Often little warning of clear-air turbulence, say aviation experts
Clear-air turbulence is caused by severe and sudden wind changes and often results in aircraft losing, or gaining, altitude.
The changes are caused by the Earth's natural jet stream coming into contact with slower-moving wind drifts, which results in churned-up air.
While generally not regarded as being as dangerous as windshear - in which bursts of strong wind buffet aircraft - clear-air turbulence is difficult to predict.
Results can range from comparatively minor incidents, such as objects being thrown around the cabin, to fatal accidents.
Passengers in yesterday's incident said they felt the aircraft drop suddenly, causing some passengers to hit their heads on the ceiling.
The Boeing 757 dropped from 6,400 metres to 5,800 metres in 10 seconds.
Chek Lap Kok airport has warning systems to counter the effect of air traps caused by surrounding mountains.
Air traffic controllers receive turbulence warnings from the Observatory every minute. However, the information is only passed on to planes within 15 nautical miles of the airport, and clear-air turbulence forecasts are often not particularly accurate, according to pilots' forums on the Internet.
The Civil Aviation Department said there was a general turbulence warning for the Hong Kong area yesterday.
While some aircraft have on-board turbulence and wind shear indicators, there is often little advance warning of clear-air turbulence, aviation experts say.
Once encountered, it can only be avoided by changing altitude.
'If people aren't strapped in sufficiently, they can get shaken around,' said Barry Grindrod, publisher and managing editor of Orient Aviation magazine.
'There have been a number of incidents in the past couple of years in the Asia-Pacific region, and it's basically one of those things that must be very hard to avoid from an airline's point of view.
'The only precaution they can take ahead of turbulence is to tell people on board to keep their seat-belts on.' Aviation expert Jim Eckes said some mainland airlines had a reputation for not following strict safety rules and the wearing of seat-belts.
'Some don't enforce these things as strictly as they should. Investigators would need to look at whether the seat belt sign was on and that sort of thing.' The Observatory had forecast fresh to strong northerly winds in the area yesterday.
