A group of employees at the Federal Aviation Administration, which operates a pay-for-performance system, say they are not being treated fairly when it comes to annual raises.

Some workers, perhaps as many as 60 by one employee's count, have filed age discrimination complaints to protest the FAA's pay practices. About 150 workers have written e-mails to FAA Administrator Marion C. Blakey and one of her aides, Gerald E. Lavey, asking that the agency treat all employees in an equitable manner, employees said.

All of the concerned employees are at the top of the pay scale for their job category. Because they have hit their pay ceiling, the employees receive their performance-based pay raises as a lump-sum payment rather than as part of their base pay. The lump sum does not count toward retirement and is often taxed at a higher rate than their regular pay, they said.

The number of affected employees is relatively small -- 829, according to data provided Mark Lash, who works for the FAA in Oklahoma. He filed a request under the Freedom of Information Act to see how many employees were affected by the pay practice.

But the data also showed that a larger number of employees -- 4,158 -- have been exempted from the lump-sum rule and are receiving an annual increase in their base pay. Some of the employees were paid above the ceilings at the time FAA launched the performance-based system and were grandfathered in as exceptions. Others are covered by union contracts that exempt them from the pay band, employees said.

"The problem is that some get an adjustment to base salary and others do not," Lash said in a telephone interview. "That part is not fair."

Tim O'Hara, who works for the FAA in Washington, said he has not received "a comparable pay raise" for the past two years. "I have lost about $6,500 in purchasing power comparable to other federal employees," he wrote in an e-mail.

FAA spokesman Laura Brown said she could not comment because some employees have filed bias complaints, which sometimes lead to litigation.

She said salary data collected by the agency show that the average FAA salary is 8 percent higher than the market rate for similar jobs.

The FAA began to move away from the 15-grade General Schedule, the primary federal white-collar pay system, in the late 1990s. Eligible employees converted to what was supposed to be a more flexible system, known as "core compensation." In the conversion, employees gave up within-grade increases and other features of the GS system.

The conversion has proved difficult, according to a report last year by the Government Accountability Office. About two-thirds of the employees interviewed by the GAO said the new pay system was unfair. The GAO also found that the number of employees joining unions increased at a significant rate, apparently because they sought guaranteed annual and within-grade increases.

FAA data obtained by Lash show that nearly 20,000 employees are covered by the core compensation system -- less than half of the agency's workforce.

Brown said the FAA conducts "regular market surveys" to set salaries. She said that the FAA has not increased the top rate in its pay bands for the past two years and that no decision has been made for the coming year.

In recent years, Congress has faulted the FAA for providing pay raises to air traffic controllers and other employees that some lawmakers believe are too generous. Last month, a report prepared by the House Appropriations Committee put the average cost of a full-time FAA employee at $130,957 -- "among the highest of all federal agencies."

But a number of the concerned employees say the issue is fairness.

One employee, in an e-mail to Blakey, said she had never complained about her treatment at the FAA in 25 years. But she said FAA pay practices have had "a disparate impact on people's salaries," especially employees over the age of 40.

"I am being left behind -- as other people's salaries increase -- yet, I am doing the same kind of work, trying to go over and above, and excel in my performance, and contribute to the success of our organization," the employee wrote.

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