APPENDIX R. ICE SHAPES (CONTINUED) R.2 UNPROTECTED SURFACES.
Aircraft will typically have surfaces that collect ice, but require no ice protection. Applicants must show their aircraft can operate safely in icing conditions with these surfaces unprotected against ice buildup (section 6.1 of this AC).
You can determine ice shapes of unprotected surfaces by tests in measured natural icing. You can also predict them by simulated icing (icing tanker) flight tests, icing wind tunnel tests, computer codes, and other analytical methods validated by test experience. You must perform flight-testing in measured natural icing conditions to verify the ice protection analyses and to check for icing anomalies (14 CFR §§ 23.1419, 25.1419, 27.1419, and 29.1419). Therefore, you must verify predicted ice shapes by flight tests in measured natural icing conditions. It is not intended that natural icing flight tests verify all the features of predicted ice shapes. Finding specific, critical conditions during natural icing flight tests may not be likely (appendix M of this AC). Use natural icing flight tests to corroborate the general physical characteristics and location of the simulated ice shapes. Also, you should corroborate their effect on airplane handling characteristics.
R.3 ICE PROTECTION SYSTEM FAILURE ICE SHAPES.
It may be necessary for an aircraft to exit icing conditions after the IPS fails. The following describe IPS failure ice accretion scenarios:
x If the IPS failure condition is annunciated and AFM procedures require the aircraft to exit icing conditions, the ice buildups on normally protected surfaces should be that resulting from normal operation of the IPS until the annunciated failure. Also include the ice that would build up during the exiting maneuver.
x If the IPS failure condition is annunciated and the AFM procedures do not require the aircraft to exit the icing conditions, ice on protected surfaces with ice protection failure may build up after the failure.
x If the IPS failure condition is not annunciated, ice buildups on normally protected surfaces should be the same as that on unprotected surfaces.
Applicants may select ice on normally protected surfaces where ice protection has failed to be half of what builds up on an unprotected surface for aircraft that are required to exit icing conditions following failure of the IPS. Alternatively, if the IPS failure is annunciated, you may select the ice that builds up on an unprotected surface during the time required for exiting the icing condition. You must calculate the time required to exit the icing conditions conservatively.
Get FAA acceptance of the icing conditions exit time.
For airplanes, you may select the time to exit the icing condition as half of a 45-minute hold, or as you verified. AC 23.1419-2C provides guidance for 14 CFR part 23 airplanes for exit times other than 22.5 minutes. AC 29-2C states that the time to exit the icing conditions for a rotorcraft is 15 minutes. For a failed IPS, ice accretion on unprotected surfaces should be R-14
APPENDIX R. ICE SHAPES (CONTINUED) unaffected and expected to build up as described in paragraph R.2, of this appendix. See paragraph R.2 for means of determining ice shapes for unprotected surfaces. You may use analyses to determine ice accretion on a failed protected surface. For certain design features (for example stall strips and vortex generators), natural icing or simulated icing conditions (tankers and icing wind tunnels) may be required.
You should determine if exiting of icing conditions is required based on results of the system failure modes and effects analysis required by 14 CFR §§ 23.1309, 25.1309, 27.1309, and
29.1309. If you select continued flight in icing conditions, you must show that the aircraft can fly safely, as described in paragraph 6.2, of this AC. Simulated ice shapes developed to evaluate the aircraft’s performance and handling qualities should reproduce the allowable failures of the IPS. If exiting icing conditions is required, you must show the aircraft has the ability to continue safe flight and landing. Also include the proper procedures in the AFM. For guidance on ways of showing safe flight after an IPS failure that requires exiting the icing condition see AC 23-8B, AC 23.1419-2C, AC 25.7A, AC 25.1419-1A, AC 27-1B, and AC 29-2C.
R.4 CRITICAL ICE SHAPES.
R.4.1 Considerations for Critical Ice Shapes.
A critical ice shape may be defined as the aircraft surface ice shape (formed within icing conditions defined by 14 CFR parts 25, Appendix C or 29, Appendix C) that results in the most adverse effects for specific flight safety requirements. The critical ice shape may differ for different flight safety requirements. For example, the critical ice shape may differ for stall speed, climb, aircraft controllability, control surface movement, control forces, air data system performance, “artificial feel” adjustments, ingestion and structural damage from shed-ice.
