There’s a whole new meaning to the term “soft landing” when it comes to air travel, and it’s not the good kind. Brutal temperatures across Europe are damaging runways — with the U.K. being hit particularly hard — making it difficult for planes to land. The British Royal Air Force was forced to divert flights away from Brize Norton air base due to extreme temperatures, according to a Royal Air Force statement. Amid reports of skyrocketing temperatures, Luton Airport, 30 miles north of London, tweeted that it was suspending flights due to a heat-induced “surface defect” on its runway.
An angry Twitter user responded to the Luton Airport statement with, “This country is a joke. Madrid it’s 40 degrees [Celsius, which is 104 degrees Fahrenheit] plus every day. Same in other countries and their runways don’t melt.”
It’s actually a great point. Why do airports in countries with the same temperatures that the U.K. is experiencing have runways that are working just fine? Asphalt is asphalt, right? Eh … not so much when it comes to runways. There’s a whole variety of ingredients that go into an asphalt mix when designing and constructing a runway — and much of it is based on an airport’s usual temperatures.
The runways that can take the heat
First off, all runways expand — even those made to stand up to the heat, said Chris Smith, a London-based aviation author and commercial pilot. “Runways are no different to anything else that expands with higher temperatures, but how they are designed and constructed can make them more or less susceptible to changes in temperature.”
It really comes down to the materials a runway is made with. Asphalt has for decades been the world’s base ingredient for runways, according to Greg White, the director of the Airport Pavement Research Program at the University of the Sunshine Coast in Queensland, Australia.
It is typically made from crushed rock and a substance called bitumen, which is a material left when you remove fuel, diesel, kerosene and other products from crude oil. White said different asphalts are made to withstand specific local weather conditions.
When airports experience extreme weather over a few days, White told Grid, their asphalt is likely not well suited to adjust. That’s what just happened in the U.K.; the country broke its record for highest temperature ever registered this week. But asphalt can also be impacted by an extreme drop in temperatures, he writes in a 2018 report. That means an airport in Minnesota would have a different runway mix than one in Sao Paulo.
And we’re talking about asphalt that has to withstand really heavy loads. “When things are hot, and aircraft or trucks or buses move slowly, then they can deform” the asphalt because it softens, White said. Alternatively, asphalt can also be impacted by an extreme drop in temperatures.
Over time, regardless of temperature, asphalt experiences wear and tear just like roadways. But extreme temperature shifts exacerbate asphalt distress. The most typical issues are depressions caused by wheel traffic, known as “rutting,” and “crocodile cracking,” when pavement distress leads to a fracture formation similar to glass.
So, are runways actually melting — a word that’s been used repeatedly in news reports? White calls that an “extreme description” for what likely took place — an isolated region simply lost firmness, he said.
“I suspect that after those two or three days of extremely hot weather in that area, the runway got soft … because this stuff’s black, so it sucks the heat up,” White said. “Then you put a high load on something that’s been softened, then these are the kinds of things that can happen. I suspect it was only in a very small, isolated area. But it would have needed to have been repaired before they could continue to use the runway.”
While recent events may cause some to worry, White assured that the risk of damage to runways is minimal at this point because runways are replaced frequently.
“Because of aging, weathering and being in the environment, generally they would require replacement every 10 to 15 years, just like the roads,” White said. “But you don’t see those things happening, because they’re generally done at night.”
The ground underneath runways can also be impacted by extreme heat, affecting a runway’s integrity.
“The temperatures around the pavement are much, much higher than the air temperature. That not only affects the asphalt layer itself, but it affects the soil layer underneath it. It takes off all the moisture,” Ghada Ellithy, assistant professor of civil engineering at Embry-Riddle Aeronautical University, told Grid. “The soil starts to become dry, and it loses its strength as well. … It becomes kind of a double problem.”
Another runway issue: Higher temperatures mean planes need longer runways (and not all airports have them)
When the temperature runs hot, air becomes less dense, and planes may require higher speeds to achieve lift.
“In other words, molecules are further apart, and so in order to fly with the same number of particles of air, you have to fly faster,” Pat Anderson, director of the Eagle Flight Research Center at Embry-Riddle Aeronautical University, told Grid. In some cases, there may not be enough runway for aircrafts to achieve the speed necessary if those airports weren’t designed for high-heat environments.
To compensate, aircrafts can lower their weight by offloading cargo, luggage or even passengers. In a 2015 study, Columbia University researchers predicted that by 2050, the number of days planes will have to do this between May and September will increase by 50 percent to 200 percent at four major airports: Phoenix Sky Harbor International Airport, Denver International Airport, New York’s LaGuardia Airport and D.C.’s Reagan National Airport.
“These performance reductions may have a negative economic effect on the airline industry,” the researchers wrote. “Planning for changes in extreme heat events will help the aviation industry to reduce its vulnerability to this aspect of climate change.”
A cool solution
Emerging technology may prevent pavement on runways and roads from becoming so hot in the future. For instance, the Environmental Protection Agency has touted what is known as “cool pavements.” It involves using materials that reflect the sun, increase water evaporation or otherwise lower the surface’s temperature. It can be created by modifying materials already used in pavement such as asphalt and concrete, as well as newer approaches such as coatings or grass paving.
For example, some cities have applied a gray reflective coating, called CoolSeal, to residential streets that reflect the sun’s rays. A study conducted by the city of Phoenix and Arizona State University found that CoolSeal pavement had an average surface temperature 10 to 12 degrees Fahrenheit lower than traditional asphalt at noon and during the afternoon hours.
“This is exactly what we were hoping for,” Phoenix Mayor Kate Gallego said in a release. “The results are promising. While there’s more work to be done, it’s exciting to see a technology that has the potential to meet the demands of a growing desert city in a world where temperatures are constantly climbing.”
Climbing temperatures are an issue in many areas of the world — desert climate or not. And the technology could be useful in airport runway design, Ellithy said.
It may be a good time for the air travel industry to take Amelia Earhart’s famous quote literally: “Some of us have great runways already built for us. If you have one, take off. But if you don’t have one, realize it is your responsibility to grab a shovel and build one for yourself and for those who will follow after you.” If planes want to get off the ground, airport designers and engineers are going to have to build new and different runways to deal with climate change.
Thanks to Lillian Barkley for copy editing this article.