B767-300 and 300 ER
Flight Crew Total 18500
Aircraft Equipment Problem Less Severe
Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution; General Declared Emergency; General Maintenance Action
Aircraft
This was the 3rd Air Interrupt in a row on this aircraft for the same problem. Prior to this flight maintenance personnel had been brought in to help contract maintenance. There were at least 3 shipments of parts in an attempt to fix problem. Prior to flight; MX replaced a loose wire connector; a T/R unit for CA instruments; left idg; an overhead electronics panel and possibly more- see logbook. Here's what happened again. About 25 min into flight at FL390; lighter aircraft because ferry without rev passengers; little chop in area; - everything associated with the Standby AC bus started cycling rapidly and violently to include: L autopilot engaged was erratic (changed autopilots); Cabin Alt Indicators bouncing rapidly back and forth; yaw damper light; flood lights cycling on off; radar cycling on off; L FMC blanking; cabin lights cycling; Status Msg 'Capt Instr Xfer'; and may have been other indications. We turned around [to departure airport; before] entering oceanic. On descent; systems returned to normal (at lower altitudes and throttles back); and so descent - approach and landing were normal; not overweight this time. ARFF checked brakes; no problem; and returned to gate uneventful.Because this problem has only occurred during flight; I recommend test pilots fly this aircraft to altitude for systems check prior to revenue flight; following further maintenance.
B767-300 Captain reported returning to departure airport after noting multiple electrical anomalies that were addressed before departure but not successfully solved.