The Civil Aviation Administration of China (CAAC) launched its investigation Sunday into China Eastern Airlines' pilots, who turned back midway on flights to airports they had set off from.
Last Monday, 18 flights returned to their departure points in southwestern Yunnan province, affecting more than 1,000 passengers.
Media reports said that the pilots, who work for China Eastern Airlines' Yunnan branch, were protesting over their pay and working conditions, but the company insisted that poor weather was the reason.
A CAAC team will talk with the pilots, passengers, air traffic controllers, ground crew and airport staff to get the truth about the incident, Xinhua News Agency said.
A CAAC spokesman said earlier that the administration required its southwest bureau to deal with the issue last Monday. By Tuesday afternoon, the airline resumed normal operations.
The spokesman said the agency will ask the authorities to impose severe penalties on the pilots if they disrupted flights on purpose.
China Eastern announced on Saturday that pilots involved would be penalized if they were found to have acted based on grievances rather than as a result of "poor weather".
According to Saturday's announcement, China Eastern's Shanghai headquarters was told by its Yunnan branch that the weather did not permit landings on Monday.
The Yunnan branch of China Eastern promised to refund affected passengers.
A source from China Eastern said that the carrier had faced a raft of complaints from passengers, some of whom had got 400 yuan ($57) to 500 yuan in compensation.
Some passengers suspected that weather wasn't the real problem. This past week, there have been reports in the Chinese media that pilots were engaging in industrial action.
A passenger surnamed Wang said that he planned to leave Xishuangbanna at 7:40 pm on Monday for Beijing but the plane did not arrive in Xishuangbanna until midnight.
"I could not get back to work on time." Wang said.
Yuan Yiting and Li Qin, from Hangzhou, Zhejiang province, went to the customer service counter at Kunming airport on Friday morning, demanding that China Eastern compensate them for losses.
They said they were among 10 passengers who came to Kunming airport on Monday to board flight MU5760 to Lijiang, but the plane did not arrive until 8 am the next morning.
When they finally took off, the plane turned around in mid-flight and flew back to Kunming. "I could even see the aircraft runway at Lijiang airport and it was really sunny that day," Yuan said.
Media reports over the past week have pointed to similar incidents.
On March 14, 40 Shanghai Airlines pilots called in sick, while at the new Wuhan East Star Airline, 11 pilots asked for sick leave on March 28.
The reports said that pilots were angered by being required to sign 99-year contracts with State-owned airlines that call for them to pay their employers up to 2.1 million yuan in compensation if they quit.
Airlines in China, which are mostly State-owned, directly pay the high costs of pilot training.
China has 12,000 civil pilots. But official figures predict that the total number of flights would increase 80 percent by 2010 and 6,500 more pilots would be required.
It generally costs a Chinese airline about 700,000 yuan to 1.8 million yuan ($100,000 to $257,000) to train a pilot.