An emergency descent by an Air China aircraft after cabin oxygen levels dropped has been linked to a co-pilot smoking an e-cigarette during the flight, state media said on Friday, citing China's aviation regulator.
The state-backed Air China Boeing 737 aircraft was flying to the Chinese city of Dalian from Hong Kong when it went down from 35,000 feet to 10,000 feet in 10 minutes, with oxygen masks deployed.
"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," it quoted Qiao Yibin, an official of the regulator's aviation safety office, as saying.
The Civil Aviation Administration of China clarified in a press conference in Beijing that the drama was triggered when the co-pilot sought to turn off a ventilation system to prevent his smoke spreading into the main cabin, the respected news site Caixin said. But he accidentally switched off air-conditioning instead, leading to a decrease in cabin oxygen levels.
That set off an emergency warning system indicating that the 737 jet may have flown too high and instructing the pilots to quickly descend. Qiao said the shut-off triggered an alarm, prompting the crew to perform an emergency pressure relief procedure, which then released the cabin's oxygen masks. The crew realised the problem after the descent and restored the air conditioning, allowing cabin pressure to return to normal, he added. After descending the flight then again ascended to its instructed flight path.
Industry experts said the decision to climb and continue the flight was unusual given the oxygen masks had already been deployed. They added there was a risk another decompression event could occur after the one-time supply of 12 to 20 minutes from the oxygen masks was used up.
The CAAC has said there were no injuries to the 153 passengers and nine crew members onboard. It added that it was continuing to investigate the incident.