Delta Air Lines and its pilots' union jointly filed for mediation Thursday after failing to agree on new contract terms.

The National Mediation Board will now lead negotiations between the two sides, as outlined by the Railway Labor Act, for the first time in 15 years.

The pilots' contract expired at the end of 2015. Management and the Air Line Pilots Association International (ALPA) at Delta previously agreed to seek assistance from the government if they were unable to produce a new contract proposal by March 31.

A federal mediator will be appointed to the case and will set a schedule for meetings. Both sides agreed to continue meeting on a regular basis, "even without the presence of the mediator," according to the union.

"We are prepared to meet our goal of delivering an agreement, one that you will strongly ratify, by this summer," said John Malone, Delta MEC chairman, in a letter to the pilots Thursday.

Former Delta union leadership had forged a tentative agreement with management last spring under expedited negotiations. About two-thirds of the voting pilots rejected the deal because it would have cut profit-sharing and levied unfavorable sick leave policies. The union's leaders quickly resigned and Malone was elected the new chairman.

The pilots sent Delta leadership an opening proposal in December asking for a 40 percent compound pay increase over three years.

Steve Dickson, senior vice president of flight operations at Delta, sent a letter to the pilots Thursday informing them about the mediation process and assuring them that management wants to continue paying its pilots the industry's highest wages.

"Delta and Delta pilots have led the industry during the last few years because we have worked together," Dickson wrote. "A new, economically sustainable pilot contract will help us build on the momentum that has set Delta apart from our competition."

The Atlanta-based airline raked in a record net profit of $4.5 billion in 2015 and the pilots hope to maintain their current slice of it. Dickson said he acknowledges the pilots' high expectations, adding that in order to maintain this momentum "over the long-term, we must take a balanced approach that is sustainable in both good times and bad."

Dickson urged the pilots to stay focused on performance and customer service and to stay engaged in the negotiations process. He stopped short of naming a target date for a new contract, but said hopes it will happen "in a reasonable time frame" and "sooner rather than later" for the benefit of both sides.

About 10 percent of Delta's 12,000 pilots are based at the airline's Minneapolis-St. Paul hub.

Kristen Leigh Painter • 612-673-4767