The former security chief at Dulles International Airport admitted yesterday he was drunk and driving early on New Year's Day -- at a time when he was supposed to be overseeing airport security during a heightened terror alert -- and he was sentenced to a mandatory 10 days in the Fairfax County jail.

However, Charles D. Brady, 50, probably will serve only parts of four days, and those will be on weekends. Defendants sentenced to time in the county jail are given one day of "good time" credit for every day served, and Brady already is credited with one day for the time he spent in custody after his Jan. 1 arrest.

In March, Brady resigned as the Transportation Security Administration's acting security director at Dulles, about two weeks after he was convicted of driving while intoxicated. In that trial, Brady conceded that his blood-alcohol level was 0.26, more than three times Virginia's legal definition of intoxication, when he was pulled over at 1 a.m. Brady was supposed to be on duty until 2 a.m., records showed, and the country was on a Code Orange anti-terror alert that night.

But Brady immediately appealed his conviction from Fairfax General District Court to the Circuit Court, where misdemeanor defendants receive an entirely new trial. Brady's attorney, Rodney G. Leffler, had argued that officers of the Metropolitan Washington Airports Authority did not have jurisdiction beyond the Dulles Toll Road, and Brady had been stopped on Route 28.

The arresting officer, Sgt. Wayne J. Miller, testified yesterday that he saw Brady's vehicle swerve onto the shoulder of the toll road three times. He pulled over Brady's Dodge sport-utility vehicle shortly after it exited the toll road.

Miller said that when Brady climbed out of the vehicle, he stumbled into the driver's door and was "noticeably unsteady." He said Brady was unable to recite the alphabet or count backward, and fell into the back of his vehicle when asked to stand on one leg. When Miller instructed Brady to stand with his feet together, the airport security director nearly fell into traffic on Route 28, Miller said.

Virginia law requires a 10-day jail sentence for anyone convicted of driving with a blood-alcohol level above 0.25.

Yesterday, Leffler said his research found that the airport police did have jurisdiction in the case and Brady withdrew his appeal. Leffler said Brady had completed an alcohol rehabilitation program and was attending Alcoholics Anonymous meetings. He asked Fairfax Circuit Court Judge Arthur B. Vieregg to allow Brady to serve his sentence on weekends.

The judge agreed and sentenced Brady to 60 days in jail, with 50 suspended. He imposed a $300 fine, ordered Brady to complete a county alcohol counseling program and revoked Brady's driver's license for a year without the ability to apply for limited privileges.

A memo issued last week by Fairfax Chief General District Judge Donald P. McDonough stated that mandatory portions of sentences would not be eligible for "good time" jail credit. But Vieregg, a circuit court judge, did not specify that Brady would be ineligible for such credit, and Sheriff Stan G. Barry said that without a specific order from a judge, a prisoner would serve half the sentence, regardless of what is mandatory.

The good-time credits are a longtime Fairfax custom, used to encourage good behavior among inmates. Serving even a small portion of a day counts as a full day, so Brady will enter jail this evening and leave Sunday, credited with three days, sheriff's officials said.