Appendix D D-4 D.4 ENGINE INLET OR ACCESSORY INLET CHANGES.
Guidance for engine or accessory inlet changes is provided in section D.2, Engine Changes, above. Section 25.1093 applies to engine oil and accessory cooling inlets as well as induction inlets.
D.5 ANTENNAS, INSTALLATIONS, OR OTHER EXTERNAL MODIFICATIONS.
D.5.1 When antennas, cameras, fairings for such installations, or other external installations such as drain masts are installed on an airplane, the applicant should show that:
D.5.1.1 The predicted ice accretion does not contribute significantly to drag.
D.5.1.2 There is no ice-shedding hazard due to impact or ingestion on downstream structure, engines, or propellers.
D.5.1.3 There is no ice-related performance reduction of lifting surfaces.
D.5.1.4 There is no ice-related effect on downstream air-data sensors or ice detectors.
D.5.2 For paragraphs D.5.1.1 and D.5.1.2 above, the applicant may first conduct a very conservative, simple analysis. If the initial analysis fails, it can be refined to determine if it was overly conservative. The conservative analysis can assume the following:
D.5.2.1 The water catch area is the full frontal area of the installation.
D.5.2.2 Collection efficiency is 1.
D.5.2.3 No runback or evaporation of impinging water will occur.
D.5.2.4 The ice shapes on blade antennae will be similar to airfoils, and the shapes on low profile antennae will be single horn shapes.
D.5.3 The applicant should determine the critical icing condition, which should include the airplane holding in continuous maximum conditions. If the analysis shows a problem, then the applicant can accomplish one or more of following:
D.5.3.1 Determine realistic collection efficiency either with an ice accretion code or with the FAA Technical Report DOT/FAA/CT-88/8-1, Aircraft Icing Handbook.
D.5.3.2 Determine the real impingement limits by using an icing code, which may reduce the collection area.
D.5.3.3 Run the full configuration in an icing code to determine if the installation is in a shadow zone.
D.5.3.4 If drag is a problem, run an ice accretion code to determine a more realistic ice shape.
Appendix D D-5 D.5.4 The applicant should conduct flight tests in measured natural icing or with simulated ice shapes to determine if the ice accretions will have any detrimental effects if any of the following conditions exist:
D.5.4.1 The installation is upstream of air-data sensors or an ice detector.
D.5.4.2 The installation is on a lifting surface.
D.5.4.3 The installation could create a wake on a lifting surface. As an example, if an external modification is large enough (e.g., dish antenna), it may interfere with the flow field around the tail, and the susceptibility to ice contaminated tailplane stall may need to be addressed.
D.5.5 The one exception to paragraph D.5.4.2 above is fairings. For fairings, an analysis to show the impact on maximum lift coefficient, in combination with flight tests with no ice accretions, may be acceptable.
D.6 GROSS WEIGHT INCREASES.
If increased gross weight requires an evaluation of the airplane’s handling qualities in icing conditions, the effect on ice accretions from the associated change in airplane attitude should be assessed. This assessment is necessary because impingement and icing limits on airfoils will change, for example, and accretion sites may form on the airframe that was previously free of ice.
D.7 CENTER OF GRAVITY ENVELOPE INCREASE.
Generally, the same guidance used for gross weight increases can be used for center-ofgravity envelope increases. The one exception is that a more forward center-of-gravity limit may make an airplane more susceptible to ICTS. For airplanes with unpowered, reversible elevators, or for airplanes with propellers, the applicant should address this by flight testing. An analysis may be acceptable for other airplanes.
D.8 FLIGHT ENVELOPE INCREASE.
D.8.1 If the applicant applies for an increase in maximum operating altitude, the applicant should demonstrate the following:
• The IPS operating pressures (for pneumatic systems) or temperatures (for thermal air) by dry air testing; and
• Stall characteristics with ice accretions.
D.8.2 The effect of increased cruise airspeeds and increased altitudes that could affect windshield ice accretion, and adequacy of the windshield heat, should be addressed.
