AC 20-197
Table 1-2. Cycling Icing Conditions for airframe mounted probes in liquid water icing conditions Test Static Air Altitude LWC (g/m3) MVD number Temperature (ft) (a) (b) (μm) (°C) SL5 -10 17,000 0.6 2.2 20 SL6 -20 20,000 0.3 1.7 20 SL7 -30 25,000 0.2 1.0 20
6.2.4.6 For both engine-mounted and airframe-mounted probes, in addition to the tests or analysis defined in paragraphs 6.2.4.1 through 6.2.4.5 of this AC, perform a critical point analysis based on appendix C to part 25 requirements to determine if there are additional critical points within the operating envelope. The critical points, w hich the engine is tested against to show compliance with § 33.68, m ay or may not be critical points for the probe. F or example, high airflow conditions like maximum continuous power may be more critical for an inlet-mounted probe than for the entire engine. T herefore, it is important to determine the critical conditions for the probe as part of this step and not rely solely on the § 33.68 points for demonstrating the proper operation of the probe. The points defined in § 33.68 and tables 1-1 and 1-2 of this AC represent conditions at the engine inlet for the installation. For a component (probe) test, the appropriate concentration factor must be determined so that the local conditions at the probe match installed c onditions.
6.2.4.7 For unheated probes in liquid water environments, evaluate the impact of ice accumulation and shedding on altitude. Consider analysis and testing at altitude. Heated probes may be tested in liquid water conditions in a non-pressure controlled (sea level or ambient conditions) wind tunnel if that would be more conservative than testing at higher altitudes
6.2.4.8 If a probe has more than one operating mode (e.g., a de-icing cycle), verify the proper operation in all operating modes.
6.2.4.9 For mixed-phase and ice crystal conditions, select test/analysis points with the TWC based on a 10 nm cloud. The TWC for a 10 nm cloud corresponds to the ‘17.4 nm’ values multiplied by a factor of 1.05, as shown in figure D3 in a ppendix D of part 33.
6.2.4.9.1 Appendix D to part 33 describes the liquid water portion of mixed-phase conditions as ≤1.0 g/m3 for clouds of less than 50 nm extents for temperatures above -20°C and zero for temperatures below -20°C.
6.2.4.9.2 SAE AS5562 assumes the LWC is per appendix C to part 25 intermittent maximum cloud, and the remainder of the TWC is ice crystals.
AC 20-197
6.2.4.9.3 When performing mixed phase condition tests, use the LWC described in appendix C to part 25 for an intermittent maximum cloud with a mean effective drop diameter of 20 microns. The ice crystal concentration is the TWC determined in paragraph 6.2.4 .9 of this AC minus the LWC described in appendix C to part 25 for an intermittent maximum cloud.
Using these test conditions is consistent with SAE AS5562 and conservatively simulates mixed-phase conditions in colder conditions.
6.2.4.10 Consider the time duration for mixed-phase icing test points. F or the maximum TWC in mixed-phase conditions, test conditions lasting 2 minutes are appropriate, as this is sufficient time for an air data probe to reach a steady state and stabilized condition. In addition, flight te sting in well-developed, large-diameter mesoscale convective systems completed as part of the FAA and European Union Aviation Safety Agency Industry High-Altitude Ice Crystals/High Ice-Water Content (HAIC/HIWC) flight test campaigns showed a low frequency of mixed-phase regions.
6.2.4.10.1 During the HAIC/HIWC flight tests, -10°C mixed phase regions accounted for less than about 5% of the total in-cloud distance traversed, and maximum average distances across mixed phase zones were about 8 nm. The frequency of mixed-phase zones decreased with decreasing temperature. Well-developed storm cells can produce large amounts of falling and recirculating ice that tend to glaciate any new updraft, thus resulting in lower LWCs and smaller regions of mixed-phase conditions.
For smaller or still developing cells with less glaciation and less circulation, regions of mixed-phase conditions with higher LWC could exist for longer times. While traversing the storm, these areas of mixedphase conditions are still likely to exist as separate regions within the storm, resulting in alternating between mixed-phase and fully glaciated conditions.
6.2.4.10.2 For engine-mounted probes, a conservative approach to represent these smaller or fresh convective cel ls is to perform a cyclic test alternating between mixed phase and fully glaciated conditions. I n addition to the test conditions for maximum TWC conditions for two minutes, test points should include cycling between mixed phase and fully glaciated conditions. For these cyclic conditions, set the TWC to a value scaled by the distance factor associated with the horizontal extent traversed in 12 minutes (3 cycles) at the aircraft operating speed. This lower TWC is justifiable as the HAIC/HIWC flight test results indicate that extended regions of liquid water seem less likely in high IWC conditions.