Engine thrust, engine control, and aeroelastic stability considerations may result in different critical ice shapes.
Critical ice shapes may vary with aircraft configuration and flight phase. The applicant may design one aircraft surface ice shape to be critical for all flight safety considerations, configurations, and flight phases. Critical ice shape determination also includes consideration of unsatisfactory effect on the aircraft’s aeroelastic stability (14 CFR §§ 23.629, 25.629, 27.629, and 29.629). Other considerations include shed-ice ingestion related to engine operation, control, and ice protection (14 CFR §§ 23.901(d)(2), 23.903, and 25.903 (which require compliance with 14 CFR § 33.77); § 33.68; and §§ 23.1093, 25.1093, 27.1093, and 29.1093).
Flight-testing with the critical ice shapes can be dangerous if approached with inadequate caution. To avoid extensive flight-testing, you can simulate the most critical unprotected and protected surface ice shapes and surface roughness as a means to show compliance with safe flight requirements. Subpart B of 14 CFR parts 23, 25, 27, and 29 provide the requirements for safe flight.
If you cannot determine a single critical ice shape, you should test each critical ice shape in the associated phase-of-flight configuration to examine the aircraft’s controllability, maneuverability, stability, performance, trim, and stall characteristics. For large turbojet air R-15
APPENDIX R. ICE SHAPES (CONTINUED) transports with large thrust margins, you should consider ice shapes that are handling-qualities critical. Also consider conservative estimates of performance effects. You should perform all flight-testing at the most critical weight, center-of-gravity, flap, and gear configuration for the aircraft characteristic of interest.
Determination of critical ice shape configurations is not straightforward and may require engineering judgment. The aircraft critical ice configuration should include expected ice buildups on all surfaces, protected and unprotected (including flow control devices such as vortex generators, stall strips, vortilons, and fences). You should consider phases of flight where ice buildups may occur and those where ice buildups may continue after exiting conditions conducive to icing. Consideration of the ice buildup effects on the aircraft should include:
x Lift, including maximum lift.
x Drag.
x Pitching moments.
x Control forces.
x Control surface movement.
x Freezing of critical seals.
x Ice ridges aft of protection surfaces.
x Runback ice.
x Vibration and aeroelastic stability.
x Stall warning.
x Stall characteristics.
x Power.
x Air induction inlet airflow.
x Stability.
x Controllability.
x Trim.
x Control surface aerodynamic balance.
x Maneuverability.
x Horizontal stabilizer and elevator stall.
x Vertical stabilizer and rudder stall.
x Wake effects on engine operability and downstream aircraft components and systems.
x Engine operability and aircraft structural damage resulting from shed-ice.
You should select the critical ice shape configuration(s) and verify that the selected ice shape configuration is most critical for the considered flight safety concern. Selection of the ice shape and its features typically requires surveying ice shapes that may build up on a surface within the required icing conditions. Select critical ice shape candidates from this survey, with an understanding that a surface ice shape that is critical for flight safety may not be that of an unprotected surface with the largest and most aerodynamically intrusive ice buildup. Protect surfaces considered critical for safe flight. You should consider:
R-16
APPENDIX R. ICE SHAPES (CONTINUED) x Preactivation ice accretion roughness.
x Protection surface intercycle ice buildup.
x Runback ice, or thin, rough ice buildups on the horizontal stabilizer (resulting from preactivation ice, intercycle ice, or from being an unprotected surface).
x Expected ice buildups on unprotected surfaces.
You can determine the shape and characteristics of ice buildup by flight in natural or artificial (icing tanker) icing conditions, icing wind tunnel tests, ice accretion codes or analysis methods, or by combinations of the different methods. You must perform flight-testing in measured natural icing conditions to verify the ice protection analysis, to check for icing anomalies, and to show the effectiveness of the IPS (14 CFR parts 23, 25, 27, and 29, § .1419). When using icing tankers, icing wind tunnels, or ice accretion codes and analytical methods, consider the comments about use of these icing simulators in paragraphs R1 and R2.
