Boeing 747-436 G-CIVP Severe Turbulence Incident in the Java Sea

Incident Overview On 13 April 1999 at 1200 UTC, a Boeing 747-436, registration G-CIVP, operating a scheduled public transport flight from Brisbane, Australia, to Singapore, encountered severe turbulence while in level flight at flight level 390 near airway intersection SABIL in the Java Sea, Indonesia. The aircraft was equipped with four Rolls-Royce RB211-524H2-19 turbofan engines and had been manufactured in 1998. ## Injuries and Operational Response A meal service was underway in the passenger cabin, and the seat belt signs were off because turbulence had not been anticipated. The turbulence lasted approximately 15 seconds. Of the 17 crew and 235 passengers on board, two crew members sustained minor injuries, while three passengers were seriously injured and 22 passengers sustained minor injuries. The aircraft commander decided to continue the flight to Singapore and requested medical services to meet the aircraft on arrival. The aircraft landed 44 minutes later, and a post-landing engineering inspection found no damage. ## Meteorological and Recorder Evidence Weather balloon observations at 1200 UTC indicated winds of about 30 kt at flight level 390, with no significant windshears or large direction changes, leading investigators to discount clear air turbulence associated with strong vertical and horizontal windshear. Significant weather charts showed no expected cumulonimbus activity, though such charts refer only to widespread or embedded cumulonimbus cloud, not isolated clouds. The incident occurred after sunset in darkness. An infrared satellite photograph for 1200 UTC showed a single very bright cell almost exactly at the turbulence location, with cloud-top temperature indications of strong vertical development. The operator's Optical Quick Access Recorder showed the aircraft at flight level 390, 264 kt calibrated airspeed, and a heading of 313°M. Initially, the wind was 18 kt from 090°. At 1200 UTC, the wind backed to about 020° and reduced to below 10 kt, with a recorded outside air temperature change of ±2°. Airspeed increased to 274 kt calibrated airspeed, then reduced to 256 kt before stabilising at 264 kt. The maximum recorded altitude was 39,285 feet, and the aircraft rolled to a maximum of 11° left wing down. Normal acceleration excursions recorded a maximum of 1.77 g and a minimum of -0.26 g over about 15 seconds. Stick shake operated for less than one second, and the autopilot remained engaged throughout. ## Discussion The aircraft probably encountered turbulence associated with a rapidly developing cumulonimbus cloud. In darkness, weather detection and avoidance would have relied on weather radar, which cannot detect rapidly rising cloud tops composed of small particles. The cloud may have been in an early stage, undetectable by radar but visible on infrared satellite imagery. Severe turbulence can occur well above the tops of cumulonimbus clouds. The incident occurred at an airway intersection, so wake turbulence was considered, but there was no evidence of another aircraft in the vicinity at the time.