U.S. Department of Transportation Federal Aviation Administration
Advisory Circular Subject: T urbine Engine and Airframe Sensor Date: AC No: 20-197 Icing Compliance Initiated By: AIR-625 Change No: N/A
1. PURPOSE.
This advisory circular (AC) describes an acceptable method for demonstrating compliance with title 14, Code of Federal Regulations (14 CFR) 33.28, 33.68, 33.75,
33.89, a nd 33.91, concerning mixed phase, ice crystal icing (ICI), and other icing conditions for engine inlet sensors. This AC may also assist applicants for part 25 airplanes in demonstrating compliance with 14 CFR 25.901(c), 25.1093(b), 25.1309,
25.1323, 25.1324, and 25.1325 c oncerning icing conditions and data obtained from engine inlet sensors on turbine-powered airplanes. This AC addresses temperature probes, combined temperature and pressure probes, and static pressure probes installed in turbine engines. Manufacturers can use this method of compliance to evaluate probe behavior in mixed phase, ICI, and other icing c onditions, and assess the resulting impact of engine probe performance on engine and airframe systems. Although the main focus of this AC is on engine inlet probes, the methodology can also be applied to evaluate temperature probes, combined pressure and temperature probes, and static pressure probe behavior regardless of installation location.
2. APPLICABILITY.
2.1 The guidance provided in this AC is for airplane manufacturers, modifiers, foreign regulatory authorities, Federal Aviation Administration (FAA) transport airplane type certification engineers, and FAA designees.
2.2 This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
AC 20- 197
2.3 The FAA will consider other means of demonstrating compliance that an applicant may elect to present. Terms such as “should,” “may,” and “must” are used only in the sense of ensuring the applicability of this particular method of compliance when the acceptable method of compliance in this document is used. If the FAA becomes aware of circumstances in which following this AC would not result in compliance with the applicable regulations, the FAA may require additional substantiation or design changes as a basis for finding compliance.
2.4 The material contained in this AC does not change or create any additional regulatory requirement, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
3. RELATED MATERIAL.
3.1 Title 14, Code of Federal Regulations (CFRs).
The following 14 CFR regulations are related to this AC. You can download the full text of these regulations from the Federal Register website at
• Section 33.28, Engine control systems.
• Section 33.68, Induction system icing.
• Section 33.89, Operation Test.
• Section 33.91, Engine system and component tests.
• Appendix D to Part 33—Mixed Phase and Ice Crystal Icing Envelope (Deep Convective Clouds).
• Section 25.901, Installation.
• Section 25.1093, Induction system icing protection.
• Section 25.1309, Equipment, systems, and installations.
• Section 25.1323, Airspeed indicating system.
• Section 25.1324, Angle of attack system.
• Section 25.1325, Static pressure systems.
• Appendix C to Part 25—(no title) Part I - Atmospheric Icing Conditions.
• Appendix O to Part 25—Supercooled Large Drop Icing Conditions.
• Appendix C to Part 29—Icing Certification.
eCFR.
3.2 FAA Advisory Circulars.
The following ACs are related to the guidance in this AC. The latest version of each AC referenced in this document is available on the FAA website at and on the
• AC 20-73A, Aircraft Ice Protection.
FAA Advisory Circulars Dynamic Regulatory System.
AC 20- 197
• AC 20-147A, Turbojet, Turboprop, Turboshaft, and Turbofan Engine Induction System Icing and Ice Ingestion.
• AC 25-28, Compliance of Transport Category Airplanes with Certification Requirements for Flight in Icing Conditions.
3.3 FAA Technical Standard Order (TSO) and Report.
The following TSO and report are related to the guidance in this AC. The latest version of each referenced in this document is available on the FAA website at and on the
• Technical Standard Order, TSO-C16b, ELECTRICALLY HEATED PITOT AND PITOT-STATIC TUBES.
• FAA Report No. DOT/FAA/AR-09/13, Technical compendium from meetings of the engine harmonization working group.
TSO, Research Reports, Dynamic Regulatory System.
3.4 ASTM International.
ASTM F3120 / F3120M – 20, Standard Specification for Ice Protection for General Aviation Aircraft, is related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. This document can be ordered online at ASTM F3120/F3120M-20.
3.5 RTCA (formerly Radio Technical Commission for Aeronautics).
RTCA DO-160G, Environmental Conditions and Test Procedures for Airborne Equipment, is related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. This document can be ordered online at RTCA.
