The flight from Vancouver to Sydney had to divert to Honolulu, Hawaii, when sudden turbulence hit the aircraft and it dropped mid-flight.
Thirty seven passengers and crew were injured. Of that number, 30 were taken to hospital and nine had serious injuries, according to Associated Press.
People onboard described passengers who were not sitting down and wearing their seatbelt shooting out of their seats and hitting the ceiling of the aircraft.
“There was a lot of blood everywhere,” passenger Llyn Williams told AP.
Fellow passenger Andrew Szucs said: “All of a sudden the plane dropped and went sideways. And that’s when the people who weren’t strapped in flew, hit the ceiling.”
Szucs added that there was no warning from the crew that the plane was about to drop.
Footage from the aftermath of the incident showed oxygen masks hanging down from the ceiling.
In a statement, the US Federal Aviation Authority said that a Boeing 777 hit turbulence at 36,000 feet 600 miles southwest of Honolulu, and medical personnel met the aircraft at the gate.
Turbulence is caused by eddies of “rough air” – a bit like waves becoming choppy at sea.
There are three main reasons they occur: thermal (warm air rises through cooler air); mechanical (a mountain or manmade structure disrupts air flow); and shear (on the border of two pockets of air moving in different directions).
This makes the aircraft rise and fall and rock from side to side.
In February, five passengers were injured when a Delta flight nosedived twice due to “crazy turbulence”.