With the days getting longer and milder temperatures just around the corner, we're rapidly approaching the best time of year for ski flying. Probably the biggest challenge of ski flying is the way our landing areas are constantly taking on different personalities. We have to accommodate the particular demands of things like deep snow, smooth ice, drifting snow, flat light and overflow, to name just a few. I've always found overflow an especially interesting topic since it puts us in the position of flying on water that is in its liquid and solid form at the same time.
Consider landing on what looks like a smooth, snow-covered frozen lake, but sandwiched between the ice sheet and the overlying snow layer is a concealed layer of wet slush. During the landing run, as the airplane slows down and settles into the snow, if the skis encounter this slush, you may feel some added drag, and in the worst case of this scenario, the skis can get bogged down to the point of getting stuck. Of course getting stuck is a nuisance under any circumstances, but it can be especially troublesome in the Alaska wilderness in the dead of winter.
The layer of slush is called overflow, and it occurs on frozen lakes when water from below the ice seeps up through cracks and rises above the surface of the ice layer. The most common cause of overflow is the weight of a snow load pressing down on the ice, which forces water up through the cracks to mix with the overlying snow to form slush. If the entire snow layer is turned to slush, the overflow condition will be short-lived because it will re-freeze quickly and just make the ice sheet thicker (assuming cold temperatures). A more noteworthy scenario occurs when the water only penetrates part of the snow layer, leaving a slush layer covered by a blanket of snow that insulates it and prevents it from re-freezing. Snow has exceptional insulating qualities, and can cause overflow conditions to persist indefinitely even when the air temperature is well below zero. For that reason, a wet, sloppy layer can exist even when the rest of the landscape is frozen solid.
Overflow does not always cause problems for ski planes. A thin layer can be easy enough to power through, or if the overlying snow layer is deep enough, the skis might not get down to the slush and it won't affect you at all. If the skis do get into the overflow, that removes the insulating snow layer in the process and this can cause other problems. If it is a cold day, there will be nothing to prevent the slushy mess of uncovered overflow from quickly freezing solid, and in the worst case you can find your skis frozen onto the lake.
Destroying the snow's insulating properties is also the key to making a surface covered with overflow suitable for ski planes. Packing down a landing strip with a snowmachine, or even by walking it with snowshoes, compresses the snow layer so that it loses its insulating properties and the packed-down area freezes into a firm surface that can be used as a runway.
Overflow can be unpredictable in its formation. While a fresh snowfall pressing down on the ice is its most common cause, it can also form for other reasons or for no discernable reason, making its existence hard to predict. Local folk knowledge suggests that changes in temperature or atmospheric pressure can be factors in its formation. In my experience, you can't count on it being there when you think it should be, nor can you count on it not being there when there is no discernable reason for it. The only thing you can really count on is that it is always a possibility, and that regardless of how cold it is, every snow-covered frozen lake may or may not have an overflow layer.
Since the blanket of snow that conceals an overflow layer can make a completely unsuitable landing area indistinguishable from a perfectly good one, you should test an unknown spot for overflow before landing on it. You can do this by making a touch-and-go landing, dragging the skis along the lake, and then fly back around to look at the tracks. If there is a significant overflow layer just below the surface, the tracks will fill in with water and turn dark, in which case you should try another part of the lake, or just go someplace else. When testing for overflow this way, if the snow is deep, you can make a second pass, putting more weight on the skis to make sure the tracks are deep enough to expose any overflow.
Overflow can be patchy, so if you find it on one part of the lake, it is possible that conditions will be better on another part of the lake. By the same token, if you land in a good spot, you can't take it for granted that there isn't overflow elsewhere on the lake, which is a good reason to stay in your tracks. I once had a vivid demonstration of that: I had laid down some tracks and landed in them, finding the conditions just perfect, but then when I taxied off my tracks to make room for someone else to land on them, the airplane started to splash through a slushy overflow layer that was just off to the side. Once I managed to get back to the tracks, all was well again, but it impressed me how much conditions can vary at different parts of the same lake. I've also noticed that you're less likely to find overflow if you stay close to the shore.
The aftermath of an encounter of overflow leaves frozen telltale tracks that serve as a hazard marker until the next snowfall wipes the slate clean. In the course of your day-to-day flying, you can take note of where other people in airplanes or snow machines have uncovered overflow. If you regularly see evidence of overflow in the same places, that can tip you off to some of the problem spots.
A little investigation can usually uncover an overflow layer before you land, but there is always the Murphy factor so be careful out there. With the mild spring days just around the corner, best wishes for some good ski flying this year.
Burke Mees flies commercially in Alaska. This article was previously published by The Alaska Airmen's Association.