3.6 Society of Automotive Engineers (SAE) International.
The following SAE documents are related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. The documents are available online at
• SAE International 2011-38-0050, An Analysis of Turbofan Inlet Water and Ice Concentration Effects in Icing Conditions, SAE Technical Paper, Liao, S., Liu, X., and Feulner, M.
• SAE International 2015-01-2086, Studies of Cloud Characteristics Related to Jet Engine Ice Crystal Icing Utilizing Infrared Satellite Imagery, SAE Technical Paper, Grzych, M., Tritz, T., Mason, J ., Bravin, M., and Sharpsten, A.
SAE International.
AC 20- 197
• SAE International 2015-01-2146, Ice Crystal Ingestion In a Turbofan Engine, SAE Technical Paper, Feulner, M., Liao, S., Rose, B., and Liu, X.
• SAE AS5562, Ice and Rain Minimum Qualification Standards for Pitot and Pitotstatic Probes.
3.7 Aerospace Industries Association (AIA).
Aerospace Industries Association/Engine Icing Working Group, AIA/EIWG Subcommittee on Engine Probe Icing: A Process for Evaluating the Performance of Temperature Probes, Combined Temperature and Pressure Probes, and Static Pressure Probes in Icing Conditions, is related to the guidance in this AC. Unless otherwise specified, use the latest FAA-accepted revision for guidance. If the document is revised after the publication of this AC, you should verify that the FAA accepts the subsequent revision or update as an acceptable form of guidance. This document can be ordered online at AIA.
4. DEFINITION O F KEY TERMS.
For the purposes of this AC, the following definitions apply.
• Ice Water Content (IWC). The amount of frozen water mass per unit volume contained in the icing environment.
• Liquid Water Content (LWC). The amount of liquid water mass per unit volume contained in the icing environment.
• Total Water Content (TWC). The amount of water mass per unit volume contained in the icing environment. In ice crystal conditions, TWC refers to the amount of water mass per unit volume contained in the ice crystals. In mixed conditions, TWC includes the water mass contained in both the liquid and frozen phases.
5. BACKGROUND.
5.1 The FAA is aware of more than two dozen examples of ice crystals blocking turbine engine inlet heated probes on turbine engine-powered airplanes. These events are discussed in the AIA/EIWG report listed in paragraph 3.7 of this AC. These ice blockages have resulted in uncommanded engine power rollbacks, lack of throttle response, and out-of-range probe data flightcrew warnings.
5.2 Some airplane manufacturers have eliminated airframe probes and instead use the turbine engine probe data for airframe systems, s uch as total or static air temperature.
Using e ngine probe data for different airframe systems requires applicants to put greater emphasis on ensuring that probes continue to function throughout the various icing conditions to support the airplane-level system safety analysis and overall aircraft safety. Eliminating the airframe probes also removes the ability of the electronic engine control system to use airframe probe signals to validate data from engine probes.
Without the use of airframe probes, data from the engine probes is substantially more critical throughout the operating envelope of the aircraft.
AC 20-197
5.3 Some engine manufacturers use heated inlet probes to prevent ice from accreting on or in the probe, while others use unheated inlet probes.
5.3.1 Heated Inlet Probes.
Heated inlet probes can effectively prevent ice from accreting on the probe while in supercooled droplet-icing conditions typically found at low to mid-altitudes. Heated inlet probes, however, can still be susceptible to ice accretion from ICI conditions.
These conditions are typically found at high altitudes but can also exist at lower altitudes. When using heated inlet probes, manufacturers should consider the reliability of the heat source when evaluating inlet probe-related loss of thrust control under § 33.28(d)(1) during icing encounters.
5.3.2 Unheated Inlet Probes.
Unheated inlet probes are less susceptible to ICI conditions but may be more vulnerable to lower and mid-altitude supercooled droplet-icing conditions. An acceptable method for unheated inlet probes in ICI conditions can be found in AC 20-147A, paragraph
9.s.(3), which allows for a similarity analysis for engines proven safe to operate in mixed-phase or glaciated atmospheric conditions. This similarity approach for engines can be extended to unheated inlet probes so long as the applicant can show that the installation effects are similar.
5.4 Generally, testing of heated and unheated inlet probes installed in the engine during certification testing has been a successful method of evaluating the engine inlet probe in supercooled droplet icing conditions. This method is effective as long as the critical point analysis includes an assessment of the critical conditions for the probe.
