China Eastern Airlines (CEA) on Wednesday suspended or demoted 13 of its pilots for flying back their planes without landing at their destinations between March 31 and April 1.

The suspensions last up to two years,and one of the pilots was expelled from the Communist Party of China too.

Plus, two top branch officials and four less senior staff of the airline were sacked for mismanagement. Two others were reprimanded.

The pilots flew back 21 planes to the airports they had taken off from in Yunnan province instead of landing at their destinations on the pretext of bad weather or technical problems.

This left more than 1,000 passengers stranded in Yunnan's airports, mostly in the provincial capital of Kunming.

But an investigation by the country's aviation regulator, the Civil Aviation Administration of China (CAAC), shows only three of the flights had been forced to fly back because of poor weather or aircraft failure.

The other pilots were suspected of flying their planes back to protest against their "poor" pay and working conditions.

The CAAC has fined China Eastern, the country's third largest airline, 1.5 million yuan ($217,400).

It has stopped it from operating flights on the routes from Kunming to Xishuangbanna and Dali, two popular tourist destinations in Yunnan, from May.

The CAAC has reduced China Eastern's overall number of flights too. Reports estimate the penalties may cost the carrier 405 million yuan ($58.6 million) in revenue this year.

China Eastern has asked its pilots to improve their professional ethics and prevent such incidents.