32 Appendix O freezing drizzle conditions can be simulated, depending on the tunnel. Icing wind tunnels have been used to evaluate ice shapes on unprotected areas and on or aft of protected areas, such as inter-cycle, residual, and runback ice. They have also been used to evaluate performance of IPSs, such as pneumatic and thermal systems. Scale models may be used in icing wind tunnels, with appropriate scaling corrections, if the scale testing on the component has been validated with full-scale testing or analysis. Hybrid models, with the full-scale leading edge extending beyond the impingement limits, may also be used. The applicant may use these models to estimate impingement limits, examine visual icing cues, and evaluate ice detection devices.
11.3.7.2 Surface temperature measurements are typically made for air-data instruments, such as pitot tubes, pitot-static pressure probes, and angle-of-attack probes, during icing wind tunnel tests to verify thermal analyses. The acceptability of the installed air-data instrument ice protection should be evaluated during natural or artificial icing tests.
11.3.8 Dry Air Wind Tunnel Tests.
Dry air wind tunnel testing using scaled models and ice shapes has been used to determine ice protection requirements. The scaling, including the effect of the roughness of the ice, should be substantiated using methods found acceptable to the authorities.
12 CERTIFICATION WITH § 25.1420.
Section 25.1420 requires that the airplane operate safely in the supercooled large drop icing conditions defined in part 25, Appendix O. For guidance on engineering tool capabilities, see Appendix E of this AC.
12.1 Certification with §§ 25.1420(a)(1) and 25.1420(a)(2).
As an alternative to the requirements of § 25.1420(a)(3), applicants have the option of complying with §§ 25.1420(a)(1) or (a)(2), which allows an airplane to be certified for a limited range of conditions, from certification to Appendix C only to certification to Appendix C and a portion of Appendix O. The icing conditions the airplane may be certified to fly through may be defined in terms of any parameters that define Appendix O conditions and could include phase of flight limits, such as takeoff or holding, in Appendix O or a portion of Appendix O. For example, an airplane may be certificated to take off in portions of Appendix O conditions, but not be certificated for holding in those same conditions. Substantiated means must be provided to inform flightcrews when the selected icing conditions boundary is exceeded. The applicant must show compliance with § 25.21(g) for exiting the restricted Appendix O icing conditions. Ice shapes to be tested are those representing the critical Appendix O icing conditions during recognition and subsequent exit from those icing conditions. Methods of defining the selected Appendix O icing conditions boundary should be considered early in the certification process, with concurrence from the appropriate ACO. See
33 section 19 of this AC for specific guidance on AFM limitations and operating procedures.
Note: While this AC provides guidance on compliance with § 25.1420(a)(2), further advancements in engineering tools may be necessary before an applicant is able to substantiate compliance with this option. A 2009 review of engineering tool capabilities found that current technology does not support distinguishing between freezing drizzle and freezing rain in flight. Therefore, certification for a portion of Appendix O will be challenging and will require close coordination with certifying authorities. Consult Appendix E of this AC for additional details on engineering tool capabilities.
12.1.1 Selection of Appendix O Boundary.
The physics of ice accretion is complex. (See FAA Technical Report DOT/FAA/CT- 88/8-1 for a detailed discussion on the icing physics process.) Some generalizations, however, can be made about the effects of supercooled water drop impingement and ice limits. The “modified inertia parameter” is a similarity parameter relating a water drop’s inertia to the drag from the airflow. The inertia of smaller drops tends to be dominated by the airflow effect, so their trajectories tend to follow the local streamlines of air.
Sufficiently small drops, depending on the size of the drop and the airplane component they are approaching, may simply flow around the airframe. (A larger airplane component increases the flow disturbance ahead of the component, deflecting airflow and the trajectories of smaller drops.) On the other hand, the inertia of larger drops tends to dominate the airflow effect and so the trajectories tend toward straight lines.
