Warming trend: As mercury rises, airline operations may be impacted

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Earlier this year, nearly 50 flights on regional jets had to be canceled at Phoenix Sky Harbor Airport because temperatures climbed to nearly 120 degrees.
Earlier this year, nearly 50 flights on regional jets had to be canceled at Phoenix Sky Harbor Airport because temperatures climbed to nearly 120 degrees.

The impact that extremely hot weather can have on commercial aviation made headlines earlier this summer when American Airlines canceled nearly 50 flights out of Phoenix because temperatures approached 120 degrees.

The cause of those cancellations was actually rather technical. The performance charts for the Bombardier CRJ regional aircraft that were set to fly that day under the American Eagle livery don't go past 118 degrees, so American and its regional partners canceled the flights as a precaution.

But research shows that the increase in especially hot days that many scientists attribute to global warming is also having a significant impact on the number of flights that must be weight-restricted at vulnerable airports. And the increase in weight-restricted flights is only expected to continue as the climate warms further, taking a bite out of airline profit margins along the way.

"In the future, weight restrictions could become more severe, or there will be more flights that have to make stops, particularly when they're going against the jet stream," said Ethan Coffel, a PhD candidate in atmospheric sciences at Columbia University who co-authored papers in 2015 and again this summer on the impact of rising temperatures on aircraft takeoff performance.

At issue is a simple matter of physics. Hot air is less dense than cool air, and that reduction in density makes it more difficult for planes to get airborne. As a result, planes must reach a faster speed to take off in hotter weather, and they must do so even though thrust performance is also diminished by the lighter air. These conditions also apply to takeoffs from high elevation airports, since air density drops with altitude.

Whether or not a plane can take off safely for a given flight is determined by a combination of air density, aircraft design, runway length and the aircraft’s carrying load. But since those first three factors cannot be changed in the run-up to departure time, airlines adapt by reducing a flight’s carrying load. This can only be done by removing either fuel, cargo or passengers from the plane.

Takeoff issues related to hot weather can be mitigated through improved aircraft designs and longer runways. But those options are expensive.

According to a 2015 study Coffel co-authored with his Columbia University professor Radley Horton for the journal Weather, Climate and Society, an increase in hot days has already led to a corresponding climb in the number of flights that have had to be weight-restricted at New York LaGuardia and Washington Reagan National airports, both of which have unusually short runways. Since 1980, Coffel said, there has been a 20% to 30% increase in weight restriction frequencies at those airports.

Other airports where climate change is leading to more frequent weight restrictions include desert locales such as Phoenix and Dubai, high-altitude locales such as Denver and hot climate airports in Southeast Asia such as Bangkok and Hong Kong, where many long-haul flights are offered, Coffel said.

With warming temperatures expected to accelerate in the coming decades, heat-related weight restrictions will likely become a more frequent phenomenon. In a follow-up study published in July in the journal Climatic Change, Coffel, Horton and another author analyzed the potential interplay between climate change and weight restrictions at 19 major airports around the world for five common commercial aircraft type.

They concluded that by 2070 to 2080 10% to 30% of annual flights departing during the hottest time of the day would require weight restrictions, with the average restrictions cutting into fuel capacity, cargo and total passengers by 0.5% to 4%. That 10% to 30% represents a threefold to fourfold increase from current weight- restriction frequency, Coffel said. 

Reducing aircraft capacity by even a small percentage can add up to millions of dollars in lost revenue for an airline over time, Coffel and Horton wrote in a recent Scientific American article.

Standard-size widebody aircraft, such as the Boeing 787 Dreamliner, will see the largest increase in weight-restriction frequencies as weather gets hotter, primarily because they often fly long routes that require a full load of fuel.

Single aisle jets, such as the Boeing 737 and Airbus A320, will be impacted less, and most frequently at short-runway airports like Reagan National and LaGuardia. The jumbo Airbus A380 will see the smallest increase in weight-restricted flights, mainly, Coffel said, because it only operates out of a limited number of major airports, which typically have long runways.

Coffel acknowledged that the expected growth in takeoff issues related to hot weather can be mitigated through improved aircraft designs and the construction of longer runways. But those options are expensive.

“The best solution is to fly at cooler times of day,” he said. “But the passenger demand presumably won’t change, so for some flights it will be more feasible than others.”

In an email, Boeing spokesman Tom Kim noted that the manufacturing giant’s airline customers can already choose what it calls the “high and hot” model for certain aircraft, which has higher-thrust engines and larger control surfaces than standard designs. In exchange for higher thrust, the customers give up a bit of fuel efficiency. Kim said that maximum takeoff weight in future aircraft will be determined by aircraft design but that Boeing isn’t specifically looking at changes due to climate change.

Paul Williams, an atmospheric scientist at the University of Reading in England who has also studied the effects that climate change is having on commercial aviation, said that he hopes Boeing and Airbus are carefully considering the interplay between climate change and takeoff dynamics as they design future generation aircraft. But he said that with planes having shelf lives of several decades, engineering improvements can’t necessarily keep up with global warming.

“You can’t design your way through the laws of physics,” Williams said.

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