Air China has placed the pilots of a flight from Hong Kong to Dalian under investigation after claims of aircrew smoking in the cockpit, causing the plane to lose altitude mid-flight. In a statement released just over 24 hours after the incident, the national airline vowed "zero tolerance" if crew members were found guilty of wrongdoing. Flight CA106 dropped 25,000 feet in 10 minutes on Tuesday night, flight data showed, triggering the release of oxygen masks from the cabin ceiling. The aircraft did not divert and continued on its journey, landing safely in the northern port city of Dalian in China's Liaoning province. At least two passengers posted photos and videos on social media showing those on board wearing masks but looking calm. Kai Tak: Hong Kong’s hair-raising airport closed 20 years ago One, writing on Chinese microblogging platform Weibo, said: “The announcement from the cockpit said the ability to increase oxygen in the cabin malfunctioned so the plane lost pressure.” According to tracking website FlightRadar24, the 159-seater Boeing 737 suddenly began descending at 7.40pm between Shantou and Xiamen, half an hour after it took off from Hong Kong International Airport . An announcement was heard over the cabin speakers saying the aircraft was carrying out an emergency descent due to cabin decompression. Similar data from aviation company Flight Aware indicated an initial drop of 14,100 feet in four and a half minutes. The Flightradar24 data shows the plane descended from 35,000 feet to about 10,000, at which point the aircraft levelled off and began climbing to cruising altitude soon after. The Weibo user posted a photo of himself in a mask and giving a thumbs-up sign, saying the issue had been resolved as the plane approached Xiamen. The aircraft then continued to Dalian, the flight lasting three and a half hours. Hong Kong’s Civil Aviation Department said it had no information on CA106 from after it left the city’s airspace. The Communist Party's media mouthpiece, the People's Daily , said the crew were suspected to have been smoking in the cockpit. Tough bans for China’s unruly plane and high-speed rail passengers Addressing the claims of misconduct, Air China confirmed the pilots were under investigation by the Civil Aviation Administration of China (CAAC). "If the investigation reveals that the crew has violated regulations, the company will seriously deal with the responsible person with zero tolerance," Air China said on Weibo. The CAAC could not be reached for comment. No reports of injuries have surfaced. Cathay Pacific Airways pilot and air safety expert David Newbery, who is also president of the Hong Kong Airline Pilots Association, said aircraft undergoing depressurisation may not be required to divert. It was sometimes safe to fly in the event of equipment failure, but a structural issue required landing at the nearest airport, he said. “The norm would usually see an aircraft suffering from depressurisation landing at the nearest suitable airport – not least to calm down the passengers, who would be severely frightened,” Newbery said. “However, if there were nothing structurally wrong with the aircraft, the commander could elect to continue to a more distant airport.” Flight attendants get cancer more than the rest of us, new study finds Another professional pilot, with more than 25 years of experience, said any depressurisation of the cabin would prompt the pilots to descend to 10,000 feet, as they did in this case. “That is what is considered a safe, breathable atmosphere for the passengers, and then the captain has to make a decision,” he said. “I would say in just about every case, you would want to land as soon as possible. But it depends on what happens in the plane and what the pilots’ checklist says. “In most cases you would have declared an emergency on the way down and then you would have landed at the most suitable airport.” The Boeing plane used for CA106 is registered as B-5851 and is four years old. As of Wednesday afternoon the aircraft remained on the ground at Dalian airport. Some Wednesday flights operated by Air China were cancelled, including the 7.45am departure to Beijing, which is usually carried out by the plane serving flight CA106. Hong Kong’s Cathay Pacific has an 18.13 per cent stake in Air China, which itself holds a 30 per cent stake in Cathay Pacific. Air China and its sister airlines operate a fleet of 655 aircraft, according to the company’s 2017 annual report.