By Stacey Leasca
July 13, 2018

On Tuesday, an Air China flight from Hong Kong to the mainland city of Dalian made an emergency descent after the plane lost significant oxygen and cabin pressure due to a pilot error — but not the kind you think.

According the Telegraph, the co-pilot of the 737 aircraft, which was carrying 153 passengers and nine crew members on the three-hour route, was allegedly attempting to smoke an e-cigarette during the flight.

"In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette," China News explained. It further quoted Qiao Yibin, an official of the regulator's aviation safety office, who explained, "Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen.”

According to CNN, a senior official from the Civil Aviation Administration of China (CAAC) revealed to reporters that the smoking co-pilot attempted to turn off the air recycling system to prevent his vape’s smoke from spreading into the cabin. However, he accidently hit the air conditioning system instead.

That move triggered internal system in the plane that then set off an alarm and released the oxygen masks, which forced the crew to descend nearly 20,000 feet in less than nine minutes. After the descent, the crew was able to restore the air conditioning and the plane’s cabin pressure. They then returned to cruising altitude and continued on their way.

"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," Hoby Sun, a passenger on the fight, told CNN. "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.”

While Air China has yet to provide an official statement, the Telegraph reported the airline did vow a "zero tolerance" approach to the investigation on its official account on China's social media site Weibo on Wednesday.