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54 Table 37. Frequency of tests generating between-site differences > 20%. Frequency (%) at Separation Distance Shortest Mid-Range Longest Fluid (4,167-5,052 ft) (7,017-13,390 ft) (27,800-28,500 ft) Type I 4 12 15 Clariant 2012 1 10 21 100/0 Octagon MaxFlo 2 12 26 100/0 Kilfrost ABC-S 1 14 28 75/25 Kilfrost ABC-S 0 38 42 50/50 Examination of Lake-Effect Snowfall tance. The extent of the differences can be worsened by lake- on HOT Differences effect snowfall. Differences in HOT generated from different sites begin to The lake-effect data, collected at a between-site distance of impact the operation when the sites are separated by mid- 8,300 ft, was compared to the other data collected at the mid- range distances (7,017 to 13,390 ft), and have a definite im- range distance. The frequency of cases where the between-site pact at long separation distances (27,800 to 28,500 ft). difference in HOT was 30% or more of the lower site value The finding of variances in precipitation rate and HOT was substantially greater for the lake-effect data. Much of the over a large airport should not be a consideration or obstacle increase showed up in the > 50% difference category. to further development of the HOTDS over the short term. Comparison of HOTDS Results to Recommendations Current Operational Practices In the short term, the finding of variances in precipitation There is considerable variance in the snow intensity de- rate and HOT over a large airport should not be a consideration rived from METAR sources and test data. or obstacle to further development of the HOTDS. This condi- The METAR report gives the pilot two alternative ways to tion should be considered only in the further development and establish a value for snow intensity. METAR reports retrieved application of the HOTDS systems for large airports where the for selected periods of testing gave conflicting intensities for taxi distance from deicing locations to the assigned departure the two alternatives, such as heavy and light snow. In some runway can be very long. Smaller airports with shorter taxi dis- cases, the corresponding intensity from collected data was tances in the order of 5,000 ft are not affected. A possible solu- moderate. tion may be to compare the accuracy in HOTs generated from The variability in snow intensity indications leads to large current processes to HOTs generated from a single HOTDS in- differences in HOT. In some cases, the METAR visibility and stallation at a large airport. If the single installation HOTDS is snow visibility charts led to no HOT availability, while the test more accurate than the current processes, then the single instal- data produced operationally valuable holdover times. The lation HOTDS may be deemed adequate. lower HOT from the two test sites generally was longer than In the longer term, a study should be conducted to com- the HOT value derived from either alternative using METAR pare the accuracy in HOT generated from a single HOTDS reports. installation at a large airport to the accuracy associated with These results suggest that a single HOTDS installation may HOT values generated from current processes using METAR be able to produce HOT values superior to those now gener- indications and pilot assessments. Two approaches may be ated through the use of METAR indications, despite the vari- considered: ance in precipitation over the airfield. 1. Install more than one HOTDS system, with the actual num- ber being dependent on each airport's layout and geography. General Conclusion This approach ultimately leads to questions as to where and In general, differences in between-site HOTs for snow can how many systems need to be installed, and subsequently be significant to the operation, and they are a function of dis- how the different indications should be interpreted: