American Airlines believes it just might be, at least for some of it smaller planes.

A dangerous heat wave is sweeping to the Valley. The National Weather Service has issued an Excessive Heat Warning for the Valley through Saturday night.

[RELATED: Weather Alerts | 7-Day Forecast]

Temperatures in the Valley will climb to about 117 Monday, 120 Tuesday and 118 Wednesday.

[READ MORE: It's going to be a scorcher! Heat wave headed to Phoenix]

Because of the extreme heat, American Airlines has preemptively canceled 20 regional flights for Tuesday and has issued a heads up that it could happen again Wednesday.

[READ MORE: American Airlines cancels 20 Tuesday flights because of heat]

[RELATED: American Airlines alerting Phoenix passengers of potential heat-related delays]

This is not the first time, heat has affected operations at Sky Harbor International Airport.

When Phoenix had its hottest day on record -- 122 degrees on June 26, 1990 -- airlines grounded all flights out of Phoenix Sky Harbor International Airport for a short time.

Many people blamed soft, "melty" asphalt on the runways. That's an urban legend. It's true that asphalt can soften in extreme heat -- it becomes liquid at about 250 degrees Fahrenheit. There was also a reported issue in Washington in July 2012 when a plane's wheels actually sank several inches into the soft asphalt.

Philip Dugaw snapped a photo of the wheels in the ruts and posted it on Imgur, saying the plane "sank into the pavement."

"The small vehicle that usually tows planes away from the gate tugged and pulled, but the plane was stuck," Martin Weil wrote in the Washington Post.

But soft asphalt was not the problem in Phoenix in 1990. No planes got stuck.

The real issue was more technical.

According to an airport spokesman, commercial airplane performance charts did not account for temperatures that high at the time. Those charts have since been updated with data that extends into the 120s, allowing pilots to calculate for the higher heat and longer takeoff requirements.

Smaller regional jets, however, have their own temperature limitations, and airlines may choose not to fly them in temps over 117. That's what American has done with its Bombardier CRJ regional jets.

Last June, a flight from Houston to Phoenix had to turn around due to heat concerns. The ground temperature on that day was 118. That plane, an Embrarer 175, seats 78 people.

Not only is extreme heat hard on planes mechanically, there is also the issue of physics. Hot air is less dense. Because the air is thin, the plane needs more distance to get the speed and lift necessary to get off the ground. It also makes the climb a little more difficult. Have you ever noticed that takeoffs feel a bit sluggish when it's hot? That's the plane working harder in the thin air.

[SPECIAL SECTION: Extreme heat]

In addition, the takeoff, climb, descent and landing can be a bit bumpy thanks to thermal (convective) turbulence.

"Turbulence can also be expected on warm summer days when the sun heats the earth's surface unevenly," explains "Certain surfaces, such as barren ground, rocky and sandy areas, are heated more rapidly than are grass covered [sic] fields. Isolated convective currents are therefore set in motion with warm air rising and cooler air descending, which are responsible for bumpy conditions as an airplane flies in and out of them."

So will commercial flights be ground en masse like they were in 1990? Probably not, but it's always a good idea to check with your airline, especially if you're on one of the smaller regional planes like the American Eagle flights.

That said, if you do find yourself delayed for whatever reason, Sky Harbor International Airport is not a bad place to be stuck.

"Sky Harbor's three terminals provide great options for eating, snacking, excellent shopping and special services," according to

[RELATED: 3 reasons why Arizona's heat wave will delay vacations]

American Airlines has cancelled 20 flights tomorrow operated by regional jets due to the heat. Travelers should contact the airline for info— PHX Sky Harbor (@PHXSkyHarbor) June 19, 2017
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