Out over the Atlantic paralleling the Eastern Seaboard of the US at night in clear conditions with only a light wind from the West we were surprised to hear the aircraft a few minutes in front of us declare that they were experiencing "extreme turbulence". 2000ft below we were in smooth air. I have to confess that I looked across at the new FO and muttered the ICAO terms "light, moderate and severe" under my breath whilst gently "tutting".
Seconds later we hit the worst turbulence that I have experienced in 41 years of flying. The aircraft was shaking so violently that it was almost impossible to read the instruments. The airspeed fluctuated wildly and the overspeed clackers were immediately followed by the stall warning stick shaker. The autopilot's altitude hold mode failed to maintain height and we got altitude warnings of plus or minus 300 feet. The EICAS (warning) screen filled with red and amber warnings which appeared and disappeared rapidly. I began to wonder about the structure of the aircraft. Just how much punishment could the tail empennage take?
I called New York and confirmed the other aircraft's "extreme" report. This was worse than severe. I recommended they reroute any aircraft planning on passing through the area. At the same time I sent an ACARS message to operations suggesting a later flight took another route. I also requested that the engineers carry out a turbulence check on the aircraft on arrival. Ops came back a few minutes later and said New York had closed the airspace.
On arrival back at base we were met by an engineer. He sat on the jumpseat and went through the reports in the CMC (central maintenance computer). Shortly after that he picked up his handset and called his supervisor saying "nothing generated in the CMC, I am coming back to the crewroom". On his way out he stopped in the galley and chatting to the galley girl asked if there were any left over bacon butties . "Fill your boots" was her response "take whatever you want. That was my last flight!" She then told him that the turbulence had terrified her and that she was never going to fly again. "Err boss, I think we are going to need a turbulence check on this aircraft...".