Initial investigations have found a co-pilot smoking an e-cigarette caused an Air China flight to drop 20,000 feet.
The CA106 flight from Hong Kong to Dalian, China went from above 10,000 meters to below 4,000 meters in less than less than nine minutes on Tuesday, triggering the release of oxygen masks.
The flight was then able to climb again and safely reach its destination.
Multiple Chinese state media outlets have since reported the cockpit crew were smoking and mistook two switches as air recycling fans and turned them off, causing the loss of cabin pressure.
However, the initial investigation into the rapid descent by Civil Aviation Administration of China (CAAC) has indicated that the co-pilot tried to turn off a fan to prevent smoke from reaching the passenger cabin, but turned off the air conditioning unit instead.
The BBC reposts the regulator’s safety officer Qiao Yibin said the crew would have had to perform emergency procedures until they could work out the problem.
“Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen,” Qiao told state-owned China News.
The pilot would have also had to bring the aircraft to a lower altitude for the safety of the crew and passengers.
Once the crew realised the problem and restored the air conditioning, the plane was able to return to its normal altitude.
Air China previously said it would terminate the contracts of the employees involved, and asked the CAAC to terminate their licences, as per the ABC.
The airline also said it had a “zero tolerance” approach to wrongdoing by any crew members.