Investigators probing the cause of the worst civil aviation accident ever in South Korea will focus on a bird strike and the unusual landing-gear failure in the final moments of the fateful flight that left all but two of the 181 occupants of the Boeing Co. 737 jet dead.

The 737-800 aircraft operated by Jeju Air Co. crashed at Muan International Airport on Sunday morning, skidding along the runway on its belly before smashing into a wall, where it exploded into a ball of fire. Only a pair of flight attendants survived.

While the aircraft was almost entirely destroyed, investigators will have valuable data to work with as they reconstruct the event. One vital key will be a readout of the two flight recorders, which were already pulled from the wreckage, though one device is damaged and may need longer to analyze.

Then there’s footage showing the aircraft during approach with one engine apparently flaming out, alongside videos of the plane coming in to the airport and sliding along the runway at high speed, appearing largely intact, before the impact with the embankment.

The accident poses several unusual mysteries, and investigators have said it’s too soon to speculate what may have caused the crash. Mid-air bird strikes are rare but not entirely uncommon and seldom deadly because aircraft can operate on one engine for some time. Why the landing gear didn’t deploy also remains unclear, or indeed if there’s a link between that malfunction and the bird strike that was discussed between cockpit and control tower just before the landing.

The pilot, considered an experienced captain with close to 7,000 hours of active duty, issued a mayday emergency call minutes after the control tower warned of a bird strike. He aborted his first landing, started a go-around and switched direction on the runway in his second attempt. The control tower granted clearance to land in the opposite direction, and officials said it’s unlikely that the runway length caused the crash.

The Boeing 737 involved in the crash is a predecessor to the latest Max variant. It’s considered a reliable workhorse that passed routine maintenance checks, in a country with deep expertise for aircraft servicing. Around the world, there are more than 4,000 planes of its type in service.

Even if one of the black boxes was damaged in the crash, the data storage units can often be reconstructed to aid the investigation. The fortified devices contain vital statistics and performance metrics of a flight, as well as taped conversations and sounds from the cockpit.

Muan’s control tower warned of the risk of a bird strike at 8:57 a.m. local time, about two minutes before the pilot declared an emergency, officials said. The airport had four staffers working to prevent bird strikes at the time of the crash, including one outside the tower.

Birds are an aviation hazard because they can be ingested into the turbine or damage other parts of the plane and cause engine failure. In 2009, an Airbus A320 landed in the Hudson River in New York after a bird strike damaged both engines, in what has become known as the “Miracle on the Hudson” because everyone on board survived.

Jeju Air’s 15-year-old plane, registered HL8088, entered service with the carrier in 2017. It was initially delivered in 2009 to Irish discount airline Ryanair Holdings Plc, according to the Planespotters.net database. The jet was configured to seat as many as 189 passengers. Founded in 2005, Jeju Air operates 42 aircraft, according to its website.

There was no sign of malfunction during regular maintenance checks, Kim E-Bae, chief executive officer of Jeju Air, said at a news briefing. The jet was returning from Bangkok overnight in a 4 1/2 hour flight. The plane, which YTN said had been chartered by a local travel agency for a Christmas holiday trip, previously left Muan for the Thai capital on Saturday evening.

Muan is a small regional airport located in the country’s south that opened in 2007. It was built to help connect cities including Gwangju and Mokpo and increased its regular service of international flights this year, including those of Jeju Air.

The two surviving flight attendants were taken to hospital, and one of the two survivors is in intensive care unit with a thoracic spine fracture, the doctor at the hospital said in a press briefing.

Boeing said it’s in contact with Jeju Air and ready to offer support. Aircraft manufacturers typically send specialists to crash sites to aid an investigation. Recovery of the victims, some of whom were ejected from the aircraft after the impact, has been completed and salvage crews are now searching the wreckage for passengers’ belongings, Yonhap said.

More than 1,500 people including police, military, coast guard and local government personnel are assisting at the crash site, the Ministry of Land, Infrastructure and Transport said. The airport’s runway will remain closed in coming days.

The accident is the deadliest passenger airline disaster in South Korea to date, surpassing the fatality toll from an Air China plane crash near Busan in 2002 that killed 129 people, according to the Aviation Safety Network. The crash is also among the worst globally this decade.

South Korea is currently experiencing a deepening political crisis after its president provoked public outrage by briefly imposing martial law earlier this month. Acting President Choi Sang-mok declared a week of mourning.

The crash is the second major air disaster in less than a week. An incident in Russian airspace led to the crash of an Azerbaijan Airlines passenger aircraft on Dec. 25, killing dozens.

After a year of not a single fatal accident among the 37 million commercial aircraft movements in 2023, this year has seen a rising number of cases. Early in January, an approaching Japan Airlines Co. Airbus A350 crashed into a small plane on a runway in Tokyo, killing five occupants in the stationary aircraft.

A few days later, a door plug blew out of an airborne Boeing 737 Max 9 flying in the U.S. Though nobody was killed in that accident, the episode threw the U.S. planemaker into deep crisis because it exposed sloppy workmanship at the company.

In August, a smaller ATR turboprop plane operated by Brazil’s VoePass crashed near Sao Paulo’s Guarulhos International Airport, killing 58 passengers and four crew members.

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(With assistance from Aradhana Aravindan.)

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