also, gelatin oil or soot slides are impractical for recording the drop diameter spectra of Appendix O type clouds. MVDs and drop diameter distributions are more practically obtained with more sophisticated equipment, such as laser-based drop measuring and recording instruments, or their equivalent. Depending on the drop size measurement capability of the instruments used, more than one instrument may be required to measure the expected range of drop diameters. If testing is performed with a calibrated icing tanker, large drop instrumentation on the test airplane may not be necessary. Some icing conditions may contain mixed phase icing (liquid plus ice). The instrumentation should provide the ability to distinguish between liquid drops and ice crystals. Equipment should be calibrated and properly maintained to obtain quality data.
11.3.5.2 Artificial Icing.
11.3.5.2.1 Flight testing in artificial icing environments, such as behind icing tankers, is one way to predict capabilities of individual elements of the ice protection equipment and to determine local ice shapes. Since the ice plume has a limited cross-section, testing is usually limited to components, such as heated pitot tubes, antennas, air inlets including engine induction air inlets, empennage, airfoil sections, and windshields. Calibration and verification of the icing cloud produced by the tanker should be accomplished as necessary for meeting test objectives. SAE ARP5904 provides recommended practices for using tanker icing simulations.
11.3.5.2.2 Use of an icing tanker can provide high confidence in local icing effects.
But obtaining small drop sizes may be difficult with some spray nozzles.
As a result, these methods could produce larger ice build-ups and different ice shapes than those observed in natural Appendix C icing conditions.
Icing tanker techniques can be used in a manner similar to icing tunnel testing with respect to ice shape development. The plume may be of sufficient size that it could be applied to sections of the airframe to examine any potential hinge moment or CLMAX effects from ice accretions behind protected areas in Appendix C or Appendix O conditions. This method also has the advantage of being able to combine the effects of thermal systems (such as runback) with direct accretion to simulate resulting ice accumulations. Tanker simulation could also be used to estimate accretion areas for use in estimates of drag from extended accretion regions (such as ice accretions extending beyond a radome) during exposures to Appendix O conditions. Atmospheric effects such as humidity and drop residence time (time required to bring the drop to static temperature) should be considered in this type of testing.
11.3.5.3 Appendix C Natural Icing Flight Testing.
11.3.5.3.1 Section 25.1419(b) requires measured natural icing flight tests. Flight tests in measured natural icing conditions are intended to verify the ice protection analysis, to check for icing anomalies, and to demonstrate that the IPS and its components function as intended. AC 20-73A provides additional information useful for planning a natural icing flight test program. The airplane should be given sufficient exposure to icing conditions to allow extrapolation to the envelope critical conditions by analysis. Test data obtained during these exposures may be used to validate the analytical methods used and the results of any preceding artificial icing tests.
11.3.5.3.2 Past experience indicates that an airplane performing flight testing in natural intermittent maximum icing conditions may be exposed to lightning strikes, severe turbulence, and hail encounters that could extensively damage it. When design analyses show that the critical ice protection design points (i.e., heat loads, critical ice shapes, accumulation, accumulation rates, etc.) are adequate under these conditions, and sufficient ground or flight test data exist to verify the analysis, a flight test in intermittent maximum icing conditions may not be necessary.
11.3.5.3.3 Flight testing in natural icing conditions should also be used to verify AFM procedures for activation of the IPS, including recognition and delay times associated with IPS activation. Such testing should verify the analytically predicted location and general physical characteristics of the ice accretions. Critical ice accumulations should be observed, where possible, and sufficient data taken to allow correlation with dry air testing.
Remotely located cameras either on the test airplane or on a chase airplane have been used to document ice accumulations on areas that cannot be seen from the test airplane’s flight deck or cabin.
11.3.5.3.4 For an airplane with a thermal deicing system, the applicant should demonstrate the effectiveness of the deicing operation either in artificial icing conditions or during a natural icing flight test certification program.
The tests usually encompass measurements of the surface temperature time history. This time history includes the time at which the system is activated, the time at which the surface reaches an effective temperature, and the time at which the majority of ice is shed from the leading edge.
The data should be recorded in the flight test report.
11.3.5.3.5 The applicant should perform a test to determine if moisture can be trapped and prevent normal operation of deicing boots or surface mounted sensors if the moisture freezes during flight. To verify proper operation of the deicing boots and surface mounted sensors, the airplane should operate on the ground in moderate rain; take off and fly through moderate rain;
30 and climb and maintain an altitude above the freezing level. During this test, all IPS should be operated in accordance with the AFM procedures.
