8.1 Compliance Means: Analysis.
a. The certification applicant should perform analyses to show compliance with the applicable regulations listed in table 5 of this AC.
b. Information defining the icing conditions in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C may be found in appendix E of this AC. Appendix F of this AC contains operational factors you should consider in your compliance analyses. For operational purposes, fixed-wing airplanes should operate safely in icing conditions for a 45-minute destination “hold” flight segment in the continuous maximum icing conditions as defined in 14 CFR part 25, Appendix C. See appendix G of this AC for guidance on the 45-minute hold in icing conditions.
Note that each turbine engine must operate throughout its flight power range (including idling) without accumulation of ice on the engine and its inlet system, or on airframe components that would adversely affect engine operation, or cause a serious loss of engine power or thrust. (See 14 CFR §§ 23.1093, 25.1093, 27.1093, and 29.1093.) (See section 10 of this AC for guidance applicable for rotorcraft.)
c. When determining compliance with the applicable regulations, you and the ACO personnel should closely review the analyses used to show compliance with the requirements discussed in section 6 of this AC. This scrutiny is necessary because verification of the ice protection equipment and system analyses are typically demonstrated over limited flight test conditions.
d. Airframe and powerplant (aircraft engine or aircraft engine and propeller) IPSs may require different analysis methods. Aircraft surfaces may be more tolerant of ice accretion than the aircraft engine and aircraft engine inlet surfaces. Airframe IPS design methods may also differ somewhat from those applied to the IPS designs for an engine. Note that although the operating envelope can be determined precisely for a specific powerplant/aircraft combination, the powerplant analysis needs to consider the installation of the engine on several airplane models with different required operating envelopes.
e. The IPS design margins are dependent on meteorological factors, airplane-engine operational, and other relevant factors. For an example, the availability of engine bleed air for airframe thermal ice protection systems must be sufficient for safe operation during descent when the engine power may be at idle.
f. Engine and aircraft type certificate applicants should determine the most critical conditions applicable to the aircraft engine inlet and propeller systems’ design, after considering all the meteorological and operational conditions within the operational envelope of the engine.
This information will assist the ACO in determining the adequacy of the method used to establish the criticality of those factors. You should define the design points in meteorological and operational terms, for example, in terms of liquid water content (LWC), temperature, drop size, and engine power setting, or propeller pitch and rpm. When you determine the most critical conditions, you should have a specific design objective in mind, with the understanding that some ice buildup may be acceptable for running-wet ice protection systems if it does not risk flight safety.
g. For compliance purposes, be aware that different areas of the aircraft will have different tolerances to ice accumulation and may require different analysis approaches.
8.1.1 Compliance Means: Airframe Analysis.
a. As the applicant, you must select the critical surfaces requiring ice protection for safe operation in icing conditions. These surfaces may include:
(1) Airframe/fuselage ice impact areas.
(2) Leading edges of wings, winglets, and wing struts.
(3) Leading edges of horizontal and vertical stabilizers, and other lifting surfaces and control surfaces.
(4) Leading edges of control surface balance areas, if not shielded (such as aileron and elevator horns).
(5) Accessory cooling air intakes that face the airstream and could become restricted due to ice accretion.
(6) Antennas and masts.
(7) External tanks and fairings.
(8) External hinges, tracks, door handles, and entry steps.
(9) Instruments, including pitot tubes (and masts), static ports, angle-of-attack (AOA) sensors, and stall warning sensors.
(10) Forward fuselage nose cone and radome.
(11) Windshields (cockpit).
(12) Landing gear.
(13) Retractable forward landing lights.
(14) Ram air turbines.
(15) Ice detection lights, if required.
(16) Vortex generators and other flow control devices like stall strips, vortilons, and fences.
(17) Other structural protuberances that are exposed to icing conditions.
(18) Fuel tank vents.
(19) Auxiliary power units (APU) inlet, exhaust, and drainpipe.
(20) Propellers.
(21) Engine air induction system.
b. Leading edges of wings are critically sensitive aerodynamically to surface roughness, especially those wings that are not equipped with leading edge high-lift devices (hard wings).
Flying in icing conditions with trailing edge flaps extended reduces the wings’ angle of attack.
