"Passengers, please find your seats and fasten your seat belts as we make our final approach to the runway. We thank you for flying..."

It's a tedious routine for passengers, but one of the most stressful parts of a flight for the pilot, as he or she navigates the aircraft out of the clouds and toward the beckoning runway ahead.

This aircraft's engineering will soon be put to the test, as tires, brakes, air surfaces and suspension work together to bring the plane to a controlled deceleration before taxiing to a terminal. However, inclement weather can and does prevent this from becoming a reality quite often. More than 132,000 U.S. flights per year are diverted to a different destination, according to a 2018 study. And per the FAA, about 75% of all flight delays of 15 minutes or longer are due to weather patterns.

Thunderstorms are the most common weather type to cause a delay. Thankfully, unless severe, interruptions from thunderstorms are fleeting. Winter weather is the second most common, and unfortunately, feet of snow and inches of ice can lock a city down for days.

To keep the planes arriving and departing, airports keep dozens of heavy-equipment plows, brooms, scrapers, snow blowers and de-icing machines to keep tarmacs as clear as possible. This requires drivers, mechanics, garages and diesel. It is not a cheap operation. And the mechanical and chemical approach slowly degrades the quality of the surface, hastening repair or replacement.

In a common refrain that both engineers and diverted passengers must frequently ask themselves — why don't airports use heated runways?

The theoretical means to defrosting runways

Heated ground surfaces have been around for centuries. Romans used underfloor hypocausts — heating systems that placed a furnace in stone passages below the floor. The ancient Ondol system in Korea channeled fireplace heat under bricks in the floors. Innovative architectural features like these suspended floors and brick channels have kept Mediterranean and Asian homes warm since before the age of Christianity began.

Of course, these same age-old heating technologies would never be scalable to the point that they can be applied under a 1,000-meter-long runway. However, if considering conventional engineering technologies, resistive wiring and heating mats have a checkered history of success.

Several snowy cities and sites around the globe have integrated snowmelt heating systems into their infrastructure, with varying results. The M4 highway in London features an elevated segment with heated roadways. Built in the 1960s, the system worked for several decades, and although it still exists, seems to no longer function. Boston's Central Artery featured heated on and off-ramps, until its demolition in the 1990s for the city's "Big Dig" project. Additionally, many steel, elevated bridges feature some type of innate de-icing technology.

These systems remain rather limited in their implementation due to their complexity, inefficiency and maintenance. Generally speaking, there have been two technologies. Hydronic systems, which pump warm antifreeze fluids throughout the length of its plumbing, or electric systems, which send voltage through a high resistance wire, just like heat trace products for roofs and pipes.

Both systems have limitations that have prevented more widespread popularity. Hydronic systems require frequent pump and solution maintenance. Additionally, they are more likely to create thermal stress in the road surface, as they tend to heat up unevenly. Electric systems are relatively easy to install, although they require a lot of electricity to heat surface areas above freezing. Not only are materials like asphalt poor conductors of heat, but at least 50% of heat is deflected down, away from the surface.

Finally, there is the cost. A recent test installation in France of a 400 ft long road segment, heated by resistive wire embedded in the road surface, allegedly cost $300,000 more than a regular road. Ultimately, these heated road technologies are stuck in the theoretical domain because they’re not financially viable. Roads and highways are hundreds of miles long and road maintenance is a big undertaking.

This is where the scale of airports can help this technology. Airline schedules are majorly disrupted, and revenues too, if flights are diverted or delayed. In an industry where safety is paramount, airports should be interested in a system that increases arrival and departure safety and reduces workers in the field under dire conditions. And while large airports may have several dozen miles of runways and taxi ways, it is much less than a civic roadways.

Exploration continues

Case in point, several airports had investigated or run small R&D tests on the technology.

After a series of harsh snowstorms snarled travel in the late 2000s and early 2010s, Heathrow Airport in London looked at interseasonal heat transfer technologies. This technology would harvest the summer heat from the solar water system embedded in the runway. That water would be stored deep underground until needed, when it would recirculate to heat the runway above freezing.

In 2013, Binghamton Greater Airport in New York opened a 3,200 ft² airplane parking ramp that utilized geothermal energy. Meanwhile, Des Moines International Airport in Iowa, actually had a small test case that began in 2016. The airport ran a multi-year study with Iowa State University and the FAA, where they embedded segments of electrically resistive conduit in a proprietary mix of concrete that included carbon fiber. The study concluded as a resounding success, although there has yet to be a commercial launch of the tech developed.

In both the Binghamton and Des Moines examples, researchers decided that heating the airport apron areas — plane parking, service vehicle areas, jetway zones — were more practical, as these areas are more difficult to service with traditional snow removal equipment. The same is true for two airports in northern Europe; Stockholm-Arlanda Airport, in Sweden, and Oslo Airport in Norway, although these systems are hydronic.

Meanwhile, research continues to improve the efficiency of currently electrical heating systems. Either by improving the thermal conductivity of the driving surface, or by reducing the current needed to heat a thin metal mesh ribbon.

Summary

Airlines and airports have a lot at stake if runways close for snowy conditions. Millions of lost dollars, upset passengers and potentially lives are in the balance.

In the near future, runway de-icing systems remain unlikely. To date, they have not proven to be economically feasible — they either require too much power or are too unreliable to be of service. And although a crew of plows might be expensive, they are time tested. Consider that Ted Stevens International Airport in Alaska has never closed due to the snow.

But the time is quickly approaching where the technology may make heated runways a worthwhile investment. There are active, ongoing developments to bring about its benefits. Snowy locations ranging from Upstate New York to Scandanavia are recognizing there is an advantage, albeit on a limited basis.

As with many technologies, it may seem far fetched or unlikely, until suddenly they are nearly everywhere.