Highly experienced Boeing widebody pilots have independently determined that an autoflight mode called Flight Level Change may explain why Asiana Flight 214 hit the sea wall ahead of Runway 28L at San Francisco International Airport July 6.

The experts, including a Boeing 777 fleet captain, tell Aviation Week that entry into Flight Level Change (FLCH) during the approach would have caused the engines to remain at idle despite the pilots having set the autothrottles to maintain 137 kt., the target approach speed. One group of pilots has concluded this based on intimate knowledge of the 777-200ER's automation systems; the other by flying scenarios in a 777 simulator

Their analyses draw in large part on information presented in four NTSB briefings after the crash from pilot interviews and the cockpit voice and flight data recorders.

The NTSB revealed that the pilots were initially high and fast on the approach but rapidly decreased speed and altitude to intercept a visual glideslope to the runway. At 500 ft. altitude, the right-seat instructor pilot said he saw three red and one white precision approach path indicator (PAPI) lights—a set of four lights located near the intended landing markers that give pilots a visual glideslope—and realized he was slightly low. His speed, at 134 kt., was close to the 137 kt. target speed.

By 200 ft., however, the same pilot said he saw four red PAPI lights (significantly below glideslope) and noticed speed was nearing stall. At that point, he realized the autothrottles had not been keeping up. By the time the pilots added power, the aircraft was too low and slow, and in its high drag state could not climb fast enough to avoid striking the sea wall with its main gear and tail.

The NTSB says the engines and flight controls were responding correctly to inputs and there were no anomalies noted in the autopilot, flight director or autothrottle systems. Switches in the cockpit showed that the left and right autothrottles were “armed,” and the flight director was “on” for the right-seat and “off” for the left-seat pilot, who was at the controls for the landing. Experts say it is not unusual for a pilot to turn the flight director off on a visual approach to reduce clutter or confusing data on the primary flight display.

Based on the NTSB's forensic data, automation decisions earlier in the approach appear to be reasonable: Descending through 4,000 ft., the right-seat pilot said the aircraft was “slightly high” and he used the aircraft's vertical-speed mode to descend at 1,500 ft. per min. with the autopilot controlling. The left and right autothrottles were “armed,” and he correctly assumed the automation system would have controlled the speed to 137 kt. Pilots use the master control panel (MCP) at the top center of the 777's panel to select autonomy modes and input heading, altitude, speed and vertical-speed commands for the autopilot and autothrottle systems.

The 777 experts verified that the pilot's assumptions were reasonable in that respect—the vertical-speed mode uses pitch to control rate of climb or descent, and throttles, via the autothrottle system, to maintain speed. In the simulator, they found that even with the autothrottles “armed” but turned off, the vertical-speed mode would not allow the aircraft's speed to decay a significant amount before autothrottles “woke up” and maintained the preset speed. Boeing recommends setting the minimum descent altitude for non-instrument approaches in the MCP altitude window to ensure the aircraft levels off and maintains speed, though some airlines will set “0 ft.” in the altitude window, the experts say.

Closer to the runway, the mode control decisions are not clear. The NTSB says that during the final 2.5 min. of the flight, “multiple autopilot modes and multiple autothrottle modes” were commanded, according to the flight data recorder. During that time frame, the aircraft was descending at approximately 180 kt. through 2,000 ft. on a straight-in visual approach to 28L. At 1,600 ft., the NTSB says the pilots disconnected the autopilot, presumably to hand-fly the approach with the 137-kt. target speed entered into the “indicated airspeed” field on the MCP.

The 777 experts say the most plausible explanation for what happened next was that the pilots, intentionally or in error, selected the FLCH mode on the MCP with the target altitude set at 0 ft. or the minimum descent altitude. In a descent, FLCH reduces thrust to flight idle. The throttles will typically reengage when the aircraft reaches an altitude selected on the MCP, or if the aircraft's speed nears stall speed at radio altimeter heights greater than 100 ft. If the altitude was selected to zero, however, the throttles would have remained at flight idle as Flight 214's pilots increased pitch to remain on the glideslope, causing airspeed to drop below preset levels.

“Boeing is aware of this shortcoming, which in some circles is known as the FLCH 'trap,' and in its training course demonstrates the danger to pilots,” says one of the 777 experts. “The danger of the FLCH trap is that if the autopilot is disengaged and the aircraft levels off early . . . or the rate of descent is reduced, then the airspeed will decay because the autothrottle is temporarily out of the loop.”