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Don Paul: There is such a thing as 'too hot to fly' – but not here

Don Paul

It will never happen here, but it can get too hot for some airplanes to fly.

You may have heard how there have been flight cancellations during the current heat wave in the Southwest. Marshall Shepherd, former American Meteorological Society president and current chairman of atmospheric sciences at the University of Georgia, has just written a refresher article for Forbes magazine on why this happens.

It’s not common for it to get quite as hot as it is in the Southwest right now, but when it does, in a broad sense, there’s not enough air around for some planes to take off safely.

Many think the oppressive heat feels heavier, and actually say “the air was heavy with heat.” In fact, the opposite is true; the hotter the air gets, the thinner it gets. When air heats, it expands. The molecules are bouncing around, getting farther apart from one another. Aircraft taking off are particularly sensitive to how much lift the atmosphere can provide to the wing surfaces. There comes a point at which the air is too thin for a safe takeoff for some aircraft.

Author and pilot Patrick Smith explained this basic principle in Business Insider:

“Hot air is less dense. This affects the output of the engines as well as aerodynamic capabilities, increasing the required runway distance and reducing climb performance. Therefore the amount of passengers and cargo a plane can carry are often restricted when temps are very high ... How much so depends on the temperature, airport elevation and the length of the available runways. And getting off the ground is only part of it: once airborne, planes have to meet specific, engine-out climb criterion, so nearby obstructions like hills and towers are another complication.”

Larger jets can handle more extreme heat than smaller jets. This was illustrated on Monday in this statement from American Airlines found in the Arizona Republic: "The American Eagle regional flights use the Bombardier CRJ aircraft, which has a maximum operating temperature of 118 degrees. Tuesday's forecast for Phoenix includes a high of 120 degrees, and the flights that are affected were to take off between 3 and 6 p.m. ... Larger jets that fly out of Sky Harbor have higher maximum operating temperatures: Boeing, 126 degrees, and Airbus, 127 degrees."

This heat effect also is relative to the altitude at airports. I recall a heat wave of lesser magnitude once caused similar flight cancellations in Denver. Obviously, Denver International is higher than Sky Harbor, so the air is thinner there to begin with. In a heat wave, the smaller jets at Denver need a lot of runway to safely take off and climb.

As an aircraft accelerates down the runway, lift is generated by the pressure gradient between the higher pressure beneath the wings and the lower pressure just above the wind. If the air is inadequately dense, the lift will be inadequate.

If you’re worried, the airlines don’t take chances with this factor. Meteorologists and engineers do the appropriate calculations, and flights are canceled when the danger threshold is near.

There is a potential warming climate connection here. Climate models predict these extreme heat waves will be occurring more often in the future. As they do, flight cancellations may become more of an issue for airlines in these situations.

As I said, this will never be an issue for Buffalo-Niagara International. But it could be an issue for you if you’re traveling to Phoenix or Tucson in the summer.

By the way, may I ask: Why in heck would you do THAT?

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