Chinese authorities are investigating why a plane was forced to drop more than 6,500m (21,000ft) due to loss of cabin pressure.
The Air China flight from Hong Kong to Dalian lost altitude rapidly and oxygen masks dropped in the cabin.
The plane then returned to cruising altitude and continued as scheduled.
There is speculation in local media that crew had been smoking in the cockpit and then mishandled the ventilation system.
Chinese aviation authorities are examining both the flight data recorder and the cockpit voice recorder to determine what caused the incident.