What is Supercooled Liquid Water? Supercooled Liquid Water (SLW) refers to water droplets in a cloud that can stay in a liquid state at below freezing temperatures. If an airplane flies into these SLW droplets, icing on the wings and airframe can occur.
SLW droplets larger than 30 microns create the biggest hazard for aircraft because they don’t freeze immediately, allowing them to “bleed” back from the leading contact edges and turn to ice. This is especially problematic at low speeds (less than 300 mph) during take-off and landing. An accumulation of ice changes the air flow, reduces lift, and increases drag and weight, making flying difficult and dangerous. The National Transportation Safety Board (NTSB) reports 78 fatalities from icing incidents between 2010 and 2014.
Can a pilot avoid SLW? In-flight icing on manned and unmanned aircraft is a serious problem. Although, de-icing before take-off is a standard procedure when icing conditions are forecast, once an aircraft is in the air there is no foolproof method to directly detect SLW and avoid icing en route. Advance notice of dangerous weather along the flight path and awareness of the visible local environment can help manned aircraft make changes in flight to avoid unsafe conditions, but rapidly changing conditions can still lead to a disaster. Unmanned aircraft are likely to simply drop from the sky without any warning, because the operator is unaware of an icing situation.
What is the solution? Current onboard systems don’t provide sufficient information concerning the size and location of SLW droplets. To provide a solution to the vulnerability of aircraft due to icing, an in-flight sensor is needed to identify cloud content and measure droplet size, from a distance, so the pilot can alter course to avoid potential ice hazards. More accurate real-time measurements of cloud characteristics and identification of icing potential using remote sensing techniques would provide advance warning of problems ahead.
How can SLW be detected and measured? The suitability of the Mini Micro Pulse LiDAR (MiniMPL) instrument is being evaluated as part of an ice cloud avoidance strategy. Flight data show that the key factors influencing icing on aircraft (particle concentration and particle size) are measurable with the MiniMPL at distances up to 15 km. The instrument offers the advantage of direct range observations that do not require ancillary data sets to calculate the cloud’s liquid water content and shows high potential for directly measuring hydrometeor particle size distributions. In addition, the MiniMPL is capable of near-real time data processing, which is crucial for making quick decisions. The initial results indicate the MiniMPL can detect and measure SLW from onboard an aircraft and provide data to help pilots avoid weather-related hazards.
For more information about Supercooled Liquid Water and how it affects aircraft, please refer to the article in Meteorological Technology International, September 2018 issue, page 114.