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.
Instead, he toggled the wrong switches, which were close to his intended target, leading to a drop in oxygen levels that triggered altitude warnings.
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 fired the entire flight deck crew and suggested CAAC revoke their pilot's licenses upon completing its investigation, the country's flag carrier said Friday night in a statement.
Air China flight CA106, en route from Hong Kong to the northeastern Chinese city of Dalian, descended from above 10,000 meters (32,800 feet) to below 4,000 meters (13,100 feet) in less than nine minutes Tuesday shortly after it reached cruising altitude, according to phone GPS data shared with CNN by a passenger on board.
The Boeing 737 jetliner was carrying 153 passengers and nine crew members on the three-hour journey and landed safely in Dalian, CAAC said Thursday.
Citing anonymous industry sources, multiple Chinese state media outlets had reported earlier that, after the abrupt drop in altitude, the plane eventually climbed back to around 7,500 meters (24,600 feet) and flew to its destination with a less-than-adequate oxygen level in the cabin.
Rapid loss of cabin pressurization can be deadly. In 2005, a Helios Airways flight from Cyprus to Greece crashed into hills outside Athens, killing all 121 people on board. A loss of cabin pressurization had incapacitated the crew, leaving the Boeing 737 flying on autopilot until it ran out of fuel.
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.
Hoby Sun, the passenger who provided CNN with the flight altitude data, said everyone was calm when the oxygen masks dropped.
"I didn't think too much of it at the time -- we didn't know what was going on, nor did the flight attendants it seemed," he told CNN on Thursday.
"I'm not physically hurt, but the psychological impact lingers. When I close my eyes, I see the oxygen masks dangling in front of me," Sun added.
Air China, headquartered in Beijing and a member of the Star Alliance global network, has a fleet of more than 600 planes. Last year, the airline and its subsidiaries carried 102 million passengers across six continents, according to company statistics.
Its last and only fatal accident was in 2002 when a Boeing 767 jetliner crashed into a hill in bad weather near Busan, South Korea, killing 129 of the 166 people on board.