An emergency descent by an Air China aircraft after cabin oxygen levels dropped has been linked to a co-pilot smoking an e-cigarette, China's aviation regulator said on Friday.
The Boeing 737 aircraft was flying to Dalian from Hong Kong when it dropped 10,000 feet with oxygen masks deployed. Then it climbed again to continue to its destination.
"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 Tuesday's incident.
"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 Yibin, an official of the regulator's aviation safety office, said.
China News added that the co-pilot had shut off the air conditioning units. That triggered an alarm, prompting the crew to perform an emergency pressure relief procedure.
The crew realised the problem after the descent and restored the air conditioning, allowing cabin pressure to return to normal, Mr Qiao said.
The CAAC said the investigation is still ongoing and was analysing the aircraft's flight data recorder and cockpit voice recorder.
Air China did not immediately respond to requests for comment. It vowed a "zero tolerance" approach towards wrongdoing by any crew, on its official account on China's Weibo platform on Wednesday.
The incident featured heavily on Chinese social media on Friday, with some commentators demanding harsh punishment and revocation of the pilot's flight license.
China's aviation regulations, which bar flight crew from "smoking on all phases of operation", also banned passengers from using e-cigarettes on flights in 2006.
Users of online airline forums have occasionally accused pilots of smoking during flights, however.
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.