Appendix D D-6 D.9 TURBOPROP CONVERSION.
D.9.1 If the IPSs use engine bleed air for operation, the pneumatic lines may accumulate more water than the current unmodified type design. This water can subsequently refreeze and block the lines, resulting in failure of some or all zones of the pneumatic system.
The applicant must show that the pneumatic deicing system will continue to function in icing conditions.
D.9.2 The pneumatic deicer operating pressure may also decrease at lower engine rpm. A minimum engine rpm for acceptable pneumatic operating pressure, which should allow for descent, should be established and published in the airplane flight manual.
D.10 MODIFICATIONS TO LIFTING AND CONTROL SURFACES.
If lifting and control surfaces are modified, critical ice accretions may have to be re-defined, especially if the changes affect operational wing angle-of-attack. This new definition includes pre-activation, inter-cycle, residual, and runback icing. Stall strips are collectors of ice and are an example of an area where leading edge ice accretions should be re-defined. If ice accretions change because of modifications, or if the modifications could affect control power or hinge moments, additional flight testing may be necessary. Refer to AC 25-25A for guidance on flight testing.
D.11 INSTALLATION OF VORTEX GENERATORS.
Depending on their location, vortex generators may accumulate ice. The applicant should provide data on expected ice accretions. Flight conditions to consider are hold, descent, and approach. Substantiation of the effects on stall speeds, stall characteristics, and stability and control should be provided.
D.12 MODIFICATIONS TO ICE PROTECTION SYSTEMS.
If IPSs are modified, critical ice accretions may have to be re-defined. This new definition would include pre-activation, inter-cycle, residual, and runback icing. If the critical levels of ice accretions are changed, or if the modifications to the IPSs could affect control power or hinge moments, the applicant should perform flight testing with simulated ice accretions. AC 25-25A provides guidance on flight testing.
Appendix E E-1 Appendix E. Capabilities of Engineering Tools for Compliance with Part 25, Appendix O Requirements E.1 ASSESSMENT OF ENGINEERING TOOL CAPABILITIES.
E.1.1 This appendix is the result of a 2009 evaluation of engineering tool capabilities for predicting Appendix O ice accretions and icing effects. This appendix provides an assessment of the engineering tool capabilities based on their state of development in 2009. In the main body of this AC, for purposes of plain language, the term supercooled large drop was spelled out. In this appendix, however, because the term appears so frequently, its acronym, SLD, will be used.
E.1.2 Table E-1 of this appendix graphically illustrates results of an assessment of the capabilities of engineering icing tools. As described in the legend to that table, the capability of each tool for the various applications is classified as green, yellow, or orange. As discussed in the main body of this AC, reliance on available simulation methods combined with engineering judgment will be required for finding compliance with the requirements of part 25, Appendix O. Even though a simulation tool type may be classified as green, an applicant should ensure that the specific tool is appropriate for its application and is used in a conservative manner, including consideration of criticalcase icing conditions. General and specific concerns that should be considered are discussed later in this appendix.
E.1.3 Section E.2 of this appendix provides guidance on the tools, their capabilities, and best practices for tool use with respect to Appendix O conditions. Section E.3 of this appendix discusses means of compliance for specific airplane components.
Sections E.4, E.5, and E.6 of this appendix discuss means of compliance for each of the three certification options given in § 25.1420. Section E.7 of this appendix discusses means of compliance for air-data sensors and windshields (§§ 25.1323, 25.1325, and
25.773). Section E.8 of this appendix defines standard roughness levels.
E.1.4 While table E-1 illustrates the state of simulation capabilities as of 2009, we expect that capabilities will improve in the future. We encourage applicants and research agencies to develop and validate the engineering tools currently classified in table E-1 as yellow and red.
