APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED) x Some means that alert the flight crew to activate the IPS before hazardous ice buildup on critical surfaces. Or, the means may alert the flight crew that icing conditions exist.
x Automated operation of the IPS by a means that detects the existence of icing conditions or ice buildup on monitored surfaces.
x Operation of the IPS based on visible moisture and temperatures provided in the AFM or RFM.
Activation of the airframe IPS should also, as necessary, adjust the stall warning and protection schedules to ensure acceptable stall margin and protection for flight in icing conditions.
Requirements have been set up to ensure pilots are provided necessary information and limits for safe flight in inflight icing conditions (14 CFR §§ 23.1525, 23.1583, 23.1585, and 23.1559;
14 CFR §§ 25.1525, 25.1583, and 25.1585; 14 CFR §§ 27.1525, 27.1583, 27.1585, and 27.1559;
and 14 CFR §§ 29.1525, 29.1583, 29.1585, and 29.1559). The applicant should provide pilots required or recommended means and procedures for operating the IPS.
Pilots may detect aircraft icing and icing conditions on some aircraft simply by seeing ice on reference surfaces. These reference surfaces include windshield wiper blades, windshield posts, and other small protuberances that are efficient ice collectors. These surfaces collect ice before ice accretion is observable elsewhere on critical aircraft surfaces. Reference surfaces used as icing cues should show the presence of icing before or simultaneously with ice accretion that has been shown to be safe on monitored surfaces of that aircraft. You must provide acceptable lighting, other than a flashlight (because of crew workload), to detect ice on critical wings surfaces at night (14 CFR § 23.1419(d), 14 CFR § 25.1403, 14 CFR § 27.1419(e), and 14 CFR § 29.1419(e)). Consider lighting ice evidence probes for flight at night. You should show the illumination provides acceptable visibility, without excessive glare, reflections, or other distractions, in and out of clouds. If cues of icing are not visible at night, aircraft approved for flight in icing conditions should not fly at night.
For some aircraft, no icing cues are visible from the normal work position of the flight crew. For those aircraft, and generally for operation of engine ice protection, you may use visible moisture and temperatures colder than those determined by the designer as indication of icing conditions.
The threshold you select must ensure that no hazardous amount of ice will build up on the engine induction system or other critical surface at a temperature above this limit. You should provide these conditions in the AFM. Alternatively, you may install approved ice detectors, airplane performance monitors, or icing conditions detectors to alert the flight crew to follow AFM procedures when the aircraft is accreting ice or when icing conditions are encountered.
The FAA has issued Airworthiness Directives (AD) requiring activation of wing deicing boots on regional air transports at the first detection of airframe ice or when ice detectors detect icing conditions. The ADs also require cycling the deicing boots once the aircraft has exited the conditions. This is a fundamental shift from previous practices, where flight crews would cycle the boots when they saw a recommended thickness of ice to avoid deicing boot ice bridging. In H-10
APPENDIX H. ICE PROTECTION SYSTEMS (CONTINUED)
flight icing accident investigations have shown that flight crews have lost control of the aircraft while judging whether the recommended thickness of ice had built up on the wing for activation of the deicing boots. Wind tunnel and flight test results have shown that continued cycling of modern deicing boots will shed the thin, first ice buildup during following cycling of the boots.
When compared with older deicing boot designs, modern boots run at higher air pressures and inflation/deflation rates, and have small tubes. Activation of the deicing boots ensures the IPS is used during icing conditions. It also ensures the ice buildup on the deicing boot surface is minimized.
The proper manual should describe the expected icing cues, the flight-crew procedure associated with seeing the cues, and when the flight-crew should activate the IPS (14 CFR §§ 23.1583,
23.1585, and 23.1559; 14 CFR §§ 25.1583 and 25.1585; 14 CFR §§ 27.1583, 27.1585, and
27.1559; and 14 CFR §§ 29.1583, 29.1585, and 29.1559). You should provide procedures such as emergency or abnormal procedures, including procedures in the AFM for when the IPS fails or when warnings or alerts occur. For fluid IPS, include information and methods for determining the remaining time of protection provided by the fluid.