This reduced angle of attack causes ice to build up further aft on the wing’s upper surface, increasing the aerodynamic sensitivity of the wing to icing. For example, ice further aft on the wing may affect the effectiveness of the ailerons and the airplane handling qualities. The aerodynamic effects depend on the airfoil’s aerodynamic characteristics. You must evaluate adverse aerodynamic effects of surface roughness that occur during normal operation of the wing’s ice protection system to ensure safe operation of the aircraft (14 CFR §§ 23.1419 and
25.1419). You should consider protecting the leading edges of high-lift devices because, in addition to ice accretion on the leading edge of the wing, the leading edges of slotted trailingedge flaps may accrete ice during approach and landing. Ice protection is typically not provided for trailing-edge flaps. Ice buildup along the leading edge of the trailing edge flaps may reduce their aerodynamic effectiveness (reduced lift at a fixed angle of attack). This buildup may be evident by airframe buffeting resulting from turbulent airflow over the flaps, and the ice on the flaps may prevent their retraction.
c. You should select the surfaces that require ice protection after careful consideration of icing and operating conditions and the icing requirements discussed in section 6 of this AC.
d. Appendix H of this AC contains information and guidance on ice protection technologies, operating modes, and associated analyses.
e. You should perform a drop impingement and water catch analysis to evaluate the impingement limits and water collection characteristics of aircraft surfaces and components.
This analysis also provides the ice collection efficiencies of aircraft surfaces and components.
The analysis should consider all the airplane’s flight configurations, phases of flight, and operating envelopes (including airspeeds, aircraft configurations, and angles of attack). The analysis should provide information to determine the extent of ice protection required to protect a surface or component from ice accretion, using the icing conditions defined in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. Also, determine in your analysis the quantity of heat required for thermal IPSs. Appendix I of this AC provides information about drop impingement and water catch analyses and determining the ice protection coverage.
f. The ice protection for air intakes must ensure satisfactory performance of essential systems that they support.
g. Table 6 shows the meteorological conditions you should consider for a typical compliance analysis for operating in continuous and intermittent maximum icing conditions.
Table 6. Continuous and Intermittent Maximum Icing Conditions Continuous Maximum Icing Conditions Intermittent Maximum Icing Conditions MVD (Pm) Temperature (qF) Liquid Water Content 3(g/m ) MVD (Pm) Temperature (qF) Liquid Water Content 3(g/m ) 15 32 0.8 15 32 2.925 14 0.6 14 2.5 -4 0.3 -4 1.925 -22 0.2 -22 1.1 25 32 0.5 -40 0.25 14 0.3 25 32 1.75 -4 0.15 14 1.45 -22 0.1 -4 1.125 40 32 0.15 -22 0.7 14 0.10 -40 0.15 -4 0.06 40 32 0.75 -22 0.04 14 0.50 -4 0.35 -22 0.25 -40 0.05 50 32 0.40 14 0.30 -4 0.20 -22 0.10 -40 0.05
h. Section 6.3 of this AC describes how to determine whether your airframe IPS can support safe operation of the aircraft in icing conditions.
i. Select pressure altitudes that cover the range of altitudes associated with each temperature from figures E-2 and E-5 in appendix E of this AC.
j. In addition to the meteorological conditions, your ice protection system design analysis should consider appropriate operational parameters such as aircraft attitude, airspeed, altitude, and engine power settings for the aircraft’s operating envelope. Those operating parameters will identify the combination(s) of meteorological and operating parameters that result in the most critical conditions. Because of the large number of variables involved, more than one critical condition may exist for both intermittent maximum and continuous maximum meteorological conditions.
k. The ice protection system design analysis should show that no hazardous quantity of ice forms on the surfaces that are critical for safe operation of the aircraft when exposed to the intermittent maximum and continuous maximum icing conditions of 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C.
8.1.2 Compliance Means: Aircraft Engine and Propeller Systems Analyses. When defining the most severe conditions for the compliance analyses of aircraft engine, propeller, and related components ice protection systems, the applicant should consider the icing envelopes in 14 CFR part 25, Appendix C, Figures 1, 2, 4, and 5, and 14 CFR part 29, Appendix C, Figures 1, 2, 4, and 5. The applicant should also consider the entire environmental and operational envelopes of the aircraft on which the propulsion system will be installed. (You should consult with your FAA ACO or ECO for guidance on the applicable environmental and operational envelopes.)
