The Influence of Snow on ILS Glide Slopes

Richard H. McFarland

Abstract:	Because the flying public depends on the reliability and accuracy of the Instrument Landing System (ILS) for safety of routine air transport, it is important to know what effects snow can have on glide slope performance. The purpose of this paper is to present a summary of knowledge and data concerning snow effects especially that which has been acquired over the past 10 years. Much of these data now serve as the documentary bases for allowing operation of image glide slopes in the United States during winter snow periods. Recently there have been several items in the technical literature suggesting that the contemporary instrument landing system (ILS) image glide slopes are vulnerable to effects of ground-plane snow cover to the extent that dangerous conditions may result. Engineering experimental data and routine practical experience do not support these claims. Quite to the contrary, the technical evidence allows confidence that the ILS glide slope is an extremely safe navigation aid as it is operated in the U.S. It is safe for continued routine use as a landing aid in all weather conditions. Supporting this is the fact that there has been no accidents or incidents due to snow effects on glide slope operation in the nearly 55 years of ILS use. Following its conservative practice, the government continues to support work aimed at investigating any anomalous glide slope performance that can be found. Several particular points are made in the paper relating to the representativeness of monitoring of image glide slopes and to the importance of the availability of the glide slope during periods of snow fall and ground plane snow cover. The principal conclusion reached is that during most periods of snow there is no significant effect of its presence, however, during unusual conditions of ground plane snow covers, path characteristics can exceed published tolerances but the public safety is protected by procedural means, i.e., through FAA maintenance policies that restrict the glide slope from use when the potentially adverse snow conditions are present.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	95 - 101
Cite this article:	McFarland, Richard H., "The Influence of Snow on ILS Glide Slopes," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 95-101.
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