Winter Operations Lessons From Norway
Winter operations are “normal daily life” at Norwegian airports. Their winter season stretches approximately from November to April. On average, the runways at Tromso, a city above the Arctic Circle, and at Oslo are contaminated for 30% of the winter.
The Norwegian aerodrome operator Avinor performed a five-year R&D project, called Intelligent Runway Information System (IRIS)*, during which it collected landing data and coupled them with reported runway condition information and weather data.
Data were collected during the winters of 2008-09 to 2012-13 at different Norwegian airports and the number involved increased from two in 2008-09 to 15 in 2012-13. Data from quick-access recorders (QARs) were obtained from all landings of Boeing 737-600, -700, -800 and -900 models operated by Scandinavian Airlines (SAS) and Norwegian Air Shuttle. Weather stations placed near the runway (within 500 m or 1,640 ft.) measured air temperature, runway temperature, dew-point temperature and wind at 1-min. intervals.
During the five winter seasons the project examined 117,849 landings, of which 5,097 involved runway surface conditions that required the aircraft to utilize its maximum attainable tire-pavement friction. From these data the researchers were able to estimate the aircraft’s braking coefficient on a variety of runway conditions including dry snow, wet snow and slush. An evaluation of the data found that 21% of the landings on wet snow produced braking conditions that were less than poor or poor. This percentage is significantly higher than on dry snow (7%) or slush (11%). This can be caused due to higher precipitation intensity during wet snowfall, or possibly because wet snow, in contrast to slush, is a compressible material that gets compacted and fills the underlying pavement texture.
The IRIS runway model evaluates seven different factors that influence the quality of surface conditions and predicts the braking action on a common scale from 1 to 5, ranging from “poor” to “good.” The first factor it evaluates is the type of contaminant, which often in the Norwegian winter can be a combination of wet snow, dry snow, slush, ice, compacted snow and/or frozen ruts.
The IRIS runway model takes into account that the contamination may vary from one end to the other, as well as the effect of runway temperature on the type of contamination. When a runway is bare and dry, there is little difference in an aircraft’s tire friction if the temperature is 0C (32F) or -10C (14F). In contrast, a runway that is covered with ice has a higher chance of being slippery around 0C (think Zamboni machine) compared with -10C. The IRIS model allows for the adjustment of the runway’s braking condition based on the type of contaminant on the runway. For instance, when ice, compact snow or frozen ruts are present on the runway, the expected braking action is downgraded when the runway temperature is warmer than -2C (28F) because of the likelihood that the melting has started or is about to occur.
The effect of contamination depth on friction is not obvious. Compacted snow is a solid contaminant whose depth is irrelevant. Loose snow that enters the contact area gets compacted, and friction is predominantly created at the snow-rubber interface, leaving a clear track of snow behind. This suggests that once the tire has lost contact with the pavement, the friction does not significantly decrease further with increasing snow depth. However, friction is created between the rubber and the pavement texture after slush has been squeezed out of the contact area.
The IRIS model also takes humidity into account. At temperatures well below 0C, runways covered with ice or compacted snow tend to be more slippery when the humidity of the air above the runway is high.
Anti-icing or deicing chemicals are often used to ensure a wet runway does not freeze, to prevent bonding of snow/ice to the pavement or to remove thin ice layers. Naturally, wet snow and slush mostly occur around 0C. However, wet snow and slush can also be present at lower temperatures due to the usage of anti-icing/deicing chemicals on runways. Generally, when chemicals are applied to a wet surface, the frictional conditions are not improved, but they prevent deterioration of the conditions. When chemicals are applied on initially dry (compact) snow or ice, a melting process starts. In such cases, the frictional conditions often get worse.
Many airports apply sand to improve pavement friction. In Norway, it is applied either dry or pre-wetted with hot water. The warm pre-wetted sand freezes to the runway, creating a sandpaper-like finish, known as “frozen sand.” It is most effective when spread on a solid contamination layer. Doing so ensures a strong bond between the sand and the ice. Runway maintenance personnel say sanding on a bare or wet runway actually reduces the friction. How? Sand particles reduce the ability of the tire’s rubber to grasp the asperities of the pavement. It is difficult to get accurate friction readings when sand is used, particularly loose sand.
At the end of the IRIS project data collection, aircraft performance engineers compared the aircraft braking performance with the predictions of the IRIS model, runway inspectors and friction measurements. The IRIS model performed better than the other predictors. In 86% of the landings, the IRIS model predicted closely to the airplane’s actual landing performance. Runway inspectors predicted the conditions in 77% of the landings. The ground friction measurement devices (GFMDs) performed less well with 61%. The IRIS model is now implemented at 15 airports in Norway.
*Reference: “Airplane Braking Friction on Dry Snow, Wet Snow or Slush Contaminated Runways.” Alex Klein-Paste, Norwegian University of Science and Technology, Department of Civil and Transport Engineering, Winter Maintenance Research Group, Høgskoleringen 7A, N-7491 Trondheim, Norway.