2010-02 · NASA ASRS report 871895
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.
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.
The airplane was experiencing +/-25 knots accompanied with minor bank angle changes and minor altitude deviations. The autopilot never kicked off. During the encounter; there were at least two; possibly 3 overspeeds. After determining that other aircraft up ahead were also experiencing severe turbulence. The Captain and I decided to laterally deviate south to steer clear of the now known and verified area. Because of a fuel gauge issue and the fuel burn down low; descent was not the first option; although we would not rule it out if needed. We immediately turned 45 degrees left and began broadcasting our maneuvers. The Captain brought another First Officer back up to the cockpit to help with communications. We sought a route deviation from Center of 1 degree further south routing. Initially it was rejected. In the meantime; using the Captain's emergency authority; we descended 500 FT and flew a 10 mile left offset of that new 1 deg further south routing; paralleling our original route. After a few minutes; Center called us back and approved our request for the new routing and altitude (unchanged at FL330). During the maneuvers; the turbulence continued; but it was never as bad as the first minute or so. It gradually diminished; and we were able to turn the seat belt sign off about 30 minutes later.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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