DriftStories – 05: One hot strip of ice

How thick does sea ice have to be for aeroplanes to safely land on it – and how do you patch cracks in the landing strip? These and other questions confronted AWI sea-ice expert Christian Haas and his team when they began constructing a landing strip on the MOSAiC floe, in total darkness. They successfully completed the task – and were the first German polar researchers to ever do so. But the project also showed them that building a runway on the ice is a science of its own.

Throughout his career, AWI geophysicist and sea-ice expert Prof Christian Haas has landed on Arctic sea ice plenty of times: to the north of Ellesmere Island (Canada) on board the AWI research aero-plane Polar 5 and with Twin Otter planes on skis; or as a passenger on a Russian Antonov cargo plane resupplying an ice camp near the North Pole. But, until the launch of the MOSAiC expedition, the 53-year-old could never have imagined that these personal travel experiences would one day help make him the first German polar researcher to lead the construction of a landing strip on the Arctic Ocean. But extraordinary expeditions call for extraordinary solutions! And that’s how, at the start of MOSAiC’s second leg, expedition leader Christian Haas, Polarstern’s captain Stefan Schwarze, and logistics specialist Hannes Laubach found themselves faced with the challenge of creating a 400-metre-long and 25-metre-wide landing strip on the ice, in perpetual darkness.

Safety considerations are what made it necessary to create a landing strip at short notice. At the time, Polarstern was over 1,000 kilometres from civilisation. If there had been a medical emergen-cy, evacuation by air would have been the only option. Canadian Twin Otter planes could have been used to fly out one or two patients.

However, the strip’s planned long-term use was for logistics purposes. During the aerial surveys planned for the spring, the AWI’s research aeroplanes could have landed near the Polarstern to refuel. This would have allowed them to penetrate far deeper into the Central Arctic than without refuelling. In addition, a crew transfer by plane was planned for April 2020: a Russian Antonov would have brought the new researchers to the Polarstern and flown the winter crew back to land. To make this feasible, the strip’s length was to be extended to 1,000 meters. As we all know, the corona pandemic put an end to all these plans, though Christian Haas and his colleagues had no way of knowing it back in December 2019. They got down to work, and began by addressing a number of key questions.

Question 1: How thick does the ice have to be for a plane to land on it?

The team found the answer in the professional literature on the bending stiffness and tensile strength of sea ice. “Using the equations and formulas we found, we determined that the ice had to be at least 80 centimetres thick for a 2.6-metric-ton Twin Otter to safely land on it – a thickness that many parts of the MOSAiC floe had already reached by late December,” explains Christian Haas.

But the much bigger problem for him and his colleagues was the fact that, by its nature, sea ice isn’t smooth, but coarse. “The surface was full of pressure ridges and snowdrifts, so we couldn’t just look for a nice, smooth spot for planes to land on,” says the expert. Accordingly, the team would have to use Polarstern’s on-board snowcat to clear away these obstacles, plus plenty of snow. But back then no-one knew what size of ridges the snowcat could handle; they’d just have to use trial and error.

But first they had to find a suitable area to clear. To do so, the team relied on ice-thickness and sur-face data that the ship’s on-board helicopters had gathered with a laser scanning system during surveying flights in the Polar Night. To be allowed to fly at night, the pilots had completed special training prior to the expedition, plus the helicopters had a host of new technical systems on board. A hefty investment, but it immediately paid off. Using the high-relief ice charts produced, the re-searchers identified two suitable areas: one a stone’s throw from the ship, and another roughly two kilometres away.

“We opted for the area closer to the ship, even though it consisted of very young ice that had only formed a few weeks earlier. Nevertheless, it was sufficiently thick throughout, and offered us a number of advantages,” the expedition leader explains. For one, having the landing strip closer would allow the team to avoid countless long and difficult treks across the ice; for another, all of the clearing and smoothing work required to make the strip could be done with the support of Po-larstern’s searchlights. Choosing the other site would have meant working in complete darkness, making every movement on the ice that much riskier. 

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Question 2: Just how powerful is the snowcat?

The next question concerned the 16-ton snowcat. Would the sea ice be able to support it? “As long as the vehicle is in motion, the ice below it only gives slightly. But when it stays in the same spot for several hours or days, then it gets problematic,” says Christian Haas. In this case, the snowcat would gradually sink as the ice around it gave way. Eventually, cracks would form in the ice, until the massive vehicle fell through. Accordingly, it was parked at a different location after every shift.