5.5 Applicants have redesigned certain heated inlet temperature probes to reduce the potential for ice crystal blockage but not eliminate it. Engine manufacturers have incorporated Full Authority Digital Engine Control logic to detect and annunciate the obstruction and utilize an alternative temperature signal to preclude thrust anomalies.
5.6 Although the need for reliable and accurate airspeed, temperature, and pressure data is addressed by §§ 33.28, 33.68, 33.89, 33.91, 25.901( c), 25.1093(b), 25.1303(b), a nd
25.1309, there is no airworthiness standard that requires applicant showings for engine probes separate from the engine control system. Historically, testing of engine inlet probes occurs during the engine’s § 33.68 induction icing system compliance testing, which demonstrates continuous operation throughout the supercooled water droplet icing environment described in appendix C to part 25. Turbine engine inlet probes are also typically subjected to RTCA DO-160 standards and test procedures during § 33.9 1 testing, but RTCA DO-160G, the latest version as of this writing, does not address the effects of ice crystals or supercooled water droplets on turbine engine inlet probes. FAA TSO-C16b describes the minimum performance standards for heated pitot (total pressure) probes and heated pitot-static (combined total and static pressure) probes.
TSO-C16b a nd SAE AS5562 describe methods to evaluate the performance of such heated probes in icing conditions, i ncluding ICI and mixed-phase conditions. This AC describes methods that assess t he performance of temperature probes, combined temperature and pr essure probes, and static pressure probes in mixed-phase, ICI, and 5
AC 20-197 other icing conditions to support design and installation requirements related to engines and airframe systems, i ncluding § 33 .68(e). If applicable, the aircraft applicant should supplement the TSO-C16b s tandard with additional data addressing specific installation aspects of the probe and the pass/fail criteria of the aircraft to show compliance with the applicable aircraft regulations.
5.7 The engine manufacturer should document probe assumptions, capabilities, and pass/fail criteria in the engine installation manual according to § 33.5(a)(4) through (6) for later use by the airplane engine installer.
6. GUIDANCE.
6.1 Probe Icing Method of Compliance.
6.1.1 This AC provides a structured method of compliance for engine and aircraft manufacturers to show how turbine engine inlet probes perform in icing conditions to support compliance with § 33.68 a nd other referenced regulations. This AC also describes a process applicants can use to target their analysis and develop tailored test points to show compliance with the requirements related to probe icing. Consider this method for any temperature or pressure probe that provides data to the engine control system or airframe systems, whether installed in the engine or on the airframe.
6.1.2 Figure 1-1 illustrates the structured method of the compliance process. A detailed explanation of each step follows. The numbered blocks in figure 1-1 are consistent with the sub-paragraphs of section 6.2.
AC 20-197
6.2.1 6.2.10
6.2.6 Provide Engine Response to Probe Icing Compare the Engine & Aircraft Finalize Test Plan To the Aircraft Manufacturer Operating Envelopes to Appendices C, D, and O Determine: 1. Probe Signal Error. 1. Which points can be covered by other test 2. Failure Indications; Fault Consider: conditions; Accommodation.
1. The Atmospheric conditions 2. Which points can be tested (and modif y 3. Change in Engine. Operating the airc
raft will operate in ; conditions to allow testing if facilit
y limitation is Characteristics.
and why test can’t be completed); and 4. Change in Thrust or Power
2. The possible failure modes of
3. Which points can be covered by Setting.
probes, including
the loss of analysis/test.
heater function.
P robe does not
6.2.7 affect engine
6.2.2 performance Complete Probe Test and Determine Ambient Operating Conditions Analysis 6.2.11 Calculate water flux as Determine Acceptability of a function of total Probe affects engine Probe Performance at Aircraft temperature. performance Level
1. Effect of signal error
6.2.8 on airframe systems. Compile and Report Pro be Test 2. Effect of engine
6.2.3 and Analysis R
esults to
Engine response (change in Manufacturer Determine Local Conditions at Probe thrust /fault Based on Ambient Conditions Quantify effect of test conditio
ns on probe: accommodation/etc.
Unacceptable 1. Signal error (temperature, pressure, Perform Scoop Factor Calculations etc.) including transient response rat e for inlet mounted probe, including any build/shed cycle effects concentrations effect, local and when ic ing conditions are velocities, etc. fo
r an airframe introduced or removed. mounted probe. 2. Accretion/shedding – size and s hape Acceptable of ice buildup.