8.1.2.1 Compliance Means: Aircraft Engine Analysis.
a. The applicant should design the aircraft engine and its IPS to protect against the most critical meteorological conditions occurring simultaneously with the most critical aircraft engine (if applicable) operational conditions. For the aircraft engine compliance analysis, you should determine critical design points for both continuous maximum and intermittent maximum conditions. The procedures for determining water catch rate, impingement data, heat available (QA), and heat required (QR) are similar to those discussed for airframe surfaces in section 8.1.1 of this AC. The flow field around engine surfaces is dependent on the mass of air flowing through the engine.
b. Engine reliability during all icing encounters is essential. The design approaches that apply to airframe and aircraft engine systems are different. Aircraft engines must not experience any serious loss of power or thrust because of induction system ice buildup. (See 14 CFR § 33.68.)
c. The aircraft engine manufacturer may know the make and model of some of the aircraft on which the engine will be installed. You cannot be sure that a future application will not differ from planned installations. Therefore, you should not limit the IPS to a specific airplane installation or to a specific airplane operational envelope.
d. Ice buildup on the aircraft’s unprotected surfaces and the aircraft operational conditions during an icing encounter emphasize the need for reliable engine performance. Unprotected aircraft engine struts, spinner cones, and inlet guide vanes may be subject to ice buildup.
Therefore, when you use heat to keep these surfaces ice-free, you must guard against the possibility of water runback refreezing inside the air induction system. You should also evaluate the first-stage fan or compressor blades of axial flow engines for possible ice buildup, with the IPS operating. Without the ice protection system operating, ice buildups on these engine components may distort or adversely affect the airflow intake and cause adverse effects on engine operability or thrust. The ice may also shed and damage downstream engine components.
Minimizing the ice buildup on an operating engine prevents possible damage from ice ingestion and helps to ensure reliable engine operation. You must demonstrate that the level of ice buildup on the engine permits acceptable engine handling and performance.
e. You should consider a buildup of ice on any aircraft engine surface unsafe, if the ice buildup:
(1) Causes a significant loss of power or thrust;
(2) Causes airflow disturbances that excite harmonic compressor or fan blade frequencies;
(3) Becomes large enough to cause serious engine damage when ingested;
(4) Causes damage to adjacent structure or engine components when shed by centrifugal force from rotating surfaces;
(5) Causes an imbalance of rotating components that produces vibrations greater than those for which the engine had been approved;
(6) Causes damage due to reduced clearance between rotating and stationary components; or
(7) Causes any other hazardous engine operation.
8.1.2.2 Compliance Means: Propeller Analysis. Aircraft type certificate applicants installing propellers on their aircraft can find more information on compliance with aircraft requirements associated with propeller IPSs in appendix J of this document. A buildup of ice on a propeller is considered unsafe if the ice causes:
a. A serious loss of thrust;
b. An unsafe engine condition;
c. Damage to adjacent structure when detached by centrifugal force;
d. Vibrations resulting in engine or propeller structural failure; or
e. Any other hazardous engine, propeller, or aircraft operation.
8.1.3 Compliance Means: Aircraft Engine Inlets Analysis.
a. Ice buildup on the engine inlet nose cowl, spinner cones, and other areas of the aircraft that may affect engine operation is generally more critical to safety of flight than ice buildup on aircraft surfaces discussed in paragraph 8.1.1 of this AC. Even though meteorological design conditions may be the same, operational conditions that affect engine operation (particularly local surface flow conditions) may vary significantly. Although a fixed-engine operational condition is assumed for certification of the airframe icing system, engine airflow may vary considerably because of changes in engine thrust or power during normal flight maneuvers.
These changes in engine thrust or power must be considered when determining the most critical conditions for these airplane areas. Nacelle inlet surfaces are susceptible to slush and ice crystal buildup during operations in glaciated or mixed phase icing conditions. Also, you must evaluate the effects of ice buildups on vortex generators or other boundary-layer control devices, if they are used in or on the engine air induction system, to ensure that these devices continue to perform their intended functions.
b. Design of the airframe ice protection is based on the most probable engine operating condition for a specific airplane operational mode, therefore you should consider the need for increased reliance on engine thrust or power during icing conditions. You should also take into consideration these needs during the design of nacelle inlet ice protection systems because the engine must be able to operate through a wide range of power settings.
c. When determining the most critical design points for engine air inlets, use guidance provided in paragraphs 8.1.1.a(21), 8.1.1.d, 8.1.1.e, 8.1.1.j, and 8.1.1.k of this AC.
