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68 Based on the results from this single test, the spot deicing procedure should clearly indicate that it is not intended for application on cold-soaked surfaces. This test result also raises a question regarding the applica- bility of the current HOT frost guidelines to the condition of frost resulting from cold-soaked wings. The current fluid endurance test in natural frost is conducted on a special frost insulated plate that has been developed to represent wings sub- jected to heated fluid spray. The heat from the fluid raises the wing surface temperature, resulting in fluid enrichment due to evaporation and in a delay to frost initiation until the surface cools to below OAT. If the wing is cold-soaked with cold fuel, the additional heat sink would cause a quite different wing sur- face temperature response, and the same endurance times may Figure 24. Simulation of a severe frost event on test not be produced. plate surfaces. Results for Spot Deicing in Active Frost Focus Group Survey In this examination of endurance times for active frost with The detailed results of the focus group survey are provided question by question in Appendix F. For multiple choice ques- cold-soaked surfaces, the degree of cold soaking is indicated by tions, the percentage of respondents selecting each response is the value of T (surface temperature minus OAT). listed. For most multiple choice questions, the responses are Of the eight test sets, only three experienced an active frost also additionally broken out by organization type (i.e., airlines, rate considered to be more than light frost (Table 44). Those deicing service providers, others). Each and every response test sets were 1, 2, and 3b. provided for the short answer questions and comments areas These three tests were examined to assess whether they are is provided. realistic representations of cold-soaked wing surfaces, and if so, Some key findings from the survey include: the measured fluid endurance times were reviewed. Test 1 is not a realistic representation of cold soaking as the Approximately half of the survey respondents (48%) are test surface temperature was essentially equal to OAT. familiar with the spot deicing for frost removal procedure; Test 3b is not realistic due to its very large T at -18F slightly fewer (43%) currently employ the procedure in their (-10C). This large T resulted in very short endurance times. operations. Test 2 is viewed as a good representation of active frost on Spot deicing for frost was believed by the respondents to be cold-soaked wing surfaces. Its T at -8.6F (-4.8C) falls one of the cheapest frost removal methodologies and also within the range of T values measured for previous cold- one of the most effective. soaked wing studies, which generally identified a maximum T The majority of respondents (89%) were aware of the AEA of -12.6F (-7C). As well, the frost rate is close to that pro- guidelines for local wing frost removal. 57% indicate they posed for fluid endurance testing in natural frost. already use a local wing frost removal methodology; 19% In Test 2, the endurance times differ by fluid strength: would consider using one; and 24% would not consider using one. For fluid strength at an 18F (10C) buffer, endurance times Of those respondents indicating they already use a local wing are in the order of mid-20 to mid-30 minutes, regardless of frost removal methodology: fluid quantity; and Most (75%) use Type I fluid mixed to a 10C buffer; the For the standard mix, endurance times are about 1.5 hrs. remainder (25%) use ready-to-mix Type I fluid for spot deicing; Results from this test indicate that applying the spot deicing All respondents (100%) indicated that the fluid used for procedure to cold-soaked surfaces may not produce endurance spot deicing is maintained at 60C; and times adequate to protect the aircraft surface until takeoff. The The majority (92%) apply fluid for spot deicing with a times produced by the 18F (10C) buffer fluid ranged from 24 regular deicing vehicle. to 36 minutes, considerably less than the current HOT guide- Training, lack of qualified individuals to make assessments lines for Type I fluid for natural frost at 45 minutes. This result about its usage, and resulting risks to safety were identified is of particular concern because more and more operators are as the key obstacles in employing spot deicing for frost introducing deicers equipped with on-board fluid blenders to removal. Lack of specific guidance in SAE ARP 4737 was also enable the use of Type I fluids diluted to the 18F (10C) buffer. mentioned.