Thus, large drops are much less likely to miss the airframe. The modified inertia parameter depends very strongly on drop size. It depends strongly on airplane (or model) size and airplane (or tunnel) velocity, and it also depends on drop density and air viscosity and density. The modified inertia parameter assumes a spherical drop and does not reflect drop distortion, drop break-up, or drop splashing. Standard impingement parameters, such as impingement limits, overall impingement, and leading edge impingement, all increase as a function of the modified inertia parameter. Other influences affecting the object’s water collection efficiency, drop impingement, icing limits, ice shape, and runback ice include splashing of the large water drops and breakup of the large water drops when forces on the distorted drop exceed the surface tension required to hold the drop together. These latter influences become more evident for the larger drops defined in Appendix O. Drop size distributions and the related distribution of water mass are important considerations for water catch, impingement limits, and ice limits, as well as for runback ice. Consideration of these influences allows comparison of critical icing conditions that can be correlated to ice accretions in aft locations, such as behind protected areas.
12.1.2 The modified inertia parameter is commonly used in icing wind tunnel testing to compare high altitude flight conditions to the available low altitude test conditions. To simulate effects of high altitudes and true airspeeds on impingement, larger drops are used at the lower altitude test conditions. As an example, an IPS designed to protect against 50 µm drop impingement at 22,000 feet and 200 knots calibrated air speed (KCAS) (290 knots true air speed (KTAS)) would be equivalently protected against
34 76 µm drops at sea level and 200 KCAS (190 KTAS) because of an equivalent modified inertia parameter (assuming a 6-foot chord).
12.1.2.1 Based on these physical processes, it is difficult to select a boundary for Appendix O icing conditions based on drop size. Consideration of the drop inertia effects throughout the flight envelope will likely be required. While technology does not currently exist to discriminate icing conditions based on specific drop sizes (exclusive of inertia effects), methods may be developed in the future to certify airplanes for flight in icing in such a manner.
12.1.2.2 Alternative methods of defining a selected boundary for Appendix O icing conditions may be considered. For example, it may be possible to develop an exit cue so that unrestricted flight in Appendix O freezing drizzle is approved but exit is required from Appendix O freezing rain.
12.1.2.3 See Appendix C of this AC for an example of a plan for certificating to Appendix C only (§ 25.1420(a)(1)) based on drop size discrimination.
12.1.3 Means for Detecting Appendix O Boundaries.
Determining whether the selected boundary for Appendix O icing conditions has been exceeded can potentially be accomplished using visual cues, an ice detection system, or an aerodynamic performance monitor.
12.1.3.1 Ice detection systems and aerodynamic performance monitors are discussed in paragraph 15.5 of this AC. For compliance with § 25.1420(a)(1), it is acceptable to use an ice detection system that detects accretions behind the airplane’s protected areas.
12.1.3.2 Substantiated visual cues can range from direct observation of ice accretions aft of the airplane’s protected surfaces to observation of ice accretions on reference surfaces. Methods used to substantiate visual cues should be agreed on with the appropriate certification authorities.
Responding to a cue should not require the flightcrew to judge the ice to be a specific thickness or size. Examples of potential visual cues are accretions forming—
• On side windshields,
• On sides of nacelles,
• On propeller spinners aft of a reference point,
• On radomes aft of a reference point, and/or
• Aft of protected surfaces.
12.1.3.2.1 Visual cues should be developed with the following considerations:
12.1.3.2.1.1 Visual cues should be within the flightcrew’s primary field of view if possible. If outside the primary field of view, the cues should be visible from the design eye point and easily incorporated into the flightcrew’s visual scan with a minimum of head movement while seated and performing their normal duties.
12.1.3.2.1.2 Visual cues should be visible during all modes of operation (day and night) without use of a handheld flashlight.
