The BBC explained that the loss of air conditioning caused the plane's other systems to react as if the cabin were losing pressure, which obliged the pilots to quickly bring the aircraft to a lower altitude to keep the passengers safe.
"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 the regulator as saying.
However, the co-pilot accidentally turned off the air-conditioning unit instead, causing oxygen levels in the cabin to fall and triggering the release of oxygen masks from the cabin ceiling.
Then it climbed again to continue to its destination.
The crew returned to normal cruising altitude once it was discovered that the air conditioning was, in fact, turned off.
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Authorities are reportedly investigating the cause "in greater detail", examining both the flight data recorder and the cockpit voice recorder to determine precisely what caused the incident.
Qiao Yibin, the CAAC official, promised to hand down "severe punishment in accordance with laws and regulations", if the regulator's final conclusion on the incident matches its initial finding.
Air China also said in a previous Weibo post it had a "zero tolerance" approach towards wrongdoing by any crew.
China bars flight crews from "smoking on all phases of operation", and passengers there haven't been able to use e-cigarettes on planes since 2006.
Now we know why this terrifying incident may have occurred.
In 2015, government-run China National Radio said four passengers on an Air China flight from Hong Kong to Beijing smelled strong smoke coming from the cabin.
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