The two pilots at the controls during the accident had also been in the cockpit for takeoff. Then they rested during the flight while a second pair of pilots took over. The two pairs swapped places again about 90 minutes before landing, giving the trainee a chance to fly during the more challenging approach phase.
The investigation is ongoing, and Hersman cautioned against speculating about the cause. 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 Wednesday. "They're there to fly, to navigate, to communicate and if they're using automation a big key is to monitor."
As the trainee pilot flew, she said, the instructor captain, who is ultimately responsible for flight safety, was tasked with monitoring. The third pilot was in the cockpit jump seat also to monitor the landing.
Crash survivor Brian Thomson, who was returning from a martial arts competition in South Korea and walked away unscathed, said he's not concerned about the pilot's lack of experience with the airliner.
"At some point you have to start at hour one, hour two. It's just natural. Everyone starts a career someway, somehow. Starts a new plane someway, somehow. They have to have training," he said.
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 flight attendants, including three thrown from the airliner during the accident.
Meanwhile, fire officials continued their investigation into whether one of their trucks might have run over one of the two summer camp bound Chinese teenagers, Wang Linjia and Ye Mengyuan, killed in the crash.