WASHINGTON — San Francisco International Airport, with its tightly spaced runways that extend right up to the water, requires more skill for landing than most of the nation's big airports, experienced airline pilots say. That challenge was further complicated by the shutdown of a ground-based instrument landing system and the movement of runway thresholds prior to the crash Saturday of a Korean airliner.
The instrument landing system, or ILS, uses radio signals to create a three-dimensional "glide slope" for planes to follow so they aren't too high, too low or too far to the right or left. The ILS for runway 28 left, where the plane crashed, had been shut down since June and the beginning of the runway was moved 300 feet to the west to accommodate construction at the airport, according to pilots who use the airport.
National Transportation Safety Board Chairman Deborah Hersman said Sunday investigators will look at what role, if any, the absence of the ILS may have played in the accident.
Airline pilots with experience flying the Boeing 777 or flying into San Francisco told The Associated Press that the Federal Aviation Administration notified pilots in June that the ILS was turned off. Pilots were also warned that the beginning of runways 28 left and right had been moved.
A white line that previously designated the end of the runway was blacked out and a new line painted further west, said Rory Kay, a training captain for a major airline who landed a plane at San Francisco the day before the crash.
The change in the runway line might have added an element of confusion to the landing, he said.
All Boeing 777s, like most modern airliners, have cockpit computers that use GPS to create a glide slope for landing that is nearly as good as the ground-based ILS, said Bob Coffman, an American Airlines captain who formerly flew the 777.
It would be standard procedure for pilots to create their own glide path before landing, but the computer's database relies on where the runway normally begins, he said. Moving the runway threshold would invalidate the computer-generated slope, he said.
Without the ILS, and with information in hand that the threshold had been moved, it's likely that the pilots of the Asiana plane were landing using other instruments and a greater reliance on visual cues, Coffman said.
It's standard procedure for pilots to refer to FAA notices on ILS shutdowns and movement of runway thresholds in a pre-landing briefing, so the Asiana pilots should have been aware that they were going to have to rely more heavily on visual cues, pilots said. The challenge of landing a wide-bodied airliner like the 777 using visual cues is greater than if an ILS or a computer-generated glide slope were available, pilots said.
The Asiana plane was flying well below its target speed of 137 knots during the landing attempt, and in the last seconds before the crash the pilots received an automated warning that the plane was about to stall, Hersman told reporters at a briefing.
Coffman said he could think of no reason why the plane would be flying that slowly unless the pilots had turned off the autopilot, which controls the aircraft's navigational systems, or the autothrottle, which controls power to the engines. That would be highly unusual, especially in a wide-bodied jet like the 777, he said.
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