12.1.3.2.2 During the certification process, the applicant should verify the ability of the flightcrew to observe visual cues or reference surfaces. Visibility of the visual cues should be evaluated from the most adverse flightcrew seat locations in combination with the range of flightcrew heights, within the approved range of eye reference point locations, if available. A visual cue is required for both the left and right seats. If a single visual cue is used, it should be visible from each seat. Consideration should be given to the difficulty of observing clear ice. The adequacy of the detection method should be evaluated in all expected flight conditions. The applicant can carry out night evaluations with simulated accretions to assess visibility in and out of clouds.
12.1.3.2.3 Visual cues should be substantiated by tests and analysis, including tests in measured natural icing, or icing tanker tests, or potentially through icing wind tunnel tests. The applicant should consider the drop distributions of Appendix O when developing the cue, and the applicant should substantiate that these cues would be present in all the restricted Appendix O icing conditions. If a reference surface is used, the applicant should substantiate that it accumulates ice at the same time as or prior to ice accumulation on the critical surfaces.
12.1.3.2.4 AC 25-25A should be reviewed for guidance on the time flightcrews need to visually detect Appendix O icing conditions.
12.1.4 Some Areas of Interest for Appendix O.
The following are some areas of interest specific to supercooled large drop conditions:
12.1.4.1 Ice Shapes Aft of Protected Areas of Lifting Surfaces.
Appendix O icing conditions may result in a ridge of ice or ice roughness aft of the protected areas. On airplanes with reversible flight controls, a ridge of ice may cause uncommanded deflections of the control surfaces, which may result in an airplane upset. Airplanes may also incur reduced maximum lift and stall at lower angles of attack, independent of the flight control system. Appendix O ice build-ups can result in an airplane upset or stall if the stall protection system activation schedules are not adjusted.
12.1.4.2 Ice Shapes on Unprotected Areas of Lifting Surfaces.
Ice shapes resulting from Appendix O icing conditions can extend further aft on the airfoil than those from Appendix C icing conditions. When portions of the airfoil leading edge are unprotected, performance and handling qualities can be affected.
12.1.4.3 Ice Shapes on Unprotected Airframe Regions.
Areas of the airframe that do not accrete ice under Appendix C conditions should be examined for impingement and accretion in Appendix O conditions. The applicant should evaluate such issues as visibility through the windshield, locations of air-data sensors, and potential airflow disturbance (for example on the airplane nose) from Appendix O accretions that could influence performance of the air-data sensors.
12.1.4.4 Drag and Power Effects.
Airplanes may exhibit reduced performance in Appendix C conditions because ice accretions cause increased drag and reduced lift. Such impacts can be exacerbated in Appendix O conditions, since the accretions may extend further aft on protected areas, extend beyond protected areas, or occur in areas not typically protected (extended accretions on the fuselage and the lower wing surface, for example). Ice accretions may be rough and cause large local drag increases. Thrust effects of bleed penalties from system operations or degradation of thrust from powerplant losses (e.g., propeller icing) should be addressed. See AC 25-25A for more information about evaluating potential effects on airplane performance.
12.1.4.5 Engine Considerations.
Ice accumulation on the airframe and on air induction system components resulting from Appendix O icing conditions should be analyzed for potential ingestion of ice by the engine (relative to requirements of § 25.1093) and potential blockage effects. For § 25.1093 compliance, analysis may be used for the radome as a potential airframe ice source.
Applicants may use qualitative analysis of the design supported by similarity to a 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. Service history for designs similar to the ones under consideration should be part of the icing certification plan.
In order to demonstrate compliance using similarity, the applicant should show successful service history of the previous design and establish confidence that the historical certification methodology is appropriate for the current certification.
12.1.4.6 Holding Ice Considerations.
Analysis of exposures to Appendix C and Appendix O conditions should consider holding operations consistent with the applicable “Holding Ice”
37 definition contained in Part II of those appendices. Appendix O, Part II, contains different holding ice conditions depending on the level of certification requested in Appendix O conditions. For airplanes not certified to § 25.1420, the applicant should choose the holding ice conditions for which certification is sought. If the component will be certified for unrestricted operations in Appendix O, or a portion of Appendix O, then holding ice is defined in Appendix O, Part II, paragraph (c)(4). If the component will be certified for “detect and exit” operations, then the holding ice is defined in Appendix O, Part II, paragraph (b)(2).
