A final airworthiness directive (AD) issued by the US FAA today calls on operators of Rolls-Royce RB211-powered Boeing 747-400 aircraft to make a modification to the aircraft's thrust reverser systems by early September to prevent its leading edge flaps, or slats, from retracting during takeoff.

The FAA directive, which calls for operators to incorporate an 8 June service bulletin (SB) issued by Boeing, is largely symbolic given that there are no Rolls-Royce RB11-powered 747-400s registered in the US.

However the AD reveals that Boeing has recommended through its SB that operators of Rolls-Royce-powered 747-400 and 747-400F aircraft make the changes by early September, the same date mandated by the FAA.

The action follows a 11 May incident in which a British Airways 747 departing from Johannesburg lost lift and came close to stalling after the leading edge flaps retracted upon rotation based on faulty signal related to the aircraft's thrust reverser system.

According to the FAA, the Rolls-Royce-powered 747-400 includes flap control unit (FCU) logic that automatically retracts the inboard leading edge flaps when either both inboard or both outboard thrust reversers are engaged, a safeguard designed to prevent debris from becoming lodged in the slats during reverse-thrust operations.

In the case of the British Airways incident, the first thrust reverser deployment signal was received by the FCU before V1, the takeoff decision speed. The second signal, which triggered the slats to retract, occurred several seconds later, after pilots had already committed to takeoff. "At rotation the flight crew reported buffeting and stick-shaker activation," says the FAA.

Troubles continued after lift off, when onboard logic determined that the aircraft was airborne and that the slats should be deployed, an action that caused buffeting and stick-shaker activation again. Pilots ultimately dumped fuel and returned to the airport for landing.

In addition to the British Airways incident, the FAA says one operator reported 12 thrust-reverser "amber" signals from individual engines during takeoff over the past three years, leading to seven rejected takeoffs. The Johannesburg incident was the first case in which signals were received from two engines, causing the slats to retract.

The AD, per the Boeing SB, calls for modifying thrust reverser control system wiring to the FCU, including rerouting and re-terminating one wire for each engine.

Source: Air Transport Intelligence news