Appendix E E-10 from FZDZ tunnel tests, and the height would be modified based on the ratio of local water catch (determined with CFD) between FZDZ and FZRA.
E.4.3.2 Other Concerns.
E.4.3.2.1 Thermal Ice Protection Systems.
Analyses to assess water catch and melting/evaporation rates are acceptable for determining the capabilities of thermal systems in FZRA, provided that the analyses are based on the validated results of the system capabilities performed for Appendix C and FZDZ. Any additional ice that may form on runback ice shapes in FZRA should be accounted for by analysis of the runback volume. Potential roughness effects ahead of the runback should be addressed.
E.4.3.2.2 Mechanical Ice Protection Systems.
For assessing mechanical IPS performance in FZRA, it is acceptable to use the same pre-activation and inter-cycle and residual ice shapes as for FZDZ. The limits of accretion should be determined using CFD tools.
E.4.3.2.3 Validation of Visual Cues.
Use of qualitative analysis is acceptable for assessing whether visual cues used for FZDZ will function in FZRA conditions.
E.5 CERTIFICATION FOR A PORTION OF APPENDIX O—§ 25.1420(A)(2).
E.5.1 Current technology does not support distinguishing between FZDZ and FZRA in flight.
For this reason, airplanes should not be certificated for compliance with § 25.1420(a)(2) based on the boundaries between FZDZ and FZRA. Certification with § 25.1420(a)(2) is discussed in the main body of this AC; however, there are concerns about the ability of applicants to define ice shapes which distinguish between the approved portions and the unapproved portions of Appendix O with the current simulation tools. As a result, certification for a portion of Appendix O will be challenging and will require close coordination with certifying authorities.
E.5.2 Certification for a portion of Appendix O that considers phase of flight (e.g., takeoff, holding), as discussed in paragraph 12.1 of this AC, may be feasible. Any method of certification for a portion of Appendix O should be included as part of the certification planning and will require approval from the appropriate certifying authority.
E.5.3 In cases where only one engineering tool or none has been validated as capable of simulating FZDZ or FZRA, the means of compliance should include flight tests in measured Appendix O icing conditions to verify ice accretions.
E.6 CERTIFICATION FOR UNRESTRICTED OPERATIONS—§ 25.1420(A)(3).
The use of simulation tools as described for detect-and-exit airplanes is also acceptable for showing compliance for airplanes certificated in accordance with § 25.1420(a)(3).
Appendix E E-11 However, in cases where only one engineering tool (or none) has been validated as capable of simulating FZDZ or FZRA, the means of compliance should include flight tests in measured Appendix O icing conditions to verify ice accretions.
E.7 COMPLIANCE WITH §§ 25.1323, 25.1324, 25.1325, AND 25.773.
E.7.1 Compliance with §§ 25.1323, 25.1324, 25.1325, and 25.773 for Appendix C and Appendix O Conditions.
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.
E.7.2 Compliance with §§ 25.1323 and 25.1324 for Mixed Phase and Ice Crystal Conditions.
Analysis of exposures to mixed phase and ice crystal conditions should consider the horizontal extents defined in the rules.
E.8 STANDARD ROUGHNESS LEVELS FOR APPENDIX O ICE SHAPES.
Ice shapes for part 25, subpart B, testing are typically based on icing tunnel tests or CFD computations or both. Current CFD programs do not provide roughness information. Roughness levels and aft extent of roughness for Appendix O ice shapes should be determined from icing tunnels or tanker testing, if the capabilities exist.
However, when the empirical capabilities do not exist, the roughness levels (or equivalents) as defined in table E-2 below may be used. Note that roughness levels for Appendix C ice shapes are discussed in AC 25-25A.
Appendix E E-12 Table E-2. Appendix O Icing Standard Roughness Ice Type Roughness Height (mm) Percent of Surface Covered Notes FZDZ and FZRA, thin accretions;
thickness < 3 mm (0.12 inch)
1.5 to 2 mm (0.06 to 0.08 inch) or 16 to 20 grit sandpaper Particle density to cover 50% to 70% of total area Use to simulate pre-detection, initial accretions, or roughness on computed ice shapes FZDZ thickness > 3 mm (0.12 inch) 3 to 6 mm (0.12 to 0.24 inch) Mean particle size ~4.5 mm Inter-cycle, residual, unprotected surfaces FZRA thickness ≥ 3 mm (0.12 inch) and ≤ 6 mm (0.24 inch) FZRA thickness ≥ 6 mm (0.24 inch) 6 to 8 mm (0.24 to 0.31 inch) Mean particle size ~7 mm Notes:
1. The simulated roughness elements should approximate roughness elements observed in icing tunnels or natural icing. Smooth and spherical elements should not be used because they may result in non - conservative aerodynamic results.
2. For computed ice shapes, the roughness simulation should be extended aft to the limits of predicted accretion (where the ice accretion thickness is calculated as 0.015").
Appendix F F-1 Appendix F. Acronyms Table F-1. Acronyms and Definitions Acronym Definition ACO Aircraft certification office AFM Airplane flight manual CFD Computational fluid dynamics CFR Code of Federal Regulations FZDZ Freezing drizzle FZRA Freezing rain ICA Instructions for Continued Airworthiness IRT Icing research tunnel ICTS Ice-contaminated tailplane stall IPS Ice protection system IWC Ice water content LWC Liquid water content MED Mean effective diameter MMD Median mass dimension MVD Median volume diameter NASA National Aeronautics and Space Administration RPM Revolutions per minute SAE Society of Automotive Engineers SLD Supercooled liquid drops STC Supplemental type certificate TC Type certificate TSO Technical Standard Order
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