A flight from China to Vietnam on Tuesday returned to the airport soon after take-off when cracks were spotted on its cockpit window, it has been reported - the second time this month a Chinese flight has made an unscheduled landing because of a cockpit window problem.

Capital Airlines flight JD421 from Hangzhou in eastern China, bound for Nha Trang and carrying 211 passengers, headed back to Hangzhou Xiaoshan International Airport an hour after its delayed 3.15pm departure, according to state-run China Central Television (CCTV).

A female passenger surnamed Fang said she had been awoken by strong turbulence before feeling a sense of weightlessness.

"The plane was descending very fast," she told The Beijing News. "It was like being on a Turbo Drop [vertical drop amusement ride]. The whole process lasted for five to six minutes."

Passengers were told that the cause was mechanical, and the Beijing-based Capital Airlines flew them to Nha Trang on a different plane at 11.35pm, as well as paying them 400 yuan (US$62) each in compensation.

A spokesperson later told CCTV that cracks had been spotted on the outer layer of the cockpit window glass, but that there were two further layers underneath that were sufficient to protect the cockpit.

The plane, an Airbus A321 model, had been in service for only about a month.

Zang Shuguang, an expert with the China Building and Materials Academy, told the China-based National Business Daily that the quality of glass, an impact from the outside or a change of temperature could all have caused the cracks.

Two weeks earlier, a Sichuan Airlines plane en route from Chongqing, in southwest China, to Lhasa, in the Tibet autonomous region, was forced to make an emergency landing in Chengdu after a windscreen, part of the cockpit window, fell out in mid-flight.

The Sichuan Airlines pilot had to fly the aircraft manually, despite the loss of air pressure and freezing temperatures, because the automatic systems were not functioning.

He was widely praised for preventing loss of life, landing the plane 20 minutes later, but questions were raised about safety checks before take-off.