An Aeroflot Boeing 777, en route from Moscow Sheremetyevo to Bangkok, encountered turbulence 40 minutes before landing at Bangkok International Airport May 1, injuring more than 20 out of 313 passengers.

According to an Aeroflot statement, the aircraft “passed through a zone of sudden strong short-term turbulence.” The Russian carrier went on to say the incident was caused by 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,” the airline said.

According to Aeroflot, there are around 750 cases of clear-air turbulence recorded in civil aviation every year.

Polina Montag-Girmes