APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED)
These curves are for typical MVDs of around 15 Pm and for flight-level temperatures between 0qC and -10qC. The LWCs are even less for other MVDs or lower temperatures.
Applicants can convert the LWC axis to icing rate for any particular airframe component or for an icing rate meter, if needed (Reference M1). You can plot measured LWC averages (or icing rates) on figure M-1 as a useful way to compare the test events with nature.
Reference M2 provides other statistical information.
M.2.2 Natural 99 Percent LWC Limits vs. Altitude.
Figure M-2 depicts the variation of the 99 percent value of LWC with altitude and the icing event distance. For events between about 6 nm and 50 nm, the largest LWC averages are expected between 10,000 ft and 15,000 ft AGL. For longer events, the largest LWC averages are found between 5,000 ft and 15,000 ft, with a slight preference for clouds with tops near 5000 ft AGL. Again, applicants can convert the LWC axis to icing rate, if needed. Plot measured averages of LWC or icing rate for the icing event distance to compare with the probable maximum values.
1 10 100 1000 Horizontal Extent (nmi) 0
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1.6 LWC (g/m3 ) 15,000 Ft AGL 10,000 Ft AGL 5,000 Ft AGL 20,000 Ft AGL 2,500 Ft AGL Figure M-2. Natural 99 Percent LWC Limit Envelopes at the Highest Icing Temperatures Available at Indicated Altitude as a Function of Cloud Horizontal Extent (for all clouds containing supercooled water drops with MVD = 15-20 Pm).
M-2
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED) M.2.3 Probability of Finding Large MVDs.
Figure M-3 shows the statistics for MVDs in stratiform clouds. There is a marked preference for MVDs close to 15 Pm. Stratiform clouds with an MVD of about 15 P m clearly are the most stable. They are more common, can contain larger maximum LWCs, and can extend over far greater distances than clouds (or portions of clouds) with larger MVDs. The larger the MVD, the less likely it is to occur. Stratiform clouds containing larger MVD drops will contain less LWC and persist over shorter horizontal distances. Since about two thirds of all random icing events contain MVDs smaller than 15 Pm, applicants will delete much of their natural icing flight test data if they use the 14 CFR parts 25, Appendix C and 29, Appendix C cutoff at 15 Pm as a lower limit. If MVD is not critical for the test objectives, you can show data for MVD less than 15 Pm.
3 LWC (g/m )
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0.1 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 Horizontal Exten t (nmi) 15µm (32%) 20µm ( 12%) 25µm (4%) 30µm (2%) 40µm (0.5%)
17.4 nmi 68% of all MVDs are < 15 µm, 77% of all MVDs are = 15 µm ± 5 µm -- -- - (Percentages by each curve give fract ion of MVD's t hat have been f ound larger t han the indicated MVD)
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0.1 0 Figure M-3. Natural Cloud Horizontal Extent Limits and 99 Percent LWC Limits for Selected, Sustained Cloud Drop MVDs in Stratiform Clouds at Ambient Temperatures of 32oF to 14oF.
M-3
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED)
If large MVDs are still of interest for test flight purposes, the statistics suggest the following strategy. Look for:
x Maritime air masses (for example, Pacific coast of North America or the North Atlantic Ocean east of the Canadian Maritime Provinces);
x Flight altitude temperatures warmer than about -17qC;
x Altitudes from 7,000 to 14,000 ft ASL; and x Convective clouds (more suitable than layer clouds).
Expect to find:
x Horizontal extents (of large-MVD conditions) less than 10 nm and x MVDs in the normal range of 10-20 Pm most of the time.
M.3 EXPERIENCE.
M.3.1 Continuous Maximum Conditions.
Continuous maximum icing conditions of 14 CFR parts 25, Appendix C and 29, Appendix C usually exist in stratiform clouds. Applicants usually need stratiform cloud decks with depths greater than 1000 ft to get worthwhile LWCs for icing flight tests. Layer clouds composed of water drops seldom exist with layer depths greater than about 6000 ft. The LWC generally increases with height in the cloud, so the larger LWCs will usually exist in the upper quarter of the layer.
In the United States, you often find wintertime stratus clouds in the Great Lakes area. But other areas are also suitable. The commercial or public weather maps on television or on the Internet (References M3 and M4) are helpful.
M.3.2 Intermittent Maximum Conditions.
