WASHINGTON, July 25 (Reuters) – Legislation to overhaul aviation programs, including billions for modernizing the aging US air traffic system, has stalled as congressional negotiators struggle to resolve stubborn issues, one of which involves US Airways Group.

Congressional and industry sources said momentum to pass the measure before lawmakers break for August vacation was interrupted this week with key lawmakers unable to reach agreement on provisions affecting long-haul air service and passenger fees that support airport operations.

Disagreements have also surfaced on how to proceed in the Senate, which is scheduled to recess Aug. 6. The agenda is crowded with finance and small business bills and a Supreme Court confirmation vote.

The House of Representatives breaks for a month next Friday.

''We're not that close to getting an agreement on outstanding issues,'' said one congressional aide not authorized to publicly discuss details while the matter was still being negotiated.

The bill is viewed by proponents as an important step toward improving the efficiency of air traffic operations, which are often blamed for delays that infuriate passengers and cost carriers millions in wasted fuel and lost productivity.

It also includes new consumer protections and safety measures sought by regulators in response to complaints of poor service and accidents.

Proponents, especially House and Senate Democrats, are not giving up, but the time constraints are now working against expedited consideration.

The Federal Aviation Administration (FAA) has been without a long-term funding blueprint for three years, a scenario that has forced congressional approval of multiple temporary spending measures so the agency can keep its airport towers and other control centers operating.

FAA operations cost more than $14 billion annually.

The bill also includes a downpayment on the multiyear $20 billion effort to transform the air traffic network from one relying on radar to a satellite-based system designed to handle more planes more efficiently.

But major sticking points include new fees that air travelers pay to support airport operations, including construction projects. Proposals would increase the charge from $4.50 to more than $5 per  ticket, which some lawmakers view as a tax increase.

Airlines oppose any increase as an ''unnecessary tax'' on passengers.