6.2.4 OEM determines acceptability of probe 6.2.12 Document Probe Results Identify Candidate Test Points response on engine control.
1. Engine OEM incorporate engine response into Select test points based o
n local 1. Corruption due to icing. final installation manual. conditions at the probe. 2. Any other failure due to icing (including 2. Airframe OEM document aircraft response to • High IWC/short duration. Turbo machinery damage f rom probe engine probe signals per the Installation
• Lower IWC/longer duration. icing build/shed cycle). Manual limitations.
• Include AC 20-147A Table Points for completeness etc. Include in safety asses § 33.28:
sment for Effect of loss of heater functio n.
6.2.5 Define Pass- 6.2.9 Fail Criteria at Determine the Probe Level acceptability Yes of probe Develop pass/fail performance criteria considering at engine engine and aircraft level requirements.
No Numbers in the upper left corner correspond to paragraph numbers in this AC.
Figure 1-1. Process Flow Chart
6.2 Method of Compliance.
6.2.1 Compare engine and aircraft operating envelopes to appendix D to part 33 and appendices C and O to part 25. The initial step requires defining the engine and aircraft operating envelopes (altitude, airspeed, temperature) relative to the environmental icing 7
AC 20-197 conditions considered for certification. Compare appendix C to part 25 and appendix D to part 33 environmental icing envelopes with the applicable airworthiness standards for the aircraft to determine the applicability of those envelopes. As required, evaluate appendix O to part 25 icing envelope to support the engine and/or aircraft certification.
6.2.1.1 Appendices C and O to part 25 a nd appendix D to part 33 apply to turbine engines except for turboshaft engines. The icing conditions depicted in appendix O to part 25 and appendix D to part 33 do not apply to turboshaft engines or their installations. Under § 33.68(b), turboshaft engines need only comply with the icing conditions depicted in appendix C to part 29.
6.2.1.2 At the aircraft level under § 25.1093(b)(3), the icing conditions depicted in appendix O to part 25 do not apply to turbine engine induction systems on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds. S ection 25.1093(b)(1) requires that turbine engine induction systems on part 25 airplanes with a maximum takeoff weight equal to or greater than 60,000 pounds comply with the icing conditions depicted in appendix C to part 25, a ppendix D to part 33, a nd in falling and blowing snow within the limitations established for the airplane for such operation. Section 23.2415(b) requires that the powerplant installation design prevent any accumulation of ice or snow that adversely affects powerplant operation in those icing conditions for which certification is requested. Guidance for part 23 airplanes is available in ASTM F3120 / F3120M.
6.2.1.3 The applicant should account for exposure to extended operations (ETOPS) if proposed as part of the engine or airplane certification basis.
The applicant should confirm there is no impact to the probe operation during a maximum length diversion since probes tend to react to ICI in significantly less than one-hour exposures, the minimum threshold diversion time for ETOPS. If the applicant finds there is an impact to probe operation during a maximum length diversion, then the applicant should account for the effect when showing compliance to § K25.1.3(a) of appendix K to part 25.
6.2.1.4 To support engine and airframe safety analysis, identify and consider the proposed probe’s possible failure modes and the failure effects at this stage. For example, if the probe is heated, identify failure modes and their effects of the heater element in the appropriate safety analysis. It is unnecessary to demonstrate these failure modes as part of the test program; however, an applicant should explain the probe failure modes and failure effects to ensure the impact of probe failure is known.
6.2.1.5 Early in development, determine the fidelity of the data provided from the probe. That way, the airframe and engine manufacturers can determine what systems the probe is acceptable for (i.e., systems with catastrophic safety effects will require a more reliable probe or multiple data sources, 8
AC 20-197 whereas systems with only minor safety effects may be able to utilize data from a less reliable probe).
6.2.1.6 Figure D1 in appendix D to part 33 shows the temperature-altitude envelope for ICI. T he envelope covers altitudes up to approximately 48,000 feet and temperatures down to -60°C, based on atmospheric and airline operation data available at publication. More recent data shows that ICI can occur at higher altitudes and lower temperatures than in figure D1.
For that reason, if an airplane manufacturer expects the airplane envelope to extend above the envelope shown in figure D1, they should consider evaluating t hat part of the envelope for ICI vulnerability.
