B737-700
Flight Crew Air Transport Pilot (ATP)
Workload; Troubleshooting; Human-Machine Interface; Confusion; Distraction
Aircraft Equipment Problem Less Severe
Aircraft Automation Overrode Flight Crew; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; General Maintenance Action
Aircraft
Enroute at FL410; the center fuel gauge started increasing and indicated a max of 29;440.  FMC showed increase (could not manually update).  Speed tapes and MCP speed indications all had maxed out readings.  Autothrottles did not seem to have control of thrust.  QRH was looked at but did not directly address this issue and known information on inop fuel gauge updates per performance data and FMC as well as test FMC to accept updated LDG WT (worked).  Maintenance Control; Dispatch; and Chief were consulted.  The concern was indications on approach/landing and possible stick shaker activation. Center fuel was assumed empty by estimating 2;000 LBS and complying with fuel pump/crossfeed requirements until Fuel Pump Low Pressure light indicated depletion. Main gauges operated normally. With Company's concurrence; we elected to update LDG WT and attempt approach/land in our filed destination with known LDG WT; fuel; and Company calculated numbers. Brief included manually setting speed bugs (if needed); stick shaker activation and landing/go around per Company's/crew input.  Center fuel decreased to 28;000 and remained there throughout the descent and landing.  The approach and landing were normal as well as indications of tapes; speeds; bugs and MCP indications.  Normal gate arrival and deplane.  Center fuel at shut down 24;550.
The reporter stated that continuous four way communications was conducted between he and his company during this event troubleshooting the problem; analyzing the indications; and discussing possible outcomes during the approach and landing phases. The complex circuit breakers arrangement examined; found normal and so left as they were. The autothrottle indication was that it was in throttle hold; without speed control capability. The SPD TAPE was yellow with no MIN-MAX; as if in a danger zone. The FMC was blinking. Some time during the descent; the SPD Tape and FMC data began to slowly return to normal so that by time the approach was started; all FMC and speed indications were apparently valid. What did not return to normal was the Center Fuel Tank quantity. A discussion with Maintenance the next day revealed that a cannon plug anomaly was the cause.
At cruise; a B737-700 FMC center fuel tank indicated 29;440; the speed tapes; MCP speed indications all had maxed out readings and the autothrottles did not seem to have control of thrust. All indications except Center Fuel Tank quantity returned to normal during descent. A loose cannon plug was implicated.