Therefore, lower IWC and the resulting lower TWC values are necessary to sustain mixed-phase conditions without transitioning to fully glaciated conditions. Because the HAIC/HIWC flight tests did not evaluate mixed phase conditions in developing storm cells, a conservative estimate of LWC should be used. T o define a conservative test, assume LWC equal to appendix C to part 25 Intermittent Maximum value for the test conditions.
AC 20-197 As discussed below, other engine or airframe level mitigation may be necessary to ensure acceptable aircraft operation, depending on the test results in these conservative conditions. Each cycle should alternate between 2 minutes in mixed-phase conditions and 2 minutes in fully glaciated conditions. To simulate an engine flying through a developing storm system or holding in such conditions, continue the cycles until the probe operates for a maximum of 3 cycles (12 minutes).
6.2.4.10.3 For heated probes, account for conditions with the heater turned off. The purpose is to address an inadvertent icing encounter where the heater is not turned on immediately or a transient power interruption. For example, some engine anti-ice systems link to nacelle anti-ice, requiring the pilot to activate the system. The timing should be coordinated with the engine and airframe manufacturers to define appropriate conditions. Paragraph 6.2.1.4 discusses the evaluation of failure modes.
6.2.5 Define test pass-fail criteria at the probe level. Considering engine and aircraft-level requirements, define the pass/fail criteria for the candidate test p oints determined in paragraph 6.2.4. Pass/fail criteria should consider a system-level analysis developed by the probe, engine, and airframe manufacturers.
6.2.5.1 The probe remaining free of ice is not necessarily the only pass criteria.
Applicants should develop acceptance criteria upon consideration of the criticality of the probe and the air data it provides to the engine or the airframe, or both. One simplified acceptance criterion is for the engine and airframe systems that utilize the data from the probe to continue to operate within an acceptable range in icing conditions. In all expected operating conditions, the probe should perform its original intended function; for example, measuring air temperature with some reasonable accuracy, rather than measuring probe heater temperature. The probe manufacturer can typically only quantify the effect at the probe level (e.g., the probe output/accuracy is ±x° when exposed to a specific condition). The engine manufacturer should translate that probe effect into the resulting system impact at the engine level (e.g., ±x° of measured temperature error equates to ±y thrust or some impact on engine operability). The airframe manufacturer needs to know the probe level effect (e.g., ±x° measured temperature error) and the installation level effect on airframe systems that use data from the probe along with the engine effect and the impact the engine effect has at the aircraft level.
6.2.5.2 The applicant should evaluate potential ice shedding from the probe as part of the pass/fail criteria to ensure no damage to downstream components occurs from ice shedding.
6.2.5.3 Applicants should also account for different atmospheric conditions and the possible failure modes of the probe when developing acceptance criteria. Some failure modes include:
AC 20-197
• In ICI conditions, a temperature probe may clog with partially melted crystals, driving it to read 0°C (the temperature of the slush) continuously.
• Smaller errors for some short periods.
• Fluctuations in the probe output with ice build and shed cycles.
• Pressure probes may clog with ice, resulting in a fixed or unchanging signal without normal signal noise.
• Damage to downstream components as a result of ice shedding.
6.2.6 Finalize test plan. Applicants should complete an analysis of the test points identified in paragraph 6.2.4. Evaluate the points to determine which points are covered by other conditions or covered by analysis supported by other test conditions. Also, evaluate the points to determine if the proposed test facility can perform the specific test conditions.
When a test facility limitation precludes a particular test condition, an applicant may use the scaling methods of SAE AS5562, s ections 3.3.2 through 3.3.4, to vary temperature, airspeed, a nd water flux to achieve defined equivalent test conditions. If a significant number of test points require modification in this manner, consider an alternate test facility capable of testing more of the proposed envelope to minimize the number of candidate points not directly tested.
