Date: 201601
Location: ZZZ.ARTCC, US
Weather elements / visibility: Turbulence
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Situational Awareness; Workload
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation - Track / Heading All Types; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Clearance
Contributing factors: Airspace Structure; Weather
Primary problem: Weather

Narrative: Departed on time; experienced light turbulence for the first 4 hours of flight. About 2 1/2 hours into flight (FL350); we had pockets of moderate chop. We were told of continuous moderate about 100 west of fix ZZZZ until well beyond ZZZZ1 at our altitude. We requested FL290; but ARINC denied us due to company flight off our right and 1;000 below. We told ATC we could see traffic and could maintain visual; but they didn't care.Not 10 minutes later we began experiencing continuous moderate turbulence with pockets of severe. It quickly migrated to continuous severe turbulence with altitude and airspeed deviations (+/- 100 feet and +/- 15 knots). We were also unable to view instruments any more.We then turned right about 60 degrees and left our offset track and descended to FL290; then FL270 as the ride at FL290 was still intolerable. At the same time; the other company flight executed a turn to the left and descended as well. They also encountered severe turbulence.We broadcast our actions on 121.5 and 123.45. As the event began to end we attempted dispatch and ATC to let them know as well and attempted to get a new clearance back to our filed track.There were no injuries. There were some sick passengers. If it weren't for the Captain staying on top of the PA announcements and a great cabin crew enforcing them I believe the outcome would have been different.We eventually landed at [our destination] 10 minutes early to a round of applause and 1;000 thank yous for a great flight.Learning what took place: the Extended Range Twin Operations (ETOPS) procedures work. They work well. ATC is a source; but with 1940's technology you may be on our own. Also; multiple seatbelt announcements are vital as there is always some passenger that thinks we are overselling the turbulence problem and feels it is okay to get up and get something out of the overhead bin. Situational awareness is key. Don't start thinking about what you want to do as the event happens. Have an escape plan in your back pocket always.




Synopsis: A Boeing 737 flight crew reported experiencing severe turbulence while on an oceanic flight plan. The captain exercised emergency authority; turned off course and descended to a smoother altitude. This action was done in accordance with established overwater emergency procedures.