Appendix E E-2 Table E-1. Assessment of Appendix O Engineering Tool Capabilities Wing Tail Radome Thermal (protected area) Thermal (Aft of protected area) Mechanical (protected area) Mechanical (aft of protected area) Fluid Freezing Point Depressant Instrument (performance) Instrument (performance) Icing Tunnels G G R * G G G G G G Y * G R * G G R * G Codes G G Y G R* * G R R R G G Y * * R* * R Tankers R R R R R R R R R* * R Icing Tunnels G G R* G G Y Y G G Y * G R* G G R* G Codes G G Y G R* * G R R R G G Y * * R* * R Tankers R R Y Y Y Y Y Y Y Y Icing Tunnels Y Y R* Y Y Y Y G R R Codes Y Y R* * Y R* * Y R R R Y R R Tankers R R R R R R R R R R Icing Tunnels R R R* R R R R R R R Codes Y Y R* * Y R* * Y R R R Y R R Tankers R R R R R R R R R R LEGEND Updated FEB 2009 Green (G) Yellow (Y) Red ( R ) * * * R R Y R R Y R G R G R G R FZRA MVD > 40µm R R R Y R G This testing capability exists today and is suitable to be an element of a means of compliance, or is readily achievable based on current experience.
This testing capability is possible, but has not been demonstrated, or there is limited or no validation.
This testing capability is unknown, or does not currently exist.
Detection Methods Air Data Sensors R R Y G G It may be possible to test small-scale installation effects, but large-scale installations are not currently feasible.
Current 2-D capabilities exist with large droplet effects, but limitations exist in the use of 3-D codes for simulation of Appendix O effects.
FZDZ MVD < 40µm FZDZ MVD > 40µm Instrument (position or installation effects) Non-lifting Surfaces (antenna, inlets, external modifications) G R G RR Y Unprotected Areas Visual Cues (Reference Surface) Instrument (position or installation effects) Protected Areas FZRA MVD < 40µm G Y R R G Y
Appendix E E-3 E.1.5 This evaluation of icing test tool capabilities is predominantly based on assessment of National Aeronautics and Space Administration (NASA) LEWICE codes (2-D and 3- D), the NASA Icing Research Tunnel (IRT) facilities, and existing icing tanker information such as SAE ARP5904, Airborne Icing Tankers. International icing simulation codes and facilities were considered where information was available. The US Air Force icing tanker that was used for SLD investigations on select commercial airplanes during the 1990s is currently not operational, and its capabilities were not assessed.
E.1.6 The main body of this AC states that flight testing in natural Appendix O icing conditions should not be necessary if at least two methods of predicting Appendix O ice accretions are shown to provide similar results. Table E-1 of this appendix illustrates that, for some Appendix O conditions, few of the engineering tools are classified as green or yellow for use as a means of compliance. Radomes, for example, have no engineering tools classified as green. In other cases, such as areas aft of protected areas, there is only one simulation tool that is classified as green.
E.1.7 For testing in freezing rain (FZRA), there are very few engineering tools classified as green. This would indicate that the primary method for showing compliance available at this time appears to be flight testing in natural Appendix O icing conditions. In some cases, engineering judgment may allow other methods of compliance, even though not validated, when used in a fashion similar to that recommended for freezing drizzle (FZDZ). However, until the engineering tools become more mature, flight tests in natural Appendix O icing conditions will likely be necessary to achieve the confidence needed for certifying airplanes for unrestricted flight in Appendix O or a portion of Appendix O conditions (i.e., compliance with § 25.1420(a)(3) or § 25.1420(a)(2)).
E.1.8 For airplanes certificated to detect Appendix O icing conditions and safely exit all icing conditions (i.e., compliance with § 25.1420(a)(1)), it may be acceptable to use one tool alone, even if classified as yellow or red, without natural icing flight tests, if that tool is used in a conservative manner. In cases where there are two green tools available, the applicant should use both tools to verify results, as discussed in the main body of this AC.
E.1.9 The number of tests may be reduced through the use of conservative test methods and critical-point analyses. However, validation of the test methods should be accomplished until confidence is provided.
E.2 CAPABILITIES AND LIMITATIONS OF ENGINEERING TOOLS AND SLD MEASUREMENT INSTRUMENTATION.
