The Impact of Temporally Varying Snowfall Rates on Holdover Time using the LWE and Check Time Systems

During winter weather, aircraft require deicing to remove any contamination before takeoff. Each particular de/anti-icing fluid provides a certain amount of protection time before the fluid freezes and fails to run off on takeoff. This time is called the Holdover Time. Pilots estimate this time by reference to the METAR precipitation intensity for a particular precipitation type. For snow, the snowfall intensity from a METAR is base on visibility, which has been shown to be inaccurate (Rasmussen et al. 1999). To address this issue, real-time systems, such as the Liquid Equivalent (LWE) system and Check Time systems, have been developed. The LWE system provides one minute estimates of precipitation type and rate using a combined sensor approach. Pilots can use these estimates at the time of deicing to determine their holdover time using tables that relate precipitation type, rate, and temperature to Holdover Time. These Holdover Times are typically assumed to remain nearly constant during the time the aircraft taxis to takeoff (instantaneous Holdover Time). However, analysis of precipitation rates on a one minute basis shows significant variations that could have significant impacts on Holdover Time. The Check Time system is designed to take these variations into account by calculating the expiration time of a fluid using one minute updates of precipitation. This paper will present the difference in Holdover Time estimation using instantaneous Holdover Times versus actual Holdover time based on using minute to minute precipitation values. The results will show that typical time variations in precipitation rate can significantly impact the actual holdover time as compared to an instantaneous value.