The union representing pilots at Frontier Airlines said Monday it has reached an agreement in principle on a new contract, likely ending a nasty multi-year battle and ensuring pilots will not walk off the job.

Pilots, who claim they were substantially underpaid compared to pilots at other U.S. carriers, will receive a big raise if they ratify the agreement, according to the Air Line Pilots Association, their union. They’ll also get “substantial” improvements to work rules, retirement, health insurance, and disability benefits, the union said.

Neither the union nor Frontier disclosed details of pay raises, but a chart posted on a message board for pilots indicated Frontier’s captains and first officers would receive raises of between 44 percent and 63 percent, per hour, as of the first day of the new contract.

Assuming union members endorse it, it’ll be a substantial step up compared to their previous contract. Frontier’s pilots are the only U.S. pilots still working under a contract negotiated while their airline was in bankruptcy.

Because of concessions they made about a decade ago to ensure the airline wouldn’t go out of business, Frontier pilots are being paid about 50 percent less than pilots at other U.S. carriers, flying the same routes and aircraft, the union said.

Strike Threat

The situation has made pilots angry — mad enough to repeatedly threaten to strike.

It is difficult for U.S airline pilots to strike. But for awhile, it seemed possible at Frontier, with the union, the Air Line Pilots Association, asking in April for federal permission to take action. In order to mount a legal strike, Frontier pilots had to persuade the National Mediation Board that their negotiations were at an impasse.

The board rarely issues permission, but it’s not unprecedented. In 2010, it allowed Spirit Airlines pilots to strike, and they walked off the job for almost a week, until management coaxed them back with a new contract offer.

The same private equity firm, Indigo Partners, controlled Spirit then as owns Frontier now, which is why it seemed possible Frontier’s pilots eventually might take similar action. At all of its airlines, Indigo is focused on cost control, including on labor.

Indigo took Spirit public one year after the strike, and then in 2013 sold its shares so it could buy Frontier and remake the airline’s model. Frontier is now among three U.S. ultra-low-cost carriers, offering no-frills service and cheap prices. Indigo had planned to take Frontier public last year, but pulled the IPO without explanation.

Frontier’s pilot union has said its members are paid less to fly Airbus A320 family aircraft than pilots at any other U.S. airline. That’s mostly because a decade ago, after Frontier filed for bankruptcy protection in 2008, pilots agreed to wage concessions to ensure the airline didn’t go bust.

The two sides have been negotiating since March 2016, and working with federal mediators since November 2016. Union-management negotiations are often contentious, but this one seemed to go further than most, with the union taking not-so-subtle shots at the company. This year, pilots have been driving what they called a “mobile strike center” around the country to try to win public support.

In July, the union sued Frontier, accusing it of engaging in bad-faith bargaining. The airline, “undermined the bargaining process and frustrated completion of a collective bargaining agreement, including making regressive proposals, delaying making proposals, reneging on prior agreements, and wasting bargaining time generally,” according to the union.

The union’s master executive council, as well as its members, still must ratify the agreement.

A spokesman for Frontier could not be reached for comment.

Photo Credit: Pilots at Frontier Airlines may ratify a new labor agreement soon. Pictured are the airline's Airbus jets in Denver. Associated Press