Former pilots of Korean Air Lines Co., South Korea's largest flag carrier, have filed a suit to retrieve money "unjustly" taken by the company related to flight training, the airline's labor union said Thursday.

Three former pilots, each of whom had worked at Korean Air for six years, filed the suit in April with the Seoul Southern District Court, according to the union. Four others later joined the lawsuit.

They are claiming that the training expenses they had to pay back to the company before leaving in the mid-2000s is inappropriate and that those fees should be returned immediately.

Under Korean Air regulations at the time, would-be pilots had to cover the costs for basic and mid-level training that took place in the United States and the airline provided high-level training held on the southern resort island of Jeju on the condition that they stay in the company for at least 10 years.

If they left the company before the required time, they had to pay back most of the training expenses to the company.

The cost of training in the U.S. and Jeju is about 100 million won($84,700) and 190 million won, respectively. Inside sources said that the financial burden that pilots-in-training have to take on has increased ever since.

The pilots who filed the lawsuit were ordered to pay back expenses ranging from 85 million won to 93 million won of the high-level training cost when they left the company. The former Korean Air pilots claimed that the requirement creates an unjust contract.

"Even though Korean Air is surely capable of providing the training necessary for work, it arbitrarily sets the amount of training expenses and has its employees take on all of that burden," they claimed.

"Forcing employees to pay back training expenses in case they do not stay with the company for 10 years is no less than a slavery contract," they added. (Yonhap)