An Air China jet that plunged 25,000ft in an emergency descent did so after a co-pilot mistakenly turned off air-conditioning systems in a bid to hide his e-cigarette smoke.
A senior official from the Civil Aviation Administration of China (CAAC) told reporters Friday that, without notifying the pilot, the unnamed co-pilot was trying to turn off air recycling fans to prevent the vapor from spreading into the passenger cabin.
In reaction to an alarm triggered by the air conditioning shut off, the flight crew dropped the plane's emergency oxygen masks and initiated a descent to a lower altitude with breathable air.
In a video of Flight CA106 obtained by the Beijing News, a flight attendant is seen walking down the aisle to check on passengers, some of who are putting on oxygen masks in response to a pre-recorded announcement in Chinese and English asking them to do so.
The aircraft, which was flying to the Chinese city of Dalian from Hong Kong, was carrying 153 passengers and nine crew members at the time. Air China says it will "adopt a zero-tolerance attitude and seriously punish those found responsible", depending on the results of the CAAC's probe.
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But he accidentally switched off air-conditioning instead, leading to a decrease in cabin oxygen levels. It expressed its appreciation for passengers and said it would "conscientiously" learn lessons from the episode to improve its safety management system and ensure such incidents did not recur.
It added that it was continuing to investigate the incident.
According to the South China Morning Post, some aviation experts reckon the airline should not have continued with the flight after its emergency oxygen supply had been used up.
A co-pilot attempted to vape in the cockpit shortly before the incident, Chinese authorities confirmed. According to FlightRadar24 it descended from 35,000 feet to 10,000 feet in 10 minutes as is standard practice in a decompression event.