Intermittent maximum icing conditions of 14 CFR parts 25, Appendix C and 29, Appendix C typically occur in cumuliform clouds. These icing conditions usually occur during vigorous convective activity that is rare when the freezing level is low. Normally you have to look for springtime or summer weather where the freezing level is above 5,000 ft to 10,000 ft AGL. A strategy that cloud physics researchers use is to penetrate the tops of isolated “feeder” cells or growing cumulus towers on the edge of existing or growing, isolated thunderstorms. This provides fairly large LWCs without having to perform risky penetrations of the main thunderstorm core.
M-4
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED) M.3.3 Glaciated Conditions.
14 CFR parts 25, Appendix C and 29, Appendix C do not include glaciated clouds.
Applicants should expect little or no icing in glaciated (snow or ice crystal) clouds. The presence of snow at ground level in wintertime (nonthunderstorm) conditions is a good hint that the clouds above the 0qC level are glaciated and have little or no potential for icing an airframe.
Glaciated clouds in any season are recognizable by their lack of texture from both the inside and outside. From the outside they look like cotton candy. The inside of most widespread, deep, stratiform-like glaciated conditions is not uniform. It may look uniformly gray in all directions outside the aircraft, but neither cloud puffs nor visible cloud may pass by the wings.
Snow may be present, yet unnoticeable unless lit up by the landing lights or wingtip strobe lights. You will see a dull, featureless overcast from underneath the cloud.
Water drop clouds are typically lumpy. They look like a cauliflower on the outside, especially on the sides and top. These clouds are denser, and puffs or cloudy parcels can be seen passing by the wing. Sometimes they partially or completely obscure the wingtip.
M.3.4 Mixed-Phase Conditions.
14 CFR parts 25, Appendix C and 29, Appendix C do not include mixed-phase icing conditions. Mixed-phase icing conditions consist of ice crystals interspersed with liquid drops. Applicants can expect to find mixed-phase icing conditions in about 40 percent of continuous and intermittent maximum icing conditions. Take care to account for these ice crystals when determining the liquid and total water content of clouds. Distinguishing between water drops and ice crystals requires special instrumentation such as a total water content probe. You can also use an instrument that discriminates between spherical water drops and crystalline frozen water (the PMS-2D-C probe). This discrimination is timeconsuming and requires detailed investigation. The investigation includes differentiation between LWC and frozen water content to determine total water content.
M.3.5 Large Drop Conditions.
Although no requirements exist for IPS design and approval in large drop icing conditions (commonly called supercooled large drop (SLD) conditions), they are discussed below for completeness. Large drop conditions include freezing drizzle and freezing rain.
Freezing drizzle (FZDZ) contains drops from 50 µm up to 500 µm in diameter. The meteorological conditions that produce these large drops are not well understood. Freezing drizzle aloft is difficult to predict. Thin layers of it have been observed at altitudes as high as 20,000 feet, at temperatures as cold as -20°C. It occurs occasionally in or below stratiform clouds. Freezing drizzle can occur as a diffuse supercooled mist aloft in some northerly maritime areas such as coastal Alaska and Newfoundland (Reference M5).
M-5
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED)
Freezing rain (FZRA), also known as ice storms, occurs when snow falls through a warmerthan-freezing layer of air where the snowflakes melt into raindrops and fall to the ground through an underlying layer of subfreezing air in contact with the ground. Aircraft (like trees, power lines, roads, and other objects) can become coated with a hard layer of clear ice.
Freezing rain will affect aircraft during approach, landing, taxiing, takeoff, and transiting in the lowest few thousand feet above ground level. In the United States, freezing rain is most common east of the Rocky Mountains. It is usually easy to forecast and its expected location will be included in the daily weather forecasts.
M.4 USING FORECASTS OF ICING CONDITIONS.
Icing forecasts have been traditionally imprecise. This is due mainly to the current difficulty of knowing when, whether, and where a given cloud interval above the freezing level will be liquid or glaciated. Forecasts typically read something like “icing in cloud and in precipitation above the freezing level.” This language allows for the fact that clouds above the freezing level may be liquid in some places and therefore capable of icing an aircraft.
Icing forecasts are improving, at least in narrowing the vertical and horizontal boundaries of existing and expected icing-prone cloudiness. New, experimental, or operational graphical presentations of icing conditions are available on the Internet at:
x http://aviationweather.gov/exp/cip/ x http://weather.noaa.gov/ x http://www.dispatcher.org/brief/adfbrief.html These can provide a quick and easy way to assess current icing conditions.
M.5 PROFESSIONAL GUIDANCE.
Applicants may find certified consulting meteorologists and forecasting services for hire in a professional directory kept by the American Meteorological Society (Reference M6). Some of those listed will be knowledgeable in aircraft icing.
