Frontier Airlines jetliners sit at gates on the A concourse at Denver International Airport in May 2017.

AP Photo/David Zalubowski

Frontier Airlines pilots are closing in on a new contract after more than two years of sometimes contentious negotiations.

The Denver-based carrier agreed Monday to significantly increase pay and benefits.

Frontier pilots earn 50 percent less than their peers due to concessions from when the airline was in bankruptcy, said Alan Christie​, a longtime pilot and senior union member.

"And frankly, at our old pay rates Frontier was having a lot of trouble attracting qualified people to fuel their planned expansions," Christie said.

Since the last contract negotiations, Frontier has expanded into three new destinations, with more on the way.

The seemed stalled this summer when the pilots unanimously voted to potentially strike. Pilots also picketed in Denver and sent a "strike bus" to cities in the airline's network over the last year to try to build public awareness for their cause.

The threat of direct action helped, Christie said.

"So that gave us a little bit of leverage going forward in negotiations," he said.

A statement from the Air Line Pilots Association said the agreement still must be reviewed by the Frontier union's executive council, which will decide whether to send it to pilots for a vote. The deal could be finalized by January.