On Friday, January 5, the door plug of a commercial Boeing 737 Max 9 came off as the plane was climbing, opening a large hole on the side of the plane, alarming passengers onboard, and raising new questions about flight safety. For now, certain models of that plane have been temporarily grounded by the Federal Aviation Administration in the US and by several aviation authorities abroad, a move that impacts Alaska Airlines, United Airlines, and other companies that utilize this aircraft.

The mishap sharpens the spotlight on air travel, which, while overwhelmingly safe, has been the subject of recent reports about outdated technology that buries important automated warnings, staffing issues leading to air traffic controller shortages, and communication failures contributing to planes nearly colliding. Additionally, it adds to specific scrutiny of the Boeing 737 Max planes, which have been involved in two past crashes and have been the subject of prior software glitches.

[Related: The Boeing 737 Max 8 crashes and controversy, explained]

The recent incident took place about 20 minutes into a flight from Portland, Oregon, to Ontario, California, startling staff and passengers onboard. As depicted in videos and described by witnesses, the door plug suddenly fell away with a loud pop, leaving a hole in the side of the plane. (A plug effectively seals a part of the plane that can be used as a door, closing off the opening if a door hasn’t been installed.) Cellphones, AirPods, a child’s shirt, and a pilot’s headset were reportedly sucked out of the plane due to the change in pressure. Oxygen masks also descended in the plane in order to help people breathe.

Ultimately, pilots were able to conduct an emergency landing back in Portland, and no serious injuries were sustained.

Since the incident, federal authorities — including the FAA and the National Transportation Safety Board (NTSB) — have launched investigations into the incident and are now requiring inspections on 171 Boeing planes before they can be used again.

Inspections of United and Alaska Airlines planes have already found that bolts and other hardware on multiple planes weren’t tight enough, both airlines said. And the FAA is now looking into whether Boeing has met design and safety standards in its manufacturing of the 737 Max 9 plane overall. Passengers on the plane that suffered the malfunction have also filed a class-action lawsuit against Boeing, with their attorney citing the “economic, physical, and ongoing emotional consequences” they say they experienced.

Experts emphasize that such incidents remain rare and that flying overall is still extremely safe compared to other forms of transportation like driving. They note, however, that careful inquiries will be vital to ensure that a concerning mishap like this one doesn’t occur again.

“I wouldn’t be terrified of this. I know it is alarming. But the thing to remember is flying is still very safe,” Dan Bubb, an aviation expert at the University of Nevada Las Vegas, told Vox. “It also underscores a very important thing, which is: Leave your seat belt on at all times.”

Why this incident adds to airline safety questions

Investigators are still looking into what caused this particular failure, including if it’s related to warnings the plane previously fielded. According to NTSB investigators, the plane’s pressurization warning light had already been triggered on three earlier flights, and the plane had been barred from flying long distances across bodies of water as a result.

A major danger in a scenario like this, in which a hole opens up in the cabin, is people being suctioned out of the plane because of the change in pressure. “Anything that is not cinched down is gonna get sucked out of the plane,” says Bubb. Additionally, planes are pressurized so people can breathe at high altitudes, and the gaping hole created by the accident depressurized the cabin, making it harder for people to do so. The oxygen masks that deployed helped address this issue, he notes.

Because people had their seat belts on and because the plane was at an altitude of 16,000 feet, the impact of the door’s loss was thankfully limited to objects, like a smartphone that was later found in a person’s yard. In past instances, that’s unfortunately not been the case. In a 2018 Southwest Airlines accident, a woman died after being partially sucked out of a window, and in a 1988 Aloha Airlines accident, a flight attendant was killed after the top of a plane was torn off.

NTSB Chair Jennifer Homendy noted that a much worse tragedy was averted due to the timing of the incident. “We are very, very fortunate here that this didn’t end up in something more tragic,” Homendy said in a statement. “No one was seated in 26A and 26B, where that door plug is.” She emphasized, too, that things could have been worse if the flight had reached an altitude that would have allowed people to walk around the cabin. Bubb added that the risks of being sucked into a vacuum would be much higher for someone who wasn’t buckled in.

In addition to wearing seat belts, which can also offer important protections in the case of turbulence, Bubb recommends passengers listen to the routine safety demonstrations that flight staff offer at the start of travel in order to be able to act quickly should an emergency occur.

Recently, there have been several high-profile air travel incidents, like an early January crash in Japan that left five people dead and a 2023 FAA system outage that grounded and delayed domestic flights, both of which have contributed to fresh scrutiny of the industry.

That doesn’t mean flying isn’t broadly safe, but it is a reminder of the importance of regulators holding companies and people accountable if there are breakdowns in equipment or staffing. Harvard researchers have found that the chances of being in a fatal flight accident are one in 11 million, compared to one in 5,000 for a car accident, an indication of how rare such incidents are.

Boeing 737 Max planes have also been the particular subject of safety concerns, including in 2018 and 2019 when countries across the globe grounded planes after Boeing 737 Max 8 planes were involved in two plane crashes that killed hundreds of people. That Boeing is facing renewed attention underscores the quality control inquiries it has faced about issues including engine construction and design flaws.

“Commercial aviation today is safe in all sorts of measurable and immeasurable ways,” Timothy Ravich, an associate professor at the University of Central Florida, told Vox, while adding that “reports of operational concerns — from runway incursions to passenger air rage to pilot training concerns to aircraft production” — have understandably heightened people’s worries about safety. Experts note that it’s vital for regulators to take an aggressive response to these concerns — in the form of groundings, inspections, and, if needed, new rulemakings — to rebuild public trust and prevent such incidents from occurring.

Update, January 12, 5 pm ET: This story was originally published on January 8 and has been updated to include information about an FAA investigation and a class-action lawsuit.