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DENVER — Frontier Airlines pilots approved a new contract in voting that ended Thursday following nearly three years of negotiations.

Of the pilots for the Denver-based budget carrier eligible to vote, 77 percent backed the deal, the Air Line Pilots Association International said. The pilots union said the contract, which will take effect next Wednesday, will provide an average 53 percent pay increase.

Frontier pilots have been working under a contract changed in 2011 to keep the airline out of bankruptcy. Negotiations began in March 2016 and soon turned into mediation overseen by federal officials.

Pilots voted to authorize a strike in 2017. However, federal law makes it difficult for airline workers to strike so pilots staged demonstrations, including staging informational pickets and sending a “strike bus” to cities in Frontier’s network.

Pilots agreed to lose pay under the 2011 contract change in order to keep prized, flexible work rules that allow them to work fewer hours when they want and to swap scheduled flights. That flexibility makes it hard to say what an average pilot makes but starting pilots, who are paid for a standard 75 flying hours a month, will jump from earning $35,000 a year to $70,000 under the new contract, union spokesman Capt. Alan Christie said.

“We’re looking forward to a much more symbiotic relationship to help Frontier reach its growth strategy,” Christie said.