E.2.1 Icing Tunnel Appendix O Simulations.
E.2.1.1 A tunnel that uses a single pressure source for spray bars cannot simultaneously produce the large and small drop distributions (bi-model distributions) defined in
Appendix E E-4 Appendix O. However, NASA has developed a technique called “sequencing” that alternates large and small drop sprays to simulate Appendix O conditions.
E.2.1.2 When icing tunnels are used as an element of the means of compliance:
E.2.1.2.1 Scale effects must be considered for tunnel blockage effects for all tests.
E.2.1.2.2 For FZDZ with a median volume diameter (MVD) less than 40 µm, the cloud drop distributions in the IRT are similar to existing Appendix C calibrations.
E.2.1.2.3 If there are concerns about the bi-modal distribution affecting performance of IPSs, sequencing should be considered. Sequencing of freezing drizzle conditions has been demonstrated in the IRT for unprotected surfaces. The sequencing technique approximates drop distributions found in natural conditions (AIAA 2005-76, Simulation of a Bimodal Large Droplet Icing Cloud in the NASA Icing Research Tunnel). It results in rougher textures than Appendix C ice shapes.
E.2.1.2.4 Ice shapes in unprotected areas are generally considered critical from the perspective of producing the largest disruption to the airflow. With a large ice shape, details on the impingement limit characteristics, which may have a less critical effect, may not be essential. However, sequencing between large and small drop conditions may be necessary if standard sprays2 do not produce the drop distribution appropriate for simulation of the conditions desired. In cases where the impingement limit characteristics are important, sequencing may be necessary.
E.2.1.2.5 In general, sequencing produces rougher textures than a standard spray.
E.2.1.2.6 Sequencing may not be an appropriate way to test thermal systems because the rate of mass flow per unit area (i.e., mass flux) of the incoming water is not the same for the small and large drop sprays.
E.2.1.2.7 If sequencing is used to test deicing systems, the ratio of sequencing time to shed cycles should be evaluated to ensure that sequencing does not inappropriately affect the ice shape.
E.2.1.3 Experimental tests of methods for designing subscale hybrid airfoils with full-scale leading edges to determine leading edge ice shapes for large-chord airfoil sections has been accomplished in icing wind tunnels.3,4 These techniques have been used successfully for Appendix C icing conditions. Adapting them to icing tests for SLD
2 The phrase “standard spray” method refers to using the IRT nozzles in off-design conditions to generate larger drops for SLD conditions.
3 Saeed, Farooq, Selig, Michael S. and Bragg, Michael B., Design of Subscale Airfoils with Full- Scale Leading Edges for Ice Accretion Testing, Journal of Aircraft, Vol. 34, No.1, 1997.
4 Saeed, Farooq, Selig, Michael S., Bragg Michael B., and Addy, Harold E. Jr., Experimental Validation of the Hybrid Airfoil Design Procedure for Full-Scale Ice Accretion Simulation, AIAA Paper No. 98-0199, Reno, NV, January 12-15, 1998.
Appendix E E-5 conditions requires evaluating the areas of interest for impingement analysis and analysis of the flow-field to determine if scale conditions aft of the leading edge can be met considering the compromises necessary in design variables on circulation, velocity distribution, and impingement characteristics. Although we anticipate that the hybrid airfoil design technique would be applicable for SLD conditions, such use has not been tested and validated to date.
E.2.2 Computational Fluid Dynamic Tools.
E.2.2.1 Computational fluid dynamic (CFD) tools—the “codes” referred to in table E-1 of this appendix—are used extensively in certification for Appendix C conditions. CFD tools can provide valuable information on impingement limits, icing limits, ice size, ice shape, and ice thickness for Appendix O conditions. Some validation of collection efficiency and ice shapes has been accomplished for FZDZ; there have been no validation exercises for FZRA.
