In the midst of a bitter contract dispute, US Airways' pilots union is suing the company, claiming that management has delayed negotiations and stalled the process for resolving grievances.

US Airways spokeswoman Michelle Mohr said the lawsuit is without merit and denied the union's accusations.

On Tuesday, the US Airways Pilots Association staged a protest outside the carrier's offices at Charlotte/Douglas International Airport. They say four pilots were being unfairly disciplined in an attempt to intimidate others, which Mohr also denied.

The two sides have been struggling to reach a new contract since 2005, when US Airways merged with America West. The pilots unions for the two companies were unable to reach a joint agreement. That has led to years of arbitration, mediation and recrimination and left the merged company functioning with separate flight crews.

The pilots claim they are in contracts drawn up while the airline was bankrupt, which pay them less than the industry rate. US Airways acknowledges the pilots are paid below what many of their peers make, but says those savings are necessary for the airline, smallest of the legacy carriers, to survive.

The labyrinthine negotiations are still grinding along, with the sides meeting for about one week every month under the auspices of a federal mediator. The union says only 10 of the 30 contract clauses have been sorted out, with all of the most contentious areas - such as pay and benefits - still unresolved.

Tempe, Ariz.-based US Airways has its busiest hub in Charlotte, with 623 daily flights and 6,900 employees based here.

In the lawsuit, filed Friday in federal court in New York, USAPA charges that the airline has deliberately delayed grievance resolution while negotiations are under way.

Union spokesman James Ray said there is now a backlog of more than 500 unresolved grievances. The lawsuit says they cover issues such as pay, unfair terminations and other disciplinary actions. The pilots are seeking an injunction forcing US Airways to speedily resolve all of the outstanding grievances brought by pilots against the airline.

USAPA also claims that US Airways is negotiating in bad faith, pretending to work on surface issues while it really has no intention of reaching an agreement.

Mohr said that US Airways has had an offer on the table since 2007 to increase pay and benefits for pilots. The proposal would increase the airline's spending on pilots by about $120 million a year. USAPA has said the proposal is unacceptable and would still leave pilots underpaid.

US Airways has previously said that the pilots are holding up negotiations by focusing on grievances that stretch back to the mid-90s.

Despite the increasingly strident talk, a strike or any deliberate disruption for passengers isn't likely anytime soon. Under the Railway Labor Act, which governs airline unions, workers can't strike until after mediation, possible arbitration and several cooling-off periods. This dispute is still in the mediation phase, which can last indefinitely.

About 150 pilots showed up to the Tuesday afternoon protest at Charlotte/Douglas according to the union; the airline pegged the number at about 60 to 70.

They were there to protest the treatment of four pilots who were called in to face possible disciplinary action over safety issues noted in FAA logbooks, Ray said.

In a statement about the lawsuit, the union told its members to expect retaliation from management. through the disciplinary system and Ray said this was an example of such treatment.

On Tuesday, Mohr denied the pilots were being punished, or even called in for a possible disciplinary hearing.

"We had some questions about some write-ups a few pilots made and were following up to see what they were noticing. This is all part and parcel of making sure we're running a safe airline," Mohr said. "And again, it's disappointing that our pilots' union is using safety as a negotiating tactic."