H.2 REFERENCES.
H1. “Aircraft Icing Handbook,” FAA Technical Report DOT/FAA/CT-88/8-1 (1991), 3 vols., FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
H2. “Ice Accretion and Droplet Impingement Codes,” SAE ARP5903.
H3. FAA Technical Report ADS-4, “Engineering Summary of Airframe Icing Technical Data.”
H4. Report No. FAA-RD-77-76, “Engineering Summary of Powerplant Icing Technical Data.”
H5. NACA Technical Note 1855 (1949), “Recommended Values of Meteorological Factors To Be Considered in the Design of Aircraft Ice-Prevention Equipment,” NASA Ames Research Center, Moffett Field, California 94035.
H6. “Calibration and Acceptance of Icing Wind Tunnels,” SAE ARP5905.
H7. “Airborne Icing Tankers,” SAE ARP5904.
H8. “Rotor Blade Electrothermal Ice Protection Design Considerations,” SAE AIR1667A, pp.
13-24.
H-11 (and H-12)
APPENIDX I. DROP IMPINGEMENT AND WATER CATCH I.1 DROP IMPINGEMENT AND WATER CATCH.
Applicants seeking FAA approval of the IPS (as discussed in section 7 of this AC) need to determine which aircraft surfaces will accrete ice, the extent of surface protection, and the energy needed to protect the surface from icing. You should supply an impingement analysis to confirm the extent of the IPS. FAA Icing Handbook (Reference I1) contains more guidance on developing this analysis.
Information needed to determine the chordwise extent of most IPSs includes drop impingement limits and the related water catch. Finding the supercooled water drop impingement limits and the resulting water catch are dependent on several variables. These variables include the surface contour, attitude, local flow angularity, local airspeed and Reynolds number, drop sizes (mass), catch efficiency, exposure time, and the LWC of the atmosphere. Perform studies for all phases of flight and aircraft configurations. These studies determine the chordwise extent of drop impingement on the surface and the required protection to prevent or remove ice accretion from the surface. Show safe flight in icing conditions for the configurations and phases of flight used to define the IPS’s coverage. Also, you should provide the configurations used with different phases of flight in the AFM. This prevents the use of configurations for which the IPS was not designed, and for which safe flight in icing conditions has not been shown. An example is using only the cruise configuration for holding in icing conditions.
Surface moisture at and below the freezing point is required for ice to accrete. Some surface leading edges, such as antennae, antennae fairing, and air data probe and vent masts may be in “shadow” areas. The cloud drops (in a certain size range) in this area do not impinge on the aircraft. The freestream airflow carries them away from the aircraft surfaces. Analysis has shown at least one case where drops in an intermediate size range did not impinge on a fuselagemounted ice detector while smaller and larger drops did impinge (Reference I2).
Also, the splashing and bouncing of cloud drops as the drops impinge on surfaces (and later ice shapes) will influence the buildup of ice. Larger cloud drops may break up as they traverse regions of high shear stress resulting from changes in the airflow speed.
Some conditions may result in runback ice buildups aft of the protected surfaces. Runback ice buildups result either through partial evaporation (by design in a running-wet IPS) or through aerodynamic heating at outside air temperatures slightly below freezing. This aerodynamic heating can result in a freezing fraction of less than one, causing water to run aft of protected surfaces, freeze, and form runback ice.
Although other analytical methods exist to determine the drop impingement limits on airfoils, computer codes are commonly used. You may use these codes to calculate the impingement area and limits, the water mass flux of the surface, the local water catch efficiency (E), and the total water catch efficiency (E). The codes also provide useful information on whether an aircraft surface is exposed to atmospheric moisture (and ice accretion). If the airflow is not separated from the surface, these tools offer the advantages of accurately determining local airflow angularities and speeds, the effects of the drop mass and aerodynamics, and the impingement for I-1
APPENDIX I. DROP IMPINGEMENT AND WATER CATCH (CONTINUED) a drop spectrum (rather than a single drop size). You may use Navier-Stokes CFD codes if the airflow is highly turbulent or separated. You should provide experimental confirmation in the form of wind tunnel, natural icing, or tanker data to support the calculated flow field and drop impingement.
