Narrative:

Encountered strong moderate turbulence (clear air turbulence with suspected wind-shear) at FL370 and speed fluctuations were extreme for the realm of flight. This occurred approximately between 90 nm to 40 nm east of 4640N on north atlantic track foxtrot. I had just come back from break and was the flying pilot. We slowed to turbulence penetration speed of .78 mach and the captain called all flight attendants to be seated with seat belts on and to lock down the cabin and stow carts. He also made a quick brief to passengers to remain seated with seat belts fastened. The aircraft very rapidly slowed below green dot speed and below vls [lowest selectable speed] and into the alphaprot/alphamax range. We started to experience low speed buffet for a few seconds (no stall warning occurred) and I disconnected the auto/pilot and simultaneously the captain and I called for and applied takeoff go around (toga) power. I kept flying the aircraft while the captain went over to gander radio on HF and requested FL350. I maintained FL370 and was never in doubt as to the ability to maintain our altitude. It took about 30 seconds to coordinate but they came back and gave us FL350. We immediately descended and ride started to get better by the time we got to FL350. The speed increased as we descended back into a safe range above green dot and we re-established normal automation for cruise flight. The captain reported the turbulence as severe to gander radio.our flight plan did call for turbulence along this portion of flight. Before I came back from my break and assumed controls as the flying pilot the crew briefed me as to why we were at FL370. This was an acceptable altitude for the performance of the aircraft especially when the crew queried shanwick radio as to ride reports before climbing to FL370. Shanwick reported that company traffic 30 minutes ahead reports mostly smooth at FL370. With the smooth report we felt that conditions must have changed sufficiently to allow a safe flight at FL370. The crew requested and climbed to FL370 at .82 mach. They started to experience some of the reported turbulence an hour later and now in contact with gander radio requested and received a change to .80 mach for the cruise speed. I think we were lulled into a false sense of security when the one company aircraft ahead reported a smooth ride. In hindsight; we could have queried dispatch in addition to ATC. What I learned from this is that we are programmed to climb as it usually takes you out of turbulence; but we would have been better off to maintain a wider buffet margin at a lower altitude; and dealt with the turbulence accordingly. More timely updates and reports from other aircraft ahead would have helped too.

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Original NASA ASRS Text

Title: A330 flight crew reported encountering severe turbulence on a North Atlantic track at FL370.

Narrative: Encountered strong moderate turbulence (clear air turbulence with suspected wind-shear) at FL370 and speed fluctuations were extreme for the realm of flight. This occurred approximately between 90 nm to 40 nm east of 4640N on North Atlantic Track Foxtrot. I had just come back from break and was the flying pilot. We slowed to turbulence penetration speed of .78 Mach and the Captain called all flight attendants to be seated with seat belts on and to lock down the cabin and stow carts. He also made a quick brief to passengers to remain seated with seat belts fastened. The aircraft very rapidly slowed below green dot speed and below VLS [lowest selectable speed] and into the alphaProt/alphaMax range. We started to experience low speed buffet for a few seconds (no stall warning occurred) and I disconnected the auto/pilot and simultaneously the Captain and I called for and applied Takeoff Go Around (TOGA) power. I kept flying the aircraft while the Captain went over to Gander Radio on HF and requested FL350. I maintained FL370 and was never in doubt as to the ability to maintain our altitude. It took about 30 seconds to coordinate but they came back and gave us FL350. We immediately descended and ride started to get better by the time we got to FL350. The speed increased as we descended back into a safe range above green dot and we re-established normal automation for cruise flight. The Captain reported the turbulence as severe to Gander Radio.Our flight plan did call for turbulence along this portion of flight. Before I came back from my break and assumed controls as the flying pilot the crew briefed me as to why we were at FL370. This was an acceptable altitude for the performance of the aircraft especially when the crew queried Shanwick Radio as to ride reports before climbing to FL370. Shanwick reported that company traffic 30 minutes ahead reports mostly smooth at FL370. With the smooth report we felt that conditions must have changed sufficiently to allow a safe flight at FL370. The crew requested and climbed to FL370 at .82 Mach. They started to experience some of the reported turbulence an hour later and now in contact with Gander Radio requested and received a change to .80 Mach for the cruise speed. I think we were lulled into a false sense of security when the one company aircraft ahead reported a smooth ride. In hindsight; we could have queried dispatch in addition to ATC. What I learned from this is that we are programmed to climb as it usually takes you out of turbulence; but we would have been better off to maintain a wider buffet margin at a lower altitude; and dealt with the turbulence accordingly. More timely updates and reports from other aircraft ahead would have helped too.

Data retrieved from NASA's ASRS site 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.