The B777 activated auto-throttle system is under close investigation by the NTSB after has been determined that the Asiana aircraft was flying too slowly before impacting the seawall at runway 28L at San Francisco airport last July 6.
Anthony Keyter, a retired senior Boeing flight instructor said on Friday that the B777 auto-throttle 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,” said Keyter in a statement.
“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”, as it was the case of Asiana 214.
“Pilots rely on the auto-throttle system as a last resort to keep the aircraft speed safe under all circumstances. Then the design flaw in the auto-throttle can thus be considered a contributing factor to the causes of the accident”, he mentioned.
Keyter added that Boeing should address the problem through changes to flight procedures, or an express note in the manual.