B747-400
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Total 20000; Flight Crew Type 2000
Confusion; Workload; Troubleshooting; Situational Awareness; Distraction
Aircraft Equipment Problem Critical
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Landed in Emergency Condition; General Declared Emergency; General Maintenance Action
Aircraft
Enroute FL310 Autothrottle rapidly went to idle thrust concurrent with EICAS message FLAP DRIVE. First Officer PFD speed tape displayed red overspeed tape down to amber under speed indicator. Disconnected A/T then followed Flight Manual Flap Drive procedure. Split flap indicators displayed with outboard in amber and left tape showing partial flaps extension. No adverse control wheel or trim indications. First Officer conducted visual inspection of flaps from aft cabin windows and noted no apparent extension. [We] established SATCOM with Dispatch to Maintenance. Data download indicated left outboard flap sensor sending erroneous flap extended signal. [We] discussed potential flap lockout due to erroneous flap position indicator and potential for split flap or no flap landing. [We] asked Dispatch to investigate long runway options enroute in event no flap landing necessary. Subsequent discussion determined our destination was the safest option due to crash/fire/rescue capability. Asked Dispatch to coordinate with ATC for us to remain at clean maneuvering speed or higher until downwind to preserve fuel and runway options should flaps fail to extend. ATC complied. Flaps Drive EICAS intermittent message but was not active on arrival. Flaps extended normally. Briefed Purser of potential for last minute cabin prep but did not advise passengers and declined equipment standing by once we had flap extension. Procedure has guidance for split flap landing but minimal information on no flap landing. The only reference noted was the abnormal landing distance chart. No maneuvers profile was found.
A B747-400 intermittent flap position sensor malfunction at cruise caused a partial flap extended signal which positioned the autothrottles to idle because of an overspeed detection. The warning was false and a normal landing ensued.