E.2.2.2 CFD tools have been devised to predict, using mathematical calculation, ice accretion on an airplane in SLD conditions and the behavior of various types of IPSs as a result of those accretions. Besides being useful for assessing effects on protected surfaces, these tools can also account for the possibility of SLD ice impingement beyond the IPS limits, as well as for possible water runback. No current CFD method, however, can identify the break-up of water into rivulets, roughness formation, or ice sliding that may occur under these circumstances. Thus analysis of the regions behind those that are protected by IPSs requires some combination of CFD results, empirical data and test results (if available), and engineering judgment. This usually consists of determining the extent of possible ice formation using some criteria from the computational analysis, such as ice extent, impingement limits, or some minimum ice thickness level. The resulting ice shape would include inter-cycle or residual ice on the protected region and ice formed aft of the protected region. This result is then combined with guidance on ice roughness levels, such as described elsewhere in this document, to produce a rough ice region that can be evaluated in wind tunnel testing or flight tests.
E.2.2.3 Information can be calculated for drop trajectories for evaluating sensor locations and potential visual cues.
E.2.2.4 Many non-lifting surfaces require testing with 3-D computer codes. At the current time, many 3-D codes do not have large drop effects (such as splashing and break-up). Even without large drop effects, however, 3-D codes can offer information on impingement limits. Although 3-D codes may generate physical models and correlations that can support analysis of large drop icing, the capabilities of performing analyses with 3-D SLD CFD tools have not yet been evaluated. There are codes that may have this potential, but no guidance can be offered at this time for their use.
E.2.2.5 Some codes have limited capabilities with short-chord geometries (e.g., antennas or struts) for Appendix C icing conditions. These limitations are expected to be similar for large drop icing and are typically addressed with empirical methods or icing tunnels.
However, for non-lifting surfaces, conservative assessments may be acceptable, such as
Appendix E E-6 assuming drop impingement on the full frontal area, assuming the collection efficiency is one, and approximating the shapes appropriate for the temperature (glaze, rime, etc.).
E.2.3 Icing Tankers.
E.2.3.1 Icing tankers have been used extensively by some manufacturers for Appendix C icing certifications. Icing tankers typically have a limited plume size and have been used primarily for localized icing effects, such as ice shedding, and for assessing the thermal performance of anti-ice systems.
E.2.3.2 Current tankers do have some limited capabilities to produce freezing-drizzle-sized drops, but they cannot produce the distribution effects. Current tankers do not produce freezing rain distributions either, and the feasibility of producing such conditions is likely limited by drop break-up (due to deceleration effects) and the ability to sub-cool the large drops within a workable flight envelope. Additionally, drop sorting effects are likely because of higher fall rates of large drops within an Appendix O distribution.
E.2.4 Instrumentation.
E.2.4.1 When making in situ measurements of Appendix O conditions, it is important to note that technology to make such measurements is rapidly changing. It is essential to consult experts in all phases of the measurement program, including those aware of the latest problems and strengths of each probe. Applicants should use instrumentation suited to the task. Instrumentation must be mounted in appropriate locations on the airplane, where the measurements are not affected by the airflow. For certification purposes, the instrumentation must be calibrated. An often overlooked aspect of a measurement program is the need to calibrate the instruments. Appropriate software and analysis techniques are also essential because complicated algorithms are often needed in the analysis.
E.2.4.2 Instruments are required to measure particle concentrations as a function of size over the complete size range—2 µm to at least 1,500 µm—including cloud drops and precipitation drops. This may require more than one probe. Liquid water content (LWC) and ice water content (IWC) measurements obtained by integrating 2-D images from spectral measurements generally have larger errors than those obtained from probes specifically designed to make such measurements. Consequently, it is recommended that probes designed to measure LWC, and if necessary IWC, be used directly, recognizing that some hot-wire devices detect larger drops (>50 µm) with reduced efficiency. Mixed phase clouds can occur frequently, so it is necessary to be able to discriminate between ice and liquid particles, especially for sizes larger than 50 µm, so that ice particles are not incorrectly identified as SLDs. For detect-and-exit airplanes (those certifying to § 25.1420(a)(1)), it may not be necessary to measure IWC directly;
but for airplanes using natural icing SLD flight tests to certify for a portion of Appendix O (§ 25.1420(a)(2)) or for unrestricted operations (§ 25.1420(a)(3)), IWC must be determined in order to assess the SLD conditions.
