• A Delta flight encountered severe turbulence last year, with 17 people injured.
• The NTSB's newly published report says none of the injured people were wearing seatbelts.
• One passenger fractured four vertebrae and was hospitalized for over a month.

Newly released details from a 2023 incident that injured 17 airline passengers highlight the dangers of encountering turbulence, and serve as a stark reminder of why keeping your seatbelt on is always a good idea.

A Delta Air Lines Airbus A350 was flying from Milan to Atlanta when it encountered severe turbulence around 40 minutes from landing.

The National Transportation Safety Board made public its investigation last Friday. It found none of the injured people were wearing seatbelts despite the seatbelt sign being on.

10 of those who were injured were flight attendants, two of whom had serious injuries.

The flight's first officer told investigators that he told the cabin crew they should be seated due to expected turbulence. A list of injuries, shared with Delta and the NTSB, said the injured crew members weren't seated at the time.

One of those who was seriously injured suffered two broken ribs and back compression fractures, while the other had compression fractures in their neck and upper back, it said.

The other injuries to flight attendants included muscle strains, a concussion, and a deep cut that required stitches.

Five of the flight attendants were taken to hospital and released within 24 hours.

Of the seven passengers who were injured, one remained in hospital nearly a month after the flight. The person fractured four vertebrae and a rib, according to the NTSB report.

Another seriously injured passenger fractured one vertebra, while the others had muscle strains — mostly in the neck.

None of the injured passengers were wearing a seatbelt, the report said.

One of those passengers reported that her head hit the ceiling, hurting her neck, back, and head, although she was later able to walk off the plane.

Guy Gratton, an associate professor of aviation and the environment at Cranfield University, previously told Business Insider that a plane encountering turbulence is like "taking a box with something in it and starting to shake the box up and down."

"And clearly, if you're the person who's inside the box, then you get thrown around inside the box, and that's where injuries happen," he added.

"Passengers are told to keep their seatbelts done up because if you're tied to the box, you're much less likely to get injured."

While the Delta incident saw four serious injuries, cases of major injuries during turbulence are rare.

Between 2009 and 2023, 184 people were seriously injured by turbulence — an average of 12 a year — according to data from the Federal Aviation Administration. The majority of those, 146, were flight attendants rather than passengers.