When an aircraft experiences winter precipitation conditions on the ground, it is required to undergo a de/anti-icing procedure involving glycol-based fluids before takeoff. In order to determine the fluid holdover time (the length of time the fluid will provide protection to the aircraft), the pilot needs to know the time deicing began, the precipitation intensity and the ambient air temperature. This information is then used with a look up chart that will tell them how long they can expect their fluid to provide protection from snow or ice building up on the aircraft surfaces under the given conditions.

NCAR Checktime display
Figure 1: (click on image to enlarge) The Checktime® display itself provides the current, up to the minute, observations from the Liquid Water Equivalent (LWE) system . It also graphically displays the rates over the past hour so a user can monitor the conditions and know if precipitation trends are increasing or decreasing. The Checktime® information is shown at the top left of the display with the fluid type and concentration, the Checktime® and the difference in time between the current time and the Checktime®. The Checktime® value is also shown on the graph as a vertical black line with sections to the right of the black line color-coded to indicate the percentage of fluid capacity left. As the snowfall intensities increase, the vertical black line moves closer to the right side of the chart, indicating shorter holdover times and vice versa. The current time on the graph is shown on the far right side with time going backwards as you move left across the graph.

There are, however, several problems with this procedure. First, the pilot’s look up chart relies on visibility to determine the intensity of the falling snow. Rasmussen et al, 1999, showed how visibility can lead to incorrect estimates of snowfall intensity and demonstrated the need for intensities to be based on Liquid Water Equivalent (LWE) measurements of snowfall rate. Secondly, this method assumes that the snowfall rate and ambient air temperature will remain constant until takeoff. In reality, snowfall rates and air temperatures are highly variable, especially at airports prone to snow squalls and lake effect snow events. Lastly, the task of looking up a holdover time means pilots have an additional task to add to their pre-flight checklists, which can further add to weather-induced departure delays.

To address this issue, an automated holdover time determination algorithm was developed by NCAR known as the Checktime® algorithm. Checktime® addresses the above issues since it’s based on a system that uses a measured LWE precipitation rate instead of visibility to determine snowfall intensity. This LWE system was developed by NCAR, and updates every minute using the real-time data provided by sensors from a locally deployed LWE instrumentation site. Measurements taken by the LWE system include temperature, pressure, humidity, wind speed and direction, and precipitation type and rate. In addition to using the rate, wind speed, precipitation type and air temperature measurements from the LWE system, Checktime® also incorporates the regression algorithms published by APS Aviation (APS) for each type of de/anti-icing fluid to give accurate estimates of holdover times for a given fluid.

Unlike holdover times, which are set in the future, Checktime® provides a wall clock time in the past (typically referred to simply as the Checktime®), which estimates the length of time a given fluid would fail by incorporating together current and past weather conditions. The Checktime® algorithm begins with the current time and integrates the LWE rate backwards in time, minute by minute, until it determines sufficient precipitation has fallen for the fluid to exceed its protection capability. Precipitation rate, wind speed, temperature and precipitation type are all inputs into the Checktime® algorithm. Fluid type and concentration can be selected by the end-user and, using the regression equations relating holdover times to precipitation rate and ambient temperature developed by APS, Checktime® then produces a time in the past. A pilot only needs to know the time their plane was de/anti-iced and as long as that time remains more recent than the Checktime®, they know their fluid is still providing protection. This gives Checktime® the unique advantage that it is aircraft independent and the only information the pilot needs to know is the time their aircraft was de/anti-iced. Additionally, because Checktime® is incorporating real-time snowfall rates, it provides a more accurate estimate of fluid failure because it does not assume a constant snowfall rate or ambient air temperature. Rasmussen et al, 2009 showed that in some cases, assuming a constant rate and temperature, holdover times can be almost twice as long as Checktime® leading to conditions that may be unsafe for aircraft departures. To further ensure a margin of safety, Checktime® was developed to incorporate a small conservative bias in the reported values such that when compared to actual fluid failures, Checktime® should give shorter holdover times.

A common question asked by pilots is “If Checktime® is based in the past, and holdover times are based in the future, how are these two related?” The following scenario can answer this question. A pilot is told s/he was anti-iced using Kilfrost ABC-S Plus fluid at 08:45, the snowfall rate is moderate and the air temperature is -8.0°C. The lookup chart tells the pilot s/he has a holdover time of 35 minutes. Using these same numbers and fluid type, Checktime® gives a time of 08:10 (35 minutes into the past). If the snowfall rates and the ambient air temperature remain constant for the next 35 minutes, the pilot will hit their holdover time at the same time Checktime® gives a value of 08:45. Thus, holdover time (35 minutes) and the difference of 35 minutes between the Checktime® (now 8:45 since we’ve gone 35 minutes ahead in time) and the current time (now 9:20) would be the same.

To demonstrate the advantage of a Checktime®-based system, the same scenario above can be used, however, instead of snowfall rate and temperature remaining constant, the snowfall rate now increases after fluid application and the temperature slowly decreases. Using the lookup chart, the pilot is still given a holdover time of 35 minutes at the time of appliation, but Checktime® incorporates the increasing snowfall rate and decreasing temperature and comes up with a Checktime® of 08:25, now only 20 minutes in the past. The pilot will now get to the Checktime® time quicker than the holdover time from their chart since the fluid will now have failed quicker than the holdover time indicates. Checktime® thus alerts the pilot to these changing conditions and automatically corrects the estimated holdover time to account for these changes.

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Please direct questions/comments about this page to:

Roy Rasmussen

Director, Hydrometeorology Applications Program