SAN FRANCISCO (TheBlaze/AP) -- A federal safety official said at a news conference Sunday that Asiana Flight 214 was traveling at speeds "significantly below" the target landing speed of 137 knots per hour, or 157 miles per hour.
"We're not talking about a few knots," she said.
This photo provided by Dawn Siadatan shows Asiana Airlines flight 214 just moments after crashing at the San Francisco International Airport in San Francisco, Saturday, July 6, 2013. (Credit: AP)
National Transportation Safety Board chief Deborah Hersman added that the plane's cockpit voice recorder showed the Boeing 777's crew calling to abort the landing about 1.5 seconds before impact. In addition, she said, the recorder showed the jetliner receiving a warning that it could stall because it was flying too slowly; an attempt was made to increase its speed before it crashed, Hersman said.
Hersman also said the aircraft's stick shaker - a piece of safety equipment that warns pilots of an impending stall - went off moments before the crash. The normal response to a stall warning is to increase speed to recover control.
There was an increase in speed several seconds before the crash, she said, basing her comments on an evaluation of the cockpit voice recorder and flight data recorder. They contain hundreds of different types of information on what was happening to the plane.
And at 1.5 seconds before impact, there was a call for an aborted landing, she said.
Pilots normally try to land at the target speed, in this case 137 knots, plus an additional five more knots, said Bob Coffman, an American Airlines captain who has flown 777s. He said the briefing raises an important question: "Why was the plane going so slow?"
The plane's Pratt and Whitney engines were on idle, Hersman said. But the normal procedure in the Boeing 777, a wide-body jet, would be to use the autopilot and the throttle to provide power to the engine all the way through to landing, Coffman said.
There was no indication in the discussions between the pilots and the air traffic controllers that there were problems with the aircraft.
Hersman earlier said investigators are looking into what role the shutdown of a key navigational aid may have played in the crash. She said the glide slope - a ground-based aid that helps pilots stay on course while landing - had been shut down since June.
In this photo provided by the National Transportation Security Board (NTSB), NTSB investigators conduct a first site assessment overnight of the Asiana Airlines flight 214 that crashed at the San Francisco International Airport in San Francisco, Saturday, July 6, 2013. (Credit: AP)
She said pilots were sent a notice warning that the glide slope wasn't available. Hersman told CBS' "Face the Nation" that there were many other navigation tools available to help pilots land. She says investigators will be "taking a look at it all."
Since the crash, clues have emerged in witness accounts of the planes approach and video of the wreckage, leading one aviation expert to say the aircraft may have approached the runway too low and something may have caught the runway lip - part of a seawall at the foot of the runway.
San Francisco is one of several airports around the country that border bodies of water that have walls at the end of their runways to prevent planes that overrun a runway from ending up in the water.
Since the plane was about to land, its landing gear would have already been down, said Mike Barr, a former military pilot and accident investigator who teaches aviation safety at the University of Southern California.
It's possible the landing gear or the tail of the plane hit the seawall, he said. If that happened, it would effectively slam the plane into the runway.
Noting that some witnesses reported hearing the plane's engines rev up just before the crash, Barr said that would be consistent with a pilot who realized at the last minute that the plane was too low and was increasing power to the engines to try to increase altitude.
Barr said he could think of no reason why a plane would come in to land that low.
"When you heard that explosion, that loud boom and you saw the black smoke ... you just thought, my god, everybody in there is gone," said Ki Siadatan, who lives a few miles away from the airport and watched the plane's "wobbly" and "a little bit out of control" approach from his balcony.
"My initial reaction was I don't see how anyone could have made it," he said.
Here's a report from NBC Nightly News: