Severe winter weather performance of Runway Visual Range (RVR) systems at five Alaskan airports

The performance of the new generation Runway Visual Range (RVR) sensor has been tested operationally at five Alaskan airports. The purpose of the tests was to determine whether clogging of either the forward scattermeter visibility sensor (VS) or the ambient light sensor (ALS) occurs during severe snow events. Three RVR sensors were located at Fairbanks, and one each at Cold Bay, Bethel, King Salmon and Nome. Data from these sensors were gathered during autumn of 1997 to the summer of 1998. The performance of these sensors were continuously monitored and recorded by microcomputers for post analysis. The latter included RVR extinction coefficients and parameters associated with window transmission loss as well as use of information derived from METAR data taken from automated weather observing systems and observers at the sites. No instances of complete loss of signal due to clogging of the ALS or VS windows were detected at any site during the period of observation. However, several instances of elevated losses in the ALS and VS windows were recorded. These occurred during conditions of light to moderate snowfall accompanied by strong winds exceeding around 20 knots. Such events usually occur rapidly and remain active until cessation of the weather event; after this time, the system recovers to its normal operating condition. There were also several instances where an apparent loss of signal occurred due to a slow accumulation or buildup of snow on the ALS or VS windows. This condition can result in an unrecoverable, long-time failure of the system due to an excessive buildup of ice and snow on the ALS or VS windows. It was also discovered that, on occasion, blowing precipitation, including snowfall, helped to clean the surface of the windows, leading to decreased window transmission loss. Overall, out of more than 700 sensor snowfall events at the five sites, only 24 cases were found to result in a significant change in the window transmission loss.