On Monday, 30 passengers on Aeroflot flight SU 270 were injured during a period of severe clear sky turbulence. The flight was headed to Bangkok from Moscow. Videos and pictures from inside the plane showed food, trash, and an array of other objects strewn on floors, as well as several passengers lying face-down in the aisles. One image showed an overhead luggage compartment streaked with blood. According to a press release from the Airport Authority of Thailand, 27 of the injured travelers were taken to the Samitivej Srinakarin hospital in Bangkok while the other three opted to seek treatment on their own.
"Some injured passengers were not wearing seat belts," said the Russian embassy in Bangkok. "All victims were taken to a local hospital with various injuries, mostly fractures and bruises. Some require surgery. Fifteen people remain hospitalized."
A passenger named Rostik Rusev described the gruesome scene to CNN. "There was blood on the ceiling, people with broken noses, babies who were hurt, it was horrible," he said.
The airline published a statement on its official website responding to the situation. "The turbulence that hit the Boeing 777 was impossible to foresee," it read. "The incident was caused by what is known in aviation as 'clear-air turbulence'. Such turbulence occurs without any clouds, in clear skies with good visibility, and weather radar is unable to alert of its approach. In such situations, the crew is unable to warn passengers of the need to return to their seats. There are around 750 cases of clear-air turbulence recorded in civil aviation every year."
Aeroflot's description of the phenomenon is accurate — clear air turbulence is nearly impossible for pilots to detect by sight and difficult for radar equipment to pick up as well. Pilots can usually tell when turbulence is up ahead by the conditions of looming clouds. Not so with clear air turbulence.
According to the Federal Aviation Administration, clear sky turbulence occurs when masses of air traveling at different speeds meet and created by "atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts or thunderstorms."
Though turbulence usually poses very little safety threat to flights, clear air turbulence can result in the kinds of injuries Aeroflot passengers experienced on Monday, due to the lack of warning those onboard get. When approaching turbulence, passengers are typically instructed to take their seats and strap on their seatbelts; crew members additionally clear up aisles, put away food carts, and take their own seats.
In 1997, a passenger on United Airlines Flight 826 traveling from Japan to Honolulu died following injuries sustained during a series of clear air turbulence waves. Fortunately, there were no fatalities on Aeroflot's flight.