You should provide justification for using a computer code to determine drop impingement information. The technical document, SAE ARP5903 (Reference I3), provides useful information on available drop impingement codes, their common use, limits, software verification, validation, and code administration. You should consider SAE ARP5903 when judging the acceptability of drop impingement analyses performed using computer codes.
Empirical evaluations have shown that 2D impingement codes are acceptable when properly used for simple, planar surfaces and drop sizes within 14 CFR part 25, Appendix C and part 29, Appendix C. Reference I3 discusses use of 2D impingement codes for impingement limit analyses of swept wings and stabilizers. At the AOA of interest, airfoil sections and pressure distributions are determined by following the direction of the airflow over the wing or stabilizer surfaces. Rather than tracking the streamlines across the surface, some manufacturers have approximated the streamline airfoil by using sections that are either normal to the surface’s leading edge or normal to the surface’s quarter-chord, and then straight aft from the quarterchord to the trailing edge. You can determine this wing streamline information accurately either empirically or by using CFD codes. Without separated flow, you may use potential flow or Euler computer codes to calculate the flow over the surface. The 2D drop impingement code is then used to determine the impingement. Streamwise sections are used for straight wings and stabilizers.
You will find limited empirical validation of impingement codes for 3D surfaces (for example, wing tips, swept wings, bodies of revolution, fairings, and junctures of bodies).
Three-dimensional codes are available to perform impingement and ice shape prediction of 3D components on airplanes. However, 3D analyses can be costly in both time and resources.
Sometimes these analyses require up to 6 weeks to calculate the drop impingement for a single 3D surface. Less costly 2D methods have been used to estimate drop impingement and ice shapes for symmetrical 3D surfaces. This is done using the centerline profile of each symmetrical surface. Examples of symmetrical surfaces include radomes, wing mounted fuel tanks, antenna fairings that form symmetric bulges in the fuselage, and light fairings.
Using 2D codes for such analyses does not address 3D flow conditions (for example, crossflow). These 3D conditions affect the drop trajectory and impingement. Three-dimensional codes, depending on their sophistication, may not fully account for cross-flow effects either.
When using 2D codes in this manner, account for the effects of nonzero pitch and yaw angles of attack. You should provide justification for the use of drop impingement and ice shape prediction codes in the form of an analysis. The analysis should show why you may ignore the 3D flow effects and should account for the effects of nonzero angles of attack. You should also validate the acceptability of your drop impingement and ice shape prediction codes. This validation may include one or more of the following:
I-2
APPENDIX I. DROP IMPINGEMENT AND WATER CATCH (CONTINUED)
x Icing wind tunnel results.
x Icing tanker results.
x Flight in natural icing conditions results.
x Similarity to other aircraft that have data from one of the above 3 validation methods.
NOTE: Reference I3 discusses the use of 2D impingement codes on swept-wing airplanes.
You should justify the drop size or drop distribution used to determine the impingement limits.
Advisory Circular (AC) 20-73, replaced by this AC, stated that an MED drop of 50 Pm had been successfully used for certification purposes. Also, the document stated that MED drop of 20 Pm had been successfully used to determine water catch rate. FAA Technical Report DOT/FAA/CT-88/8-1 (Reference I1) states that for design purposes, a drop size of 40 Pm is often used for determining impingement limits. However, the largest drop size within Appendix C will determine the maximum impingement limits.