6.2.6.1 As noted in SAE AS5562, s ection 3.3.1, there is no acceptable method to scale the altitude for ICI conditions at this time. Therefore, test ICI conditions at the most critical altitude corresponding to one of the candidate test points determined from paragraph 6.2.4 of this AC. Test facilities capable of testing probes in ICI conditions at altitude are available. It may be possible to change the probe design such that any untested points are no longer critical. Alternatively, it may be possible to impose limitations on the probe’s operating e nvelope. H owever, this may require engine or aircraft level design changes or limitations. Identify and communicate any limitations on the probe installation to the engine and aircraft manufacturers so those manufacturers can adhere to the limitations as installed.
6.2.6.2 As stated in paragraph 6.2.4.10.3, you should account for the engine and aircraft level effects of loss of the heater function as appropriate. Address the loss of the heater function either in a test matrix or by analysis. This activity documents the effect on the probe with the heater inoperative in the worst-case icing conditions.
6.2.7 Complete probe test and analysis. Complete testing of all conditions identified in paragraph 6.2.6 in a facility capable of meeting the test conditions. The configuration of the test article should match the intended installation, including orientation, as closely as possible, and the probe itself should match the type design configuration, except as necessary to install instrumentation or other test equipment. Meet the following criteria for each test condition.
AC 20-197
6.2.7.1 SAE AS5562 test criteria. The following test characteristics should be consistent with the requirements defined by SAE AS5562:
• Probe Mounting Location.
• Probe Mount Heating Requirements.
• Installation Heat Sink Effects.
• Probe Power for Electrically Heated Probes.
• Tunnel Blockage.
• Data Collection Sample Rate.
• Electrically Heated Probe Test Unit Selection.
6.2.7.2 Non-electrically heated probes. For heated probes using some method other than an electrical heater, set the heat source to the minimum allowable value expected for the installation. For example, test a probe that is heated using bleed air with the bleed air supply at the minimum expected regulation pressure and temperature. Provide the specific air supply conditions tested to the installer to support in stallation approval. It is essential to characterize this air supply fully, including in-line pressure drop, piping clearances, local heat transfer characteristics, a nd any control orifices in the supply line or sensor. P erform probe qualification tests on a unit having the lowest acceptable performance on a production article as defined by the acceptance test procedure.
6.2.7.3 Angle of attack (AOA). A ccount for the effect of AOA in the intended installation. Conduct a critical point analysis as specified in paragraph
6.2.3.1 to determine the local probe AOA for each test case. Establish a clear relationship between the aircraft or engine AOA and the local probe AOA. Consider the impact of various flight attitudes within the approved flight envelope on the probe AOA. Specific flight conditions, such as side slipping, can influence local probe conditions due to flow interference from wings or other aircraft features.
6.2.7.4 Test particle size distribution. The particle size median mass dimension for the test conditions must match that defined by appendix D to part 33 (50-200 microns equivalent spherical size) unless justified that a different size will not significantly affect the test.
6.2.7.5 Test duration for steady state ice crystal icing tests. Run tests at the peak TWC values for 2 minutes and run tests completed at one-half of the peak TWC value run for the encounter time calculated from the horizontal extent and the airspeed.
6.2.7.6 Test duration for mixed-phase tests. Run tests conducted at the maximum TWC values for 2 minutes. R un the tests conducted at the reduced TWC values as cyclic tests. Each cycle should alternate between 2 minutes in 17
AC 20-197 mixed-phase conditions and 2 minutes in fully glaciated conditions.
Continue the cycles until the probe operates for a maximum of 3 cycles (12 minutes).
6.2.7.7 Test duration for airframe mounted probes in liquid water icing conditions. Identify test/analysis points per the conditions stated in tables 1-1 and 1-2 of this AC. This testing requires 15-minute durations per continuous maximum cloud concentrations, 5-minute duration per intermittent maximum cloud concentrations (table 1-1), and either a 30- minute or 10-minute duration cyclical liquid water concentrations (table 1-
2).
6.2.7.8 If the probe data will be used by the engine, proceed to paragraph 6.2.8. If the probe data will be used by the aircraft only, proceed to paragraph
6.2.11.
6.2.8 Compile and report probe test and analysis results to engine manufacturer. Produce the test report. The test and analysis results should quantify the effect of the conditions on the probe. T he following topics should be covered, if applicable:
• Signal error (temperature, pressure, etc.), i ncluding transient response rate, a ny build/shed cycle effects, and when icing conditions are introduced or removed.
• Measurement accuracy and any changes upon introduction or removal of icing conditions, and any changes when a probe heater is turned on or off.
• Accretion/shedding - size and shape of ice buildup.
• The frequency of build/shed cycles, i mpact on signal error, and probe transient response to shedding.
