Thai budget carrier Nok Air has firmed up purchase rights on two Bombardier Q400s, valued at $66 million, taking the program to over 500 orders.

Nok Air placed the original order for two Q400s in November 2013, taking purchase rights on four more aircraft. It firmed two options in March 2014 and has now confirmed two of the purchase rights, taking its total commitment to six aircraft.

“We are very pleased to confirm that Thai low-cost carrier Nok Air has converted two purchase rights, taking us over the 500 Q400 [orders] milestone,” Bombardier Commercial Aircraft president Mike Arcamone said, announcing the order at the Farnborough Airshow.

Nok Air is the launch customer for Bombardier’s high-density Q400, which can accommodate 84 passengers. Nok Air CEO Patee Sarasin said the aircraft will be used to support the airline’s growth strategy.

“The Q400 NextGen airliner will play a major role in our domestic development, but also our international expansion as we look at new destinations such as Myanmar, Laos and Cambodia—areas with airports not serviceable by jets but easily accessible with the Q400 turboprop,” he said.

With the Nok Air order, Bombardier has now accumulated 501 Q400 orders.