11.3.5.4 Appendix O Natural Icing Flight Testing.
Section 25.1420(a) requires airplanes with a certified maximum takeoff weight less than 60,000 pounds or equipped with reversible flight controls to operate safely in the supercooled large drop icing conditions defined in part 25, Appendix O.
11.3.5.4.1 Unless shown by other means, such as discussed in paragraph 11.3.5.4.2, flight testing in measured natural Appendix O icing conditions may be necessary to—
11.3.5.4.1.1 Verify the general physical characteristics and location of the simulated ice shapes used for dry air testing and, in particular, their effects on airplane handling characteristics,
11.3.5.4.1.2 Determine if ice accretes on areas where ice accretion was not predicted,
11.3.5.4.1.3 Verify adequate performance of ice detectors or visual cues,
11.3.5.4.1.4 Conduct performance and handling quality tests as outlined in § 25.21(g), and
11.3.5.4.1.5 Evaluate effects of ice accretion not normally evaluated with simulated ice shapes (on propeller, antennas, spinners, etc.) and evaluate operation of each critical airplane system or component after exposure to Appendix O conditions.
11.3.5.4.2 Flight testing in natural Appendix O icing conditions should not be necessary if similarity is shown to an airplane that has shown compliance with § 25.1419 and has a successful service history, or if—
11.3.5.4.2.1 The design analyses show that the critical ice protection design points (i.e., heat loads, critical ice shapes for performance and handling qualities, accumulation, accumulation rates, etc.) are adequate under Appendix O conditions and various airplane operational configurations, and
11.3.5.4.2.2 Except where Appendix E of this AC allows the use of one method to perform the analysis in paragraph 11.3.5.4.2.1 above, at least two different methods of predicting Appendix O ice accretions must be shown to provide similar results (ice accretion thickness, location). One method should be either an icing wind tunnel or icing tanker test.
Note: Icing tunnels and analysis with CFD codes are considered two different methods, but two different CFD codes are not.
11.3.5.4.2.3 Determination of the appropriate analyses and/or tests should include consideration of the need to evaluate more than one airplane component simultaneously.
11.3.5.4.2.4 One consideration for evaluating more than one component simultaneously could include evaluating airplane performance with propeller and airframe ice accretion or with asymmetric ice accretions from propeller wash.
11.3.5.4.2.5 Another consideration for evaluating more than one component simultaneously could include evaluating engine performance with inlet (including cooling ducts for engine accessories) ice accretions.
11.3.5.4.2.6 Another consideration for evaluating more than one component simultaneously could include evaluating the stall warning system and characteristics with ice accretion that affects air data used by stall protection systems.
11.3.5.4.3 The need for flight testing in natural Appendix O icing is reduced for airplanes limited to exiting from Appendix O conditions according to § 25.1420(a)(1) as a result of reduced exposure to such conditions.
11.3.5.4.4 Flight testing in natural Appendix O icing conditions should be accomplished for airplane derivatives whose ancestor airplanes have a service record that includes a pattern of accidents or incidents due to in-flight encounters with Appendix O conditions. For derivative airplanes whose ancestor airplane is free of accidents or incidents recorded in icing conditions, flight testing in natural Appendix O icing conditions should not be necessary.
11.3.6 Fluid Anti-Icing/Deicing Systems.
Flight testing should include evaluation of fluid flow paths to confirm that adequate and uniform fluid distribution over the protected surfaces is achieved. Fluid flow paths should be determined when the fluid is mixed with impinging water during system operation. A means of indicating fluid flow rates, fluid quantity remaining, etc., should be evaluated to determine that the indicators are plainly visible to the pilot and that the indications provided can be effectively read. A shutoff valve should be provided in systems using flammable fluids. Fluid anti-icing/deicing systems may be used to protect propellers and windshields as well as leading edges of airfoils. The fluid for windshield fluid anti-ice systems should be tested to demonstrate that it does not become opaque at low temperatures. The AFM should include information advising the flightcrew how long it will take to deplete the amount of fluid remaining in the reservoir.
11.3.7 Icing Wind Tunnel Tests.
11.3.7.1 Icing wind tunnels provide the ability to simulate natural icing conditions in a controlled environment. A variety of Appendix C icing conditions and