Date: 201601
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Turbulence
Light: Night
Aircraft: B777 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Situational Awareness; Time Pressure; Physiological - Other
Second reporter function: Captain; Pilot Not Flying
Anomaly: Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: General Physical Injury / Incapacitation
Contributing factors: Human Factors; Weather
Primary problem: Weather

Narrative: Flying to the mainland; I was at the controls.  We were on track E at 380.  Two other company flights had gone before us at 350; and 370 with only some light chop.  Our ride had been smooth for quite some time and it appeared to be clear.  We were not in any SIGMET area.   So with all that information; the seat belt sign was off.  I noticed a sparse green radar return right at the tip of the airplane triangle on my Navigation Display (ND) which just popped up. When I reached over to tilt the radar lower; I felt the airplane nose being drawn down and the airspeed increasing.  I immediately turned the seat belt on and put my hands back on the controls to help the airplane arrest the rapid increase in airspeed.  The autopilot was on. It was at that moment that we experienced a jolt of MED/SEV turbulence.  It lasted about a second and then dissipated quickly.  I chose to leave the autopilot on and help it ride out the turbulence.  While I was doing all that; the other First Officer was trying to communicate to the flight attendants and the passengers to sit and be safe.  We found out a bit later that the flight attendants had be thrown around; especially the ones in the back of the airplane.  The passengers were all fine.  We reported the incident to Dispatch and suggested a severe turbulence inspection of the airplane was unwarranted. As we got close to the mainland; the Flight Attendants requested [a] Supervisor meet the aircraft for as time went on they became more sore.  [Our] Flight Attendants did an outstanding job.  The rest of the Flight was smooth and uneventful. As an aside; the radar pattern that popped up was something we normally see at or south of the equator; not between Hawaii and the mainland.  Don't know if the weather patterns are changing.


Second narrative: The relief pilots went thru small area of level one (green) precipitation at night. I was in bunk and Pilot Flying first officer was in first class seat.  The aircraft experienced jolt that picked me off bunk.  Three flight attendants in back of aircraft fell but said they were no injured according to relief officers. They stated they were ok to continue to work but sore.  I talked to one later and they wanted to have aircraft met by [a] supervisor in case they were injured since they were much more sore now. After the incident dispatch did send a report of turbulence in the area.


Synopsis: A B777 Relief Pilot at the controls and Captain resting in the bunk reported a brief; severe; turbulence jolt which injured several flight attendants. The pilots were surprised by the lack of warning and mid north latitude at which the event occurred.