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53 The availability of accurate information on rate of precip- 20 to 30% are of potential operational interest, and between-site itation, along with true indications of temperature and pre- differences greater than 30% are operationally significant. cipitation type, is the key to the generation of reliable HOT values. The current use of METAR indications and subjective Base Case Examination of Between-Site Differences assessments of weather conditions does not take full advan- tage of the accuracy and consistency provided by the scien- For all fluids examined, there was no statistical difference in tific approach used to generate HOT Guidelines. HOTs for the two sites in about 40% of the data sets collected. In contrast, the HOTDS measures actual precipitation For all fluids examined, there was no statistical difference (LWE). These data are used along with temperature, precip- in HOT values in approximately 40% of the data sets col- itation type, and the regression curves and coefficients gener- lected. Between-site differences in HOT values varied by fluid ated during the fluid endurance testing, to generate HOT val- type and fluid strength: ues. Subjectivity is removed and the complete process is scientifically based. In addition, HOT values can be updated · For thickened fluids at full strength and in a 75/25 mix, every 10 minutes. between-site HOT differences greater than 30% were seen Implementation of a single HOTDS system at any airport, 5 to 7% of the time and differences greater than 50% were regardless of size, may potentially produce HOT values seen 2 to 3% of the time. superior to those now generated through the use of METAR · For Type I fluid, between-site HOT differences greater indications. than 30% were seen 11% of the time and differences greater than 50% were seen 5% of the time. · For the 50/50 fluid strength case, between-site HOT differ- Conclusions and Recommendations ences were larger than for the other fluids, with HOT dif- This task of ACRP Project 10-01 was conducted to deter- ferences greater than 20% about 29% of the time, greater mine if a single location precipitation sensor can reliably re- than 30% about 19% of the time, and greater than 50% port precipitation intensity for an entire airport. The conclu- about 14% of the time. sions and recommendations resulting from this task are presented in this section. CAR Exemption Case Examination of Between-Site Differences Conclusions In comparison to the base case, there was a decrease in the Test Methodology frequency of between-site differences in the range of 20 to 30% and an increase in the range of 30 to 50% when looking The approach to collecting test data was effective, and the at HOTs using the CARs exemption conditions. data provided a suitable base for comparing HOTs generated A major reason for this shift was the stepped augmentation from two separate test sites at an airport. The test methodol- of measured rates in accordance with the CARs exemption. In ogy developed and applied in the collection of data proved the case of the Octagon fluid, for example, of the 18 data set satisfactory. The repeatability of precipitation rates measured pairs falling in the 30 to less than 50% difference range, 10 ex- amongst the four samples collected at each site proved to be perienced a differential in augmentation, where the measured better than for rate collection during fluid endurance time rates of one site were slightly below 10 g/dm2/h and thus were tests. augmented by 6 g/dm2/h, while the rates of the other site were Two sets of analysis were conducted. One was based on the over 10 g/dm2/h and thus were augmented by 14 g/dm2/h. data as collected (base case) and the second was based on the precipitation rate data adjusted by the CAR exemption con- Examination of Site Separation Distance ditions. In each case, HOTs were calculated for a selection of currently active fluids at specific strengths. Sorting the base case data into three separation-distance ranges showed a distinct relationship between site distance and HOT difference. The longest separation distances Operational Significance showed a considerably higher frequency of occurrence of of Between-Site Differences large between-site differences in HOT. The extent of the between-site difference in HOT and its level The frequency of tests generating a between-site difference of impact on the operation varied greatly. Examination of the greater than 20% varied by shortest distance separation, mid- absolute size of between-site differences led to the conclusion range, and longest distance as shown in Table 37 (note that that between-site differences in holdover times on the order of lake-effect data has been removed).