Appendix E E-7 E.3 COMPONENT EVALUATIONS.
E.3.1 Lifting Surfaces.
E.3.1.1 This paragraph is applicable to anti-icing systems aft of protected areas and to deicing systems both on and aft of protected areas.
E.3.1.2 For detect-and-exit airplanes (§ 25.1420(a)(1)) in freezing drizzle conditions, icing tunnels alone may be used to develop ice shapes, provided that the model appropriately represents the airfoil beyond the FZDZ icing limit. Roughness may be evaluated in icing tunnel testing and replicated on the ice shapes for flight testing. The standard spray2 method should be used for anti-icing systems because of the varying mass flux of incoming water associated with sequencing. For deicing systems, it is acceptable to use either the standard spray or the sequencing technique.
E.3.2 Radomes.
E.3.2.1 Most radomes are too large to fit into existing icing wind tunnels. Additionally, computational analysis of radomes typically would require 3-D codes. Many 3-D codes do not have large drop effects and if they do not, freezing drizzle ice shapes cannot be simulated. However, all 3-D codes have capabilities for testing impingement limits.
Radome ice shapes have been developed in the past for Appendix C icing conditions using analysis and observed ice shapes from Appendix C flight tests (typically holding ice shapes).
E.3.2.2 For detect-and-exit airplanes (§ 25.1420(a)(1)) in freezing drizzle conditions, one method of compliance would be to modify Appendix C ice shapes to account for the larger impingement regions produced in FZDZ as predicted by the 3-D codes. You may use CFD codes to predict ice thickness. Ice roughness should be in accordance with the section E.8, Standard Roughness Levels for Appendix O Ice Shapes, of this appendix.
The radome ice tested should reflect the total mass associated with the icing exposures for § 25.1420(a)(1) airplanes, which are defined in part II of Appendix O.
E.3.3 Non-Lifting Surfaces (Antennas, Enhanced Vision Cameras, Struts, Auxiliary Inlets).
E.3.3.1 For non-lifting surfaces that do not require use of 3-D codes, 2-D codes in combination with icing tunnels are available as a means of compliance. However, many non-lifting surfaces require the use of 3-D codes. At the current time, many 3-D codes do not have large drop effects, although some codes may have this potential.
E.3.3.2 For detect-and-exit airplanes (§ 25.1420(a)(1)), if the non-lifting surface is not critical from an engine ingestion or airframe damage perspective, 3-D codes may provide sufficient information for compliance. However, for more critical surfaces, until large drop effects in 3-D codes are validated, icing tunnels alone may be used to develop ice shapes.
Appendix E E-8 E.3.4 Ice Detection Methods.
E.3.4.1 Different types of ice detection require assessment of their capabilities in large drop conditions, based on their sensing technology. Vibrating probe type ice detectors, which detect ice accretion on a probe through a decrease in the probe’s vibration frequency, may experience increased response time in large drop exposures because splashing and aerodynamic forces, particularly in near freezing temperatures, can cause water to shed from sensing surfaces. This physical behavior may also occur with other types of probes.
E.3.4.2 While CFD can determine whether the large drops impact the ice detection surface, available CFD codes cannot accurately predict aerodynamic forces that cause drop shedding, or freezing fraction effects that may delay freezing. Therefore, use of CFD alone is not acceptable for showing that ice detectors function in large drop conditions.
When possible, effects of installation position should be evaluated with a combination of codes and icing tunnels. Devices mounted on smaller surfaces could be assessed in an icing tunnel. However, if the device is mounted on the fuselage, and tunnel blockage effects would preclude a meaningful icing tunnel test, then CFD codes that adequately predict the shadowing and concentration effects may be used to verify that the equipment is properly located.
