B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Aircraft Aircraft Damaged; Flight Crew Took Evasive Action; Flight Crew Exited Penetrated Airspace; Flight Crew FLC Overrode Automation
Human Factors; Weather
Large area of reported severe turbulence was over Greenland. We were on track Alpha west bound when SIGMET was issued. I had just returned to the cockpit after crew rest break and was briefed by other pilots of what was forecast. [We had descended to] FL260 that put us below the altitudes included in the SIGMET. Just prior to entering the SIGMET area I requested a speed reduction to .78 Mach. Just prior to the SIGMET area the Captain had briefed the flight attendants and also made a PA to the passengers about possible turbulence. The flight was smooth till about halfway into the SIGMET area. We [then] had a smooth vertical up and down like a mountain wave but no turbulence. Shortly there after we entered an area of severe turbulence at 6243N Lat. Engine igniters and anti-ice were turned on and we also disconnected the autothrottles. The Captain made another PA while I monitored the flight controls and throttles. First turbulence was short lived followed quickly by a second hit of longer duration and harder turbulence. We were below the Tracks. The Captain elected to descend to get out of turbulence and set FL240 in the window. We slowed to 290 knots and descended to FL 240. Capt made radio calls on all frequencies to advise and entered a CPDLC free text about turbulence and altitude change. Gander returned a CPDLC message approving FL 240. Capt also alerted Dispatch. Severe turbulence lasted about 30-40 seconds which ended passing FL250. Even though the Captain had advised the cabin crew of the turbulence area and to remain seated until clear [of the forecast area]; the carts were out and set up for second service in the aft galley. All carts were thrown and contents emptied throughout the aft galley. No crew member or passenger was injured during this event.
A B767-300ER encountered a SIGMET forecast area of severe turbulence enroute over the Atlantic Ocean and descended without clearance to FL240 where the turbulence abated.  Despite advance notice from the flight crew passenger service carts; left unsecured; were upset and emptied of their contents.  No injuries or damage to the aircraft itself were sustained.