Boeing defends design philosophy for 777 autothrust mode
is vigorously defending an automation design that appears to be at the center of the accident investigation of an -200ER. On July 6, the aircraft crashed on approach to .
The issue relates to the interaction between the 777's auto-flight and auto-thrust modes, in particular the flight-level-change (FLCH) speed mode that typically is used for varying the aircraft's vertical speed between two altitudes. Boeing does not recommend using FLCH during the final phases of an instrument approach, but does not address what modes pilots should use during a visual approach.
During an investigative hearing on Dec. 11, therevealed that the Asiana pilots on the visual arrival to Runway 28L that morning had selected FLCH for the final portion of the descent. By design, the FLCH mode allows the autothrust system to enter into a “hold” or “sleep” state during descents if pilots manually move the throttle levers for more than 1.2 sec. Flight recorder data show that the pilots did retard the throttle levers after entering FLCH. For practically all other autoflight modes, the autothrottle will reengage, or “wake up” and command thrust if speeds get too low.
In the Asiana crash, the left-seat pilot turned off the autopilot and was hand-flying the aircraft for the final portion of the approach, but descended below the airport's precision-approach-path indicator and tried to climb. He told investigators he expected the autothrottle would keep the aircraft from going “below minimum speed,” which it did not. Despite a full-thrust and a go-around initiated seconds before impact, the 777's tail hit the seawall ahead of the runway. Three passengers were killed and nearly 200 passengers and crew injured in the accident.
Aviation Week reported soon after the crash that the lack of an autothrottle wake-up capability in FLCH was a known problem in the industry; it is sometimes referred to as the FLCH “trap.” Boeing states it is unaware of the expression.
Company officials tell the NTSB that some concerns about the lack of a wake-up function were raised during the 787 certification program, but that there have been no reports of in-service events regarding the mode in other Boeing aircraft, nor have operators asked for a change to the mode.
“We were conducting a flight test [during the 787 certification program], and there was a FLCH initiated,” says Stephen Boyd, manager of the's airplane and flight-crew interface branch. “The FLCH was interrupted by another event, in this case there was a traffic-avoidance event, and as a result of the logic of the airplane, the autothrottle went into a hold mode.” Boyd says an FAA pilot noticed airspeed was decaying, and allowed it to “decay further” to see what would happen. “Our test pilot was expecting at that time [that] the autothrottle would wake up, not realizing that the autothrottle system was on [but in hold mode],” says Boyd. The FAA raised the issue with Boeing as a “Response Item,” a process the agency uses “to document a potential concern and to get a response from the applicant,” he says.
The FAA ultimately concluded that it was not a safety issue, and additional information about the hold mode was added to the flight manual. Boeing says the same basic information has been included in 777 manuals for 15 years. “In the end when he understood that this design has 210 million flight hours, he felt it was not as critical an issue,” says John Cashman, retired chief test pilot for Boeing, of the FAA test pilot's concerns.
The, during its evaluation of the 787, also questioned whether it was wise to include an autothrottle mode that differs from other modes in that it does not provide low-speed protection. “Inconsistency in automation behavior has been, in the past, a strong contributor to aviation accidents,” the regulator said.
Despite no formal complaints to Boeing, incidents have occurred and are known to the pilot community. An Asiana Airlines ground school instructor who helped train the accident pilot, told the NTSB at the start of the investigation that he had discussed “the fact that when the 777 autothrottle mode goes to 'hold,' it will not automatically reengage when in a descent using the FLCH speed mode.” He says he emphasizes this in training because he had “personally experienced, in flight, an unexpected activation of [the mode] and thus the failure of the autothrottle to reengage.”
Boeing says it considered changing the mode to automatically wake up. Bob Myers, flight deck chief engineer, says—for this particular design—they were faced with a choice. “If we did a wake-up or some sort of mode transition of the autothrottle and hold mode for a low-speed condition, we would have had a mode transition without the pilots authorizing that change.” He says the change would have resulted in two systems trying to control speed—the autopilot and the autothrottle. “We looked at the two situations and felt that the less confusing was the design we chose.”