6.2.2 Determine ambient operating conditions. Appendices C and O to part 25 and a ppendix D to part 33 describe ambient or free stream concentrations of liquid water and ice crystals. Use these concentrations and the aircraft operating speeds to calculate water flux as a function of total temperature. For glaciated conditions, the highest water flux cases tend to be the most severe. In contrast, the highest water flux cases are not necessarily the severest cases for super-cooled liquid water or mixed phase conditions.
Other parameters to consider when selecting critical points include the local concentration factor at the probe, highest total cooling load, minimum predicted surface temperature, maximum water-to-air mass flux ratio, etc.
6.2.2.1 Within the mixed phase and ice crystal envelope described in appendix D to part 33, TWC in g/m3 is based upon the adiabatic lapse defined by the convective rise of 90% relative humidity air from sea level to higher altitudes. The TWC is scaled by a factor of 0.65 to a standard cloud length of 17.4 nautical miles (nm). In-service experience shows that several temperature and pressure probe icing events in glaciated conditions have occurred outside appendix D to part 33 envelope altitude and outside air temperature limits. E vents have occurred in conditions outside of those considered by appendix D to part 33, encompassing International Standard Atmosphere (ISA) +30°C conditions and down to a minimum temperature of -70°C. E vents have also happened outside appendix D to part 33 envelope down to ISA -5°C conditions above 25,000 ft and down to a minimum temperature of -70⁰C. Therefore, if the applicable aircraft envelope extends beyond the appendix D to part 33 envelope, the applicant should consider evaluating the icing environment corresponding to the portion of the aircraft operating envelope that lies outside appendix D to part 33, such as that described in SAE AS5562.
6.2.2.2 In addition, data suggest that the standard cloud of 17.4 nm and the associated scaled TWC values described in a ppendix D to part 33 m ay not provide an appropriately conservative set of conditions for air data probe testing. Service data experience suggests that peak ice-crystal concentration values (not average values) are critical to probe ice freeze over or blockage. T he ‘max’ or ‘peak’ TWC values occur at shorter distances than shown in figure D3 of appendix D to part 33. The applicant 9
AC 20-197 should use the max or peak TWC values instead of the ‘17.4 nm’ values provided by appendix D to part 33 if the peak TWC values are higher.
These ‘max’ or ‘peak’ adiabatic values are depicted in FAA Report No.
DOT/FAA/AR-09/13 and correspond to the ‘17.4 nm’ values multiplied by a factor of 1.538 (1/0.65). See the discussion on mixed-phase values in paragraph 6.2.4.9 of this AC.
6.2.3 Determine local conditions at the probe based on ambient conditions. Based on the ambient operating conditions determined in paragraph 6.2.2, define the local conditions at the probe. Probes are typically mounted a sufficient distance from the mounting surface (e.g., the fuselage skin or engine inlet) to accurately sense the freestream parameter of interest (total temperature, total pressure, etc.). However, when flying through particles such as supercooled water droplets, ice crystals, or rain, there can be a concentration effect near the mounting surface. This concentration effect is primarily due to inertia and drag effects, but large particles that bounce, splash, or break up yet remain in proximity to the mounting surface can also affect the results. This effect is highly installation-dependent and can vary significantly d epending on probe location and probe design.
6.2.3.1 Determining local conditions involves reviewing the probe's installed position (e.g., in the engine inlet or on the airframe) and determining local flow velocities and concentration effects for airframe-mounted probes or scoop factor effects for inlet-mounted probes.
6.2.3.2 AC 20-73A, appendix I, presents some methods used to calculate drop impingement and water catch at the location of interest for supercooled liquid water cases.
6.2.3.3 For ICI conditions, there is currently no standardized method for calculating ice crystal trajectories or the effects of ice crystals bouncing or breaking up. Both ice crystal events significantly affect the local concentration levels.
6.2.3.4 Preliminary research indicates the ice crystal concentration can be significantly higher than ambient conditions (see SAE Technical Papers 2011-38-0050 and 2015-01-2146), so conservative assumptions should be made at the probe regarding local IWC for ICI conditions using state-ofthe-art analytical tools. Document the assumptions regarding concentration factors and provide a rationale for those assumptions.
6.2.3.5 Static testing of an engine inlet probe outside of the engine inlet system may not replicate the same build and shed behavior due to variations in airflow and vibration levels. An integrated inlet/engine test can produce different results due to changes in vibration and local airflow, which may affect shedding behavior differently than when tested in isolation within an icing tunnel.
AC 20-197
6.2.4 Identify candidate test points. Based on the local conditions at the probe, identify candidate test points, including peak and cyclic conditions. Test a r ange of temperatures representative of the icing envelopes for the probes.
