An airliner was forced to make an emergency landing in southwest China after part of the cockpit window fell out in mid-flight.

The Sichuan Airlines aircraft was flying from Chongqing to Lhasa on Monday morning when the incident happened.

Pictures shared online show the co-pilot's side of the windshield gone and it emerged that he was almost sucked out of the window by the sudden loss of pressure.

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Flight 3U8633 landed safely at Chengdu Shuangliu airport at 7.42am, about 20 minutes after the accident happened.

The co-pilot was cut on the face and suffered an injury to the lower back. A cabin attendant also suffered a minor injury as the plane suddenly lost altitude.

Twenty-seven passengers received medical checkups in a hospital in Chengdu, but no injuries were detected.

The rest of the passengers resumed their trip at noon, according to statements by the airline.

Pilot Liu Chuanjian told the Red Star News, a local news portal, that the windshield had given way without warning around 150km from Chengdu, the capital of Sichuan province.

"The windshield cracked suddenly and gave a huge bang. I looked aside and found half of the co-pilot's body was already outside the window. Fortunately his seat belt was fastened," Liu said.

The pilot added that the cabin equipment malfunctioned as a result and it was so noisy he could not hear the wireless.

The aircraft was shaking vehemently and he could not read the metres.

"The sudden loss of pressure and low temperature made me very uncomfortable and it was very difficult to make a single move when the aircraft was flying at 900 kilometres an hour and at such a high altitude," Liu said.

Flying data showed the aircraft was on the cruising altitude of 9,750 metres (32,000 feet) but then dropped suddenly to 7,300 metres (24,000 feet).

Liu said he had to fly the aircraft manually because the automatic systems were not functioning.

"I have flown this route a hundred times and know everything very well," Liu said.

One veteran pilot who did not wish to be named said the crew had handled the incident masterfully.

"As pilots we receive such training twice a year, but it was one thing to train on a simulator and another when you are affected by the sudden loss of pressure and oxygen when the temperature drops to minus 40 degrees.

"The pilot stayed calm, responded quickly and correctly to drop the aircraft to an altitude where emergency oxygen is not needed and handled the situation with strong skills. That's very professional."

Passenger Zhao Shihai told the China Youth Daily that he was sleeping when he suddenly felt the strong turbulence.

"I was thrown up in the air and fell for several times. The oxygen masks on the plane all dropped out", he said.

Zhao, who was sitting in the middle of the aircraft, felt a draft of cold air and saw the cockpit door flew open several minutes later.

He added that several passengers had fallen over but the turbulence reduced after crew pushed the door closed.

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