12.2 Certification with § 25.1420(a)(3).
For compliance with § 25.1420(a)(3), if the AFM performance data reflects the most critical ice accretion (Appendix C and Appendix O) and no special normal or abnormal procedures are required in Appendix O conditions, then a means to indicate when the airplane has encountered Appendix O icing conditions is not required.
13 PNEUMATIC DEICER BOOTS.
Some existing AFMs specify a minimum ice accumulation thickness before activation of the deicer boot system. Such AFMs are associated with airplanes certified prior to § 25.1419, Amendment 25-129. This amendment added § 25.1419(e), (f), (g), and (h), which do not allow activation of airframe IPSs based on flightcrew visual observation of a specific thickness of ice. See section 15 of this AC for additional guidance on compliance with § 25.1419(e), (f), (g), and (h).
14 EMERGENCY AND ABNORMAL OPERATING CONDITIONS.
The applicant should conduct flight tests to demonstrate continued safe flight and landing following failures of the IPS using appropriate AFM abnormal and/or emergency operating procedures. Such demonstrations should be conducted with anticipated ice accumulations on normally protected surfaces. See part 25, Appendix C, and AC 25-25A for subpart B testing with failure ice shapes.
15 ICE AND ICING CONDITIONS DETECTION SYSTEMS.
Sections 25.1419(e), (f), (g), and (h) specify requirements for activation of the IPS.
These requirements are only applicable to Appendix C icing conditions.
Section 25.1420(c) requires compliance with § 25.1419(e), (f), (g), and (h) for the selected portion or all of Appendix O as applicable. Sections 25.1420(a)(1) and (a)(2), respectively, require a means to alert the flightcrew that they are in Appendix O icing conditions or have reached Appendix O icing conditions from which they must exit.
15.1 Compliance with § 25.1419(e)(1) and (e)(2).
These sections of the rule provide alternatives to operation of the IPS based on icing conditions defined in § 25.1419(e)(3). These alternatives require either a primary ice
38 detection system or substantiated visual cues and an advisory ice detection system.
Section 25.1419(e)(2) requires defined visual cues for recognition of the first sign of ice accretion on a specified surface combined with an advisory ice detection system that alerts the flightcrew to activate the airframe IPS. An acceptable means of compliance is described in the preamble of that rule, which states that the acceptability of visual cues combined with an advisory ice detection system is contingent on the following conditions:
15.1.1 The advisory ice detection system annunciates when icing conditions exist or when the substantiated visual cues are present.
15.1.2 The defined visual cues rely on the flightcrew’s observation of the first sign of ice accretion on the airplane and do not depend on the pilot determining the thickness of the accretion.
15.1.3 The flightcrew activates the IPS when they observe ice accretion or when the ice detector annunciates ice, whichever occurs first.
15.1.3.1 Ice Detectors.
15.1.3.1.1 Primary Ice Detector. A primary ice detector must either alert the flightcrew to operate the IPS using AFM procedures or automatically activate the IPS before an unsafe accumulation of ice on the airframe, engine components, or engine air inlets occurs. The primary ice detection system must perform its intended function for the airplane configurations, phases of flight, and within icing envelopes of Appendix C and the applicable portion of Appendix O. Compliance for Appendix O conditions may be shown through qualitative analysis of the design, and supported by similarity to previous designs that have shown successful service history.
Laboratory tests should demonstrate the ice detector’s ability to function properly within all of the required icing conditions and airplane operating envelope. If similarity is not shown because of the lack of engineering tools for freezing rain, primary 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. If the ice detector cannot detect ice at low freezing fractions and ice accretes on the airplane in such icing conditions, the applicant should show that the airplane can be operated safely with the ice accretions. If safe operation cannot be substantiated, installation of an icing conditions detector may be required. Approval of the primary ice detector should include flight tests in measured natural icing conditions to verify analyses and laboratory test results, as well as to verify that the ice detector system performs its intended function. The primary automatic ice detection system should be designed to prevent frequent cycling up and down of engine thrust in intermittent icing conditions. However, small thrust changes will occur with the opening and closing of the engine bleed valves, resulting in a possible nuisance to the flightcrew.