The Society of Automotive Engineers (SAE) has an active Aircraft Icing Technology committee (AC-9C) of icing practitioners who produce helpful, icing-related SAE publications (Reference M7).
The FAA publishes an Advisory Circular listing Designated Engineering Representatives (DERs) who are available for consulting work (Reference M8). Some of these have experience in aircraft icing applications.
M-6
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES (CONTINUED) M.6 REFERENCES.
M1. “Icing Design Envelopes (14 CFR-25, 29, Appendix C) Converted to a Distance- Based Format,” Richard K. Jeck, FAA Technical Report DOT/FAA/AR-00/30 (2000), FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
M2. “A New Database of Supercooled Cloud Variables for Altitudes up to 10,000 Feet AGL and the Implications for Low Altitude Aircraft Icing,” Richard K. Jeck, FAA Technical Report DOT/FAA/CT-83/21 (1983), FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
M3. Aviation Weather Center: http://aviationweather.gov M4. National Weather Service: http://weather.noaa.gov M5. “An Inferred Climatology of In-Flight Icing and SLD for North America,” Bernstein, B.C., Proceedings of the 10th American Meteorological Society Aviation Range and Aerospace Meteorology Conference, Portland, Oregon, May, 2002, pp. J21-J24.
M6. American Meteorological Society, 45 Beacon Street, Boston, MA 02108-3693.
Tel: 617-227-2425. Internet: http://www.ametsoc.org.
M7. Society of Automotive Engineers:
http://forums.sae.org/access/dispatch.cgi/TEAAC9C_pf_ M8. “Designated Engineering Representatives Consultant Directory,” Advisory Circular No. 183.29-1HH (2000), Federal Aviation Administration, 800 Independence Ave. SW, Washington, DC 20591.
M-7 (and M-8)
APPENIDX N. INSTRUMENTATION N.1 INSTRUMENTATION.
Applicants must measure icing conditions during certification flight-testing in natural and simulated icing conditions (14 CFR §§ 23.1419, 25.1419, 27.1419, and 29.1419).
Measurements should characterize the icing conditions that are useful for understanding the IPS’s performance and effectiveness. The measurements should:
x Verify the IPS analysis;
x Provide information to understand icing anomalies; and x Allow extrapolation of the IPS performance and effectiveness to other conditions within 14 CFR parts 25, Appendix C and 29, Appendix C.
The FAA considered the variables selected to define 14 CFR parts 25, Appendix C and 29, Appendix C important for the design of thermal IPSs. These variables include liquid water content (LWC), mean drop diameter, temperature, cloud extent, and altitude. They are typically measured during natural and simulated icing flight tests to characterize the icing conditions.
Modern flight tests have used hot-wire type probes for LWC measurement, and a laserbased (electro-optical) drop size spectrometer for counting and sizing cloud drops (References N1 and N2).
You may use a research icing rate sensor, such as the Goodrich model 871FA, as a part of the instrument suite. Appendix P of this AC discusses use of the icing rate meter information.
You should continuously record the selected variables with 1-second resolution during the icing events. Spot measurements or long-distance averages are less desirable.
The older methods of measurement (rotating cylinders for LWC, and coated slides for dropsize analyses) are less desirable because of their serious limitations. These limitations include measurement errors, spot sampling, and the difficulty of data analysis (Reference N2). But hot-wire and electro-optical instruments often present problems, too. Experienced technicians should calibrate, install, and run this specialized instrumentation. You should install the icing rate meters at proper locations on the aircraft (appendix K of this AC).
Only those who know how to recognize subtle errors in the probe performance or have experience in using these modern instruments should analyze their data. If necessary, several commercial enterprises are available to supply this service.
You should use video cameras with adjustable zoom lenses to document ice buildups on the wings, probes, or other surfaces of interest. A video view of the wing tip area also provides a good qualitative record of the density and continuity of cloud conditions during icing events. A cloud that is dense enough to obscure the wing tip on a general aviation airplane N-1
will usually be a convective (or intermittent maximum) cloud. Stratiform clouds are usually less dense. You may also use the video for recording the presence of precipitation that is lit up by the wingtip strobe lights or landing lights.
N.2 REFERENCES.
N1. “Aircraft Icing Handbook, Volume 1 of 3,” FAA Report number DOT/FAA/CT-88/8-1 (1991), FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
N2. “ Cloud Physics Instruments for Measuring Icing Conditions,” Richard K. Jeck, FAA Technical Report DOT/FAA/AR-02/120 (2003), FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
N-2
APPENIDX O. WAYS TO EVALUATE ICING EXPOSURES RELATIVE TO
APPENDIX C O.1 BACKGROUND.
