FAA Wednesday published a direct final airworthiness directive (AD FAA-2010-0221
) ordering Boeing 777 operators to upgrade autopilot flight director computer (AFDC) software to prevent unsafe rejected takeoffs and address another unsafe condition.
According to a Boeing Alert Service Bulletin issued January 22, 777s have been involved in several rejected takeoffs at speeds above takeoff decision, including two this year. Explains FAA:
Since 1995, there have been nine reported instances of a rejected takeoff because of higher than normal control column forces at takeoff rotation due to the pilot inadvertently engaging the autopilot in lower-speed takeoff roll. When the flightcrew inadvertently engages the autopilot on ground during the lower-speed phase of the takeoff roll, the control column forces will be higher than normal when the pilot attempts to initiate the takeoff rotation.
Boeing also found an error that can cause problems during "performance-limited" (engine out) takeoff climbs "caused by an error in the pitch command law of the" AFDC.
The new software will disengage the autopilot whenever the flaps are extended and the airplane is on the ground and not in rollout mode. Autopilot engagement is inhibited when the flaps are extended and the airplane is on the ground or below 50 feet above ground level. The new software also maintains the initial engine-out (performance-limited) takeoff pitch target for a longer duration to minimize altitude loss during the takeoff, and targets a velocity to maximize climbout performance.
Operators of the affected aircraft have until July 1, 2010, to make the changes. The service bulletin recommended 12 months; FAA deemed that too long.