B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; General Declared Emergency
Company Policy; Weather
I was the pilot on a rest break in the cabin during the severe turbulence event.  Below is the narrative from Captain and First Officer; pilots at the controls and on the flight deck during the event.   We were level at FL290 for the Oceanic Crossing on Track X. CPDLC and SATCOM were both operational. Reports indicated moderate turbulence at FL310 and above. We encountered some chop and had the seatbelt sign on. A flight about 30 miles in front at FL310 started into an area of Mod Turb and advised us on 123.45. We had already discontinued the service and I made a PA to the passengers advising them that we had asked the flight attendants to be seated for their safety. Shortly afterwards; we started picking up the turbulence and it quickly turned Severe for approximately 2 minutes. Airspeed fluctuated +/- 30kts and altitude deviations about 200 feet. On the later TAPS report [Turbulence Auto-PIREP System] it indicated MOD to SEVERE with Plus 1.573 to Plus 0.396G forces. The Max Flag threshold was not exceeded. We sent an Emergency request for FL270 by CPDLC and were promptly cleared. The ride improved considerably. We checked with the flight attendants and no injury or damage. The aircraft operated normally the remainder of the flight. We did not need any further assistance. Dispatch was very helpful and provided all of the SIGMET reports before and after. This was unplanned in both intensity and altitude. The flight plan indicated 0's for this portion of the flight; although we were expecting some turbulence. We wrote up severe turbulence in the Aircraft Log. If the passengers or crew were not seated and strapped in; there definitely would have been serious injuries. At some points during the event; we couldn't even read the instruments. We were very lucky. The First Officer was also very helpful as the pilot monitoring with coordinating and radio calls. Despite excellent tailwinds; choose an alternate path with any moderate turbulence forecast.  Our flight was dispatched into the jet stream on a NAT track at a lower than normal altitude to 'hopefully' miss the significant turbulence areas.  However; the area was known by Dispatch; ATC; and other aircraft to have extremely poor rides.  Despite the desire to save time; gas; and money by riding the significant winter jet stream on the NAT we would have been much better off flying further north out of this area.  The flight plan had our flight arriving over 45 minutes early due to the strong winds.  Planning a flight path with less wind would most likely cause us to arrive closer to our schedule and provide a much more comfortable and safer ride.
B767-300 flight crew reports encountering severe turbulence where only moderate had been forecast. An emergency request for altitude change is quickly granted by ATC.