Applicants should document measured natural icing flight test conditions. You may use the methods in paragraph O.2, of this appendix, to compare the measured natural ice test conditions to 14 CFR parts 25, Appendix C and 29, Appendix C.
The 14 CFR parts 25, Appendix C and 29, Appendix C envelopes, as shown in figures D-1 and D-4 of appendix D of this AC, are valid only for averaging distances of 17.4 nm and
2.6 nm, respectively. Most of the time, icing encounters in flight will cover shorter or longer distances. To compare the flight data with the envelopes, you should average the inflight data over the same distances for which the envelopes are drawn. This is not always possible, especially if the encounters are shorter than the design distances. It is improper to use the F-factor curves in figures D-3 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 3) and D-6 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 6) of appendix D of this AC to adjust the encounter-averaged LWC to equivalent values over
17.4 nm or 2.6 nm. Use the F-factor curves to adjust the LWC curves in 14 CFR parts 25, Appendix C and 29, Appendix C to match each of the actual averaging distances obtained from the test flights.
The 14 CFR parts 25, Appendix C and 29, Appendix C envelopes treat LWC and MVD as principal variables with exposure distance treated as a constant. Distance may be a more useful variable for comparison with test data. You may redraw the envelopes for fixed values of MVD. Statistical analysis of MVD measurements in the atmosphere suggest that MVDs are much less variable than may be generally realized . You should provide redrawn MVD envelopes for each icing encounter when presenting certification data.
O.2 APPENDIX C CONVERTED TO DISTANCE- OR TIME-BASED ENVELOPES.
Applicants can use the F-factor curves to convert figures D-1 and D-4 of appendix D of this AC to an equivalent LWC versus Horizontal Extent (HE) format (Reference O1).
Figure O-1 shows conversion of figures D-1 and D-4, 14 CFR parts 25, Appendix C and 29, Appendix C icing envelopes in a distance-based format for temperatures of 0qC, -10qC, -20qC, and -30qC and for an MVD of 15 Pm. As explained in Reference O1, after you enter the basic coordinates into a computerized spreadsheet, you can create variations or customized versions of the envelopes easily using the spreadsheet charting capabilities. For Statistics compiled at the FAA William J. Hughes Technical Center from
12,000 nm of measurements in stratiform icing conditions revealed that about 75 percent of all MVDs are within r5 Pm of 15 Pm in stratiform clouds.
O-1
APPENIDX O. WAYS TO EVALUATE ICING EXPOSURES RELATIVE TO APPENDIX C (CONTINUED) example, in figure O-1 a logarithmic HE scale has been chosen to accommodate both the short HEs of the Intermittent Maximum envelopes and the long HEs of the Continuous Maximum envelopes.
You can plot natural icing atmospheric measurements on Figure O-1 for MVDs near 15 ȝm,
or on similar graphs for other MVDs.
LWC (g/m3)
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0.1 1 10 100 1000 Horizontal Extent (nmi) 0 C -10 C -20 C -30 C 0 C -10 C -20 C -30 C (Intermittent Maximum) (Continuous Maximum) ` Figure O-1. 14 CFR Parts 25 and 29, Appendix C Icing Envelopes Converted to a Distance-Based Format (for MVD=15 Pm).
O-2
APPENDIX O. WAYS TO EVALUATE ICING EXPOSURES RELATIVE TO
APPENDIX C (CONTINUED) You may define measured inflight exposures by the distance flown in the icing condition or by elapsed time. Icing wind tunnel exposures and computer simulations are typically reported as timed exposures. You can convert the distance-based format to a time-based format by dividing the distance scale by the airspeed, provided the flight speed is approximately constant during the cloud penetration. For example, at 200 knots, the 200 nm mark is also the 60-minute mark. The 20 nm mark is also the 6-minute mark, and so on.
In using the time-based format, you must renumber the time scale for each airspeed in use.
If the airspeeds are similar, then using this time-based format allows you to plot wind tunnel, computer simulated exposures, and flight test averages on the same time-based 14 CFR parts 25, Appendix C and 29, Appendix C envelopes.
O.3 REFERENCES.
O1. “ Icing Design Envelopes (14 CFR-25, 29, Appendix C) Converted to a Distance- Based Format,” Richard K. Jeck, FAA Technical Report DOT/FAA/AR-00/30 (2000), FAA William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405.
O-3 (and O-4)