15.1.3.1.2 Advisory Ice Detector. The advisory ice detector, in conjunction with another cue, such as visible ice accretion on referenced or monitored surfaces, should advise the flightcrew to initiate operation of the IPS using AFM procedures. An advisory system is not the only means used to determine if the IPS should be activated. When there is an advisory system installed on an airplane, the flightcrew has primary responsibility for determining when the IPS should be activated. Although the flightcrew has primary responsibility for determining when the IPS must be activated, if the airplane is certificated in accordance with § 25.1419(e)(2), the advisory ice detector should be part of the type design and not be provided as optional equipment. The advisory ice detector must also perform its intended function for the airplane configurations, for its phases of flight, and within the icing envelopes of Appendix C and the applicable portion of Appendix O. Analyses and tests similar to those performed for a primary ice detector should be performed for an advisory ice detector to understand its characteristics, limitations, and installation. If the advisory ice detector cannot detect ice at low freezing fractions and ice accretes on the airplane in such icing conditions that may go undetected by the flightcrew, installation of an icing conditions detector (i.e., one that detects both moisture and temperature), or additional substantiation with the resulting undetected ice accretions may be required.
15.1.3.1.3 Certification Plan. The applicant should present an icing certification plan, as suggested by section 7 of this AC, to the appropriate ACO. This plan should include the method selected for demonstrating the ice detector system’s compliance with §§ 25.1301, 25.1309, 25.1419, and all other applicable sections. Appendix K of AC 20-73A provides guidance on certification of ice detector systems. That appendix provides guidance on ice detection response times as the freezing fraction drops below 1. That guidance is equally applicable to advisory and primary ice detection systems.
15.1.3.1.4 Performance of the Ice Detector System when Installed. The applicant should accomplish a drop impingement analysis and/or tests to ensure that the ice detector(s) are properly located. The applicant should show that under the various airplane operational configurations, phases of flight, airspeeds, and associated angles of attack, the ice detector is exposed to free-stream water drops. The ice detector should be located on the airframe surface where the sensor is adequately exposed to the icing environment. The applicant should conduct flow field and boundary-layer analyses of candidate installation positions to ensure that the ice detector sensor is not shielded from impinging Appendix C and Appendix O water drops. The ice detection system should be shown to operate in the range of conditions defined by Appendix C and the applicable portion of Appendix O. Sections 25.1419 and 25.1420 also require a combination of tests and analysis to demonstrate performance of the ice detector as installed on the airplane. This could include icing tunnel and icing tanker
40 tests to evaluate ice detector performance. The applicant may use drop impingement analysis to determine that the ice detector functions properly over the drop range of Appendix C and the applicable portions of Appendix O when validated through natural or artificial icing tests (e.g., tanker, icing tunnel). The applicant should demonstrate that the airplane can be safely operated with the ice accretions formed up to the time the IPS becomes effective, following activation by the ice detector. The detector and its installation should minimize nuisance warnings.
15.1.3.1.5 Ice Detector System Safety Considerations. The applicant should consult AC 25.1309-1A for guidance on complying with § 25.1309. In accordance with the method of compliance in that AC, the applicant should accomplish a functional hazard assessment to determine the hazard level associated with failure of the ice detection system. The probability of encountering the icing conditions defined in Appendix C to part 25 should be considered to be 1. The unannunciated failure of a primary ice detection system is assumed to be a catastrophic failure condition, unless characteristics of the airplane in icing conditions without activation of the airframe IPS(s) are demonstrated to result in a less severe hazard category.
