Hot planes can be dangerous for fliers. Why are they so hard to cool?

On a recent delayed flight in Las Vegas, Delta passengers stuck on the plane experienced such intense heat onboard that people reportedly vomited and passed out. According to the airline, two passengers were taken to the hospital and others were treated on-site.

The incident sparked outrage, a Transportation Department investigation and an apology from Delta. But the issue of travelers being subjected to uncomfortable temperatures dates back years — and may only get worse as record heat continues to seize the U.S. and Europe.

In the summer of 2017, a mother reported that her infant son overheated and lost consciousness on a delayed United flight in Denver. In 2013, passengers on an Allegiant flight were stuck in Phoenix with no air conditioning for more than two hours.

In recent weeks, several people have aired their complaints on social media as extreme heat has gripped the nation. Crooked Media co-founder Tommy Vietor tweeted that his Delta flight had sat on the tarmac in Atlanta with no power or air for 30 to 45 minutes earlier this month.

“We literally had to ice down our 7 month old who was bright red and screaming,” he wrote. “Indefensible and unsafe for all kids/elderly passengers.”

How extreme heat may wreak havoc on your flight

Lauren Moses, a digital sports reporter who lives in Austin, complained in a series of tweets on Friday that she wasn’t able to get off her delayed American Airlines flight “with minimal AC” for two hours. Her phone’s weather app showed a temperature of 99 degrees outside.

“It was very, very uncomfortable,” Moses, who was flying to Los Angeles for her friend’s baby shower, told The Washington Post. She said a flight attendant who handed out water after about an hour and a half was “dripping in sweat.” The water, she noted, was room temperature, “which means it’s hot. Everything is just hot.”

Loose standards for protecting passengers

Federal regulations require airlines to provide “comfortable cabin temperatures” if planes are delayed on the tarmac. But there’s no definition for what “comfortable” means.

“It’s very subjective,” said Taylor Garland, spokeswoman for the Association of Flight Attendants-CWA.

The Transportation Department described its process for deciding what a violation is in an order finding that Allegiant had failed to provide comfortable temperatures for passengers on multiple flights in 2016 and 2017.

An enforcement officer “considered written passenger complaints, crewmember statements, temperature readings, reported medical incidents, operational considerations such as the use of external cooling units or air carts during the delays, and decisions to deplane passengers,” the order says.

Flight attendants have been asking for a more cut-and-dry approach: standards that would set the target range during flights and on the ground between 65 and 75 degrees, with a maximum on the ground of 80 — or 85 if all in-flight entertainment systems are operating.

“Extreme temperatures clearly impose hardships on passengers and crewmembers,” the Association of Flight Attendants-CWA wrote in a petition to the Transportation Department in 2018.

Since then, Garland said, “there’s been no movement. It always becomes an issue in the summer.” The union launched an app, 2Hot2Cold, for crew and passengers to report extreme temperatures. Between the 2018 launch and August 2022, the app had gotten more than 3,417 reports.

The FAA reauthorization bill passed by the U.S. House of Representatives last week includes a provision to review the issue and potentially study unsafe cabin temperatures and conditions as well as provide recommendations to improve temperature management.

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Why planes are so hard to cool

Air-conditioning systems on commercial airplanes are designed to work best in the sky, which can lead to problems on the ground.

“When you talk about airplanes of that type, they’re usually anticipating needing to be at 35,000 [feet] cruising for hours, and that’s where they’re optimized to work the best,” said Robert Thomas, an assistant professor of aeronautical science at Embry-Riddle Aeronautical University in Florida.

Air-conditioning units on planes known as “packs” work by taking hot, pressurized “bleed air” from the plane’s engines and cooling it down by using the cold temperature outside the plane at cruising altitude — which can be minus-40 or lower — before pushing it through the cabin.

When an aircraft is grounded and in extreme heat, that same heat exchange isn’t possible.

Robert W. Mann, an airline industry analyst and consultant, said the temperature regulation systems are powered by each of a plane’s main engines or a small jet engine at the plane’s tail called the auxiliary power unit, or APU.

