B777 flight crew reported encountering severe turbulence at FL330 over the north atlantic. The capta...

Narrative:

Enroute on random route encountered severe turbulence; approximately 3 distinct episodes; lasting about a minute each; causing overspeeds up to M.90. Aircraft 100 miles ahead at FL360 reported continuous severe encounters and bank angles left and right up to 30 degrees. We attempted to get reroute left of course and center was unable to approve. We declared emergency; squawked 7700 and deviated left of course 60 NM and descended to FL325. Ultimately we did get clearance to continue flight 60 nm left of track at FL330. Sigmet 5 was out for severe turbulence; but we were well ahead of and 1000 feet below lowest forecast altitude. Later found out that aircraft in the middle of the sigmet block at FL360 had encountered only moderate turbulence for about 15 minutes. It appears that the sigmet was inaccurate in detailing the area of concern and it had shifted further south. At the time all passengers were seated with the seatbelt sign on. I had notified all the flight attendants of possible turbulence ahead;and they had done a cabin walk through checking belts. When the severe turbulence hit we had been encountering only light turbulence up to that point. Two flight attendant's were not seated. One got to a seat right away; and the other sat down in the aisle. No injuries reported. Outside weather was widespread high cirrus clouds. Great assistance from dispatch; and ATC.

Original NASA ASRS Text

Title: B777 flight crew reported encountering severe turbulence at FL330 over the North Atlantic. The Captain exercised her emergency authority to deviate 60 NM south when a timely clearance cannot be obtained from Center.

Narrative: Enroute on random route encountered severe turbulence; approximately 3 distinct episodes; lasting about a minute each; causing overspeeds up to M.90. Aircraft 100 miles ahead at FL360 reported continuous severe encounters and bank angles left and right up to 30 degrees. We attempted to get reroute left of course and Center was unable to approve. We declared emergency; squawked 7700 and deviated left of course 60 NM and descended to FL325. Ultimately we did get clearance to continue flight 60 nm left of track at FL330. Sigmet 5 was out for severe turbulence; but we were well ahead of and 1000 feet below lowest forecast altitude. Later found out that aircraft in the middle of the sigmet block at FL360 had encountered only moderate turbulence for about 15 minutes. It appears that the sigmet was inaccurate in detailing the area of concern and it had shifted further south. At the time all passengers were seated with the seatbelt sign on. I had notified all the Flight Attendants of possible turbulence ahead;and they had done a cabin walk through checking belts. When the severe turbulence hit we had been encountering only light turbulence up to that point. Two Flight Attendant's were not seated. One got to a seat right away; and the other sat down in the aisle. No injuries reported. Outside weather was widespread high cirrus clouds. Great assistance from dispatch; and ATC.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	871895
Time
Date	201002
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Aircraft 1
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Person 1
Function	Captain Pilot Not Flying
Experience	Flight Crew Last 90 Days 180 Flight Crew Total 16000 Flight Crew Type 3000
Person 2
Function	First Officer Pilot Flying
Experience	Flight Crew Last 90 Days 250 Flight Crew Total 9000 Flight Crew Type 1730
Events
Anomaly	Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

B777 flight crew reported encountering severe turbulence at FL330 over the north atlantic. The captain exercised her emergency authority to deviate 60 NM south when a timely clearance cannot be obtained from center.