The variation of atmospheric cloud water drop diameters is high. Since the MED or MVD is the median drop diameter, water drops having larger diameters than the MED or MVD exist. NACA researchers used the Langmuir distribution in table I-1 to calculate MED for the data used to define 14 CFR parts 25, Appendix C and 29, Appendix C icing conditions (Reference I4). So, the Langmuir distribution is used to describe the variation of 14 CFR parts 25, Appendix C and 29, Appendix C cloud drop diameters. Since drops larger than the MVD exist, consider using the Langmuir D distribution with the 50 Pm maximum median drop diameter of 14 CFR parts 25, Appendix C and 29, Appendix C to determine impingement limits.
Table I-1. Langmuir Distributions Fractional Liquid Water Content in Each Size Group ~ % Langmuir Distribution ~ (Drop Diameter)/Median Drop Diameter
A B C D E 5 1.00 0.56 0.42 0.31 0.23 10 1.00 0.72 0.61 0.52 0.44 20 1.00 0.84 0.77 0.71 0.65 30 1.00 1.00 1.00 1.00 1.00 20 1.00 1.17 1.26 1.37 1.48 Fractional Liquid Water Content in Each Size Group ~ % Langmuir Distribution ~ (Drop Diameter)/Median Drop Diameter
A B C D E 10 1.00 1.32 1.51 1.74 2.00 5 1.00 1.49 1.81 2.22 2.71 If you fail to consider drop diameters larger than the maximum mean value and ignore water catch below a selected local water catch efficiency (E) (for example, 0.10), you may be ignoring I-3
APPENDIX I. DROP IMPINGEMENT AND WATER CATCH (CONTINUED) the buildup of thin, rough ice at the impingement limits that may cause adverse aerodynamic effects. Provide justification when s electing local water catch efficiencies other than zero for the impingement limits. Also provide justification when choosing not to consider the drop spectrum around a median diameter of 50 Pm for determining the chordwise extent of the ice protection surface.
Analyze each ice-protected surface for different drop sizes and phases of flight. This analysis finds the drop diameter associated with the largest water catch. The water catch may determine the maximum heat requirements for thermal anti-icing IPSs. See the earlier discussion on median drop diameter and drop distributions. It may be proper to consider other drop diameters when addressing other IPS design requirements.
You may also determine drop impingement information empirically by using natural icing conditions flight tests, icing wind tunnels, or icing tankers. These methods may provide information on ice accretion limits, which should approximate drop impingement limits in rime conditions. Impingement and icing limits may differ substantially in glaze icing conditions. You may use these data to validate results from code-derived drop impingement limits.
During natural icing flight tests to determine icing limits, you should measure the icing cloud characteristics (including LWC, temperature, drop size), airplane airspeed, angle of attack, and altitude. Get enough information to ensure that icing limit results address the drop impingement issues and the phases of flight and associated airplane configurations being approved for flight in icing conditions.
You may use icing wind tunnels to determine drop impingement information. Techniques have been developed to calibrate water catch with the color intensity of captured dyed water. When possible, icing wind tunnels are useful for determining drop information for full-scale, 3D configurations. Carefully examine the use of scale models for impingement analysis since effective impingement scaling requires matching the modified drop inertia parameter, K0.
Appendix R of this AC contains details on using scaled wind tunnel models to determine impingement limits, water catch, and ice shapes.
Get FAA approval for use of scale models for impingement. Acceptance of scale model data may be based on repeating ice wind tunnel shapes, or by correlating ice shapes for models of different scales. This approach gives better results with rime ice accretions.
You should consider SAE ARP5905 (Reference I5) when judging the acceptability of icing tunnel drop impingement investigations.
You may also use icing tankers to get drop impingement limits, even though the drop MVDs may exceed 50 Pm. Tanker tests allow testing of full-scale configurations, but are limited by the spray array’s plume size and ability to match 14 CFR parts 25, Appendix C and 29, Appendix C drop sizes and LWC—especially at an MVD of 50 Pm. However, drop impingement limits determined with MVDs exceeding 50 Pm under rime icing conditions have been considered to provide conservative estimates of the impingement limits for some applications. You should justify and conservatively use icing tanker data. You should show the icing tanker, associated I-4
APPENDIX I. DROP IMPINGEMENT AND WATER CATCH (CONTINUED) instrumentation, and icing cloud plumes are calibrated. You should also show the impingement evaluation was performed using commonly accepted practices. You should consider SAE ARP5904 (Reference I6) when judging the acceptability of icing tanker drop impingement investigations.
