The nation's air traffic controllers are turning gray.

More than 25 percent of them will reach retirement age by the end of next year. And union officials fear their replacements cannot be adequately trained in time, putting the flying public at risk.

The Federal Aviation Administration insists replacements will be ready when needed.

"We are re-examining the controller work force structure," said Geoffrey Basye, FAA deputy administrator for communications. "I cannot discuss changes in the work force numbers that are coming. We will . . . bring on the number necessary."

Those in the present generation of controllers - hired after President Reagan fired about 11,000 striking workers in 1981 - are approaching their 50s. Ironically, the FAA counts almost 500 of the controllers fired in 1981 as possible replacements for the retiring controllers.

Controllers are eligible to retire at 50 or after serving 25 years. They face mandatory retirement at 56.

The average, base salary of an air traffic controller is $113,600. Retirement pay is based on the highest salary in the final three years before retirement.

There are two retirement systems, depending on when the controller joined the FAA.

Most controllers (about 75 percent) would retire under the Federal Employee Retirement System. A 53-year-old controller with 20 years of service would receive $40,800 a year.