Par 3-10 Page 15 NOTE: If the accepted practice for HOT table development with respect to HOT caps is modified, these modifications should be applied to HOTDS and CTDS criteria.
3-11. PROVISIONS FOR LWES APPROVAL FOR SUPERCOOLED LARGE DROPS (SLD).
NOTE: These provisions pertain to a system that can identify Supercooled Large Drops (SLD) (i.e., freezing rain or freezing drizzle), but cannot distinguish between freezing rain and freezing drizzle.
a. Performance Standards for Identification of SLD. The following provision should be met if the system is to be approved for identification of SLD. If precipitation of an LWE rate of at least 2.0 g/dm2/h (as determined by a precipitation rate pan measurement for an LWE sampling time of at least 5 minutes) has been detected and identified as SLD by a human observer with credentials the FAA finds, the system should identify it as SLD at least 95 percent of the time.
b. Performance Tests for Identification of SLD. A detailed protocol for performance of the tests should be included in the test plan described in Chapter 2. A minimum of 25 data points for an LWE sampling time of at least 5 minutes in SLD, distributed over freezing drizzle and light freezing rain, and documented as described in the test protocol, should be collected to demonstrate compliance with performance standard in subparagraph 3-11a. The data set should include a range of temperatures and wind speeds representative of those in which the system may be used.
c. Performance Standards for Determination of LWE Rate for SLD. These provisions should be met if the system is to be approved for determination of LWE rate for SLD:
(1) LWE rate measurements should be analyzed in reference to simultaneously recorded precipitation rate pan measurements or in reference to simultaneously recorded measurements by another method acceptable to the FAA. The testing period should be the same period used by the system operationally to determine HOT or CT, and cannot be less than 5 minutes.
(2) The LWES precipitation rate measurements should conform to the precipitation rate pan measurements (refer to SAE ARPs 5485 and 5945) or other measurements the FAA finds acceptable, within the tolerances or alternate tolerances in Table 3-4 at a 95 percent confidence level:
Par 3-11 Page 16 TABLE 3-4. THE LWES SLD RATE MEASUREMENT TOLERANCES LWES Rate g/dm2/h Acceptance Tolerance g/dm2/h Alternate Acceptance Tolerance From 2 to 13* -3.0 -4.0 g/dm2/h Above 13 to 25 -6.0 -40% Above 25 -14.0 * 0 g/dm2/h may be used as the lower limit of this range at the discretion of the applicant.
(3) If the system also meets the stated criteria in Table 3-4 when a minus (-) sign is replaced with a plus or minus (±) sign, then the system meets not only the stated (1-sided) criteria, but also a more stringent (2-sided) criteria. In its response to the report and data submission by the manufacturer, the FAA can provide a statement that this more stringent standard has been met.
d. Performance Tests for Determination of LWE Rate for SLD. A detailed protocol for performance of the tests should be included in the test plan required in paragraph 2-5.
A minimum of 25 data points for the LWE sampling time in SLD, distributed over freezing drizzle and light freezing rain, and documented as described in the test protocol, should be collected to demonstrate compliance with the performance standard in paragraph 3-11c.
e. Determination of HOT or CT Using LWE Rate. These provisions should be met if the system is to be approved for determination of HOT or CT using LWE rates for SLD:
(1) HOT or CT determinations for SLD should be based on computing the associated HOT or CT for freezing rain as specified in subparagraph 3-9e(1) and freezing drizzle as specified in subparagraph 3-10e(1) and using the resulting lower value.
(2) HOT or CT determinations must not be provided in SLD conditions exceeding 25 g/dm²/h. This threshold is to be compared to the LWE rate determined by the system plus the tolerance (within a 95 percent confidence level) that has been demonstrated for the appropriate range.
(3) In accordance with the currently accepted practice for HOT table development, HOT and CT determinations from the LWES for Type II and IV anti-icing fluids must be capped at 3 hours for SLD.
