A co-pilot has been accused of sending his plane into a rapid emergency descent – by having a sneaky vape on his e-cigarette.
The Air China plane plunged 6,500m (21,000ft) after oxygen levels dropped in the cabin on Tuesday.
Investigators said the co-pilot tried to turn off a fan to stop the vape aerosol reaching the passenger cabin.
He made the attempt without informing the captain – and accidentally shut off the plane’s air conditioning – with severe consequences.
The plane’s oxygen masks were deployed as it plummeted down to an altitude of 3,000m (10,000ft) before stabilising.
It eventually returned to cruising altitude and continued its journey from Hong Kong to the port city of Dalian in northwest China.
An initial investigation by China's Civil Aviation Administration (CAA) said the vape aerosol seeped into the passenger cabin, which can carry up to 215 passengers.
"The relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen," said CAA official Qiao Yibin.
He said the crew had to perform emergency measures until they could figure out the problem.
He said that whenever a plane loses pressure, the pilot is obliged to drop to a lower altitude.