Icing Research Making Skies Safer

Aircraft icing has caused two major airplane disasters in North America in the past five years. Although most planes are designed to withstand routine icing, hazardous conditions occur when they encounter large supercooled drops, which freeze on contact and can jam control mechanisms, decrease lift and cause stalling.

Environment Canada scientists have conducted dozens of flights into winter storms on the East Coast and over the Great Lakes to learn more about what causes icing, and to find better ways of predicting hazardous icing conditions. Considered among the top groups in the world in their field, they were one of the first to extensively document from the air the microphysical characteristics of freezing precipitation. This information is being used to improve forecasting and should lead to better engineering standards for aircraft certification.

Research flights have taken place over St. John's, Newfoundland, and the Great—Lakes icing-prone regions of the country that receive an average of 150 and 75 hours of freezing precipitation per year, respectively. Special instruments on board the aircraft collect data on the microphysical parameters of the clouds, including pressure, temperature, horizontal winds, altitude, liquid water and ice content, and droplet size and concentration. Scientists use these data to develop statistical probabilities for worst-case icing scenarios in different conditions, and to assess the accuracy of numerical forecast models.

It is only recently that these models have been able to predict such parameters as liquid water and ice content, and most are still unable to forecast droplet size. These parameters are important in determining icing severity, because the higher the liquid water content of a cloud and the larger the size of its droplets, the more likely these droplets are to collide with and, therefore, freeze to the hard surface of a plane. Small cloud droplets—less than 30 microns ( 30 millionths of a metre) in diameter—are less likely to collide with an aircraft surface because they are lighter and tend to follow the air flow around the plane. Larger drops, such as freezing drizzle (100-500 microns) or freezing rain (500-3 000 microns), are more likely to collide because of their greater momentum.

In addition to characterizing icing environments, scientists are studying the way freezing precipitation forms. The classical method, which can be fairly accurately predicted, is for ice crystals or snowflakes to fall through a warm layer of air, melt, and then supercool to sub-freezing temperatures as they pass through a colder layer near the ground.

Environment Canada scientists focused on a second formation method while conducting research flights near St. John's, where 75 per cent of freezing precipitation forms as a result of supercooled cloud droplets growing from condensation and collision with other droplets. This non-classical or collision-coalescence method is of particular concern because it produces large precipitation that collides efficiently, and cannot yet be predicted by numerical models. However, the data from these research flights are being used to develop a new numerical forecast model that will be able to predict non-classical cases explicitly.

Environment Canada, in partnership with the National Research Council of Canada, Transport Canada, the National Aeronautics and Space Administration, the U.S. Federal Aviation Administration and others, is currently involved in a study to assess the usefulness of remote-sensing instruments, such as radars, satellites and microwave radiometers, in determining aircraft icing conditions aloft. This real-time information would be invaluable in informing pilots of conditions at takeoff and landing, when most icing-related accidents occur, and would also improve the accuracy of long-term forecasts.

As a follow-up to this work, the Department is proposing the development of a prototype aircraft-icing detection/warning system that would integrate data from a variety of sources to generate detailed warnings for hazardous icing conditions both on the ground and in the air. Such a system would be a major step forward in making air travel safer in Canada's severe winter climate.

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