(1) The inlet design compliance analysis should show the engine inlet IPS prevents ice formations that adversely affect continued safe engine operation or cause serious power loss. In your analysis, use the meteorological conditions described in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C and engine operations that meet the aircraft operational needs.
(2) Engine inlet IPSs are often designed to “run dry” (impinging moisture is evaporated) under continuous maximum icing conditions and “run wet” under intermittent maximum icing conditions. Service experience shows that this approach is satisfactory when the formation of hazardous runback ice is prevented.
d. Your analysis should include evaluations of ice collection on inlets for the APU, accessory cooling, and fuel vent areas. The analysis should determine whether ice will collect on the inlets and affect the APU’s performance. Improperly located NACA inlet scoops may collect ice, so you should include their location and ice collection potential in the analysis. Results obtained during natural icing flight tests of NACA inlet scoops range from small amounts of ice collection on turboprop engine cooling inlet lips (located on the engine cowling or nacelles) to complete blockage of electronic cooling inlets located on the radome.
e. 14 CFR part 23 airplane type certificate applicants can find guidance on the approval of turbine engine induction system ice protection in AC 23-16A. This AC also includes guidance for testing IPS in falling and blowing snow and in freezing fog.
8.1.4 Compliance Means: Ice and Icing Conditions Detection Systems Analyses. Aircraft instrumentation may alert the flight crew or aircraft systems (or both) to the presence of icing conditions or ice buildup on aircraft surfaces. These ice detection systems may be primary or advisory. Appendix K of this document contains guidance for airframe type certificate applicants on obtaining approval of ice detection systems.
8.1.5 Compliance Means: Windshield Analysis. Airframe type certificate applicants must provide a means to prevent, or to clear accumulations of, ice from the windshields during icing conditions to ensure pilots’ field of vision is not restricted during flight into icing conditions (14 CFR §§ 23.775(f), 25.773(b)(1)(ii), and 29.773(b)(1)(ii)). These surfaces are usually protected by electric heat elements. Your analysis should confirm that the windshield surface temperature is sufficient to prevent ice accumulation without causing structural damage to the windshield. Appendix L of this AC provides added guidance for windshield ice protection.
8.1.6 Compliance Means: Air Data System Sensors and Probes Analyses.
a. Airframe type certificate applicants should provide ice protection for flight essential instrument sensors that may be affected by flight into icing conditions. Confirm that instrument surfaces and drainage cavities are protected against ice buildups and blockage by frozen drained water that may adversely affect the instrument.
b. You must provide compliance analyses showing static and pitot pressure ports are either protected against or not affected by ice buildup, frozen runback water from forward surfaces, and water and slush from the landing gear during takeoff and landing. Slush and water ingested into static and pitot pressure ports during takeoff or at a lower altitude might freeze when the airplane climbs to higher altitudes and lower temperatures. Instruments that could be affected include pitot tubes, engine-pressure-ratio total pressure probes, and certain types of stall warning system sensors. These instruments are typic ally electrically heated.
c. When drafting your compliance analyses for instrument sensing elements or probes located on or near the fuselage, consider varying the water drop concentration to account for a variety of icing conditions. As the aircraft flies through icing clouds, small cloud drops may not impinge directly on the fuselage, but may be carried over the fuselage surface by the airflow.
While these diverted drops reduce the collection efficiency in the stagnation regions of the fuselage, the diverted drops can increase water concentration near the fuselage’s surface, and significantly increase the water catch of probes, such as pitot tubes and other sensor probes. The resultant ice buildup may adversely affect the probe’s function.