Inflight icing accident investigations and experience suggest that glaze and high intensity icing conditions during descent may result in the most critical ice shapes. Glaze icing conditions will exist at near-freezing temperatures (about -5°C or warmer). The largest ice mass occurs at conditions of maximum water catch. Hazardous ice accretions may also occur at colder mixed and rime ice conditions for intercycle ice and for running-wet thermal IPSs.
You may use experience from previous certifications of similar aircraft designs as guidance for determining the icing conditions that produce candidate critical ice shapes. Unprotected critical surface ice shapes should reflect the icing exposure time associated with the respective phase of flight. For the holding configuration, consider the 45-minute holding period. Guidance material contained in the JAA ACJ 25.1419, states that from service experience ice on the most critical unprotected main airfoil surfaces do not usually exceed a pinnacle height of three inches in a plane towards flight (Reference R4).
A 1962 airframe manufacturer’s magazine article is often referenced as giving credence for accepting critical ice shapes with peak heights of no more than 3 inches. This article discussed elimination of the B707/720 airplanes’ horizontal and vertical stabilizers ice protection (Reference R4). The magazine article provided an analysis of a “maximum possible ice (based on CAR icing conditions envelopes)” mission profile that resulted in a maximum ice shape of
3.31 inches, following:
x A dispatch hold of 10.4 minute at 250 KTAS;
x Climb of 4.5 minutes at 290 KCAS;
x Cruise at an altitude of 15,000 feet for 22 minutes at 360 KIAS;
x A destination hold of 31 minutes at 15,000 feet with at 250 KTAS.
Flight tests showed that simulated 3-inch ice shapes did not hamper these Boeing airplanes.
Minimum control speeds and other stability and control characteristics were explored. The article only addressed the horizontal and vertical stabilizers of the B707/720 models. It also R-17
APPENDIX R. ICE SHAPES (CONTINUED) stated that the analysis only applied to turbine-powered airplanes. The 3.31-inch analysis ice shape and the flight test three-inch ice shape discussed in the magazine article may lead to the conclusion that service experience shows that ice on critical unprotected wing and stabilizer surfaces need not exceed a peak height of three inches. Certification experience has shown that peak heights on the critical unprotected surfaces may exceed three inches, such as on the unprotected leading edges of the horizontal or vertical stabilizers or the wing of airplanes smaller in scale than the B707. Therefore, you should not truncate the peak height of ice shapes to three inches.
Also, the magazine article’s combination of a 15-minute dispatch takeoff hold and a 30-minute destination hold may be correlated to the 45-minutes destination hold (based on operational fuel requirements) ice shape guidance. You should determine the preactivation and intercycle ice shapes for your aircraft. However, you may use sections R.1.1 and R.1.2 of this appendix as guidance. Castings of these ice accretions, which you may use as a guide for producing simulated ice roughness, are available at the FAA William J. Hughes Technical Center. For larger ice shapes that build up on unprotected surfaces, the ice shape surface texture should either reflect the empirically determined ice shape texture or be about 3 mm in height with a particle density of 8 to 10 per cm2. Thin, rough layers of ice, commonly called “sandpaper ice” should reflect empirical results. (Carborundum paper no. 40 has been used during certification for ice contaminated tail stall.) However, you should verify use of the 40-grit Carborundum paper to simulate intercycle ice roughness (as discussed in sections R.1.1 and R.1.2 of this appendix).
Runback ice and ice ridges that may occur aft of deicing or running wet protection surfaces may form critical ice shapes. You should determine these critical shapes empirically by using icing tankers or icing wind tunnels. Use these empirical methods until the ability of an ice accretion code or analysis method to perform this task is validated.
Determining the critical aircraft ice shape configuration requires integrating knowledge about all the aircraft surfaces’ critical ice shapes into one overall configuration. The integration may reveal that the critical ice shape for one surface may not be consistent with developing critical ice shapes on other surfaces. For example, sandpaper ice roughness on the horizontal stabilizer may be critical for contaminated tail stall testing. The wing’s extended holding and intercycle ice configurations may be critical for airplane performance and handling qualities. Also, when showing stall recovery and drag for aircraft with low power margins, you should consider propeller ice contamination.
