APPENDIX D. COMPONENT ICE PROTECTION SYSTEM DESIGN AND
CERTIFICATION PROCESS FLOWCHARTS (CONTINUED) Figure D-6. Typical Inflight Icing Certification Process for Air Data Probes and Sensors Regulatory Compliance Demonstrated NO Ice Accretion Effects on Component Function Shed-Ice Trajectory and Structural / Engine Damage Tolerance Analyses Struclural Analysis IPS Energy Over Appendix C Air Data System IPS Design Dry Air Test lo Evaluate Ice Shape Effects On Air Data System Calibrations Natural Ice Flight Test to Check Thermal Analvses. Simulated Ice Shapes, Dry Air Test Results, and Shed Ice Trajectory Empirical Validation of Heat Required Over Appendix C Demonstrations and Documentation Component Ice Accretion from Ground and Flight Operations Analyses / Evaluations Critical Ice Shape Selection, e.g. Radome Ice Shed-Ice Trajectory and Structural /Engine Damage Tolerance Analysis Weat Required/Available Over Appendix C Analysis Regulatory compliance Manufacturer's Design Requirements and Oblectives Requirements and Guidance, Including Safety Assessment •AC 23.1419-2A •AC 25.1419-1 •AC 29-2 • FAA Icing Handbook •AC 20-73 *AC 23.1309-1B •AC 25.1309-1A Regulatory Requirements and Guidance, Including Safety Assessment A/C Conformity to Type Design Drawinas Demonstrated Air Dala System (Pilol, Static Pressure, Angle-of-Attack) *** Section 23/25/27/29.1419 • Section 23/25/27/29.1323 • Section 23/25/27/29.1325 • Section 23/25.1326 • Section 23/25/27/29.1301 • Section 23/25/27/29.1309 Application for IPS Catifratior YES Certification Plan Accepted YES Flight in Icing Conditions Approval Desired A/C Limitation Against Flight in Known or Forecast Icing Conditions * • Section 23.1583 Protection Required Certification Plan Certification Plan Revision D-14
APPENDIX D. COMPONENT ICE PROTECTION SYSTEM DESIGN AND
CERTIFICATION PROCESS FLOWCHARTS (CONTINUED) Figure D-7. Typical Inflight Icing Certification Process for Antenna, Tanks, Fairings, Masts, Struts, and Other Miscellaneous Protuberances Antenna, Tanks, Fairing's, Masts, Struts and Protuberance's IP Approved Regulatory Compliance Ice Protection Required NO Ice Accretion Analysis Ice Effect on Pertine System Function Shed Ice Trajectory and Engine / Structure Damage Tolerance Analyses A/C Flying Qualities Effects Evaluation Flutter Analysis IPS Effectiveness Evaluation Over Appendix C Regulatory Compliance Demonstrations and Documentation Ice Effect on Pertinent System Function Ice Accretion Analysis Shed-Ice Trajectory and Engine / Structure Damage Tolerance Analyses IPS Energy Requirements A/C Flying Qualities Effects Evaluation Flutter Analysis A/C Confommity to Type Design Drawings Demonstrated Manufacturer's Design Requirements and Objectives Regulatory Requirements and Satey Assessming Section 23/25/27/29.1419 •AC 20-73 •AC 29-2 •AC 25.7A •AC 23.629-1A •AC 25.629-1A •AC 23.8A, Ch1 •AC 23.1309-1B •AC 25.1309-1A Certification Plan Accepted Pan Revison Antenna, Tanks, Fairing's, Masts, Struts and Protuberance's • Section 23/25/27/29.251 • Section 23/25/27/29.1301 • Section 23/25/27/29.1309 •AC 23.1419-2A •AC 25.1419-1 • FAA Icing Handbook Certification Plan YES Flight in Icing Conditions Approval Desired NO A/C Limitation Against Flight in Known or Forecast Icing Conditions * • Section 23/25/27/29.1583 Regulatory Requirements Application and Guidance, for IPS Certification Including Safety Assessment*:
D-15 (and D-16)
APPENIDX E. METEOROLOGICAL CONDITIONS Aircraft must be able to operate safely in the continuous maximum and intermittent maximum icing conditions of 14 CFR part 25, Appendix C to be certificated with ice protection provisions.
The following provides background about the development of 14 CFR part 25, Appendix C and guidance about its proper use.
E.1 ICING CLOUD SPECIFICATIONS.
