Investigators expect to spend weeks or months determining what caused Asiana Airlines Flight 214 to crash-land at the San Francisco airport Saturday, killing two of the 307 passengers and crew members aboard.
The National Transportation Safety Board and South Korean investigators are interviewing the flight’s pilots and crew this week. Two South Korean investigators are also scheduled to arrive in Washington Tuesday to examine the plane's black box.
As the probe continues, here are five key factors known so far that may have contributed to the crash landing:
1. The plane was approaching the landing too low and too slowly.
Flight 214 was on a straight, 17-mile approach to the San Francisco International Airport on Saturday, according to NTSB Chairman Deborah Hersman, when it quickly lost too much speed and height to make a safe landing.
Data show that the plane’s autopilot was turned off 82 seconds before the crash, when the plane was about 1,600 feet in the air, Ms. Hersman said Monday. At 34 seconds before the crash, the plane was at 500 feet and its speed dropped below the level required for a safe landing.
Seven seconds before collision, the cockpit recorder captured a call from somebody in the cockpit to increase the plane’s speed, Hersman said, and three seconds later, the control yoke started vibrating to warn of the potential to stall. At 1.5 seconds before the impact, a crew member issued a call to abort the landing, but it was too late to regain speed and height. The tail of the plane appears to have clipped the sea wall there, sending the plane slamming into the runway, where it skidded about 2,000 feet before catching fire.
In the last three seconds, the plane was traveling at least 37 miles per hour below the minimum recommended approach speed of 137 knots (about 158 miles per hour).
2. The control pilot and supervising pilot were new to their positions.
The pilot at the controls during the attempted landing, Lee Gang-guk, had logged only 43 hours at the controls of a Boeing 777, the aircraft he was flying Saturday. Mr. Lee had more than 10,000 hours of experience on other planes, including the Boeing 747, according to an Asiana Airlines spokeswoman, but was making his first landing at San Francisco with the larger 777 model.
The pilot serving as a training supervisor for Lee was also in a new position. Although the supervisor, Lee Jeong-min, had landed 33 times at the San Francisco airport in a Boeing 777, he had become a training supervisor in June, and Saturday’s flight was his maiden flight as a Boeing 777 supervisor, an Asiana official told The Wall Street Journal Saturday.
“Lee Jeong-min has experience landing at San Francisco airport 33 times in a 777," Mr. Yoon said. "And as a trainer, while 500 hours of experience is required, he has more than 3,200 hours of experience."
Multiple questions have been raised about the actions of the pilots, including why cockpit voice recordings show the two didn’t communicate until less than two seconds before the plane struck the sea wall and why the supervising pilot didn’t call for an aborted landing sooner.
Investigators are on their second day of interviewing the four pilots aboard the flight. Whether all four pilots were in the cockpit during the landing, as is normal procedure, or just the two known pilots has not yet been publicly released.
3. An automated landing system was out of use due to airport construction.
An automated navigation system, known as Glide Path, was turned off at the San Francisco airport Saturday because of airport construction.
The landing system is meant to help planes land in bad weather, but pilots have grown increasingly reliant on it, according to Cass Howell, a former military pilot and now a human factors expert at Embry-Riddle Aeronautical University in Daytona Beach, Fla.
"If your last dozen landings were autopilot landings and here you are faced with nothing but visual (cues) to deal with, your rust factor would be greater," Dr. Howell told the Associated Press. "Too much automation can undermine your flying skills."
Oscar S. Garcia, CEO of InterFlight Global, a consulting firm in Miami, and a former 777 pilot with a major Asian carrier, told the The New York Times that in Asia, “there is high reluctance to hand-fly the airplane.”
But aircraft safety experts told Reuters that Glide Path is “far from essential for routine landings,” because of other systems and visual clues, and that it was common to turn off the system in good weather or during airport construction. According to a notice from the airport on the Federal Aviation Administration's website, San Francisco International Airport has turned off the system for nearly the entire summer because of construction. The notice showed the system out of service June 1-Aug. 22, Reuters reports.
4. San Francisco can be a tricky airport to navigate.
“Many pilots and safety experts consider San Francisco International Airport a particularly tricky place to land and takeoff, due to nearby peaks, closely spaced parallel runways and the area’s frequent thick fog,” The Wall Street Journal reports.
Fog was not a factor in the Asiana crash. Rather, it was an unusually clear day.
5. Engine or system failure isn’t thought to be a factor, for now.
Both engines on the Asiana Boeing 777 were operating normally when the plane crash-landed on Runway 28 Left at the San Francisco airport, the NTSB said Monday.
That statement came a day after Yoon, the Asiana president, said preliminary reports indicated there were no mechanical failures.
"For now, we acknowledge that there were no problems caused by the 777-200 plane or (its) engines," he told a media conference Sunday at the company headquarters in Seoul.
But Hersman, the NTSB chair, refused to rule anything out on Monday.
“We are certainly looking at pilot performance, and we’re looking at communication between the two crew members,” she said. “But everything is still on the table.”