An Airbus A330-243 aircraft registered A6-EYN was being operated by Etihad Airways as a passenger scheduled flight EY474 from Abu Dhabi International Airport (OMAA), Abu Dhabi, United Arab Emirates to Soekarno-Hatta International Airport (WIII) Jakarta, Indonesia. The flight departed at 01:30 UTC and scheduled to arrive at Soekarno-Hatta International Airport Jakarta, at 07:46 UTC. The prognosis chart issued by World Area Forecast Center (WAFC) London which was the part of briefing material for the pilots prior to departure, showed possible thunderstorms and development of cumulonimbus clouds up to above FL470 at around Sumatra Island. Onboard the flight were 274 occupants: three pilots, nine cabin crew members and 262 passengers. The flight was uneventful since departure until about 15 minutes commencing to descend when the cabin crews were preparing for arrival procedure and some passengers were not wearing the seatbelt. At 06:40:10 UTC, the aircraft was on cruising at FL 390 and encountered severe turbulence for 22 seconds. The severe turbulence caused one cabin crew and six passengers sustained serious injuries, while three cabin crew and 14 passengers sustained minor injuries. Several aircraft interior parts were damaged such as detached ceiling, broken passenger service units and dropped oxygen mask containers. Approximate 30 seconds after encountering the severe turbulence, the aircraft was in stable flight and the flight was continued to Soekarno-Hatta International Airport Jakarta. The injured occupants were transported to the nearest hospitals for treatment. The investigation determined the contributing factor was: ? The aircraft was flying outside the visible cloud which was within the turbulence area of the thunderstorm. ? The turbulence was not anticipated and no warning provided to the cabin crew and passengers.