Wind speeds cause turbulence for Hong Kong Air and Etihad over Indonesia

Medical officers rescue injured passengers at Bali’s airport from a Hong Kong airliner jolted by turbulence. (TEMPO/*)

The Meteorology, Climatology and Geophysics Agency ( BMKG ) has claimed wind speeds triggered turbulence for Hong Kong Air and Etihad over Indonesian skies recently, updating initial claims that the airplanes had entered rainstorm clouds. 

"According to Himawari 8 satellite imagery, the planes did not enter cumulonimbus clouds during the flight route," the head of flight meteorology, Mustari Heru Jatmika, said on Saturday.

Instead, he added, the turbulence happened because of waves from the Bukit Barisan Mountains in South Sumatera and the cumulonimbus clouds around the route, calling the phenomenon a form of clear air turbulence ( CAT ) 

Etihad Airways EY-474 from Abu Dhabi to Jakarta experienced severe turbulence at 37,000 feet causing 31 passengers to suffer light injuries, including broken bones. The same case happened to Hong Kong Airlines HX-6704 from Denpasar to Hong Kong at 41,000 feet, causing 17 passengers to suffer light wounds, and three passengers to suffer severe injuries.

CAT commonly occurs at the upper layer of the atmosphere, around 30,000 to 50,000 feet, because the water vapor level is not high compared to the cumulonimbus clouds.

Mountain waves interact with the cumulonimbus clouds and cause a wave to break in the upper layer of the atmosphere, causing turbulence.

"In Hong Kong Airways HX-6704, the CAT happened at a severe level, but the scale was not big enough to be detected. At that time, the wind speed difference was increasing at the tropopause level, causing severe turbulence," he said.

During the flight, there was the potential for CAT in East Kalimantan. However, the percentage probability was just 4 percent, and no cumulonimbus clouds were detected around the area.

"Since it is hard to identify the location of CAT, we hope the airlines will increase awareness and send an AIREP," he said.

AIREP is a report for CAT and other forms of turbulence sent to the Air Terminal Service ( ATS ) so it can be directly broadcast without delay, Mustari said. ( anh/dan )