The Air Force's B-1 bombers are grounded with maintenance problems far more than the aging planes they are supposed to replace, says a congressional study.

The General Accounting Office study said the time that B-1s weren't available at Dyess Air Force Base in Texas due to maintenance problems ranged from 47 to 66 percent.For systems that have been flying longer, such as the FB-111 and the B-52, the Air Force expects that the total of "not mission capable" rates won't exceed 25 percent of the available hours.

The Air Force responded to the figures by saying it won't have enough experience with the B-1 until 1994 to consider the plane "mature," as the FB-111 and B-52 are now considered.

Rep. Les Aspin, D-Wis., chairman of the House Armed Services Committee, disagreed. "When the GAO suggested standards by which to judge the reasonableness of breakdowns, they were told the plane isn't mature yet. But its maturity is being pushed back further by breakdowns," he said.

The GAO study, released Sunday, concludes that "the B-1B has not been mission capable because of maintenance a significant percentage of the time."

In Air Force parlance, "mission capable" means the ability to carry out an assigned job. In the case of the B-1 - which includes the first four prototype B-1As, which are no longer flying, and the next generation of 100 B-1Bs - that would mean a nuclear strike on the Soviet Union.

"The Air Force says the B-1B has been ready to go to war for two years, but when it comes to pinning the service down on some of the plane's problems, the answer is the plane is still just a kid," said Aspin, one of the plane's chief critics.

Capt. Jay DeFrank, a spokesman for the Air Force, said, "This weapon system is not fully mature so it is not valid to compare it to those that are."