Iberia flight IB6740 from Buenos Aires to Madrid was overflying the equator over Atlantic international waters at flight level 340, when the aircraft encountered sudden moderate-to-strong turbulence which caused serious injury to one passenger. Some eleven other passengers and one flight attendant suffered light concussions or anxiety attacks. The aircraft was not damaged. The aircraft diverted its course to Las Palmas airport, where the seriously injured passenger with signs of hemiplegia was disembarked to be taken to a hospital for medical assistance. There was no forecast turbulence in the area where the accident occurred. There were no visible clouds. No turbulence announcements were given and the fasten seat belt signs remained off. Causes The accident was caused by the penetration of the aircraft in an area of turbulence when passengers were not secured with their seat belts. The seat belt signs were not lit, because turbulence was not expected. The turbulence was probably of a convectively-induced turbulence (CIT) nature and was neither detected nor were people warned.