At a press conference Tuesday, the National Transportation Safety Board said it had interviewed three of the four pilots on the plane, which crash-landed Saturday. Two people were killed and scores were hurt.
Hersman said the training pilot, who served as the leader of the cockpit crew, noticed soon before the crash that the plane was going in too low. The pilot said the crew thought the auto throttle was maintaining speed, but it was not. The crew tried to abort the landing but it was too late, she said.
The flight crew was "very cooperative and forthright" with investigators, she added.
Lee Kang-kook was at the controls of the flight. It was his first time landing a Boeing 777 at the San Francisco airport, and with a key part of the airport's automated landing system not working, he was forced to visually guide the massive jetliner onto the runway.
Officials said Lee and the more-experienced instructor pilot sitting next to him didn't discuss the predicament. Cockpit voice recordings show that the two didn't communicate until less than two seconds before the plane struck the sea wall and then slammed into Runway 28L.
Officials said the Asiana jetliner had fallen more than 30 knots below its target landing speed in the seconds before it crashed, even as the crew desperately tried to apply more engine power.
But even before that, the aircraft had departed from a stable and planned approach to the runway, failing to keep up with its intended speed of 134 to 137 knots at 500 feet over the bay.
Michael L. Barr, an aviation safety expert and former military pilot who teaches at USC, said that at 500 feet the pilots should have had a stable approach in which the aircraft was on its proper glide slope, on course to the center line of the runway and at its proper airspeed. Otherwise, the landing should have been aborted and a "go around" taken for another attempt.
Pilots can be reluctant to abort a landing, even when the approach is unstable, Barr said. Although pilots' willingness to abort a bad approach has improved, it remains a problem in the industry.
The Washington, D.C.-based Flight Safety Foundation, which advocates for airline safety, said in a recent published report that 97% of the time, pilots do not abort a flight from an unstable approach. The reasons they most often cite are their experience and competency to recover.
But Lee had only 43 hours of experience in that type of jet, although he had many thousands of hours in other Boeing aircraft, including the 747. He was being supervised by the more experienced Capt. Lee Jung-min, though he too did not call for a go-around until 1.5 seconds before the crash — far too late to abort. By then, the aircraft's systems were already warning that it was near stall, a condition in which it does not have enough lift to continue flying.
Only seconds earlier, Lee Jung-min had called for more engine power, but that also came too late.
Barr said the powerful engines on big jetliners can take up to 10 seconds to go from idle to full thrust.
"Ten seconds when you are low to the ground is like a lifetime," he said.
At three seconds before impact, the jet's speed dropped to 103 knots and the engines were spooling up but still at only 50% of full power. The jet's aft fuselage clipped the sea wall and the plane slammed into the ground.
Investigators were combing through the wreckage Monday. The lower portion of the plane's tail cone is on the rocks at the sea wall, officials said, and a "significant piece" of the tail is in the water. More pieces of the plane are visible in the water when the tide goes out. At the edge of the tarmac, investigators found the horizontal stabilizer, the vertical stabilizer and the upper portion of the tail cone.
Farther down Runway 28L, investigators have documented pieces of the landing gear and fractured pieces of the aft fuselage, as well as sea wall debris several hundred feet away.
The San Francisco airport's glide path instruments were taken out of service in June for construction, though the crew had two other automated systems to help make a smooth landing. But flight crews have become increasingly reliant on the automated systems, and in many cases jetliners execute fully automated landings. In the process, crews are at risk of losing their proficiency to handle the complex jobs with their own skills.
The lack of the automated systems should not have been a problem, said Jared Testa, chief flight instructor at Embry-Riddle Aeronautical University's Arizona campus.
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