Instead, the wingman accidentally triggered a drop in oxygen levels, prompting an altitude warning that caused the almost fatal plunge - from 32,800 to 13,100 feet in less than nine minutes, Civil Aviation Administration of China officials told the station. Data from flight-tracking website FlightAware.com shows the Air China jet heading northeast at 35,000 feet when it began to descend at around 7:39 pm.
"In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette", state-owned China News said, citing a news conference by the Civil Aviation Administration of China (CAAC) investigating the incident.
"At present, we are investigating the cause in greater detail, and if the investigation proves it is true, we will handle it according to the law and regulations and deal with it seriously", said Qiao Yibin, from the aviation authority's safety office, as quoted by the South China Morning Post.
The CAAC said the aircraft dropped down to as low as 3,000 metres (10,000 feet), without specifying its original altitude.
The crew returned to normal cruising altitude once it was discovered that the air conditioning was, in fact, turned off.
The incident featured heavily on Chinese social media, with some commentators demanding harsh punishment and revocation of the pilot's flight licence.
The flight landed safely in Dalian at 10:29 pm.
He went on to make this attempt without previously informing the captain - and accidentally shut off the plane's air conditioning while doing so.
Air China's website says smoking is strictly prohibited on all its flights.
In 2015, government-run China National Radio said four passengers on an Air China flight from Hong Kong to Beijing smelt strong smoke emitted from the cabin.
In 2016, the United States prohibited the use of e-cigarettes on commercial flights.