WASHINGTON (CNN) -- A manufacturing defect blamed for the mid-air breakup of an F-15 Eagle fighter may cause the Air Force to ground a quarter of its fleet of those warplanes permanently, a top general said Thursday.

The mid-air breakup of an F-15 fighter jet is being blamed on a a manufacturing defect.

Gen. John Corley, the head of the U.S. Air Combat Command, said about 160 of the jets may never return to service after an investigation into the November 2 crash that left the plane's pilot seriously injured.

The single-seat F-15C broke up in a 500-mph turn during a combat training mission over Missouri, with its fuselage breaking in half behind the cockpit, an Air Force probe of the crash determined.

Investigators concluded that a critical piece of the jet's airframe broke during the flight because of a manufacturing defect. A defective longeron -- a metal strut that runs lengthwise down the fuselage -- was cut improperly by the manufacturer, Boeing, and led to a series of cracks over the plane's lifespan, Corley said. Watch a report on the plane and its problems »

"Some of these airplanes will never return to flight," Corley said. "The age, the fatigue on these airplanes has been manifest as we looked under the hood extensively over these last two months."

The Air Force has been flying the twin-engine, supersonic F-15 since the early 1970s. The C model involved in November's crash is credited with 34 of the 37 "kills" credited to Air Force pilots in the 1991 Persian Gulf war, according to Thursday's report on the accident.

The service has about 700 F-15s in its fleet, all of which were grounded after the November crash. Most were returned to service after being checked out, but about 40 percent of the Air Force's 442 F-15 models A through D remain grounded.

"I flew these airplanes 30 years ago," Corley told CNN "This is a fleet of airplanes that's 25-plus years old on average. That constant pulling and pushing and twisting has also caused fatigue."

If the grounded planes are retired, the Air Force would still have about 240 of the older fighters and nearly 300 of the newer F-15E, a two-seat version used for ground-attack missions in Afghanistan and Iraq.

Investigators released the results of the Missouri crash at a news conference Thursday in the St. Louis suburb of Bridgeton, home of the Air National Guard wing involved in the accident. The pilot, 37-year-old Maj. Stephen Stillwell, told reporters that his plane broke up in a turn that produced about eight times the force of gravity.

"I had no idea what was happening," he said. "I knew something bad was happening, but I didn't know what it was."

Stillwell suffered a broken arm and still has problems with his shoulder. He credited his survival to the training he received. "You always prepare for the worst-case scenario," he said.

"I think luck played a small part in it, but a large part of it was due to the training I received and my faith in God," said Stillwell, who is also a pilot for Northwest Airlines.

Col. Bob Leeker, the wing's commander, said the first four of his F-15s took off Thursday after receiving clearance. Six other planes in the 131st Fighter Wing have not been released, but three are expected to be once additional examinations are completed, he said.

The F-15 was first built by McDonnell-Douglas, and it's now manufactured by Boeing. The service is trying to determine whether Boeing would be liable for the defect after 30 years.

The A through D models are used in the United States for air-defense missions. After the initial grounding, the service had to move F-16s to cover for F-15 missions, and Canadians had to help cover missions over Alaska, according to Air Force officials.

The defect was discovered as the Air Force continues to fight for more advanced F-22 Raptors, seen as the future of the service's fighter fleet. Congress allowed the purchase of only 183 of the almost 400 the Air Force wanted, but the service continues to ask for another 200.

Corley said suggestions that the service is trying to use the problems with the F-15 as leverage to get more of the Lockheed-built F-22s "makes me just outraged, because it's just flat wrong."

"I'm the one who looks into the eyes of the moms and dads, the sons and daughters, the husbands and wives that I put in that airplane," he said. "To think that I would put one of those individuals at risk, to almost kill one aviator and to risk other aviators, that is beyond my possible belief." E-mail to a friend

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