Just how dangerous is turbulence?

A Singapore Airlines flight was the latest trip to injure passengers, after it encountered “sudden extreme turbulence.”

Intense turbulence can force pilots to change altitude, cause flights to divert for emergency landings and send passengers to the hospital. In the most recent high-profile incident, during a flight from London to Singapore, one man died and dozens were taken to the hospital.

A passenger identified as a 73-year-old from the United Kingdom may have suffered a heart attack aboard Singapore Airlines Flight 321 when it encountered severe turbulence, an airport official said Tuesday. After the pilot declared a medical emergency and diverted the plane to Thailand, 30 people among the 211 passengers and 18 crew members were either hospitalized or being treated at a hospital, the airline said.

Turbulence is classified by four levels of intensity, with “extreme” being the worst and “severe” the second-worst. According to the National Weather Service, turbulence is extreme when an aircraft is “violently tossed about and practically impossible to control.” Singapore Airlines initially described the turbulence on Flight 321 as “severe” but in a later update called it “sudden extreme turbulence.” (The airline did not immediately clarify whether it used the term “extreme” in reference to the intensity scale.)

Turbulence itself is a frequent occurrence, as any regular traveler can attest — and it typically isn’t cause for alarm, experts say. But examples of passengers suffering serious injuries keep piling up.

What causes intense turbulence?

The Federal Aviation Administration describes turbulence as movement of air that usually can’t be seen and that often happens unexpectedly. It can be created by factors including atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts, or thunderstorms.

“It can be unexpected and can happen when the sky appears to be clear,” the FAA says on its website. “Turbulence can give an airplane a sudden jolt that can injure passengers and flight crewmembers who aren’t buckled in.”

Between 2009 and 2022, 163 people on U.S. air carriers suffered serious injuries due to turbulence, the FAA says.

Bob Thomas, a pilot, former weatherman and assistant professor of aeronautical science at Embry-Riddle Aeronautical University, told The Washington Post in 2022 that there are many different types of turbulence.

“The bad kind is always unexpected,” he said. What’s called “clear-air” turbulence happens without visual cues.

“You can have turbulence from a thunderstorm 20 miles away from the actual worst part of the storm,” Thomas said. “Thunderstorms will create these huge up-and-down movements of air, and when you get that, you get these big waves that come through, and you can just fly through it.”

“It can be associated with almost any kind of weather,” said Patrick Smith, a commercial pilot for 30 years who runs the Ask the Pilot blog. “It doesn’t always matter, and it’s not always predictable.”

Is turbulence a serious safety threat?

Smith said every flight every day “encounters some form of rough air. For crews, by and large, we look at it as a comfort issue, not necessarily a safety issue.”

He said the tools that pilots have in the flight deck are “amazingly good” at predicting where, when and how bad turbulence might be. They can alter their route or altitude to try to avoid the rough air or, if that’s not possible, give flight attendants plenty of warning to prepare the cabin.

“But in a lot of ways, it’s more art than science, and sometimes you just don’t know,” Smith said. “It can get bumpy when you just don’t expect.” One place that will probably be slightly less bumpy: the middle of the plane over the wings, he said. The bumpiest place to sit is in the tail area of the plane, Smith said, though it doesn’t make a lot of a difference.

Smith said the fear that a plane might flip upside down or lose a wing is, “at best, science fiction.”

Although worst-case-scenario fears are extreme, experts say turbulence still poses a risk — especially if people are not buckled in.

“You want to be cautious, because the aircraft itself is going to survive,” said Mark Baier, CEO of AviationManuals, which provides safety information and systems to smaller flight operators. “You’re going to get thrown around the cabin, or loose objects are going to be thrown around the cabin and cause injury.”

Can turbulence cause a plane to plunge?

The Singapore Airlines flight was cruising at 37,000 feet over Myanmar’s Irrawaddy basin when it dropped to 31,000 feet in minutes, flight data captured by Flightradar24 shows.