8.1.7 Compliance Means: Ice Protection System Failure Analysis. Applicants must perform system failure analyses as a means of showing compliance with the applicable certification rules (14 CFR §§ 25.901(c), 27.901(b)(1), 29.901(c), and 14 CFR §§ 23.1309,
25.1309, 27.1309, and 29.1309). You should use the guidance provided in AC 23.1309-1C, AC 25.1309-1A, AC 27-1B, AC 29-2C, or AC 33-2B, to determine the hazardous system failures for your product. You must perform failure modes and effects analyses to ensure that single failures do not result in unsafe conditions. Perform fault-tree analysis of system failures to ensure safety objectives of combined failures are met. Separation, cascading, and common cause analyses ensure system independence if there are various airplane failures. For example, you should consider leading edge wing slat failure conditions and bird-strike conditions for a wing leading edge thermal anti-ice system. Also, under certain failure conditions, the hot air of a thermal IPS may approach temperatures that can affect the integrity of nearby structure and may ignite leaking fuel. You should evaluate these and similar conditions to ensure aircraft safety.
When system failures result in unsafe operations, the aircraft must exit icing conditions and be capable of continued safe flight and landing.
8.1.8 Compliance Means: Flutter Analysis. Aircraft must be free of flutter and control reversals. (See 14 CFR §§ 23.629(a), (b), (c), or (d), 25.629(a), 27.629, and 29.629.) You may show compliance with the applicable certification rules by following the guidance contained in AC 23.629-1A, AC 25.629-1A, AC 27-1B, and AC 29-2C. For transport category airplanes not approved for operation in icing conditions, you must consider the maximum likely ice accumulation expected, as a result of an inadvertent encounter, to show compliance with 14 CFR § 25.629(d)(3).
8.1.9 Compliance Means: Power Sources Analysis. Applicants should evaluate the power (energy) source needs of their IPS (electrical, engine bleed air, pneumatic pumps, etc.).
Compliance analyses or tests should show that each power source requirement is adequate for each IPS. You must ensure an IPS component failure does not affect the power available to operate other essential equipment (14 CFR §§ 23.1309, 25.1309, 27.1309, and 29.1309). All power sources that affect engine or engine IPSs for multiengine aircraft must comply with engine isolation requirements contained in 14 CFR §§ 23.903(b) and (c), 25.903(b), and 29.903(d).
8.2 Compliance Means: Tests.
a. Applicants must perform tests to show their aircraft can operate safely in the icing conditions as specified in 14 CFR §§ 23.1419, 25.1419, 27.1419, and 29.1419, and 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. You must perform flight and ground tests to verify the analyses, and you may use other means to support test data that show your aircraft will perform satisfactorily in icing conditions. You must determine that there are no icing anomalies associated with your design. Flight testing must be performed in measured natural icing conditions. Flight testing may also be performed in dry air, dry air with simulated ice shapes, or in measured simulated icing conditions. (See AC 23-8B, AC 23.1419-2C, AC 25-7A, AC 25.1419-1A, AC 27-1B, and AC 29-2C.)
b. Typically, applicants perform icing flight tests in three stages: (1) dry air tests with the IPS operating, (2) dry air tests with predicted simulated ice shapes, and (3) natural icing flight tests.
8.2.1 Compliance Means: Natural Icing Flight Tests.
a. Applicants must show the effectiveness of the ice protection equipment and systems by flight testing in measured natural atmospheric icing conditions. (See 14 CFR §§ 23.1419(b),
25.1419(b), 27.1419(b), and 29.1419(b).) During the natural icing flight tests, you should target the critical design point icing conditions you identified in your compliance analyses. Because natural icing characteristics are probable, performing natural icing tests at each critical design point may be impractical (See appendix M of this AC.) Icing tests should also investigate the effects of runback ice buildups aft of protected surfaces and the buildup of ice on unprotected surfaces to determine whether the ice poses a hazard to continued safe flight. Engine operational characteristics and the aircraft handling qualities and performance should be acceptable when runback ice, buildup ice on unprotected surfaces, or intercycle ice roughness exist on deice protected surfaces.
b. You should use data from the natural icing tests to validate analytical methods used for showing compliance with the icing regulations. You should also use the natural icing tests data to validate the results of simulated ice shape and simulated icing tests used during your icing certification.
c. You must test in natural icing to evaluate the effectiveness of the propeller IPSs and to check for icing anomalies not addressed by laboratory tests and analyses. You should provide a means of measuring aircraft performance with the propeller, and propeller and engine vibration. Also, record propeller ice accretion and shed-ice trajectories.
