While the two men had years of aviation experience, this mission involved unfamiliar duties, and it was the first time they had flown together.
Hersman said the pilot trainee told investigators he was blinded by a flash of light at about 500 feet, which would have been 34 seconds before impact and the point at which the airliner began to slow and drop precipitously. She said lasers have not been ruled out. It was unclear, however, whether the flash might have played a role in the crash.
Hersman also said a third pilot in the jump seat of the cockpit told investigators he was warning them their speed was too slow as they approached the runway.
Hersman stressed that while the trainee pilot was flying the plane, the instructor was ultimately responsible and, thus, the way they worked together will be scrutinized.
Details emerging from Asiana pilot interviews show the captains thought the airliner's speed was being controlled by an autothrottle set for 157 mph.
Inspectors found that the autothrottle had been "armed," or made ready for activation, Hersman said. But investigators are still determining whether it had been engaged. In the last two minutes, there was a lot of use of autopilot and autothrottle, and investigators are going to look into whether pilots made the appropriate commands and if they knew what they were doing, she said.
When the pilots realized the plane was in trouble, they both reached for the throttle. Passengers heard a loud roar as the plane revved up in a last-minute attempt to abort the landing.
Hersman cautioned against speculating about the cause of the crash. But she stressed that even if the autothrottle malfunctioned, the pilots were ultimately responsible for control of the airliner.
"There are two pilots in the cockpit for a reason," she said. "They're there to fly, to navigate, to communicate and if they're using automation, a big key is to monitor."
The flight originated in Shanghai and stopped over in Seoul before making the nearly 11-hour trip to San Francisco.
A dozen survivors remained hospitalized Wednesday, half of them flight attendants, including three thrown from the jet. Meanwhile, other survivors and their family members visited the crash site, where some shed tears and others stood in disbelief, passenger Ben Levy said. They were kept about 50 yards away from the wreckage, which was surrounded by metal railing.
"What I think I really came for was to meet other fellow passengers and share a bit of our stories," Levy said. "How we felt and how we got out of that plane."
Associated Press writers Joan Lowy in Washington, Terry Collins in San Francisco, Haven Daley in Scotts Valley and Peter Banda in South San Francisco contributed to this report.
Follow Martha Mendoza at https://twitter.com/mendozamartha .
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