If visual cues are the primary means of ice detection, the pilots retain responsibility to monitor and detect ice accretions when an advisory ice detection system is installed. However, the natural tendency of flightcrews to become accustomed to using the advisory ice detection system elevates the importance of the detector and increases the need to make flightcrews aware of an advisory ice detection system failure. Therefore, an undetected failure of the advisory ice should be considered as at least a major hazard unless substantiated as meriting a lower failure condition classification.
15.1.3.2 Visual Cues.
Visual cues can be either direct observation of ice accretions on the airplane’s protected surfaces, or observation of ice accretions on reference surfaces. The first indications of accretions forming on any of the following are examples that could potentially be used as visual cues:
• The windshield wiper posts (bolt or blade).
• Propeller spinners.
• Radomes.
• The protected surfaces.
15.1.3.2.1 If accretions on protected surfaces cannot be observed, a reference system would be necessary if compliance with § 25.1419(e)(2) is desired. The applicant should consider providing a reference surface that can be periodically deiced to allow the flightcrew to determine if the airframe is continuing to accumulate ice. Without a means to deice the reference surface, compliance with § 25.1419(e)(2) would require operation of the
41 IPS as long are there is ice on the reference surface, even when additional ice is not accumulating. As the freezing fraction drops below 1, although some reference surfaces may not build up ice, ice may begin to accumulate on protected surfaces of the airplane. The applicant should show, for all the icing conditions defined in Appendix C and the applicable portions of Appendix O that the reference surface accumulates ice at the same time as or prior to ice accumulating on the protected surfaces.
15.1.3.2.2 Field of View. Visual cues should be developed with the following considerations.
15.1.3.2.2.1 Visual cues should be within the flightcrew’s primary field of view, if possible. If cues are outside the primary field of view, they should be visible from the design eye point and easily incorporated into the flightcrew’s vision scan with a minimum of head movement while seated and performing their normal duties.
15.1.3.2.2.2 Visual cues should be visible during all modes of operation (day, night, and in a cloud).
15.1.3.2.3 Verification. During the certification process, the applicant should verify the ability of the flightcrew to observe the visual cues. Visibility of the visual cues should be evaluated from the most adverse flightcrew seat locations in combination with the range of flightcrew heights, within the approved range of eye reference point locations, if available. A visual cue is required for both the left and right seats. If a single visual cue is used, it should be visible from each seat. The difficulty of observing clear ice should be considered, and the adequacy of the visual cue should be evaluated in all expected flight conditions. The applicant can carry out night evaluations with simulated accretions to assess visibility in and out of cloud. Visual cues should be substantiated by tests and analysis, including tests in measured natural icing.
15.2 Compliance with § 25.1419(e)(3).
This paragraph of § 25.1419 provides an alternative to the primary ice detection system and visual cues, plus the advisory ice detection system as defined in § 25.1419(e)(1) and
(e)(2). This alternative requires operation of the IPS when the airplane is in conditions conducive to airframe icing during all phases of flight.
15.2.1 Temperature Cue.
The temperature cue used in combination with visible moisture should consider static temperature variations due to local pressure variations on the airframe. If the engine and airframe IPS are both activated based on visible moisture and temperature, a common conservative temperature for operation of both systems should be used. For example, if the engine IPS is activated at +5 °C static air temperature or less, the airframe IPS should be activated at the same temperature, even if it is substantiated that the airframe
42 will not accrete ice above +2 °C static air temperature. This would ease the flightcrew workload and increase the probability of procedural compliance.
15.2.2 Either Total or Static Temperatures are Acceptable Temperature Cues.
If static temperature is used, a display of static air temperature should be provided to allow the flightcrew to easily determine when to activate the IPSs. As an alternative, a placard showing corrections for the available temperature, to the nearest degree Celsius, can be used, so the flightcrew can determine the static air temperature in the region of interest (that is, around 0 °C).
15.2.3 Airplane Flight Manual.
The Limitations section of the AFM should identify the specific static or total air temperature and visible moisture conditions that must be considered as conditions conducive to airframe icing and should specify that the IPS must be operated when these conditions are encountered.
