Almost five months after seeking federal mediation in contract talks, Delta Air Lines and its pilots union remain at odds on pay, vacation and other issues.

The Air Line Pilots Association at Delta said it is proposing a pay increase of 37.25 percent over three years, while the company is proposing a 27.3 percent boost over four.

“We are still apart on scope, vacation and other work rules,” union chairman John Malone wrote in a letter to pilots.

Delta said it “is committed to reaching a timely new agreement that is market-based, sustainable, and that also ensures Delta pilots have an industry-leading package of pay, benefits and work rules.”

As Delta reports billions of dollars in profits, pilots have said they want to recover from past pay cuts.

Malone wrote that during Delta’s power outage earlier this month, pilots and other employees “true to tradition – stepped up and did our best to mitigate impact on the traveling public… That action has long been a hallmark of the Delta pilot, something that greatly contributes to the corporate bottom line, and should be reflected in our contract.”

The two sides recently took a recess from mediation, and were preparing to resume talks.

The union in July sought to ratchet up pressure by opening a center in Atlanta to organize activities “in the event a strike becomes necessary.” Pilot contract negotiations fall under the Railway Labor Act, which lays out a number of procedural hurdles before a union can strike.

The current talks follow an initial round that produced a tentative deal last year that was rejected in a vote by pilots.