Major equipment failures and shortages have grounded as many as one-third of the Air Force's B1 bombers, leaving the service with no crews trained to use the full capability of the bomber and sharply restricting the number of planes it can put on alert until 1990, according to a new government report.

The equipment breakdowns -- far more than expected -- have hampered training and will almost double the bomber's projected repair bills to more than $895 million, according to the General Accounting Office.

The report by the GAO, a congressional investigative agency, provides the most detailed public account yet of how equipment failures have affected the capability and readiness of the $28.3 billion long-range bomber. The study comes as the Air Force has engaged in a major publicity effort to rebut earlier disclosures of shortcomings in the plane, a cornerstone of the administration's strategic buildup.

Air Force officials said that despite the problems, in a national emergency all of its B1s could be mission-ready within a few days and all of its crews could fly the planes.

But the equipment and training problems will leave the Air Force unable to meet its criteria for keeping up to 30 percent of the long-range bombers on alert until 1990 -- shortly before the Stealth bomber is scheduled to become operational, according to the GAO. The B1, billed by the Air Force as the most sophisticated and capable bomber in the world, was intended to bridge the technical gap between the aging B52 bombers and the Stealth, or Advanced Technical Bomber.

Only one of the 51 B1s now in the force is kept on alert, fully ready to fly with minutes' notice in case of emergency, according to an Air Force spokesman. Air Force officials said keeping more than one plane on alert now would further hinder their ability to repair existing problems and improve crew training. Air Force officials also note that the last of the 100 B1s is not scheduled to enter the force until next spring.

Faulty equipment and spare parts shortages have at times temporarily grounded up to one-third of the bombers, including some planes that were cannibalized to keep other planes flying, according to the survey. The report said the Air Force, as of March, was requesting about 200 parts a day to replace faulty or worn-out equipment. Backlogs on 60 percent of those requests have been responsible for grounding the planes, the report said.

The bomber, which officially became operational last fall, has needed five times the number of design changes as expected to correct flaws and improve the plane -- 162,000 thus far compared to the 34,250 estimated in 1985, according to the GAO.

"Officials expect several years of intensive technical support to adequately correct problems and deficiencies with the complex avionics and software of the weapon system, the mechanical systems, weapons compatibility and support equipment," the report stated.

The contractor engineering bill alone for that effort is expected to more than triple from the 1988 budget request to an estimated $382.9 million, the GAO said.

Those problems also have set back Air Force plans to take over much of the repair work from its contractors. The lack of repair manuals and test equipment is forcing the service to allow contractors to make repairs for an extra two years until 1990 at a taxpayer cost of $513 million, more than double what the Air Force said it would cost to do the repairs in-house, according to the report.

The GAO said that as of the end of April, the Air Force had only 13 mission-ready crews for the 30 B1s then assigned to the strategic bombardment wings. No crews have been trained to "use the full, planned capability of the B1," said the report, prepared for House Armed Services Committee Chairman Les Aspin (D-Wis.).

Training has been limited because of mechanical equipment failures as well as problems with the B1's terrain-following radar and defensive avionics, which are critical to its mission of darting into hostile airspace, hitting a target and escaping. The Air Force said that despite limitations on the planes, it is training with flight simulators.

The Air Force also said that although "all aspects of B1 operations are somewhat affected by parts shortages," it has exceeded its training sortie requirements from March through June.