The wing’s lower surface impingement limit analysis typically suggests the ice protection should extend over a large percentage of the lower wing surface to encompass the entire impingement area. Structural (front spar location) and IPS energy considerations typically determine the extent of the IPS on the wing’s lower surface. Airplane drag is usually the most significant aerodynamic effect of lower surface ice accretion roughness aft of the airflow’s stagnation point (line) at airplane stall. Aircraft manufacturers will typically accept the drag penalty on performance rather than extending the IPS too far aft on the wing’s lower surface.
You should consider the aerodynamic effects of ice accretion aft of the selected lower surface aft ice protection limit when showing safe flight of the aircraft in icing conditions. Simulated ice shapes are typically used. Since the aircraft horizontal stabilizer may lift in both directions, the design philosophy used for the ice protection on the wing may not be suitable for the horizontal stabilizer. These considerations reflect the use of engineering judgment and experience in design of IPSs. You must show safe operation of the aircraft in natural icing conditions with the proposed IPS (14 CFR § 23.1419, 14 CFR § 25.1419, 14 CFR § 27.1419, and 14 CFR § 29.1419). You may supplement this demonstration by analyses and laboratory tests (as necessary) to verify adequacy of the IPSs for icing conditions throughout the required icing environment.
I.2 REFERENCES.
I1. “Aircraft Icing Handbook,” FAA Technical Report DOT/FAA/CT-88/8-1 (1991), 3 vols., FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
I2. “Certification and Integration Aspects of a Primary Ice Detection System,” D. Jackson, D.
Owens, D. Cronin, and J. Severson, paper no. AIAA-2001-0398, AIAA 39th Aerospace Sciences Meeting, Jan. 8-11, 2001. Reprints available from American Institute of Aeronautics and Astronautics, Reston, VA 20191-4344.
I3. “Ice Accretion and Droplet Impingement Codes,” SAE ARP5903.
I4. “Mathematical Investigation of Water Droplet Trajectories,” The Collected Works of Irving Langmuir, New York, Published by Pergamon Press [c 1960-62], p. 348-393.
I5. “Calibration and Acceptance of Icing Wind Tunnels,” SAE ARP5905.
I6. “Airborne Icing Tankers,” SAE ARP5904.
I-5 (and I-6)
APPENIDX J. PROPELLER ICE PROTECTION J.1 PROPELLER ICE PROTECTION.
When the airplane’s powerplant installation includes propellers, the propellers’ IPS must meet the applicable requirements of 14 CFR §§ 23.929, 23.1419, 23.1301, 23.1309, 25.929, 25.1091,
25.1419, 25.1301, and 25.1309. Each installed propeller must have means to prevent or remove hazardous ice accretion within icing conditions for which the applicant is seeing airplane certification. With the IPS installed, the propeller assembly must meet the requirements of 14 CFR part 35. The propeller IPS should prevent or remove hazardous ice buildup that could put engine performance at risk and enable satisfactory functioning without significant loss of thrust. If you are showing compliance with propeller ice protection requirements, you should consider the following:
x Airplane performance loss.
x Thrust requirements for airplane handling qualities evaluations, such as when showing stall warning and stall recovery.
x Vibrations.
x Engine ice ingestion.
x Airframe damage from ice shed from the propeller.
x Propeller structural behavior (integrity) resulting from functioning or failure of the propeller IPS.
x Power requirements for ice protection.
x Margins for operation in icing conditions, which may exceed certification requirements.
