While the NTSB's final analysis will likely take a year or more to complete, preliminary information from the cockpit voice and flight data recorders and pilot interviews indicate that distractions and automation surprises appeared to cause the pilots to lose altitude and airspeed awareness.
“I don't know how the whole crew could take their eyes off the speed,” a 777 fleet captain for a major carrier tells Aviation Week. “One of the basic tenets of a stabilized approach is speed.”
Internationally accepted guidelines call for airspeed and thrust to be stable and the aircraft on the proper glide path by 1,000 ft. above ground level for an instrument approach and 500 ft. for a visual approach. The 777 captain's airline recommends that pilots have the landing gear down by 2,000 ft. altitude and final flaps no lower than 1,500 ft. If the approach becomes unstable at any time below the entry altitude, pilots are advised to perform a go-around.
Complicating the arrival for the Asiana crew was an air traffic control request to maintain 180 kt. until 5 nm from the airport during the final leg of the visual approach, requiring pilots to bleed off 20 kt. airspeed before lowering the flaps to 30 deg. The tower later cleared Flight 214 to land when it was 1.5 nm from the runway. Some carriers allow for pilots to land with 25 deg. flaps, which can be deployed at 185 kt. or below.
The left-seat pilot, the “pilot-flying,” was not officially the pilot-in-command as he was roughly midway through the airline's initial operating experience phase for 777 checkout. The instructor pilot in the right seat was pilot-in-command, on his first flight as an instructor pilot. He had never been paired with the left-seat pilot, who himself had never flown a 777 into San Francisco. At press time, it was not clear what roles the two front-seat pilots and a relief first officer in the jump seat had set in terms of crew resource management.
Though GPS-based vertical guidance was likely available to replace Runway 28L's inoperative instrument landing system glideslope, the Asiana crew may have elected to fly a visual approach using the four-light precision approach path indicator (PAPI) located near the touchdown point on the runway. When centered in the PAPI's 2.85-deg. glideslope, pilots will see two white and two red lights. All red indicates a position significantly below the glideslope, while all white lights indicates one well above the reference glideslope.
The instructor pilot told investigators that at 500 ft. altitude, he realized the aircraft was below the PAPI's visual glideslope and told the left seat pilot to “pull back” on the control yoke. “He had set the speed at 137 kt. and assumed the autothrottles were maintaining the speed,” the NTSB says. Depending on the auto-flight mode selected, autothrottles, if armed and turned on, should automatically control engine thrust to maintain a preset speed, in this case 137 kt., the reference landing speed for the 777-200ER that day.
There are caveats, however. In the takeoff/go-around or flight-level change (FLCH) auto-flight modes, the autothrottle will not automatically activate to maintain the selected speed. FLCH is pitch mode used to climb or descend at a constant airspeed using the elevator for pitch control. There are also “mode surprises,” certain conditions in which modes will transition without the pilot's knowledge, potentially putting the automation into a mode like FLCH without the pilot's knowledge.
By late last week, the NTSB had not said whether the pilots had purposefully or mistakenly entered a mode that inhibited the autothrottles, or if the autothrottle system failed. During interviews, the instructor pilot told officials the aircraft was “slightly high” when it descended through 4,000 ft. on the approach and he set the auto-flight system's vertical speed mode for a 1,500 ft./min. descent rate. NTSB Chairman Deborah Hersman said July 10 that during the final 2.5 min. of flight, the flight data recorder shows transition to “multiple autopilot modes and multiple autothrottle modes.” “We need to understand what those modes were, if they were commanded by pilots, commanded inadvertently, and if the pilots understood what the mode was doing,” she says.