The pilot at the controls of an Asiana plane that crashed-landed was guiding a Boeing 777 into the San Francisco airport for the first time, and tried but failed to abort the landing after coming in too slowly, aviation and airline officials said.
It was unclear if the pilot's inexperience with the aircraft - he had only 43 hours of flight time on a 777 - played a role in Saturday's crash, in which two Chinese students, Ye Mengyuan and Wang Linjia, both 16, died.
Officials were investigating whether the airport or plane's equipment could have also malfunctioned before the plane slammed into the runway, broke apart and burst into flames.
Remarkably, 305 of 307 passengers survived and more than a third did not need admission to hospital. Only a small number were critically injured.
The head of the National Transportation Safety Board, Deborah Hersman, said on Sunday the slow speed of Flight 214 on the final approach triggered a warning that the plane could stall, and an effort was made to abort the landing but the plane crashed barely a second later.
Hersman told a news conference the aircraft was travelling at speeds well below the target landing speed of 137 knots, or 253km/h. "We're not talking about a few knots," she said.
Hersman described the frantic final seconds of the flight as the pilots struggled to avoid crashing.
Seven seconds before the crash, the pilots recognised the need to increase speed, she said, basing her comments on an evaluation of the cockpit voice and flight data recorders.
Three seconds later, the aircraft's stick shaker - a piece of safety equipment that warns pilots of an impending stall - went off. The normal response to a stall warning is to boost speed and Hersman said the throttles were fired and the engines appeared to respond normally.
At 1.5 seconds before impact, there was a call from the crew to abort the landing.
The details confirmed what survivors and other witnesses said they saw: an aircraft that seemed to be flying too slowly just before its tail apparently clipped a seawall at the end of the runway and the nose slammed down.
Pilots normally try to land at the target speed, in this case 137 knots, plus an extra five knots, said Bob Coffman, an American Airlines captain who has flown 777s. He said the briefing raised an important question: "Why was the plane going so slow?"
The airline said yesterday in Seoul the pilot at the controls had little experience flying that type of plane and was landing one for the first time at that airport.
Asiana spokeswoman Lee Hyomin said Lee Gang-guk, who was at the controls, had nearly 10,000 hours flying other planes but only 43 hours on the 777, a plane she said he was still getting used to flying. Another pilot on the flight, Lee Jeong-min, had about 12,390 hours of flying experience, including 3,220 hours on the 777, according to the Ministry of Land, Infrastructure and Transport in South Korea. Lee Jeong-min was the deputy pilot, tasked with helping Lee Gang-guk get accustomed to the 777, according to Asiana Airlines.
Among the questions investigators are trying to answer was what role, if any, the deactivation of a ground-based landing guidance system due to airport construction played in the crash.
The flight began in Shanghai, China, stopped in Seoul, South Korea, before making the nearly 11-hour trip to San Francisco. The South Korea-based airline said four South Korean pilots were on board, three of whom were described as "skilled".
Among the travellers were citizens of China, South Korea, the United States, Canada, India, Japan, Vietnam and France. There were at least 70 Chinese students and teachers heading to summer camps, according to Chinese authorities.
Fei Xiong, a Chinese passenger, was travelling to California so she could take her eight-year-old son to Disneyland. The pair were sitting in the back half of the plane. Xiong said her son sensed something was wrong.
"My son told me: 'The plane will fall down, it's too close to the sea'," she said. "I told him: 'Baby, it's OK, we'll be fine'."