You may use simulated critical ice shapes to show safe flight in icing conditions. To minimize the number of configurations necessary to show safe flight in icing with simulated ice shapes, you should show that the selected configurations are most critical. Wind tunnel testing at fullscale Reynolds and Mach numbers, or a verified Navier-Stokes full-configuration CFD analysis may be necessary to determine the critical ice shape configuration without applicable information from a similar certificated aircraft, and for other than the simplest aircraft configuration. (Section R.4.2, of this appendix contains CFD codes guidance.) Also, when incrementally adjusting flight test data for other critical ice shape considerations by using wind tunnel data (like drag), paying attention to the effects of Reynolds and Mach number on those increments is important. Ice buildups may be difficult to reproduce at a small-scale. The ratio of R-18
APPENDIX R. ICE SHAPES (CONTINUED) the ice buildup height to the model’s boundary-layer height may be different at low Reynolds number from that at flight Reynolds number. Also, scaling the ice shape roughness may be difficult.
You may simulate three-dimensional (3D) characteristics of critical ice shapes for flight as long as you provide enough empirical information to verify the simulated shapes. For surfaces whose chord and leading edge shape significantly vary across the span of the unprotected surface, the critical ice shape may reflect the effects of the varying surface geometry on ice accretion. You may create a 3D critical ice shape by first lofting 2D critical ice shapes at enough spanwise locations. Then apply suitable surface texture, as discussed above. Low Reynolds number wind tunnel test data, shown in figure R-10, shows that a continuous ice shape produces conservative aerodynamic effects when compared with those for ice shapes segmented spanwise. These results are for the model, ice shapes, and testing conditions tested. You may use segmented ice shapes to simulate known spanwise variations in ice buildups.
The FAA will closely review the use of simulated ice shapes that you assume conservative when compared with critical ice shapes reflecting ice buildups. This is also true for the use of devices that make surfaces aerodynamically ineffective differently than how the airflow would behave with critical ice shapes. The objective of showing safe flight in icing conditions is to have the aircraft behave as if it has collected critical ice shapes in those icing conditions. Arbitrarily developed aircraft surface flow conditions may not produce aircraft flying qualities that would occur with natural icing critical ice shapes. Without knowing the aircraft flying qualities with natural icing critical ice shapes, you may have difficultly verifying that arbitrarily developed flow-disturbance devices are acceptable.
Simulated critical ice shapes used to show safe flight should include collateral icing that may collect on aircraft surfaces. Figure R-6(a) shows an aircraft that experienced heavy icing before landing, showing that collateral icing can be significant. Ice accretion codes may not predict icing that may collect at aircraft surface discontinuities. Although drag may be the most significant effect of the collateral ice, address these ice buildups when showing safe flight (14 CFR §§ 23.65, 23.67(e)(2), 23.67(e)(3), 23.75, 23.77, 25.119, 25.121, and 25.123).
R-19
APPENDIX R. ICE SHAPES (CONTINUED) Figure R-10(a). Segmented 0.25-inch, forward-facing, quarter-round simulated ice shapes on the upper surface of an 18-inch chord NACA 23012m airfoil 2D model.
Figure R-10(b). Aerodynamic effects of continuous and segmented 0.25-inch, forward-facing, quarter-round simulated ice shapes on the upper surface of an 18-inch chord NACA 23012m airfoil 2D model (Re = 1.8x106, M=0.185).
Figure R-10. Aerodynamic Effects of Continuous and Segmented 0.25-inch, Forward-Facing, Quarter- Round Simulated Ice Shapes on the Upper Surface of an 18-inch Chord NACA 23012m 6Airfoil 2D Model (Re = 1.8 x10 , M=0.185). (Reference R5).
R-20
APPENDIX R. ICE SHAPES (CONTINUED) R.4.2 Aerodynamic Considerations for Determining Critical Ice Shapes.
Aerodynamic effects of ice accretions result mainly from the effects of the ice accretion on the surface’s boundary-layer behavior. Ice accretions that occur in areas favorable to keeping the boundary layer attached to the aircraft surface will result in effects that are less aerodynamically adverse than ice accretions that occur in areas less favorable to attached boundary-layer conditions. Ice shapes that build up in areas of local airflow deceleration (positively increasing surface pressure), or result in conditions unfavorable to keeping attached flow conditions, as the airflow negotiates the ice surface, will result in the most adverse effects.