14 CFR parts 25, Appendix C and 29, Appendix C provide the cloud parameters and the ranges of values required for the approval of aircraft inflight ice protection equipment (Reference E1).
CAR 4b Amendment 4b-2 dated August 25, 1955, introduced Appendix C envelopes for 14 CFR parts 25 and 29. The charts were derived from tabulations presented in Reference E2. One of the principal investigators stated that the icing conditions in 14 CFR parts 25, Appendix C and 29, Appendix C do not present physical relations among the variables used to define icing conditions (Reference E3). Instead, they represent combinations of the three variables (liquid water content (LWC), drop size, and temperature) believed to occur often enough to warrant consideration in design.
Drop diameter correlates only weakly correlated with either water content or temperature.
Efforts to define standard design icing conditions, as described in Reference E1 of this appendix, began with the use of wing heated anti-icing systems. Most of the flight measurements of LWC and effective drop diameter were made by the rotating-cylinder technique. Theoretical relations involving drop diameter, cylinder diameter, collection efficiency, and airspeed were used to derive average LWC and effective drop diameter. The method also yielded a rough measure of the spread in the size distribution. More dependable information on the distribution could have been obtained from counts of drops captured in oil. There were few determinations of drop diameter distributions.
When selecting design conditions, the applicant should consider the ability of thermal anti-icing system to recover from severe, temporary overloads. 14 CFR parts 25, Appendix C and 29, Appendix C icing conditions are envelopes of maximum-severity icing that occur in winter stratiform and cumulus clouds. The FAA chose conditions for each icing condition category based on the belief that thermal anti-icing systems should provide full protection in roughly 99 percent of icing encounters. Also, slight, temporary impairment of ice protection performance would be accepted. Rare occasions of exceptional severe icing might require evasive action.
Figures E-1 through E-6 reproduce Figures 1 through 6 of 14 CFR parts 25, Appendix C and 29, Appendix C. Figures E-1 and E-4 (14 CFR parts 25, Appendix C and 29, Appendix C, Figures 1 and 4) estimate the “probable maximum” (99th percentile) value of cloud water concentration (commonly known as LWC) that is expected. The LWC is provided as an average over a reference distance, for a given temperature and drop size (Reference E4). In defining the envelopes combining LWC, MED, and T, the investigators of Reference E5 determined that the condition that 99 percent of cases lie within the envelopes was roughly equivalent to a probability of 1/1000 that all three variables represented by a single point would be exceeded E-1
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED) simultaneously. For figure E-1 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 1) this reference distance is 20 statute miles (17.4 nm) in stratiform icing conditions. For figure E-4 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 4) it is 3 statute miles (2.6 m) in convective icing conditions. These are arbitrary reference distances but are based on the NACA research flights in the late 1940s. NACA and Weather Bureau researchers estimated these probable maximum values of LWC in the early 1950s. They proposed them as the basis for the present-day 14 CFR parts 25, Appendix C and 29, Appendix C (References E2 and E5). Review of recent icing cloud characterizations that used modern instrumentation shows the 1940 data used for 14 CFR parts 25, Appendix C and 29, Appendix C, acceptably accurate.
Continuous Maximum (stratiform Clouds) Atmospheric Icing Conditions Liquid Water Content Vs. Mean Effective Drop Diameter
1. Pressure altitude range, S.L. - 22,000 ft.
0.9 2. Maximum vertical extent, 6,500 ft.
Liquid Water Content - Grams Per Cubic Meter Horizontal extent, standard distance of
17.4 Nautical Miles
SOURCE OF DATA NACA TN No. 1855 CLASS 111-M CONTINUOUS MAXIMUM AIR TEMPERATURE +32 F
- +14 F -4 F
0.4 -22 F 15 20 25 30 35 40 Mean Effective Drop Diameter - Microns Federal Aviation Administration, DOT Pt. 25, App. C Figure E-1. 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Liquid Water Content Vs. Mean Effective Drop Diameter (14 CFR Parts 25 and 29 Appendix C, Figure 1).
E-2
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED) Continuous Maximum (Stratiform Clouds) Atmospheric Icing conditions Ambient Temperature Vs. Pressure Altitude Source of Data NACA TN No. 2569 32 26 Ambient Temperature - °F 20 14 -10 -1 -22 4 12 16 20 24 Pressure Altitude - 1000 Ft.
