Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; MEL; Procedure
Flight crew noted a slight rumbling vibration on climb out.  The vibration increased with airspeed and as we reach cruise at FL300 and increased speed we noted the increased vibration and that there was a control yoke displacement of approximately 2 units right in straight and level flight with all other trim neutral.  After discussing the situation as a crew; we decided to contact [Operations and Maintenance] for further discussion and possible solutions.  Due to a known history and MEL addition affecting spoilers; we selected the flight control schematic page and noted that the right flaperon indicated a near full upward deflection in level flight. Through our discussion with [a] tech pilot; it was suggested that we try utilizing rudder trim to determine how much was needed to neutralize the control yoke; and we determined that 4 units of rudder trim was required.  Without being able to fully determine what was causing the vibration we decided that a 13 1/2 hour flight and oceanic crossing would not be prudent; and most likely would not be possible due to fuel over burn.  We discussed possible alternate airports and [Operations] ultimately requested that we return to ZZZ.  We were North West of ZZZ1 when we coordinated with ZZZ2 Center for our return to ZZZ.  We coordinated with ZZZ1 Center to jettison fuel in order to be below max landing weight.  The fuel jettison of 78;400 lbs. was accomplished just south east of ZZZ3 VOR at FL200.  Arriving back into the ZZZ area; we coordinated for runway 36C.  As we configured the aircraft for landing; we noted a drastic increase in vibration as the flaps were extended from 20 to the planned setting of 30.  We adjusted to flap 25 for landing and flap 25 speed.  Although the vibrations and roll control were abnormal on final; the landing was uneventful.  After landing we elected to leave the flaps at 25 until mechanics at the gate could verify that retraction would not cause further damage.  After block in it was discovered that the number 6 spoiler was up.  We could not see indications for the number 6 spoiler on the flight control page due to the MEL deactivation.  Initial indications are that the number 6 spoiler which was deactivated under MEL was 'floating' or extended when it should have been down.
On taxi out; during the control check a 'spoilers' EICAS message displayed. PDM (Programmed Depot Maintenance) procedures were attempted; but we had to return to the gate for maintenance; as it was a 'flagged' item.  Spoilers were MELed.  After takeoff; we felt airframe vibrations that got worse at higher altitudes and airspeeds. We found the right flaperon was indicating all the way up; the left flaperon was indicating slightly down.  Additionally; the control wheel was right wing down about 2 degrees.  After consulting Maintenance and [Operations]; everyone decided that the best course of action was to RTB (Return to Base) ZZZ.  Fuel jettison checklist was executed in order to land on 36C.  Due to excessive airframe vibrations at flaps 30; a flaps 25 landing was made.Uncommanded asymmetric spoiler deployment.
B777 flight crew reported uncommanded asymmetric spoiler deployment in flight.