6.2.4.1 The probe design should be analyzed to determine whether the probe is more susceptible to higher TWC for short durations, to lower TWC for longer durations, or to cyclic conditions, such as those defined in Test Condition 3 of § 33.68, Table 1 (14 CFR part 33, A mendment 34). As noted in paragraph 6.2.2 of this AC, the maximum ambient TWC should account for peak TWC conditions corresponding to the values of appendix D to part 33, multiplied by 1.538.
6.2.4.2 In addition to the peak adiabatic TWC conditions discussed in paragraph
6.2.2.2 of this AC, test points with reduced ambient TWC and extended duration should be included. Testing at different TWC provides evidence that the installed probe will work throughout the operating envelope and not just at the maximum level. Scale back the peak concentration to a standard (17.4 nm) cloud by dividing the peak T WC values by 1.538.
Testing at a concentration of one-half of the peak value is appropriate.
From appendix D to part 33, f igure D3, the scale factor for the standard cloud is 1, and the scale factor for one-half of the peak value is ½*1.538 =
0.769. Based on appendix D to part 33, figure D3, this scale factor corresponds to a horizontal extent of approximately 215 nm. A review of in-service engine ICI events, as documented in SAE paper 2015-01-2086, shows that the 99th percentile horizontal extent length for events where engine damage occurred is 354 nm (657 km). Most engine events occur within horizontal extents of less than 215 nm (400 km) in length. Since probes typically respond to ICI faster than engines, it is reasonable to reduce the maximum horizontal extent for a probe to 215 nm and use a TWC corresponding to one-half the peak value.
6.2.4.3 Consider the time duration of each test point. In general, 2 minutes is sufficient to evaluate the probe’s behavior when demonstrating compliance with appendix D to part 33. However, for cases where ice builds up on external probe surfaces, extend the test point duration as required to quantify the size of ice that may accrete and thoroughly evaluate any build/shed cycles. For the reduced concentration points, run the tests for a sufficient time to traverse a 17.4 nm or 215 nm cloud as appropriate to the test condition. For a typical transport airplane at cruising airspeed, traversing a 17.4 nm cloud takes about 2 to 3 minutes. For these cases, the conditions completed with the peak TWC for 2 minutes is a more severe test than 2 to 3 minutes at a reduced TWC. Therefore, you can eliminate the lower TWC case from consideration. For a typical transport airplane at cruising airspeed, a 215 nm cloud takes approximately 25 minutes to traverse.
AC 20-197
6.2.4.4 For liquid water icing testing of engine-mounted components, test or analysis points should be identified according to the conditions in table 1 of § 33.68. T his testing requires 10-minute duration glaze ice and rime ice tests at various power settings (airflows) as well as 45-minute glaze and rime ice holding conditions.
6.2.4.5 For airframe-mounted pr obes in liquid water icing conditions, identify test or analysis points per the conditions stated in tables 1-1 a nd 1-2 of this AC. Table 1-1 requires steady-state 15-minute durations representing continuous maximum cloud concentrations and 5-minute durations representing intermittent maximum cloud concentrations. Table 1-2 requires cyclical tests at the liquid water conditions at either of the repetitions per section 6.2.4.5.1 or 6.2.4.5.2 of this AC.
6.2.4.5.1 For 15.1 nm in the conditions of table 1-2, column (a), under column LWC (g/m3), appropriate to the temperature, followed by 2.7 nm in the conditions of table 2, column (b), under column LWC (g/m3), a ppropriate to the temperature, for a duration of 30 minutes.
6.2.4.5.2 For 3.2 nm in the conditions of table 1-2, column (a), under column LWC (g/m3), a ppropriate to the temperature, followed by 2.7 nm in the conditions of table 1-2, column (b), under column LWC (g/m3), appropriate to the temperature, for a duration of 10 minutes.
Table 1-1. Stabilized Icing Conditions for airframe mounted probes in liquid water icing conditions
Test Static Air Altitude Range LWC (g/m3) Duration MVD number Temperature (ft) (min) (μm) (°C) SL1 -20 0-22,000 0.22 to 0.3 15 15 to 20 SL2 -30 0-22,000 0.14 to 0.2 15 15 to 20 SL3 -20 4,000-31,000 1.7 to 1.9 5 15 to 20 SL4 -30 4,000-31,000 1 to 1.1 5 15 to 20 12