NOTE: If the accepted practice for HOT table development with respect to HOT caps is modified, these modifications should be applied to HOTDS and CTDS criteria.
Par 3-12 Page 17 (and 18)
3-12. LWES LATENCY. The LWES should generate an updated HOT or CT every 10 minutes or less.
a. LWES Inputs. The inputs the system uses to generate a HOT/CT are Air Temperature, Precipitation Type, LWE, and Wind Speed (optional) and should not be more than 10 minutes old. These inputs should also be reported every 10 minutes along with the updated HOT/CT.
b. Onset of Winter Precipitation. The LWES should determine and report a HOT/CT within 10 minutes of the onset of a winter precipitation event as identified in paragraphs 3-8 through 3-11c.
Appendix 1 Page 1 (and 2) APPENDIX 1. ACRONYMS USED IN THIS ADVISORY CIRCULAR AC Advisory Circular AFS Flight Standards Service ARP Aerospace Recommended Practice AWOS Automatic Weather Observation System CFR Code of Federal Regulations CT Check Time CTDS Check Time Determination System FAA Federal Aviation Administration g/dm2/h Grams per Decimeter Squared per Hour HOT Holdover Time HOTDS Holdover Time Determination System LWE Liquid Water Equivalent LWES Liquid Water Equivalent System RMSE Root Mean Square Error SAE Society of Automotive Engineers SLD Supercooled Large Drops
Appendix 2 APPENDIX 2. DEFINITIONS
a. Anti-Icing Fluid. A fluid applied to an aircraft as a precautionary procedure that provides protection against the formation of frost or ice and the accumulation of snow on treated surfaces of an aircraft for a period of time.
b. Check Time (CT). A time in the past that indicates whether an aircraft’s anti-icing fluid is still providing protection in the given conditions. Incorporating real-time snowfall rates, the algorithm begins with the current time and integrates the fluid’s loss in protection capacity rate backwards in time, minute by minute, until it determines sufficient precipitation has fallen for the protection capability to be exhausted. The time at which this occurs is displayed to the user as check time and is updated every minute. The check time is compared to the time the aircraft was anti-iced. As long as the time the aircraft was anti-iced remains more recent than the check time, the fluid is still providing protection.
c. Deicing Fluid. A fluid applied to an aircraft to remove contamination in the form of frost, ice, or snow.
d. Endurance Time. A time of protection determined from a test of a fluid of specified dilution at a given temperature for a given type of precipitation and LWE rate. (Based on the endurance time testing, estimated protection times for use in operations, called HOTs, are determined.)
e. Endurance Time Regression Analysis. A data analysis protocol used to analyze fluid endurance time data for the generation of HOTs for anti-icing fluids.
f. Glycol Pan Measurement. A process to determine precipitation rate by using a glycol-wetted pan that is weighed precisely before and after a timed exposure to precipitation.
This methodology has been included in SAE Aerospace Recommended Practice (ARP) 5485 and 5945.
g. Holdover Time (HOT). The estimated time that deicing/anti-icing fluid will prevent the formation of frost or ice and the accumulation of snow on the critical surfaces of an aircraft.
HOT begins when the final application of deicing/anti-icing fluid commences and expires when the deicing/anti-icing fluid loses its protection effectiveness.
h. Holdover Time Tables. Tables of HOTs for anti-icing fluids published as HOT guidelines by the FAA.
i. Liquid Water Equivalent (LWE) Rate. The rate of LWE accumulation of freezing or frozen precipitation in g/dm2/h.
j. Liquid Water Equivalent System (LWES). A near-real-time system that in precipitation conditions determines precipitation type (if required) and LWE rate. The system can use the LWE rate to determine a HOT or CT.
Appendix 2
k. Liquid Water Equivalent (LWE) Sampling Time. The period of LWE sampling used by the LWES to determine the LWE rate. This system sampling time must not be less than 5 minutes, and will ordinarily be 5 or 10 minutes.