8.2.1.1 Compliance Means: Natural Icing Flight Test Icing Conditions Considerations.
a. You should record the parameters that measured the icing conditions encountered during certification testing in the same units as those used in 14 CFR part 25, Appendix C and 14 CFR part 29 Appendix C. This will assist you in understanding the effectiveness of the IPS. You may use these measurements to verify analyses of the IPS, to understand icing anomalies, and to extrapolate the IPS effectiveness to icing conditions you did not encounter. The parameters (LWC, mean drop diameter, temperature, cloud extent, and altitude) used in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C are important for thermal IPS designs. The water catch and the required heat density on thermally protected surfaces are dependent on these icing condition parameters. Measure these parameters and airspeed during natural and simulated icing flight tests to characterize the icing environment. You may also use the icing cloud drop median volume diameter (MVD) to determine IPS coverage. (See appendix I of this AC.) You should evaluate the operation of deicing boots at cold temperatures. The LWC and icing intensity are important parameters for defining preactivation and intercycle ice accretion on deicing boots.
You should refer to appendix N of this AC for information on measuring natural and simulated icing conditions.
b. Appendix M of this AC contains information on finding meteorological conditions for natural icing flight-testing.
c. Perform flight tests in icing conditions that are within icing condition envelopes described in 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. Appendix O of this AC provides procedures for converting the contents of 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C icing envelopes to a distance-based format. The quality of an icing event includes the size of the aircraft ice buildups and the rate of ice buildup (icing intensity).
Appendix P of this AC describes how to determine the icing intensity of an icing event.
d. Flight testing in convective air currents of cumulus clouds may lead to severe turbulence and hail encounters, and may cause structural damage to test aircraft. Flight testing in intermittent maximum icing conditions may not be necessary when your compliance analyses show the critical ice protection design points in these icing conditions (heat loads, critical ice shapes, ice accretion, and ice accretion rate, etc.) are acceptable and enough test data exist to verify the analyses.
8.2.1.2 Compliance Means: Natural Icing Flight Test IPS Evaluation.
a. For non-automatic IPS systems, the flight crew must activate the IPS according to the Airplane Flight Manual (AFM) procedures. You should conduct icing flight tests with delayed IPS activation to simulate icing events when the pilot may unintentionally delay activation of the IPS. You must determine the delay interval based on the icing recognition means and crew procedures. Ice buildup resulting from delay intervals greater than 2 minutes may shed and be ingested into an engine. Per 14 CFR § 33.77, you must ensure acceptable engine operation after ingesting the maximum in-service ice mass that can be shed from the airframe. Determine the delayed-activation ice buildup in the continuous maximum icing conditions of 14 CFR part 25, Appendix C and 14 CFR part 29, Appendix C. Include the time required for the IPS to perform its intended function as part of the delayed activation interval. Aircraft handling qualities with 18
the delayed activation ice buildup should remain acceptable and the aircraft must operate safely.
You are not required to test for delayed activation of IPSs that use a primary automatic ice detection system.
b. You should evaluate the effects of preactivation ice accretion (ice accumulated before the IPS becomes fully effective). Preactivation ice includes ice buildups that occur before thermal IPSs achieve their operating temperatures, fluid IPSs clear ice accretions, and ice buildups that occur before completion of a deicing cycle. Aircraft handling qualities should remain acceptable, and the aircraft must operate safely with the preactivation ice accretion.
c. Applicants for certification of fluid IPSs should show acceptable distributions of the system’s fluid over the protected surfaces. Acceptable distributions of the fluid determined in dry air may result in freezing of the IPS fluid at cold temperatures. You should test windshield fluid anti-ice systems to show the fluid does not become opaque at cold temperatures. A clear and legible means should be used to show fluid flow rates and the fluid quantity remaining. You should provide shutoff valves for systems using flammable fluids. The AFM should include information describing how long it will take to exhaust the fluid.
8.2.1.3 Compliance Means: Natural Icing Flight Test Ice Detector Evaluation. Applicants should perform natural icing flight tests of primary and advisory ice detectors to show they detect ice before hazardous ice builds up on aircraft surfaces. The flight test should also demonstrate the intended functions of the ice detector, such as automatic operation of the IPS.