For example, an ice accretion at the same chordwise location on different airfoils may result in different aerodynamic effects. Figure R-11 shows the surface pressure distributions for uncontaminated NACA 23012 and NLF 0414 airfoils for about the same lift. The NACA 23012 airfoil upper pressure is forward-loaded and shows a severe pressure recovery downstream of the suction pressure peak near the airfoil’s leading edge. The severe pressure recovery results from deceleration of the local airspeed to produce more positive pressures. The NLF 0414 airfoil’s pressures and local airspeeds are much more constant, with a pressure recovery occurring about
0.7c. This delayed pressure recovery provides conditions conducive to keeping a well attached, laminar boundary layer over most of the airfoil’s upper surface.
Figure R-12 shows low Reynolds number wind tunnel test results. These data resulted from installing a ¼-inch simulated ice shape (quarter-round) at various chordwise locations on 18-inch models of each airfoil. Figure R-13 shows the maximum lift values. Although these aerodynamic characteristics are Reynolds number dependent, the data show where the ice builds up is important and show the influences of the ice buildup on the behavior of the boundary layer and the airfoil aerodynamic performance.
Figure R-11. Compar ison of NACA 23012m and NLF 0414 Clean Airfoil Model Experimental Pressure Distributions (Cl ~ 0.6, Re = 1.8x106, M = 0.185) (Reference R5).
R-21
APPENDIX R. ICE SHAPES (CONTINUED) NACA 23012m NLF 0414 Clean x/c = 0.02 Clean x/c = 0.10 x/c = 0.02 x/c = 0.20 x/c = 0.10 x/c = 0.40 x/c = 0.20 15 a ( ) NACA 23012m NLF 0414 Figure R-12. Effect of an Upper Surface Simulated Quarter-Round Ice Shape at Various Chordwise Positions on Lift for Forward- (NACA 23012m) and Aft-Loaded (NLF 0414) Airfoils (k/c = 0.0139, Re = 1.8x10°, M = 0.185) (Reference R5).
1.6 -
1.4 .....
1.2 Clean (NACA 23012m) Ice Shape (NACA 23012m)
• • Clean (NLF 0414)
1.00;
Ice Shape (NLF 0414)
0.8
0.6
0.4 -
0.2 -
0.0 - 0.00 0.05 0.10 * 0.15 0.20 0.25 "0.30" 0.30 Ice Shape Location (x/c) Figure R-13. Variation of C I max for NACA 23012m and NLF 0414 Airfoils With and Without an Upper Surface 0.25-inch, Forward-Facing, Quarter-Round Simulated Ice Shape at Various Chord Positions (Re = 1.8x10°, M = 0.185) (Reference R5).
R-22
APPENDIX R. ICE SHAPES (CONTINUED) The critical location, for maximum lift, for the simulated ice accretion on the NACA 23012 airfoil is about 0.125c. Comparatively, the lowest maximum lift occurs at 0.40c (not shown) for the NLF 0414 airfoil. In both cases, the critical ice accretion location occurs near the area of the most severe pressure recovery (or flow deceleration) for their respective stall AOA. (Note the pressure distributions shown in figure R-17 are for normal operation attitudes.) Visual flow studies reveal the separation bubble that forms downstream of the simulated ice shape on the NACA 23012 airfoil fails to reattach immediately downstream of the ice shape. This is because of the adverse pressure recovery gradient, and results in a much longer separation bubble and a larger loss in maximum lift. Because of the less severe pressure recovery gradient on the NLF 0414 airfoil, the separation bubble is not as long. The resultant maximum lift loss is not as severe as that of the NACA 23012 airfoil.
For drag, the critical ice shape location for the forward-loaded NACA 23012 airfoil correlates with the area of highest local airspeed, not the area of most severe pressure recovery.
Figure R-14 shows this. However, the NACA 23012 ice shape location correlation does not apply to the results for the NLF 0414 airfoil, as shown also in figure R-14. For the NLF 0414 airfoil ice shape drag increment, the critical ice shape location suggests a sensitivity of the ice shape boundary-layer disturbance to the ability of the boundary layer to remain well attached during the pressure recovery at the aft part of the airfoil.
