Date: 202109
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Aircraft: B787 Dreamliner Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
ATC / advisory: Center ZZZZ
Aircraft component: Speedbrake/Spoiler
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Troubleshooting; Training / Qualification
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: MEL; Incorrect / Not Installed / Unavailable Part; Human Factors; Equipment / Tooling; Aircraft; Procedure
Primary problem: MEL

Narrative: We were briefed by inbound crew that they had spoiler float.  When we got to aircraft; we found out that the number 10 spoiler had been deactivated.    Everything was normal below 10;000 feet.  As our speed increased above 250 knots; we had a very slight yaw/roll to the right and auto pilot put in left rudder.  As we climbed our speed increased further and so did left rudder.  Finally; we got over 320 knots; rudder went left with foot pedals to 3.5 degrees left.  Simultaneously; we received the Yaw/Roll Asymmetry EICAS Warning.  The checklist had no procedure.  It just said to land at the nearest suitable airport. Knowing that we had a deactivated spoiler at takeoff; I sent the First Officer back to look at it.  He was unable to see it even with his flashlight.  We asked to hold at present position because we were about to go over mountains.  We reread the deferral to see if this is one of the expected messages we could get as a result of the spoiler being deactivated.  It was not.  We then called Dispatch and got a phone patch to Maintenance Control.  I told him our situation and that I thought the spoiler was floating.  I asked if the Roll/Yaw Asymmetry EICAS was a possible message for our deferral to which he replied no.At this point; unable to determine the cause of the EICAS; we stopped our climb at 30;000 feet; [notified ATC]; found a new place to hold; got permission to dump fuel; and proceeded back to ZZZZ.  Dump altitude was 30;000 feet down to 18;000 feet.  As our speed decreased; the rudder slowly centered.  By the time we reduced to 250 knots; it was zeroed.  Landing was uneventful at max certified weight except we had hot brakes. After we got back to the hotel; I found the inbound crew.  They said they only had 2.5 units of left trim.  This makes sense because at altitude; the IAS is only around 270 KTS and do to speed restrictions at ZZZZ they never went over 300 knots on descent.  After talking to the inbound crew; I then called Maintenance Control and gave them a full in-depth review of what happened.  I asked him to call me when they found out something so we could prepare our rest cycle.Five hours later; Maintenance Control called me and said the rod on the PCU Actuator was incorrect.  Apparently; our maintenance manual has two ways to lock out a spoiler.  One is to use a Rear End Cap and the second is to use lock out plates.  Our Maintenance manuals don't specify which one to use and apparently; they used the wrong method.  After maintenance talked to Boeing; Company is going revise their manuals to reflect the preferred lockout method for each specific situation.  They suspect we had a sheared shaft but won't know until we bring the aircraft home with the spoiler locked out using the Lock Out Plate Method.  Note: We used all 4 pilots in this event.  We had one flying the entire time and that was his only job.  We had one in the back of the aircraft checking out the spoilers and helping with checklists including the diversion guide.  We had another rereading the Maintenance Required Document and looking up the possible EICAS messages we might receive with a spoiler deactivated and assisting with checklists like fuel dumping and giving the flight attendants the brief.  I was overall in command talking to Dispatch and Maintenance Control and coordinating a plan.


Second narrative: We were briefed by the incoming captain that they had a flight control problem coming into ZZZZ.  Upon getting to the airplane; we noticed that the flight control problem had been deferred.  After reviewing the maintenance papers; the Captain gave a brief and we talked about what we could expect during the flight. After takeoff passing through 10;000 feet and starting acceleration for the climb; I noticed pressure on my rudder pedals and EICAS indication that rudder inputs were being activated without my input. After accelerating through approximately 300 knots; we received a roll /yaw asymmetry message. I believe the trim indication was 3.5 units at that time. We ran the checklist and it said land at the nearest suitable airport. Everyone then worked together after talking with maintenance to return to ZZZZ. We ended up dumping about 110;000 pounds of gas and came in for [a] max gross weight landing.


Synopsis: B787 flight crew reported roll and yaw controlability issues and elected to jettison fuel and perform an air turn back.