By David Shepardson

WASHINGTON (Reuters) -Critical U.S. air traffic control (ATC) facilities face staffing challenges and the Federal Aviation Administration "lacks a plan to address them," a government audit released on Friday found.

The Transportation Department Office of Inspector General said the staffing issues pose risks to the continuity of air traffic operations. U.S. airlines are expecting record summer air travel and have urged more air traffic-control hiring.

The report said the FAA has made limited efforts to ensure adequate controller staffing at the busiest air traffic control facilities, adding that 77% of critical ATC facilities are staffed below the FAA's 85% threshold.

COVID-19 led to training pauses over a period of nearly two years, "significantly increasing controller certification times," the audit added. "Due to these uncertain training outcomes, FAA cannot ensure it will successfully train enough controllers in the short term."

Managers told auditors facilities are not adequately staffed and many do not have enough supervisors. At several facilities, controllers are working mandatory overtime and six-day work weeks to cover staff shortages, the report found. Of the FAA's 13,300 total controllers, 26% are trainees.

The FAA released a controller workforce plan in May and said in response it "fully understands that adequate staffing at its critical facilities helps ensure the safety and efficiency of the National Airspace System and is committed to getting to adequate staffing levels."

Jacksonville Center overtime costs in 2022 exceeded prepandemic costs by 27%. The center in busy Florida has had 300 staffing triggers - events when workforce issues require reducing air traffic, the report said.

The Transportation Department in May sought $117 million to hire 1,800 air traffic controllers next year, in addition to 1,500 being hired this year. Many critical facilities have more trainees than average and it can take more than three years to train new controllers.

The report found the FAA had 10,578 certified controllers in 2022 - virtually the same as 2021 -- and down 10% from 2012.

In March, the FAA agreed to requests by Delta Air Lines and United Airlines to return up to 10% of summer slots and flight timings at congested New York JFK, LaGuardia, Newark and Washington National airports, citing ATC shortages.

Last summer ATC staffing was a factor in delays of 41,498 flights from New York airports, the FAA said in March. The report added the FAA "may occasionally overstate the number of controllers who are available to conduct air traffic operations because it includes those that are absent for administrative reasons."

The audit said New York Terminal Radar Approach Control (TRACON) staffing was at 54% compared with optimal levels; it had eight supervisors but was authorized for 30. New York TRACON has had 170 staffing triggers.

Miami Tower was at 66%.

(Reporting by David Shepardson in WashingtonEditing by Chizu Nomiyama and Matthew Lewis)