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 conceal his e-cigarette smoke.
Smoking is not allowed aboard Chinese commercial passenger flights.
The incident, which resulted in the deployment of passenger oxygen masks, occurred Tuesday on a flight by the Chinese flag carrier from Hong Kong to the city of Dalian in northeastern China.
The Civil Aviation Administration of China (CAAC) said 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.
Chinese media reports quoted passengers and flight-tracker sites as saying the plane may have descended as much as several thousand metres.
The CAAC said the aircraft dropped down to as low as 3,000 metres (10,000 feet), without specifying its original altitude. According to FlightRadar24 it descended from 35,000 feet to 10,000 feet in 10 minutes as is standard practice in a decompression event.
It then ascended and continued the flight at a peak altitude of 26,600 feet rather than heading to a nearby airport to land.
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.
Former Qantas head of safety, Ron Bartsch, said: “The crew would not have been able to accurately assess the amount of emergency oxygen available.
“In my opinion the pilot in command should have landed at the nearest suitable airport.”
Video from aboard the flight that was later posted on social media showed oxygen masks hanging down from the ceiling throughout the cabin, but no signs of panic.
The CAAC has said there were no injuries to the 153 passengers and nine crew.
It added that it was continuing to investigate the incident.
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