In cases where airplanes are delayed in a long line, the main engine — the best source of air conditioning — will often be shut down. These planes would have to rely only on the auxiliary power unit to power the plane and keep air cool. And while taxiing, planes often use a single engine to cut down on fuel usage and costs. Capacity for air conditioning is reduced throughout.

Though flight captains, “as final arbiters of flight safety,” can, in theory, override either of these standard procedures, “everything is a trade-off,” Mann said. “Engine runs during protracted delays consume fuel planned for use to destination, cutting into reserve safety margins, which could in the worst case cause a return to gate for refueling.”

Airplanes are also heavily sealed and pressurized, “so, when you’re on the ground and the doors are closed … you’re basically the equivalent of the car sitting there in the parking lot with all the windows closed,” Thomas said. “You’re kind of baking in the sun.”

Even without the doors closed, airplanes absorb energy from the sun and from the hot ground. When 150 or more passengers are loaded onto the plane, they produce more heat and humidity.

Preboarding, aircraft will often use external units to pump air preconditioned to a temperature considered acceptable to passengers, usually around 70 degrees. On an especially hot day, though, 70-degree air might not be enough to bring cabin temperatures down to more comfortable temperatures.

Billy Nolen, a former FAA acting administrator and chief safety officer at the aircraft company Archer, said that preconditioned air technology can bring down cabin temperatures about 20 degrees. But 20 degrees below 110 is still plenty hot.

“At some point on the temperature scale, the very best unit can only do so much,” Nolen said.

According to Mann, the maximum temperature considered comfortable for passengers, preboarding, is generally as high as 90 degrees Fahrenheit.

Thomas said maximum safe temperatures can depend on both airplane manufacturers and airlines. Boeing, he said, will have its own set of rules for “what the airplane can handle.” But so can American Airlines, whose rules and procedures will dictate things like how long planes are allowed to wait on the ramp before they have to go back to the gate — or try to.

Flight attendants want to do something about cabin temperatures that run hot or cold

Under U.S. tarmac delay rules set by the Department of Transportation, delayed domestic flights departing from a U.S. airport must begin moving to a location where passengers can safely deplane before the three-hour mark. The problem is getting to that location.

“If you leave the gate, then you need to find a way to get back to the gate,” Thomas said. “And if there’s no gates available, then you’re kind of stuck out in nowhere land until a gate becomes free.”

The result, Mann said, is that “you may be in a situation where the aircraft is sitting, baking in the sun. … And this could go on for half an hour, an hour, or longer, and there’s really no place for the airplane to go.”

Extreme heat can cause other problems on the runway, too. Hotter temperatures can affect the density and barometric pressure of the air, two factors that, combined with the altitude of the airport, can affect how well a plane works. Warm, less dense air can make it harder for planes to take off, for example.

“That means that, depending on how hot it is, you might have to … restrict the number of passengers that could fly, or you might have to restrict the number of luggage that you can carry or you might have to even restrict the number of maybe excess fuel that you’d want to carry,” Nolen said.

Thomas said that increased problems with extreme heat and delayed flights may have to lead to a change of airline operating procedures in hot weather: “When something like that happens, you want to be kind of proactive in the aviation world, not reactive.”

Delta has said it is “looking into the circumstances that led to uncomfortable temperatures inside the cabin” of the plane in Las Vegas last week.

In an email, United spokesman Charles Hobart said the airline works with flight attendants and crew on a flight-by-flight basis to keep the cabin comfortable, using measures like preconditioning and the auxiliary power unit connection before departing.

American Airlines has procedures for crew members and agents to close window shades and open air vents on the cabin any time the temperature climbs above 80 degrees. CEO Robert Isom spoke about the challenges of operating in high temperatures during an earnings call this month, saying the carrier is having to use its hot-weather practices “more often and longer throughout the year.”

That includes getting jet bridges with conditioned air to planes at the gate as quickly as possible, doing preventive maintenance on auxiliary power units to keep them from breaking down and only boarding when planes have appropriate air conditioning.

“We’re really taking this seriously, and we’re going to have to as we go forward,” Isom said.

correction

A previous version of this article misstated the temperature outside airplanes at cruising altitude. It is typically minus-40 or lower. This version has been corrected.