WASHINGTON - Air Force inspectors have discovered major structural flaws in eight older-model F-15 Eagle fighters, sparking a new round of examinations that could ground all of the older jets into January or beyond, senior Air Force and defense officials said.

The Air Force's 442 F-15A through F-15D planes, the mainstay of the nation's air-to-air combat force for 30 years, have been grounded since November, shortly after one of the airplanes broke into large chunks and crashed in rural Missouri.

Since then, Air Force officials have found cracks in the main support beams behind the cockpits of eight other F-15s, and they fear that similar problems could exist in others.

Current and former Air Force officials said that the grounding of the F-15 fleet is the longest that U.S. fighter jets have ever been kept out of the air and that even if the jets are cleared for flight, it could take six months to get the pilots and aircraft back to their normal status.

The F-15A-Ds - on average 25 years old - are responsible for defending the United States, including flying combat air patrol missions over Washington, a job at least for now filled by F-16 Fighting Falcons.

"This is going to be a major problem, and it's going to be a difficult one to recover from," said retired Air Force Gen. Dick Hawley, who led the Air Force's Air Combat Command from 1996 to 1999. "You could basically be without the nation's primary air superiority capability for an extended period of time, which puts us at risk."

The disclosure of the cracks comes amid intense Air Force lobbying for the purchase of additional new fighter jets. The Air Force wants to replace its aging F-15s with 200 more F-22 Raptors beyond the 183 already approved by Congress and the Defense Department.

Significant cracks have been found in the longerons that support the F-15 fuselage, Air Force officials said.

The damage is believed to be connected to the intense stresses placed on the planes during decades of high-speed maneuvers. The crash last month happened after the aircraft disintegrated behind the cockpit during a 500-m.p.h. dogfight and the entire back of the plane was ripped off.