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28 CHAPTER 3 Holdover Time Variance Across an Airfield Introduction multaneously using a stringent data collection protocol. The data collected were then compared to evaluate the variance in Holdover time determination systems (HOTDS), such as rates attributed to distance. D-Ice A/S Deicing Information System and NCAR Check- In general, the results indicated that the data on rate of pre- time, measure meteorological parameters at airport sites cipitation was similar; therefore the overall variance on the that then are used to calculate expected de/anti-icing fluid resulting HOT values provided by the HOTDS positioned at HOT, thus facilitating better fluid selection. HOT is di- these sites would be minimal. That said, the preliminary re- rectly dependent on precipitation intensity, so it is vital that sults indicated that the rate variance increased as the distance the intensity measured and used by the determination sys- between the rate collection sites increased and therefore that tem reflects the highest intensity to which the aircraft may additional data should be gathered at sites separated by longer be exposed during its departure taxi. The key question, distances. A full statistical analysis of the data collected as part then, is whether a precipitation sensor at a single location of this task was completed in August 2008. The results indi- at an airport can provide data with sufficient reliability for cated that a single HOTDS positioned at a central location at this application. an airport with small surface area would likely be sufficient to This part of the report presents the results, findings, and con- provide accurate information for the entire airport site. How- clusions of an experiment to determine if a single location pre- ever, at airports with large surface area, such as Denver Inter- cipitation sensor can reliably report precipitation conditions for national Airport, the distances from a central location at the the entire airport. Precipitation intensity was measured at sev- airport to a departure runway may exceed 16,000 ft. No data eral locations at an airport simultaneously. Tests were conducted for similar distances were collected in 200708, and it was over two winter seasons, 200708 and 200809. therefore decided to conduct additional testing to verify whether airports with large surface areas would require addi- Preliminary Testing (Winter 200708) tional HOTDS installations to provide reliable data. In the winter of 200708, Montreal-Trudeau airport (YUL) Additional Testing (Winter 200809) was selected as the primary location for testing. Montreal- Mirabel airport (YMX) was selected as an alternative site; how- During the winter of 200809, the work effort was ex- ever, no data collection was gathered at that airport during panded to collect data at three additional airports. This work the first season. satisfied two requirements: (1) the collection of data from sites Testing was performed during 11 natural precipitation with larger separation distances, approximately 25,000 to events at YUL, and approximately 140 comparative data points 29,000 ft and (2) the collection of data to examine between- were collected during this period. For each event, data collec- site differences in precipitation rates as a result of lake-effect tion teams were separated by distances ranging from 4,200 to snowfall. 13,300 ft at the airport. Data collected by each team included The examination of variance at distances ranging from ap- precipitation rate and other relevant meteorological param- proximately 25,000 to 29,000 ft required data collection at eters affecting fluid HOT. The procedure consisted of collect- Mirabel Airport (YMX) and at Denver International Airport ing the precipitation rate data (as well as the other relevant (DEN). The investigation of between-site differences in pre- data) over a 10-minute period at two sites at the airport si- cipitation rates was conducted at Syracuse Hancock Interna-