This photo combination of mobile images, courtesy of passenger Jazmin Bitanga, shows the interior of a Hawaiian Airlines plane flying from Phoenix to Honolulu after severe turbulence rocked the flight, Sunday, Dec. 18, 2022. Image Credit: Courtesy of Jazmin Bitanga via AP

Washington: US federal safety officials will launch an investigation into a Hawaiian Airlines flight after severe turbulence injured 36 people aboard, while five people were hospitalised in Houston on Monday after a second flight was rocked by turbulence en route from Brazil.

There was no indication that the two instances of turbulence were related. Air travel is exceptionally busy around the world this week due to the winter holidays.

The Hawaiian Airlines flight departed Phoenix and was nearing Honolulu around 10:35 a.m. HST (2035 GMT) on Sunday when it hit the turbulence, according to the US Federal Aviation Administration (FAA).

On Monday, the US National Transportation Safety Board announced it would investigate the incident.

Hawaiian Airlines said in a statement on Monday that 36 people had been evaluated at the airport upon landing, and that it was "continuing to support" the 17 passengers and three crewmembers who were transported to area hospitals for treatment, 11 with serious injuries.

Turbulence on United Airlines flight, 5 injured

Meanwhile, two passengers and three crewmembers on a United Airlines flight from Rio de Janeiro were injured early on Monday by turbulence en route to Houston, Texas. Medical personnel brought them to a local hospital with "minor injuries," United Airlines said in a statement.

An ABC News affiliate captured video of ambulances responding to Bush Intercontinental Airport when the United Airlines flight landed around 5:30 a.m. CST (1130 GMT). It reported that the flight hit turbulence while passing over Cancun, Mexico, citing air traffic radio scanners.

Of the 20 people hospitalised after the Hawaiian Airlines flight, 11 were brought to emergency rooms in serious condition, according to Honolulu Emergency Medical Services.

Injuries included head laceration, bruises, and loss of consciousness, the EMS said in a statement. A spokesperson said they had no further updates to provide.

Hawaiian Airlines said in its statement on Monday that it was conducting a thorough inspection of the Airbus A330 aircraft before returning it to service.

In video of the inside of the plane shown on CNN, debris was scattered on the floor and there were multiple cracks in the ceiling where people or objects apparently hit with force.

The airline declined to comment beyond the statement, citing the impending safety board investigation.

'Flew up and hit the ceiling'

Tiffany Reyes had just gotten back to her seat from the bathroom and was about to buckle her safety belt when Hawaiian Airlines Flight 35 dipped.

This mobile photo courtesy of passenger Jazmin Bitanga shows the interior of a Hawaiian Airlines plane on its flight from Phoenix to Honolulu. Image Credit: Courtesy of Jazmin Bitanga via AP

In an instant, Reyes found herself on the aisle floor, staring up at caved-in ceiling panels and a cracked bathroom sign that was hanging.

“I asked everyone around me, ’Was that me?” Reyes said in an interview Monday. “They said I had apparently flown into the ceiling and slammed into the ground.”

Reyes, 40, was among 20 people on the flight — passengers and crew — taken to hospitals after turbulence struck their plane flying from Phoenix to Honolulu without warning Sunday.

Eleven people were in serious condition. In all, 36 people received medical treatment for bumps, bruises, cuts and nausea, said Jim Ireland, director of Honolulu Emergency Medical Services.

I asked everyone around me, ’Was that me? They said I had apparently flown into the ceiling and slammed into the ground.

- Tiffany Reyes, Passenger

Reyes was heading home after picking up her daughter Kaylee from college. She initially thought something had hit the plane and that it was crashing. She briefly thought they were going to die because she had never encountered anything so violent on a flight before.

'Most terrifying experience'

“That’s the most terrifying experience I’ve been through in my whole 40 years of life,” Reyes said.

Reyes wasn’t bleeding. And the adrenaline surging through her dulled the pain that would eventually come. She crawled back into her seat. And her daughter, who was buckled up and escaped injury, “just held me the whole time.”

Others had it much worse, Reyes said. She saw a woman walk off the plane with gashes in her head and blood on her face and clothes.

An ambulance took Reyes to an emergency room where she received X-rays, had her blood taken and various other screenings. After five hours there, she and her family – her daughter, son and husband – went home to decompress.

It was just rocky, and then, it quickly just escalated to the point where we’re shaking so much that we were pretty much like floating off of our chairs.

- Jacie Hayata Ano, Passenger

She had a headache which began to fade Sunday night. But the left side of her body started to ache.

“I can’t even move around in bed,” Reyes said. “So I have to sleep right on my back without even moving.”

The full flight had nearly 300 people aboard and carried many passengers traveling to Hawaii for the holidays, like Jacie Hayata Ano, who was heading home.

“It was just rocky,” she told KHON-TV . “And then, it quickly just escalated to the point where we’re shaking so much that we were pretty much like floating off of our chairs."

Just how dangerous is turbulence

The accounts from the Hawaiian Airlines flight were dramatic: Passengers hit the ceiling, objects went flying, people were crying. Sunday's incident was the result of "severe" turbulence, the airline said, the third of four levels on a scale from light to extreme.

But turbulence itself is a frequent occurrence, as any regular traveler can attest - and it typically isn't cause for alarm, experts say.

"Turbulence is normal; it's part of the sky," said Patrick Smith, a commercial pilot for 30 years who runs the Ask the Pilot blog. "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."

Turbulence can get bumpy when you don't expect it. One place likely to be slightly less bumpy: the middle of the plane over the wings. The bumpiest place to sit is in the tail of the plane though it doesn't make a lot of difference.

- Patrick Smith, a commercial pilot

The Federal Aviation Administration describes turbulence as movement of air that usually can't be seen and often happens unexpectedly. Roughly 58 people are hurt because of turbulence every year while not wearing their seat belts, the agency says.

"It can be created by many different conditions, including atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts or thunderstorms," the agency says on its website. "Turbulence can even occur when the sky appears to be clear."

Bob Thomas, a pilot, former weatherman and assistant professor of aeronautical science at Embry-Riddle Aeronautical University, said 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."

In the Hawaiian Airlines case, weather officials told media, there were storms in the flight path. But Snook said there had been no warning of the specific patch of air that the Airbus A330 encountered.

"It can be associated with almost any kind of weather," Smith said. "It doesn't always matter, and it's not always predictable."

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 the 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 likely to be slightly less bumpy: the middle of the plane over the wings, he said. The bumpiest place to sit is in the tail of the plane, he said, though it doesn't make a lot of difference.

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

While worst-case scenario fears are extreme, experts say turbulence still poses a risk - especially if people are not buckled in. Snook, the Hawaiian Airlines executive, said Sunday that it wasn't yet clear how many people on the flight were not wearing seat belts, but that the seat belt sign was on.

"You want to be cautious because the aircraft itself is going to survive," said Mark Baier, CEO of Aviation Manuals, 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."