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Have you felt like it took your airplane an especially long time to lift off of the ground? That might seem normal for an A380 or a Boeing 747, both very heavy aircraft, but the phenomenon is noticeable even in smaller aircraft such as the Boeing 737, especially at airports like Denver, Phoenix or Mexico City.

What makes these airports unique?

Each of these fields are high-altitude and hot temperature airports (or in the case of Phoenix, just hot). This affects the amount of pavement needed for the plane to lift off. Or more precisely, it affects the air rushing around the wings to generate sufficient lift and into the engine to provide sufficient thrust.

I connected with Kaitlin Doherty, United Airlines’ Station Operations Control Manager in Denver, to understand why flying out of high and hot airports can be tough. Indeed, in 2017, American Airlines had to cancel some 50 flights from Phoenix SkyHarbor International Airport, whose regional jets had a maximum temperature rating of 118 degrees. On a 120 degree day, those planes won’t go.

“Aircraft performance is affected by a combination of weight, outside air temperature, altitude and speed,” Doherty explained.

The Denver Takeoff Roll

Planes fly through a fluid: air. And the density of that fluid affects the performance of an aircraft. The hotter the temperature, the less dense the air. And the same goes for higher altitude.

“Similar to how people have a harder time breathing at higher altitude, our aircraft must fly faster or carry less weight to get the lift they need to take off. The combination of hot and high makes Denver and other high-altitude stations uniquely challenged when it comes to weight restrictions,” Doherty said.

Denver provides an instructive example; it has several 12,000-foot runways and and a very, very long 16,000-foot runway.

Above is a video shot by a business class customer aboard a United Airlines Boeing 787 Dreamliner departing from Denver’s 12,000 foot long runway 17R in the summertime. That airport is already at an elevation of 5,378 feet above sea level.

The takeoff roll is 55 seconds, and the aircraft leaves the ground attaxiway M4, which means it rolled approximately 8,730 feet before taking off. (For perspective, LGA’s runways are 7,000 feet long.) That still leaves some 4,000 feet of runway for our Boeing 787, plenty of safety margin.

When it’s very, very hot, an aircraft operating out of LGA, which is close to sea level, might perform as if it were already flying 1,800 feet above sea level. The air-density change is that dramatic.

How Airlines Like United Deal

Several airports qualify for high and hot operations during many months of the year. Other than Denver, Phoenix and Mexico City, these airports include Las Vegas, Reno and  Albuquerque in the US; Calgary, Canada; Addis Ababa, Ethiopia; and La Paz, Bolivia.

Dealing with weight and balance coupled with high, hot airport is a challenge for the airlines. Ethiopian Airlines, for example, which flies out of Addis Ababa, often schedules departures later in the evening when temperatures are cooler and the air more dense.

“Sometimes we have to restrict the number of passengers, cargo or fuel to reduce the weight of the aircraft thereby improving the aircraft performance to remain within limits,” United’s Doherty explained.

“Typically at the warmest time of day — early to mid-afternoon — is where we often see the restrictions start popping up,” she said.

“We do prioritize the payload when we get into a restricted state,” she said. “We have a weight and balance team in our Network Operations Center that are reviewing the temperatures and fuel load for each flight, among other things. And our dispatch team is monitoring the passenger, bag and cargo counts associated with each trip. This team will advise the station if there are any restrictions in place ahead of time when possible,” she said.

“Our flight crew also reviews the weight and balance once in the flight deck to ensure no changes have taken place… temperature increase after planning timeframe occurred, change in cargo weight, bag or passenger count. If a flight is affected by the hot temps and or altitude, our teams coordinate to see if there is any way to find a solution to accommodate as many passengers, bags and cargo as possible while remaining safe and within compliance,” she explained.

There are other factors that also must be considered for every flight, and each additional factor can compound another: shorter runways, obstacles or construction equipment near runways.

“In Denver, despite having long runways, most often it is the climb performance of the aircraft once it is airborne that has the most effect at limiting our takeoff weight.”

And It’s Getting Hotter, Too

Are we going to see more weight-restricted days in the future? It appears so. One research paper has studied weight-restricted days at Phoenix, Denver, LaGuardia and Washington DCA, the latter two being affected by their relatively short runways. Not surprisingly, weight-restricted days have increased over time as temperatures have risen. That could cause increased delays in the future.

But, know that there’s a team of professionals behind the scenes like United’s Doherty, who handle high and hot operations as a matter of course.

Featured image of Denver International Airport by George Rose/Getty Images

Know before you go.

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