WASHINGTON - The head of the independent board investigating the Columbia disaster said yesterday that NASA has not given enough clout to engineering and safety units within the agency meant to ensure the reliability of the shuttle fleet.

Harold W. Gehman Jr., the retired admiral who chairs the Columbia Accident Investigation Board, told Congress that the National Aeronautics and Space Administration appears to be "ill-served by an imperfect system of checks and balances."

He said engineering specialists may be too closely tied to NASA's shuttle program to effectively offer an independent view of the condition of the orbiters and that another group responsible for safety lacks enough personnel to give it heft.

"The safety organization is, on paper, perfect," Gehman said. "But when you bore down a little deeper, you don't find any there there."

Gehman's comments, his first public testimony on Capitol Hill since the Feb. 1 shuttle disaster, underscored that the Columbia probe is likely to result in recommendations that would shake up NASA management.

Board members are focusing on damage to the leading edge of Columbia's left wing that apparently allowed superheated gases to penetrate the shuttle - causing its disintegration during re-entry into the atmosphere.

Some lawmakers expressed dismay at Gehman's analysis. Sen. Bill Nelson, a Florida Democrat who flew on Columbia in 1986, said he had thought NASA's safety teams, which were beefed up after the Challenger disaster 17 years ago, were adequate for the job.

"That makes my blood boil," Nelson said.