Air Force investigators said yesterday they believe a B1B bomber crash that killed three crew members last September was caused by a large pelican that ripped through the skin of the plane above the engine, severing hydraulic lines and sending flames through the craft.

An Air Force official said that if a bird or an artillery shell hit the bomber at the right point, the plane would "have a problem." The Air Force has temporarily changed the low-level training routes of the bomber -- designed to fly at 200 feet to avoid enemy radar -- until it can make safety modifications.

The investigation also revealed that the copilot's automatic ejection seat malfunctioned. The copilot and two crew members, who were sitting in instructor's seats and did not have enough time to bail out manually, were killed. Three survived.

The findings that a bird apparently caused such catastrophic damage to the bomber, which was on a high-speed, low-level bombing training mission, raises new questions about the controversial plane's ability to survive its intended missions in combat.

As a result of the accident, the Air Force has changed some of its low-level training routes and schedules to "reduce the risk of large bird strikes while operating at high speeds," according to Brig. Gen. James W. Meier, assistant deputy chief of staff for operations at the Strategic Air Command which operates the B1 fleet.

"If you track bird populations around the world, we do have a problem . . . the mass of that bird is like a bowling ball operating at the speed of sound," Meier said.

The bomber, designed to dart into enemy territory at terrain-hugging levels, has been restricted from flying below 5,000 feet on training missions until the Air Force can modify the planes to make them less vulnerable to bird strikes, according to Meier.

Air Force officials say they expect those improvements, which will involve reinforcing vulnerable spots on the planes, to cost $62.5 million. Officials said the first repairs will be made by this spring, but will not be completed until the end of the year. The Air Force also is now making improvements to the automatic ejection system, Meier said.

The restrictions on low-level flights, coupled with major problems in the plane's defensive electronics system, drastically hits pilot and crew training for the bomber's primary missions. Air Force officials say that the flight-level limits apply only to peacetime training and would be lifted during wartime.

The Air Force bought 100 bombers, the last of which rolled off the assembly line in Palmdale, Calif., yesterday, for about $28 billion.

The investigation of the Sept. 28 crash found no bird parts in the engine or inside the plane, but said something large apparently had smashed through the thin aluminum skin of the plane in the four-inch space between the engine and the wing while the plane was flying at about 580 mph.

Air Force officials reported that large flocks of white pelicans had been sighted near the flight.

Student commander Capt. Lawrence Haskell, one of three who survived, said just before the crash he spotted "a white blur coming from slightly left of the aircraft centerline at an angle leading just right of the aircraft fuselage."

"The crew heard a loud bang or explosion and the aircraft began to shudder and made a groaning and grinding sound," the report said.

The investigators reported that the bird probably tore through electric wires, hydraulic lines and fuel lines located in the overwing area. They speculated that a break in the hydraulic line caused a fire that melted or broke a fuel line. The fire apparently damaged or destroyed lines from at least three of the four hydraulic systems used to control the aircraft, the report said.