Thomas Guinn, chair of the Applied Aviation Sciences Department at Embry-Riddle Aeronautical University, said Tuesday that he could not speculate on what happened with the Singapore Airlines flight but that the first thing a pilot will do to get out of turbulence is to try to change altitude. Guinn, an expert in aviation meteorology, described turbulence regions, outside of thunderstorms, as “kind of pancake-shaped.”

“If you can adjust altitude either up or down, you can get out of the turbulence pretty well,” he said.

Paul Williams, a professor of atmospheric science at the University of Reading who studies turbulence, said in an email that pilots will try to escape it “quickly.”

“The rapid drop was almost certainly pilot maneuvering rather than the direct effect of the turbulence,” he wrote.

There have been at least two incidents in recent years in which commercial airplanes frightened passengers with rapid plunges, though turbulence was not to blame.

In December 2022, a United Airlines flight from Hawaii dove 1,400 feet toward the Pacific Ocean, coming less than 800 feet from the water and causing people onboard to scream. According to the National Transportation Safety Board, miscommunication between pilots was responsible.

Last year, an American Airlines plane dropped nearly 15,000 feet in about three minutes, then another roughly 4,000 feet from its 30,000-foot cruising altitude. The airline cited a possible pressurization issue, and one passenger described it as “the flight from hell” after his ears popped, oxygen masks dropped and a burning smell filled the cabin.

How common is bad turbulence?

The FAA does not track general incidents of turbulence, though the National Transportation Safety Board requires U.S. airlines to report serious injuries and fatalities.

According to the National Center for Atmospheric Research, pilots report an average of 65,000 encounters with moderate or greater turbulence every year over the United States, and 5,500 encounters considered severe or greater.

Williams wrote to The Post that “0.1 percent of the atmosphere at cruising altitudes contains severe turbulence, and the fraction containing extreme turbulence will be even smaller.”

“Extreme turbulence is so rare that we actually don’t usually include it in scientific studies,” he said.

But Williams co-wrote a study that found that clear-air turbulence has been on the rise because of climate change. In a statement Tuesday, Association of Flight Attendants-CWA President Sara Nelson said instances of severe and clear-air turbulence have been increasing as the climate changes. Nelson called it a “serious workplace safety issue” for flight attendants and urged passengers always to follow crew instructions and keep a seat belt on whenever seated.

“While details of Singapore Flight 321 are still developing, initial reports seem to indicate clear-air turbulence, which is the most dangerous type of turbulence,” Nelson said. “It cannot be seen and is virtually undetectable with current technology. One second, you’re cruising smoothly; the next, passengers, crew and unsecured carts or other items are being thrown around the cabin.”

Can turbulence induce a heart attack?

Erica Spatz, associate professor of cardiology at the Yale School of Medicine, said the dangers of turbulence are far more likely to involve banged heads or bruised limbs than cardiovascular problems.

“I don’t know that turbulence has been identified as a direct risk for any cardiac conditions,” said Spatz, who is also director of Yale’s Preventive Cardiovascular Health Program. “We usually think about turbulence with respect to injuries: people falling or hitting their head or the overhead bins coming down and luggage falling on top of them.”

But Spatz explained that “different acute stressors,” such as fear for one’s life, can increase one’s blood pressure, heart rate and stress hormones. The fight-or-flight response could cause plaque in the artery to rupture or induce an irregular heartbeat, experts said.

That said, “there’s no evidence that a patient with underlying heart disease should not fly,” said Saurabh Sharma, a preventive cardiologist and a member of the Prevention of Cardiovascular Diseases Council at the American College of Cardiology.

Sharma said he tells his patients with underlying heart disease to carry nitroglycerin tablets with them, to relieve chest pains temporarily and get blood flowing in a stressful situation, like flying can be. Spatz also recommended that passengers stick to their medication regimen and not skip a dose, especially on international flights. Incorporating relaxation techniques can help alleviate the stress of travel as well.

“The psychological trauma is just as risky as the potential for physical trauma,” Spatz said.

Andrea Sachs, Ryan Bacic and Gabe Hiatt contributed to this report.

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