Because aircraft move so quickly, they can experience sudden unexpected accelerations or 'bumps' from turbulence, including CAT - as the aircraft rapidly cross invisible bodies of air which are moving vertically at many different speeds. Although the vast majority of cases of turbulence are harmless, in rare cases cabin crew and passengers on aircraft have been injured when tossed around inside an aircraft cabin during extreme turbulence (and in a small number of cases, killed, as on United Airlines Flight 826 on December 28, 1997). BOAC Flight 911 broke up in flight in 1966 after experiencing severe lee-wave turbulence just downwind of Mount Fuji, Japan.