Commercial aviation has suffered its deadliest year since 2018 after the Jeju Air Co. disaster in South Korea and last week’s downing of an Azerbaijan Airlines plane.

Fatalities onboard passenger aircraft jumped to 318 this year with the two recent accidents, data compiled by Cirium shows. That’s by far the highest death toll since more than 500 people died in 2018, a year marked by the first of two Boeing Co. 737 Max crashes.

This year started and ended with tragedies in Japan and South Korea during attempted landings, among the most dangerous phases of flight. Fatal aviation accidents remain very rare, and one major incident can suddenly turn a statistically safe year into one of the worst.

“The recent spike falls into the margins of unpredictability,” said Darren Straker, a former head of air accident investigation units in the United Arab Emirates and Hong Kong. He suggested airline crews could be better trained to respond to so-called outlier events.

Investigators have yet to determine why a Jeju Air Boeing 737-800 — a predecessor to the Max — careered down the runway at Muan International Airport early Sunday with no landing gear deployed and smashed into a concrete wall. All but two of the 181 people on board died as the wrecked jet exploded into a ball of fire.

The fatal Jeju Air crash was the airline’s first, and ranks as the worst civil air accident ever in South Korea.

As the plane neared the airport, one of the pilots reported a bird strike, declared a mayday and initiated a fly-around procedure, according to Korean transport ministry officials. The plane then turned back and attempted to land in the opposite direction, with the barrier in its path at that end of the runway.

While it seems increasingly evident a bird strike was “an initiator of this tragic chain of events,” what happened next is less clear, said Jeff Guzzetti, a former accident investigation chief for the U.S. Federal Aviation Administration.

Key questions include why flaps weren’t extended that could have slowed the plane down, and why the landing gear wasn’t lowered, Guzzetti said. If birds damaged both of the engines, that could have caused a loss of power affecting those other systems, he said.

“If you have both engines out — or even one engine — there’s going to be lots of cautions and warnings and bells and whistles going off in the cockpit,” he said. “And that can really challenge a flight crew.”

While the Jeju Air aircraft was almost entirely destroyed, investigators will have valuable data to work with as they reconstruct the event. The damaged flight data device may take longer to analyze than the cockpit voice recorder.

After another Jeju plane experienced a landing-gear issue, Korean authorities on Monday ordered maintenance record checks of 101 other 737-800s in operation among local airlines. Boeing shares fell 1.8% as of 1:08 p.m. in New York, bringing their 2024 decline to 32%.

The wider inspections came as investigators started to analyze the cockpit voice recorder and the damaged flight data device, which hold crucial clues into the movements of the jet and the actions and state of the pilots.

In January, a huge Japan Airlines Co. jet collided with a coast-guard plane at Tokyo’s Haneda Airport. Everyone aboard the commercial jetliner survived, but five people on the smaller aircraft were killed. The year was also marred by a fatal case of severe turbulence in May on a Singapore Airlines Ltd. jet flying over Myanmar.

In July, Nepal’s Saurya Airlines crashed after taking off from Kathmandu, killing 18. And in August dramatic footage of a plane operated by Brazilian airline VoePass captured the turboprop freefalling from the sky after encountering icy weather conditions. That accident killed 62.

Global conflicts may also have contributed to this year’s aviation fatalities. The Azerbaijan Airlines Embraer SA 190 had nearly completed a scheduled flight from the capital city of Baku to Grozny, Russia, on Dec. 25 when it was suddenly diverted across the Caspian Sea. The damaged plane crashed about 3 kilometers (1.9 miles) from its destination of Aktau, Kazakhstan. Azerbaijan’s president has said the plane was accidentally shot down by Russia.

The spike in fatalities marks a reversal from 2023, which became the safest year ever in aviation with no deaths among large passenger jetliners — typically planes made by Airbus SE and Boeing.

Airline safety has been improving for decades as regulators, manufacturers and carriers incorporate lessons learned to minimize accidents and improve the chances of survival when things go wrong. Fire-repelling designs on the carbon-hulled Airbus A350 involved in January’s Tokyo crash, for example, combined with near-flawless execution by the JAL crew and rescue team to allow all 379 people onboard to survive.

It’s best to look at aviation safety over decade-long periods, said Jan-Arwed Richter, an aviation safety consultant who runs the Jacdec research site in Hamburg, Germany.

“I’m pretty confident that 2024 may be regarded as an outlier in years to come,” he said.

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(With assistance from Heejin Kim and Allyson Versprille.)

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