Conduct flight tests at near-freezing ambient conditions to evaluate the ice detector’s functional reliability. This testing should also evaluate the possible accumulation of hazardous airframe ice in airplane (local) surface areas that have high airspeeds (for example, airfoil or engine inlet surfaces) before the ice detector detects icing conditions. You may determine temperatures for this testing by evaluating conditions when the water catch on critical aircraft surfaces freezes because of high local air velocities while only a small fraction of the water catch on the ice detector’s sensor surface freezes. Determining the critical temperatures may require testing the ice detector and critical aircraft surfaces in an icing wind tunnel.
8.2.1.4 Compliance Means: Natural Icing Flight Test Ice Shedding.
a. Ice shed from the airframe may be ingested into the engine and may cause engine damage that may result in unsafe operability and thrust loss. This ice may also damage downstream aircraft structure and components. Ice shedding from the forward fuselage and wings may cause significant damage to fuselage-mounted engines and pusher propellers. Airframe type certificate applicants should show by analysis or flight testing that ice shedding from aircraft components (including antennas) does not cause engine or propeller damage that would adversely affect engine or propeller operation. (See 14 CFR §§ 23.1093 and 25.1093.) You should also consider airframe damage that may result from propeller ice shedding.
b. Currently available trajectory and impingement analyses may not accurately predict damage caused by ice shedding. Experience has shown that analyses are unable to accurately predict the trajectories of ice shed from aircraft forward areas, such as from the radome, wing, wing fairings, and control surfaces. Therefore, trajectory analyses are unacceptable predictors of shed ice trajectories. Your damage analysis should conservatively consider the most critical ice 19
mass that shed and its impact with the areas of concern. Also, when predicting the ice trajectories, assume all shed ice is ingested into the engines or propellers, independent of the trajectories. In lieu of a conservative analysis, flight testing has been used for aircraft certificated under 14 CFR part 23 to show that ice shedding does not impact critical areas (see AC 23.1419-2C). Appendix Q of this AC provides information and guidance on ice shedding.
AC 33-2B provides guidance on flight-test procedures for engine induction system icing and engine ice ingestion test procedures.
c. You should evaluate failures of the IPS and the resultant hazards from ice shedding (14 CFR §§ 23.1309 and 25.1309). Perform this analysis using the probability of the failure occurrence versus resultant hazard levels.
8.2.1.5 Compliance Means: Natural Icing Flight Test Additional Guidance and Information. Information on natural ice testing is in AC 23-8B; AC 23.143-1; AC 23.1419-2C;
AC 25-7A; AC 25.1419-1A; AC 27-1B; AC 29-2C; AC 33-2B; and appendix M of this AC.
8.2.2 Compliance Means: Dry Air Flight Tests.
a. Dry air flight tests are usually conducted before natural icing tests to evaluate the aircraft with the IPS operating. The initial dry air tests are conducted to verify the IPS (for example deicing boots) operates as intended and does not affect the aircraft’s flying qualities, and to measure thermal characteristics for verifying analyses of thermal IPSs.
b. You may use dry air flight tests to verify significant portions of the IPS analyses and to demonstrate the aerodynamic effects of predicted ice shapes. You should perform these tests before conducting natural icing tests. These tests check the function, performance, and compatibility of all systems components. During the dry air flight tests, you can verify the effects of engine bleed air used by the IPS on engine thrust settings. Data collected during dry air tests allow you to analyze heat requirements and availability during various operational conditions. You can also get information about the effects of the local environment on the installed components of the IPS during these dry air flight tests. Examples of the local environments are propeller wash on the wing and empennage and wing downwash on the horizontal stabilizer. You should use these tests to evaluate the aircraft’s handling qualities using simulated ice shapes on unprotected and protected surfaces to show compliance with Subpart B of 14 CFR parts 23, 25, 27, and 29.
c. Compliance means for several commonly used IPSs and components are discussed below. They illustrate typical dry air flight test practices. However, you should evaluate your IPS for compliance as your IPS design dictates.
8.2.2.1 Compliance Means: Dry Air Flight Test Thermal Ice Protection Systems Evaluation.
a. Dry air flight tests are conducted to verify the system design parameters and thermal performance analysis.