The captain flying, identified by Asiana as Lee Gang Guk, and the captain instructor, Lee Jeong-min, realized as they passed 4,000ft on approach to Runway 28 Left that they were "slightly high", says Deborah Hersman, chairman of the National Transportation Safety Board.
At that point, the crew set the vertical speed mode on the visual approach at about 1,500ft per minute, she says. But that descent rate brought the 777 down too fast.
As they passed 500ft, Lee Jeong-min, who was making his debut flight as a 777 instructor, noticed the three glowing red lights on the airport's precision approach path indicator that signaled they were slightly too low, Hersman says the captain told the NTSB.
The instructor told the captain flying to pull-up, Hersman says. At roughly the same time, the aircraft yawed off the centreline, forcing the crew the to make a quick series of corrections in two directions.
"They were making corrections vertically because they knew they were too low," Hersman says, "and they are making lateral corrections to line up on the centreline."
Amidst this burst of activity, both crew members lost track of the aircraft's perilously slowing speed, with the aircraft climbing slightly to regain altitude and the engines stuck on idle. At least one of the crewmembers, Lee Jeong-min, believed he was under a fatal misapprehension.
"They had set speed at 137kt (254km/h) and he assumed the autothrottles were maintaining speed," Hersman says, based on the interview with Lee Jeong-min.
But the automatic speed protection system, for reasons that are still unclear, did not maintain the aircraft at 137kt.
"He went to push the [throttles] forward, but he stated that the other pilot had already pushed the throttles forward," Hersman says.
Unfortunately, it was already too late. The aircraft slowed to a low speed of 103kt, then accelerated to 112kt as the nose crossed the threshold of the runway. But the 777 was still too low and the main landing gear clipped the edge of the sea wall separating Runway 28 Left from San Francisco Bay. That caused the aircraft to sink further and the tail to strike the runway, ripping Section 47/48 aft of the pressure bulkhead off the airframe.
The pilots reported seeing the aircraft "balloon" upward, yaw left and spin in a 360 degree circle as the aircraft crashed down on the runway again. The collision tore open an oil tank on the No. 2 engine, igniting a fire that eventually would burn through the upper section of the forward fuselage.
Remarkably, the crash killed only two people, but injured more than 180, including several critically.
The NTSB is now working to solve a number of puzzles related to the crash, especially why the autothrottles did not perform in the manner that the crew expected.
After the crash, the NTSB entered the cockpit and found the autothrottles in the armed position, Hersman says. Her investigators are now verifying that discovery with the flight data recorder. If he recorder corroborates the on-scene finding, the NTSB will then explore how the autothrottles function in different operational modes.
"Armed means that they are available to be engaged, but depending on what mode is used we really need to understand that a little better," Hersman says.
The NTSB also is making inquiries with the US Federal Aviation Administration about the how air traffic on approach to Runway 28 Left has been managed since 1 June, when airport officials deactivated the glideslope indicator for three months to complete a construction project at the other end of the runway.
"They are asking for this information because it provides us some baseline information and they can identify trends," she says.
A new concern has appeared about the aircraft's crashworthiness. Crew members and witnesses have reported that at least one of the escape slides deployed within the aircraft, trapping one flight attendant until it was removed by the relief first officer, Hersman says.
The NTSB also is interested in possible human factors related to the backgrounds and experience levels of both the captain flying and the captain instructor.
Lee Gang Guk's 43h of flying time in the 777 has been well documented, but Hersman also revealed that he had been flying Airbus A320s for eight years until only a few months ago.
"Immediately prior to his initial operating experience on the 777 he was flying as a captain on theA320," she says.