NLF 0414 NACA 23012m Figure R-14. Drag Increase for NACA 23012m and NLF 0414 Airfoils With a 0.25-inch, Forward-Facing, Quarter-Round Simulated Ice Shape Located on the Upper Surface at Various Chord Positions (Re = 1.8x106, M = 0.185) (Reference R5).
R-23
APPENDIX R. ICE SHAPES (CONTINUED) The comparison of rime and glaze ice shapes that have about the same mass is another example of how important it is to understand the ice accretion shape’s influence on the boundary-layer behavior when determining critical ice shapes. A rime ice leading edge ice shape that conforms with the profile of the surface may effectively improve the surface’s camber and aerodynamic performance. The rime may improve the local boundary-layer conditions and increase the lifting chord length. Conversely, the rime ice accretion may also thicken and weaken the boundary layer because of the rime ice roughness. This causes the boundary layer to become turbulent or separated earlier than a contamination-free boundary layer. However, the resulting increased drag and reduced maximum lift will usually be much less than that of a glaze ice shape of similar mass. The glaze ice is usually characterized by upper and lower horns that the airflow and boundary layer must negotiate. In fact, complete separation of the flow often occurs behind the horns. Also, the character of the boundary layer and its wake (for example, turbulence intensity, tendency to remain turbulent, tendency to remain attached or to separate from the surface, and the periodicity of separation from the surface) may vary. These viscous flow effects will vary, depending on:
x Reynolds number;
x The size, shape, and surface roughness of the ice accretion;
x The heights of the ice accretion protuberance and its surface roughness when compared with the surface’s characteristic length;
x The boundary-layer characteristics of the uncontaminated surface (that depend on the surface’s camber, thickness distribution, surface finish, and aoa); and x The location of the ice shape.
The boundary-layer effects resulting from the ice accretion typically reveal themselves in a small loss of lift within the normal operating envelope of the aircraft. However, there is a significant loss in maximum lift and a significant decrease in the AOA at which maximum lift occurs (figure R-8). The loss in maximum lift may affect stall-speed margins, maneuverability margins to buffet, and stall warning margins unless maneuvering airspeeds and the stall protection AOA schedules are adjusted. Significant drag increase will occur at operational angles-of-attack. The drag increase will increase with increasing AOA as the aircraft approaches maximum lift. The boundary-layer effects will adversely affect rotary wings and propeller thrusting efficiency. Changes in the contaminated airfoil’s pressure loading usually increase the airfoil’s nose-down pitching moments (more negative pitch stability). These effects are obvious in figure R-8. The increased negative stability may adversely affect airplane stability and control, stall characteristics, and trim. The boundary layer effects may also adversely affect rotary wing and propeller performance.
Figure R-15 shows the effects of Reynolds and Mach numbers on the performance of an airfoil with an ice accretion for a 36-inch NACA 23012 airfoil with and without intercycle ice. For the clean airfoil, increasing the Reynolds number favorably influences the behavior of the boundary layer. The boundary layer remains attached to the airfoil at higher angles of R-24
APPENDIX R. ICE SHAPES (CONTINUED) attack. Also, for the uncontaminated airfoil, the improved boundary-layer behavior reduces drag at higher Reynolds numbers. However, for the airfoil with a simulated ice shape, the Reynolds number effects are much less significant. Figure R-15 shows the effect of Mach number on airfoils with and without the simulated ice shape. Aerodynamic characteristics of contaminated surfaces do not always show large variation with Reynolds number, as shown in figure R-15. Applicants should provide an evaluation of Reynolds number effects when using low Reynolds number testing for the analysis of the aerodynamic effects of ice buildups.
Consider the effects of Mach and Reynolds numbers when determining the critical ice shape for a specific flight safety concern. For example, when comparing the coefficients of lift and drag for clean and contaminated airfoils, Reynolds and Mach numbers significantly affect the aerodynamic differences.
Critical ice shapes may result in reduced control surface authority, hinge moment and control force anomalies, uncommanded control surface movement, and may affect downstream control surface pressure loading. The change in surface pressure loading and periodic separation of the surface airflow may also affect surface vibration and tendencies for surface flutter.
Clean NACA 23012 airfoil NACA 23012 airfoil with Ice Shape 290 (figure R-5).
Figure R-15(a). Reynolds number effect on lift for a NACA 23012 airfoil with and without a simulated ice shape.
R-25