Provide the testing and analysis results to the engine manufacturer so that the acceptability of the probe response can be determined. T he engine manufacturer should evaluate the effect on the engine of variations in the probe output signal due to icing and any other failure modes observed during the testing.
6.2.9 Determine acceptability of probe performance at engine level. Based on the testing and analysis results, the engine manufacturer should determine whether the effect of the probe response to the icing conditions meets the requirements of § 33.68. It is important to note, do not change the pass/fail criteria to match the results. I nstead, repeat the system analysis step to ensure correctness and ensure the design is reviewed or changed as necessary.
6.2.10 Provide engine response to probe icing to the aircraft manufacturer. T he engine manufacturer should document the response of the engine based on the probe response to the icing conditions tested and provide that response to the airframe manufacturer.
The engine response evaluation should address:
• Probe Signal error.
• Failure indications/Fault accommodation.
AC 20-197
• Changes in engine operating characteristics (surge/stall, flameout, etc.) due to signal error.
• Change in thrust or power setting.
• Change in displayed parameters.
6.2.11 Determine the acceptability of probe performance at the aircraft level. As part of the aircraft certification and compliance demonstration activities, the aircraft manufacturer should review the data provided by the engine manufacturer in paragraph 6.2.10 of this AC to determine the impact on the aircraft. To complete this review, the aircraft manufacturer may need information on the probe location in the engine, etc., from paragraph 6.2.7.1. B ased on the testing and analysis results, the aircraft manufacturer should determine whether the effect of the probe response to the icing conditions is acceptable at an aircraft level, as part of the aircraft certification and compliance demonstration activities. If the probe response is not acceptable at the aircraft level, the aircraft manufacturer should review the pass/fail criteria to ensure aircraft-level requirements are clearly defined and repeat the process from paragraph 6.2.1. The revised system analysis may necessitate a review of the candidate test points and/or design changes to the probe or installation, and/or re-evaluation of the aircraft response.
6.2.11.1 Engine probes usually are part of the engine type design and therefore, the engine manufacturer is primarily responsible for evaluating the probe.
When the engine utilizes an aircraft probe, the aircraft manufacturer is primarily responsible for assessing whether the probe meets aircraft-level requirements (such as § 25.1323 or 25.1324). R egardless of primary responsibility, however, i t is vital that both the aircraft and engine manufacturer communicate requirements and capabilities early and throughout the certification process.
6.2.11.2 When the engine probe is part of the aircraft type design, the aircraft manufacturer will still need information on the engine installation. The aircraft manufacturer should coordinate with the engine manufacturer to determine what installation information is required under § 33 .5. The aircraft manufacturer should provide the engine manufacturer with appropriate information to support the engine manufacturer’s design and compliance demonstration.
6.2.11.3 The aircraft manufacturer should also ensure that the engine manufacturer provides the necessary information about the engine probe and engine type design to facilitate demonstrating compliance to relevant aircraft certification requirements, such as §§ 25.901(c), 25.1093(b), 25.1309,
25.1324, and 25.1325.
6.2.11.4 When the aircraft probe is part of the aircraft type design and utilized by the engine manufacturer, the aircraft manufacturer should coordinate with the probe manufacturer and engine manufacturer as necessary. The aircraft 19
AC 20-197 manufacturer should refer to AC 25-28 for additional information on certification requirements for flight in icing conditions for aircraft probes.
6.2.12 Document probe results. Complete the final documentation of the probe, engine, and aircraft response to the icing environments, to show compliance with relevant engine and aircraft requirements. The final documentation should include the engine manufacturer incorporating information about the engine response into the engine installation instructions provided under § 33.5. This information should consist of the data provided to the airframe manufacturer in paragraph 6.2.10, a description of the icing environments evaluated, and any other pertinent data from the safety assessments required by §§ 33.28(e) and 33.75(a), describing the probe and engine response. As part of the aircraft certification and compliance demonstration activities, the airframe manufacturer should document the response and compliance with any requirements included in the engine installation instructions as required. Aircraft-level documents that might be affected include the Airplane Flight Manual, the system safety documentation for the aircraft, compliance documents, etc.
7. AC FEEDBACK FORM.
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DANIEL J.
Digitally signed by DANIEL J. ELGAS ELGAS Date: 2025.06.18 09:15:35 -04'00' Daniel J. Elgas Director, Policy and Standards Division Aircraft Certification Service 20
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