15.3 Compliance with § 25.1419(f).
This paragraph of § 25.1419 states that requirements of § 25.1419(e)(1), § 25.1419(e)(2), or § 25.1419(e)(3) are applicable to all phases of flight unless it can be shown that the IPS need not be operated. To substantiate that the IPS need not be operated during certain phases of flight, the applicant should consider ice accretions that form during these phases, without the IPS operating, and establish that the airplane can safely operate in the continuous maximum and intermittent maximum icing conditions of Appendix C and the applicable portions of Appendix O.
15.4 Compliance with § 25.1419(g).
15.4.1 This paragraph of § 25.1419 requires that after the initial activation of the IPS—
15.4.1.1 The IPS must operate continuously,
15.4.1.2 The airplane must be equipped with a system that automatically cycles the IPS, or
15.4.1.3 An ice detection system must be provided to alert the flightcrew each time the IPS must be cycled.
15.4.2 The following are some examples of systems that automatically cycle the IPS so that the pilot does not have to manually cycle the IPS after initial activation:
15.4.2.1 A system that senses ice accretion on a detector and correlates it to ice accretion on a protected surface. This system then cycles the IPS at a predetermined rate.
15.4.2.2 A system that uses a timer to cycle the IPS. The applicant should substantiate that the airplane can safely operate with the ice accretions that form between the time one deicing cycle is completed and the time the
43 next cycle is initiated. If more than one cycling time (a choice of 1- or 3-minute intervals for example) is provided, it should be substantiated that the flightcrew can determine which cycle time is appropriate.
15.4.2.3 A system that directly senses the ice thickness on a protected surface and cycles the IPS.
15.4.3 Some types of ice detection systems that alert the flightcrew each time the IPS must be cycled could operate in a manner similar to the automatic systems discussed above, except that the flightcrew would need to manually cycle the system. Flightcrew workload associated with such a system should be evaluated. Because of flightcrew workload and human factors considerations, a timed system without an ice sensing capability should not be used to meet this requirement. The ice shedding effectiveness of the selected means for cycling the IPS should be evaluated during testing in natural icing conditions. All inter-cycle and runback ice should be considered when showing compliance with § 25.21(g). See paragraph 15.1.1 of this AC for guidance on installed ice detector performance and safety considerations.
15.5 Compliance with § 25.1420(a)(1) or (a)(2).
These paragraphs of § 25.1420 require a means be provided to allow the flightcrew to detect when Appendix O conditions are encountered or when the selected portion of Appendix O conditions is exceeded. Means for determining when the selected portion of Appendix O icing conditions is exceeded may include visual means, ice detectors, or an airplane performance monitor. The substantiation of visual means is discussed in section 12 of this AC.
15.5.1 Ice Detectors.
An ice detector system installed for compliance with § 25.1420(a)(1) or (a)(2) is meant to determine when conditions have reached the boundary of the Appendix O icing conditions in which the airplane has been demonstrated to operate safely. For compliance with § 25.1420(a)(1), it is acceptable to use an ice detection system that detects accretions behind the airplane’s protected areas. The applicant should accomplish a drop impingement analysis and/or tests to ensure that the ice detector is properly located to function during the airplane operational conditions and in Appendix O icing conditions. The applicant may use analysis to determine that the ice detector is located properly for functioning throughout the drop range of Appendix O conditions when validated with methods described in SAE ARP5903. The applicant should ensure that the system minimizes nuisance warnings when operating in icing conditions.
15.5.2 The low probability of finding conditions conducive to Appendix O ice accumulation may make natural icing flight tests a difficult way to demonstrate that the system functions in conditions exceeding Appendix C. The applicant may use airplane flight tests under simulated icing conditions (icing tanker). The applicant may also use icing wind tunnel tests of a representative airfoil section and an ice detector to demonstrate proper functioning of the system and to correlate signals provided by the detectors with the actual ice accretion on the surface.