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.”