Figure E-2. 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Ambient Temperature Vs. Pressure Altitude (14 CFR Parts 25 and 29, Appendix C, Figure 2).
Liquid Water Content Factor, F-Dimensionless 1.34 Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions Liquid Water Content Factor Vs. Cloud Horizontal Distance Source of Data NACA TN No. 2738 310 5 6 78 910 20 30 40 50 100 200 300 Cloud Horizontal Extent - Nautical Miles Figure E-3. 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Liquid Water Content Factor Vs. Cloud Horizontal Distance (14 CFR Parts 25 and 29, Appendix C, Figure 3).
E-3
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED) Intermittent Maximum (cumuliform Clouds) Atmospheric Icing Conditions Liquid Water Content Vs. Mean Effective Drop Diameter
• Pressure altitude range, 4,000 - 22,000 ft.
Liquid Water Content - Grams Per Cubic Meter 2.5 Horizontal extent, standard distance of 2.6 Nautical Miles
DASHED LINES INDICATE POSSIBLE EXTENT OF LIMITS.
SOURCE OF DATA
2.0 CA TN No. 18 ASS 11-M INTERMITTENT MAXIMI AIR TEMPERATURE +32 F +14 F
- 4 F
- -22 F -40 F
0.5 25 30 35 40 Mean Effective Drop Diameter - Microns Federal Aviation A tration, DOT Pt. 25, App. C Figure E-4. 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Clouds) Atmospheric Icing Conditions, Liquid Water Content Vs. Mean Effective Drop Diameter (14 CFR Parts 25 and 29, Appendix C, Figure 4).
Intermittent Maximum (Cumuliform Clouds) Atmospheric Icing conditions Ambient Temperature Vs. Pressure Altitude Indicates Possible Source of Data Extend of Limits. NACA TN No. 2569 32 26 Ambient Temperature - °F -10 -16 -22 -28 -34 -40 16 20 24 28 30 Pressure Altitude - 1000 Ft.
Figure E-5. 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Clouds) Atmospheric Icing, Ambient Temperature Vs. Pressure Altitude (14 CFR Parts 25 and 29, Appendix C, Figure 5).
E-4
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED)
Figure E-6. 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Cl ouds) Atmospheric Icing Conditions, Liquid Water Content Factor Vs. Cloud Horizontal Distance (14 CFR Parts 25 and 29, Appendix C, Figure 6).
The FAA extended the Intermittent Maximum envelopes below -22°F (-30°C) “for completeness” in the original data source (Reference E2). The exact values of LWC are uncertain because of the lack of data at these low temperatures. Nevertheless, these extensions appear in the regulations (Reference E1) and applicants should consider them for design o f IPSs.
NOTE: Figure E-4 shows LWC for a temperature of 32qF. The warmest temperature in figure E-5 is 26qF. The inconsistency in 14 CFR parts 25, Appendix C and 29, Appendix C may be becaus e of the lack of data at temperatures above 26qF at the time of the investigation. Consider the LWC shown in figure E-4 for 32qF as the figure E-5 LWC value at 32qF.
In these icing applications, a single variable called the drop median volume diamete r (MVD) (or, in the older use, an equivalent variable called the MED) represents the drop size distribution (typically 10-30 Pm diameter) in clouds. The MVD is the midpoint of the LWC distribution over the range of cloud drop sizes that happen to be present at the time. The MVD varies with the number of drops in each size category. The overall average for layer clouds is about 15 Pm and about 19 Pm for convective clouds. This is based on data reviewed by the FAA. The MVD has proven useful as a simple substitute for the full drop size distributions in ice accretion estimates.
For certification of rotorcraft IPSs, you may truncate the icing conditions in 14 CFR part 29 , Appendix C to a pressure altitude of 10,000 feet or the altitude limit of the aircraft. Advisory E-5
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED)
Circular (AC) 29-2C states that applicants who elect to certify rotorcraft with a 10,000 feet pressure altitude limit based on equivalent safety may select icing conditions envelopes in AC 29-2C. AC 29-2C states that you must use 14 CFR part 29, Appendix C icing condition s for pressure altitude above 10,000 feet.