The snowcat was put to the test before it even reached the area where the landing strip was to be built: on the way from the ship to its destination, massive pressure ridges blocked its way, meaning the snowcat would have to clear a path to proceed. How easy this was came as a surprise to the expedition leader and logistics experts alike. As Christian Haas recalls: “With sheer brute force, the snowcat levelled a one-metre-tall pressure ridge effortlessly.” This was chiefly due to the ice’s age: “The pressure ridges consisted of young ice. In other words, they were basically loosely stacked piles of ice blocks, which fortunately for us hadn’t yet become frozen together.” If the ridges had already survived a summer, the blocks would have already condensed into a compact mass that wouldn’t have been so yielding.

Question 3: How can we patch cracks in the landing strip?

While the snowcat was clearing the path to the construction site, the researchers were busy stak-ing out a straight line for the future landing strip with flags. Not an easy task in the dark. With the aid of positioning lights, which they used like small lighthouses for orientation, the team was able to ensure that the ‘straight and narrow’ would ultimately live up to its name. After that, the actual task of clearing and levelling the strip with the snowcat only took a day – mission accomplished, right?

“In February we noticed the first cracks in the landing strip. This immediately raised the question of how we could patch them, how close the snowcat could safely get to them without breaking through the ice, and how wide they could be for the snowcat to even be able to repair them,” Christian Haas recalls.

Back then, the ice was still more than a metre thick, easily thick enough for the snowcat to drive to the ice edge. According to the available literature, the cat shouldn’t be used to repair cracks with a width exceeding one-third of the vehicle’s length – so no more than one to three metres wide. And the cracks could be filled with snow and blocks of ice. “The advantage of snow is that it’s the same temperature as the air, making it far colder than the ice. If you pack snow into a crack and then seal it, the snow mass almost immediately freezes into a cement-like mortar, and the crack is patched. So it was really quite simple; after all, we had snow to spare,” the expedition leader re-ports.

At the same time, during helicopter flights over the MOSAiC floe, the researchers had made an interesting discovery: in thermal imagery of the floe, the snow-free landing strip glowed a brilliant orange, indicating it was a ‘hot strip’. But why? According to Christian Haas: “The rest of the ice was so well insulated by snow cover that hardly any heat was released from the ocean into the atmos-phere. But on the landing strip, that insulating layer had been removed.” For the same reason, the ice below the landing strip (snow-free – no insulating effect, heat loss from the ocean, more freez-ing) grows significantly faster than the surrounding ice (snow-covered – insulating effect produced by the snow, less heat loss, less freezing).

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Question 4: What do we do with all the snow?

Driven by their own success, in February 2020 the team decided to expand the strip, originally meant to accommodate Twin Otter aircraft, to suitable dimensions for an Antonov (1,000 metres long, 60 metres wide). But where could they put all the snow? One-metre-tall walls of snow had already piled up on the sides of the landing strip – and, as if by magic, were steadily growing thick-er. “On sea ice, anything that projects above the surface is an obstacle, and the drifting snow ac-cumulates behind it. That’s why the snow shouldn’t really be piled up, but instead scattered over a broad area,” Christian Haas explains. Plus there’s the weight of the snow masses to consider: “If you plough the snow into a pile, all the weight is concentrated at one point, pushing down on the sea ice. And that can lead the ice to crack or break, or the area can flood if the weight of the snow pushes the ice below water level and seawater begins rising through the porous spaces and cracks.” In contrast, wherever snow is removed, the weight of the snow layer is lost. As a result, the snow-free ice rises somewhat, which can produce further cracks and reduced stability.

As you can see, constructing a landing strip on the Arctic sea ice is a science in its own right, and far more complex than one would imagine. But in retrospect, for Christian Haas it was one of the most exciting sea-ice projects at the MOSAiC Ice Camp, even though he wasn’t there in person when the two Twin Otter planes landed there in April 2020. By that time, the team for Leg 3 had commenced operations in the Central Arctic, and Christian Haas was – like the majority of AWI staff – doing home office to keep safe from corona.

Prof Christian Haas is head of the AWI’s Sea Ice Physics section. As expedition leader for the sec-ond leg of MOSAiC he was, together with all of the researchers involved, responsible for the success of the research efforts. He was on board RV Polarstern from mid-December to early March.

Text: Sina Löschke
Translation: Matthew Fentem (gonative.de)