The state-backed Air China Boeing 737 aircraft was flying to the Chinese city of Dalian from Hong Kong on July 10 when it dropped to 10,000 feet (3,048 metres), with oxygen masks deployed.
According to the outlet, the unnamed male co-pilot "tried to hide the fact that he was smoking" in the cockpit and attempted to turn off a fan to prevent smoke from reaching the passenger cabin "without telling the captain", but accidentally turned off the air-conditioning unit instead.
The official said the shut-off triggered an alarm, prompting the crew to perform an emergency pressure-relief procedure, which released the cabin's oxygen masks. That's the lingering anxiety of a passenger on a recent Air China flight that plummeted thousands of feet after oxygen levels dropped.
Then it climbed again to continue to its destination.
If a plane loses cabin pressure, it must fly at a lower altitude to give crew and passengers breathable air.
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Authorities are reportedly investigating the cause "in greater detail", examining both the flight data recorder and the cockpit voice recorder to determine precisely what caused the incident.
Now we know why this terrifying incident may have occurred.
Representatives for Air China did not immediately return Fox News' request for additional comment.
Air China said it would terminate the contracts of the employees involved in the emergency descent incident, and suggested the CAAC cancel their licences, it said on its official account on China's Twitter-like Weibo yesterday.
In a statement to the South China Morning Post, Air China initially said if an "investigation reveals the crew violated regulations, the company will deal with those responsible with zero tolerance".
The airline prohibits smoking, including e-cigarettes, on all its flights.