Speaking at a news conference to discuss the crash of Flight 214, which killed two teenagers from China and injured scores more, Hersman said the aircraft reached its lowest speed of 103 knots three seconds before impact, and at that moment the crew was throttling the engines in a last-second attempt to speed up.  
  
The plane lost so much speed, Hersman said, that a vibrating ‘‘shaker stick’’ in the cockpit signaled an impending stall, a condition in which the wings lose lift and a plane can’t be controlled.  
  
Shocked

Aviation experts said they were shocked the jet had lost so much speed and that the crew apparently made no effort to speed up until the final seconds. The crew first sought to accelerate 7.5 seconds before impact, investigators said, and with 1.5 seconds left, someone in the cockpit said the crew would try to pull up and circle around.  
  
‘‘That’s astounding,’’ said Kevin  Hiatt, a former chief pilot for Delta Airlines who heads the Flight Safety  Foundation in Alexandria, Va. ‘‘If 137 (knots) is the designated approach speed and they got to 103, there’s a wow factor there. Your tolerance on an approach is generally plus or minus five knots on a clear day.’’  
  
Hiatt said the slow speed raised the question of whether the pilot - who was in training to fly the 777 - thought he had an automatic throttle engaged to set the plane at a targeted speed. The safety board said the throttles were set to idle during the approach.  
  
Hiatt also wondered how it was possible for the pilot and his training captain to not notice the loss of speed.  
  
‘‘I would expect they would feel it, and I would expect that one of the other pilots would have pointed it out a lot sooner,’’ Hiatt said. ‘‘With the check pilot being there, I’m surprised it wasn’t revealed earlier. It’s unusual, there’s no doubt about it.’’  
  
Hiatt said the plane likely stalled before crashing, essentially floating through the last few seconds. Had the plane cleared the seawall and made the runway in such a stall, he said, the crew still might not have avoided the disaster.  
  
Hersman said it was too early to discuss possible causes of the crash or whether the pilot or the other three members of the crew might be at fault. The flight originated in Shanghai, stopped over in Seoul, and was carrying 307 people on its 11-hour leg to San Francisco.  
  
But Hersman said everything seemed to be going fine as the plane began its approach to the airport -- a straight 17-mile path on a clear day.  
  
She said the descent was not unusually steep. The autopilot was disengaged at 1,600 feet, or 82 seconds prior to impact, which was a standard move. Nine seconds later, at 1,400 feet, the plane was going 173 knots, Hersman said, but it slowed rapidly from there.

50 per cent power  

When the plane reached its lowest speed, she said, the engines were at about 50 per cent power and the engine power was increasing. According to aviation experts, airplane engines that have been set at idle do not respond immediately to being throttled back up.

Hersman said the two 16-year-old girls who died - Ye Mengyuan and Wang Linjia, who were on a summer educational trip from Jiangshan City, China - were sitting near the back of the plane, which suffered more structural damage than the front.  
 
‘‘It’s an area where we’re seeing a lot of the serious injuries as well,’’ Hersman said.  
  
Hersman said her team was looking into reports from survivors of the crash that one or more escape chutes deployed into the aircraft instead of outside of it. ‘‘We need to determine why that happened,’’ she said.  
  
Hersman also said the San Mateo County Coroner is leading an investigation into whether one of the two teenagers who died was run over by an emergency vehicle. Both of their bodies were found outside the aircraft.  
  
Ninth flight

An Asiana spokeswoman, reached in Seoul, identified the chief pilot as Lee Jeong-min, who joined Asiana in 1996. She said Lee Kang-guk, who has been with the airline since 1994 and has logged more than 10,000 flying hours, was at the controls during the landing.  
  
Saturday’s flight was the ninth time Lee Kang-guk had flown a Boeing 777, and it was his first time landing that type of plane at SFO, said spokeswoman Hyomin Lee. She said that technically Lee Kang-guk was still training on the 777, but given his years of his experience, ‘‘training is an unsuitable word to describe him.’’  
  
He had landed a Boeing 747, another large-capacity plane, at SFO ‘‘many, many times,’’ she said. ‘‘He was very experienced.’’  
  
Hiatt said the pilot’s experience would be looked at closely, but added, ‘‘That’s not an uncommon scenario. You’ll have a check pilot with you until you they determine you are capable to fly without a check pilot.’’

San Francisco Chronicle