Did Asiana Pilots Misunderstand the Flight Level Change Mode?
San Jose’s NBC affiliate is reporting that a “trap” in the Flight Level Change mode (FLCH) of the plane’s autothrottle system may have contributed to the July 6 crash. According to the report, the “trap” exists if a pilot is using the Flight Level Change mode to decrease altitude during a final descent. At some programmed altitude point, the engines go into an “idle” mode. Then, in a last minute emergency, if the plane is in FLCH mode and the pilot tries to quickly use the autothrottle to add power, it won’t add any power to the idle engine.
More discussion regarding the Flight Level Change mode can be found at CockpitChatter.com. An article at the website also points out weaknesses in the operation of the automated system. Anthony Keyter, a retired senior Boeing flight instructor said that the B777 autothrottle system has a design flaw in its speed control system, suggesting that it may be the culprit of the accident.
“There is an inconsistent functioning of auto-throttle and stall protection systems in the Boeing 777 aircraft. This weak point has been discussed with grave concern among some B777 pilots. The flaw becomes evident when the aircraft is descending with autopilot in the – FLIGHT LEVEL CHANGE MODE. If the autopilot is disconnected and the auto-throttle is left in – ARMED – position, the throttles will remain in IDLE position and would not automatically increase thrust to maintain the target speed. If a speed decrease is not noticed and corrected manually by the pilot, it would continue to bleed off to the point of stick shaker.”
Additional reading on how the autothrottle functions can be found at AirSafe.com.
The official NTSB report is expected to be released within 12 months.