You should base compliance with regulatory requirements on flight tests in measured natural icing conditions and necessary analyses, or laboratory tests (14 CFR § 23.1419, 14 CFR § 25.1419, 14 CFR § 27.1419, and 14 CFR § 29.1419). You should verify analyses used to support compliance with icing requirements by test.
Perform analyses to determine:
x The most critical flight icing condition.
x The required zones of ice protection.
x The use of and operational modes for electrothermal ice protection. Include heat transfer characteristics, power requirements assuming an atmospheric temperature of J-1
APPENDIX J. PROPELLER ICE PROTECTION (CONTINUED) -30°C, intercycle intervals and critical ice shapes, if required, and their effects, runback ice and its effects, and deicer timer cycle.
x AFM limits for fluid systems, to prevent exhaustion of the fluid before exiting the icing conditions.
x The maximum size and trajectory of the ice that sheds from the propeller (propeller inter-cycle ice). This ensures that the energy of the shed-ice does not damage the airframe structure, resulting in unsafe conditions.
x The integrity of the propeller and the propeller IPS when subjected to high temperatures due to hot weather operation or failure of the thermal regulator.
Other concerns include:
x The effect of deicer boot installation on the propeller blade.
x The effect of the slip ring or equivalent accessories on the hub structural integrity.
x Blade surface temperatures.
x Voltage and current transmission losses from the airplane to the deicer boot.
x Timer and other control system component reliability.
x Spinner ice buildup.
x Brush block or slip ring hydroplaning when exposed to moisture.
x Reliability of wiring, brush blocks, slip rings, and other components.
The propeller must not adversely affect engine operation throughout the engine’s power range (including idle). There must be no loss of thrust throughout the full rotational speed and pitch range expected during icing conditions that would result in unsafe airplane performance or handling qualities. Propeller ice shedding should not cause structural damage. You must confirm this performance in flight test, targeting the predicted critical icing conditions within 14 CFR part 25, Appendix C.
Because of possible hazards, you should pay attention to propeller ice accretion with an operative propeller IPS in icing conditions that temporarily exceed 14 CFR part 25, Appendix C icing conditions. Figure J-1 shows a coating of ice along the leading edge of the propeller blade, underscoring the need for this attention. The ice is on top of and beyond the protected area of the blade. This type of propeller ice buildup may severely affect thrust. In addition, icing tunnel testing and flight testing of propellers’ runback have shown propeller performance losses of up to 20 percent. Airframe and propeller ice accretion in conditions that temporarily exceed 14 CFR part 25, Appendix C icing conditions may impact the ability of the airplane to accelerate out of stall warning and stall entry events by increasing propeller power.
J-2
APPENDIX J. PROPELLER ICE PROTECTION (CONTINUED)
Figure J-1. Propeller Ice Accretion During an SLD Encounter With the Propeller Ice Protection System Operating (Reference J1).
Examine any significant vibrations while testing. Also consider the maximum temperatures which a composite propeller blade may experience during operation of the IPS. This includes unintentional operation on the ground during hot day conditions or during dry air flight. Check electrically protected propellers, the IPS current, brush block voltage (between each input brush and the ground brush), and IPS duty cycles during tests to ensure that proper power is applied to the deicer protection surfaces. You should also check the IPS during flight in rain to ensure proper operation.
The propeller must not adversely affect engine operation throughout the engine’s power range, including idle. There must be no loss of thrust throughout the full rotational speed and propeller pitch angle range expected during icing conditions that would result in unsafe airplane performance and handling qualities. Propeller ice shedding should not cause structural damage.
You must show compliance with these requirements during flight tests, targeting the predicted critical 14 CFR part 25, Appendix C icing conditions.
J.2 REFERENCES.
J1. “SLD Encounter, MU-2B-60, March 5, 2000, ABQ-ABQ, Alt = 16,100 ft, IOAT = -7C.” Timmons, Larry, FAA In-Flight Icing/Ground Deicing International Conference and Exhibition, June 16-20, 2003.
J-3 (and J-4)