The AC 29-2C icing conditions envelopes resulted from a 1985 FAA William J. Hughes Technical Center analysis of the data used to establish 14 CFR part 29, Appendix C. The AC 29-2C icing conditions envelopes are significantly different from those in 14 CFR par t 29, Appendix C. For example, the AC 29-2C icing conditions envelopes for altitudes of 10,00 0 feet and lower show that you do not need to address temperatures colder than -10°F and 0°F for continuous and intermittent icing conditions, respectively. Comparative temperatures in 14 CFR part 29, Appendix C are -22°F and -6°F for continuous maximum and intermittent maximum icing conditions, respectively. Also, the AC 29-2C intermittent icing conditions minimum altitude is truncated at 4,000 feet, compared to sea level for 14 CFR part 29, Appendix C intermittent maximum icing conditions. Since AC 29-2C icing conditions are less rigorous and differ from that required by 14 CFR § 27.1419 and 14 CFR § 29.1419, you must consult w ith the FAA before using them.
E.2 USING 14 CFR PARTS 25 AND 29, APPENDIX C.
There is no comprehensive guidance available for using 14 C FR parts 25, Appendix C and 29, Appendix C icing conditions for design purposes. Engineers typically select a drop MVD, temperature, and altitude of interest. They use these parameters to determine the probable maximum LWC from figure E-1 or E-4 (14 CFR parts 25, Appendix C and 29, Appendix C , Figures 1 or 4). Other suggested or conventional practices are in this AC and in other refer ences (References E6 through E10).
You may select temperatures fr om the corners (extremes) of the temperature vs. altitude envelopes (14 CFR parts 25, Appendix C and 29, Appendix C, Figures 2 and 5 and figures E-2 and E-5 of this AC). Or you may select from average or extreme values for the flight alti tude under consideration. Reference E9 contains several example applications.
The values of LWC gotten directly from figures E-1 or E-4 (14 CFR parts 2 5, Appendix C and 29, Appendix C, Figure 1 or 4) are valid only for the reference distances of 17.4 nm or 2.6 nm, respectively. NACA researchers recommended these distances as “appropriate” for ice protection system design considerations when designing an IPS (Reference E2).
For different IPS design considerations with different exposure distances, you ma y change the originally selected LWC by a factor from figures E-3 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 3) or E-6 (14 CFR parts 25, Appendix C and 29, Appendix C, Figure 6).
This is because for both types of clouds, longer averaging distances will result in lower maximum values of LWC. To account for this behavior, the NACA researchers developed LWC adjustments (F-factors). (See 14 CFR parts 25, Appendix C and 29, Appendix C, Figures 3 and 6 and figures E-3 and E-6 of this AC.)
E-6
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED)
The choice of exposure distance depends on the application. One application is to estimate ice buildup amounts on unprotected surface s during a long exposure, perhaps 100 or 200 miles (Reference E9, page 4.1-10). In this case, you reduce the LWC from figure E-1 by an amount gotten from figure E-3 for the selected exposure distance. For example, to find the maximum probable LWC during flight through 100 nm of stratiform icing clouds, use the multiplying factor (0.46 in this example) from figure E-3. Thus, for stratiform clouds in which the MVD is 15 Pm and the temperature is -10qC (+14qF), the maximum average LWC over 100 nm is expected to be 0.46 x 0.6 g/m3 = 0.28 g/m3. This procedure raises or lowers the LWC curves in figure E-1 or E-4, depending on the exposure distance.
Another use is to estimate ice buildups on unprotected surfaces during a 45-minute hold. For the 45-minute hold discussed in section 8 of this AC, use th e LWC from Figure 1 of 14 CFR part 25, Appendix C at full value. Guidance for the 45-minute hold assumes the conservative case when the holding pattern remains within 17.4 nm and the LWC is changed from that shown in Figure 1 of 14 CFR part 25, Appendix C.
Note the only valid use of the F-factor curves is for adjusting 14 CFR parts 25, Appendix C and 29, Appendix C LWCs to get probable maximum values of LWC for other distances. Do not use the F-factor curves any other way, including applying the F-factors to measured flight test LWCs to:
a. Extrapolate available flight-test LWCs to values they supposedly would have if the exposures had been over the design distances of 17.4 nm or 2.6 nm;
b. Compensate in flight for smaller-than-desired LWCs by extending the exposure dist ances by some calculable amount to achieve an exposure that is supposedly equivalent to the wanted LWC over the design exposure distance; or
c. Create some rating system for comparing available test exposures to design exposures.
Appendix N of this AC provides acceptable ways of documenting variable length icing exposures without using the F-factor.
