The US Federal Aviation Administration (FAA) has issued a notice of proposed rule making which would require operators of Boeing 747-400s to modify the wiring of the aircraft’s flap control unit (FCU) following cases of inadvertent flap retraction on some 747s.
The proposed airworthiness directive comes from an investigation into an uncommanded flap retraction of a 747-400 powered by Rolls-Royce RB211 engines which occurred shortly after the aircraft had taken off. The investigation found that a signal from a faulty sensor in the engine control unit which indicated that the aircraft’s thrust reversers had been deployed led to the automatic retraction of the flaps.
The proposed AD will require US operators of 747-400s with Rolls-Royce, GE and Pratt & Whitney engines to enact a Boeing service bulletin to modify the wiring related to the FCU and the thrust reverser control system. The AD is currently out for comment and will not become final until late September, at which time it is expected that CASA will issue a similar AD for Australian registered 747s.