E.3.4.3 Because of the lack of engineering tools for FZRA, primary and advisory ice detectors used for compliance with § 25.1420(c) will require validation in natural large drop conditions to substantiate that the detectors function in all Appendix O conditions.
However, if ground testing can be substantiated with both FZDZ and FZRA drops representative of Appendix O distributions and temperatures, then validation in natural large drop conditions is not needed. If visual cues are certificated as the primary means of compliance with § 25.1420(a)(1)(i), and the airplane is equipped with an ice detector system that is not required for compliance with § 25.1420(a)(1)(i) or § 25.1420(c), then the ice detector need not be tested in natural Appendix O conditions. Certification of visual cues for detect-and-exit airplanes is discussed later in section E.4, Certification for Detect and Exit—§ 25.1420(a)(1), of this appendix.
E.3.5 Air Data Sensors.
E.3.5.1 Air Data Sensor Position Installation Effects.
When possible, applicants should evaluate the effects of air data sensor installation position with a combination of CFD codes and icing tunnels. Devices mounted on smaller surfaces could be assessed in an icing tunnel. However, if the device is mounted on the fuselage and tunnel blockage effects would preclude a meaningful icing tunnel test, then codes that adequately predict the shadowing and concentration effects are acceptable as the only method for compliance with installation location requirements.
E.3.5.2 Air Data Sensor Performance Effects.
E.3.5.2.1 Icing tunnels alone may be used to determine the performance of air-data sensors in Appendix O icing conditions in compliance with §§ 25.1323,
Appendix E E-9
25.1324, and 25.1325. For sensors with collection efficiencies approaching “1,” if performance has been shown in FZDZ conditions, then a qualitative analysis based on water-catch ratios may be used for extrapolation to FZRA conditions.
E.3.5.2.2 If the Appendix C or mixed phase or ice crystal conditions are shown to be more critical for air data sensors than the SLD environment, the number of tests may be reduced. However, the test methods should be validated until there is confidence with the results. In some cases, such as that of wing leading-edge-mounted lift transducers, icing tunnel tests may still be necessary.
E.4 CERTIFICATION FOR DETECT AND EXIT—§ 25.1420(A)(1).
E.4.1 Detect-and-Exit Freezing Drizzle Less than 40 µm.
E.4.1.1 FZDZ conditions with an MVD less than 40 µm are similar to existing Appendix C distributions with the exception of a small percentage of the mass in drops larger than typical Appendix C conditions. As a result, many of the current Appendix C simulation methods can support compliance. The small percentage of large drops in this distribution can affect the impingement limits and increase the water catch.
E.4.1.2 If visual cues are used for compliance with § 25.1420(a)(1)(i), it may be possible to use codes in combination with icing tunnels to verify the visual cues. Visual cues should not be based on only one engineering method; a second, correlating method should be used.
E.4.2 Detect-and-Exit Freezing Drizzle Greater than 40 µm.
Where the capabilities of the tools available for FZDZ>40 µm are the same as for FZDZ<40 µm, the applicant may use similar means of compliance that are adjusted for the FZDZ>40 µm icing conditions. The tool capabilities are different for mechanical deicing system protected areas and areas aft of the protected areas.
Note: Icing tunnels are classified as yellow when used for testing for FZDZ>40 µm because use of the tunnels appears feasible but has not been demonstrated. Icing tunnel tests alone are acceptable for the development of ice shapes for deicing system protected areas and areas aft of the mechanically protected areas. Sequencing or standard distributions are acceptable, but the ratio of sequencing time to shed cycles should be examined.
E.4.3 Detect-and-Exit Freezing Rain (MVD<40 µm and MVD>40 µm).
E.4.3.1 The capabilities of the tools for FZRA are limited. To simulate accretions on unprotected surfaces and aft of protected areas, CFD codes may be used to determine the difference in impingement region between FZRA and FZDZ. The increase in impingement area can then be simulated using a standard roughness in that region. For areas where a ridge of ice may form, a simulated ridge may be developed using a height developed analytically based on local water catch. Ridge location could be developed