E.3 REFERENCES.
E1. Code of Federal Regulations, Title 14, Parts 1-59; Office of the Federal Register, National Archives and Records Administration, Washington, DC 20408; (full text available at http://www.access.gpo.gov/nara).
E2. “Recommended Values of Meteorological Factors to be Considered in the Desi gn of Aircraft Ice-Prevention Equipment,” NACA Technical Note 1855 (1949), NASA/Ames Research Center, Moffett Field, California 94035.
E3. “Review of Icing Criteria,” Lewis, William, Aircraft Ice Protection, Report of Symposium , Engineering and Manufacturing Division, Flight Standards Service, Department of Transportation, Federal Aviation Administration, dated April 28-30, 1969.
E-7
APPENDIX E. METEOROLOGICAL CONDITIONS (CONTINUED)
E4. “Review of Icing Criteria,” Lewis, William, Aircraft Ice Protection, Report of Symposium, April 28-30, 1969.
E5. “A Probability Analysis of the Meteorological Factors Conducive to Aircraft Icing in the United States,” NACA Technical Note 2738 (1952), NASA/Lewis Research Center, Cleveland, OH 44135.
E6. “Certification of Part 23 Airplanes for Flight in Icing Conditions,” Advisory Circular
23.1419-2B (1998); Federal Aviation Administration, Washington, DC 20591.
E7. “Certification of Transport Category Airplanes for Flight in Icing Conditions,” Advisory Circular 23.1419-2C (2004); Federal Aviation Administration, Washington, DC 20591.
E8. “Certification of Transport Category Rotorcraft,” Advisory Circular 29-2C (1999); Federal Aviation Administration, Washington, DC 20591.
E9. “Engineering Summary of Airframe Icing Technical Data,” D.T. Bowden et al., Technical Report ADS-4 (1963); Federal Aviation Administration, Washington, DC 20591.
E10. “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.
E-8
APPENIDX F. OPERATIONAL FACTORS F.1 AIRPLANE OPERATIONAL FACTORS.
The applicant should consider the airplane’s flight envelope when determining the most critical conditions for designing an IPS. Explore flight regimes such as climb, cruise, hold, and descent at various altitudes. For airplanes with high-lift devices, the flight regime during which these devices are used may be the most critical. In other cases, the cruise configuration may be the most critical because of the lift, drag, or control problems associated with the buildup of ice on critical surfaces. Service experience shows that you should consider holding in icing conditions for as long as 45 minutes (appendix G of this AC).
The airplane’s attitude affects the buildup of ice on critical surfaces. The airplane’s airspeed and attitude affect the type and shape of ice buildups that occur (appendix R, sections R.1.1 and R.4 of this AC).
For a given set of meteorological conditions, the rate and extent of ice buildup is a function of flight speed as well as airfoil or body geometry. Small bodies moving at high speeds encountering large liquid water drops will have high collection efficiencies. The flow field associated with larger bodies deflects small drops. This results in lower collection efficiencies for larger bodies.
The ice accretion on the airplane depends, in part, on the airplane’s speed and the horizontal extent of the icing cloud. However, icing tunnel tests show that the ice buildup rates on a surface are not always linear – especially for glaze ice. Ice buildups change the airfoil characteristics and cause a continuous change in collection rate above the initial rate. So, extrapolating ice buildups linearly with time can be misleading.
Airplanes not approved for inflight icing may encounter icing unintentionally. Therefore, the FAA requires the powerplants of these airplanes be protected against hazardous ice effects.
Continued exposure of these aircraft to icing conditions may cause them to become incapable of flight. See 14 CFR § 23.901(d)(2) for engines and engine installations. For engine induction systems, see 14 CFR § 23.1093 and 14 CFR § 25.1093.
F.2 ROTORCRAFT OPERATIONAL FACTORS.
Rotorcraft may unintentionally encounter icing conditions even though they are not approved for flight in icing conditions. Therefore, the powerplants of these helicopters must be protected against hazardous effects of ice. However, continued exposure to icing conditions may cause the helicopter to become incapable of flight (section 10.1 of this AC and AC 29-2C).
F.3 ENGINE OPERATIONAL FACTORS.
Considerations for determining the most critical icing conditions for engines are related to the aircraft’s operating procedures. This relationship exists because changes in engine power requirements usually accompany changes in airplane speed and attitude. Important F-1