WASHINGTON - The US Air Force has grounded its entire fleet of F-15s, the service's premier fighter aircraft, after one of the planes disintegrated over eastern Missouri during a training mission, raising the possibility of a fatal flaw in the aging fighters' fuselage that could keep it out of the skies for months.

General T. Michael "Buzz" Moseley, the Air Force chief of staff, ordered the grounding Saturday after initial reports showed that the Missouri Air National Guard fighter plane broke apart Friday in midair during a simulated dogfight.

Although the 688 F-15s in the Air Force's arsenal gradually are being replaced by a new generation of aircraft - the F-22 - they remain the nation's most sophisticated front-line fighters. US officials said the F-15s are used heavily for protecting the continental United States from terrorist attack, as well as for combat missions in Iraq and Afghanistan.

Lieutenant General Gary L. North, the Air Force officer in charge of military aircraft in the Middle East, issued a statement yesterday saying he would be able to fill the gap with other fighters and bombers. But another Air Force official said the F-15 grounding will have a "significant impact" on operations in Iraq and Afghanistan.

"They will clearly have to work hard to pick up the slack," the official said.

The health of the F-15 fleet has long been a concern for Air Force brass, who repeatedly have warned that the two-engine fighter had exceeded its expected life span and was straining under the workload imposed by the counterterrorism duty.

In addition, Moseley repeatedly has raised concerns that the plane is inadequate for increasingly sophisticated air defense systems being developed by potential adversaries such as China and Iran.

"The F-15s . . . they're very capable airplanes," Moseley told a congressional hearing in October. "But against the new-generation threat systems, they don't have the advantage that we had when they were designed in the late 1960s and built in the 1970s."

In May, another Missouri Air National Guard F-15 crashed in southern Indiana during a similar training exercise. The pilots in Friday's crash and the May accident survived.

The F-15 that crashed Friday was 27 years old. Of the five versions of the F-15 used by the Air Force, four versions average 24 to 30 years of age. The F-15E, the newest version, is 15 1/2 years old, but has been grounded with the other versions because it has a similar airframe.

Air Force leaders frequently have cited the age and growing obsolescence of the F-15 as the main reason to buy the new, stealthier F-22, the most expensive fighter ever made.

Critics of the F-22, which was first designed to fight a generation of Soviet MiGs that never materialized, say it is an overpriced Cold War relic, but the Air Force insists it has adapted the plane to meet more modern threats and missions.

Lieutenant General David Deptula, a former F-15 pilot who is now the Air Force's head of intelligence, said his son now flies the same F-15 aircraft that Deptula flew while based in Japan in the late 1970s.

"They have become serious maintenance challenges as they get older, and now I'd suggest that we may be facing a crisis," Deptula said. "We must recapitalize our aging fighter forces - and fast."

Loren Thompson, a military analyst with the Lexington Institute who has consulted for aircraft manufacturers, said the accident probably was caused by metal fatigue, corrosion, or faulty maintenance.

If maintenance problems turn out to be the culprit, Thompson said, the F-15 fleet could be returned to flight relatively quickly. Similarly, corrosion could be fixed by examining other aircraft for similar problems.

If the Missouri crash was the result of metal fatigue, however, it could lead to a much more extended grounding, as it would suggest that time and intense use of the aircraft since the Sept. 11 attacks have caught up with the aging fighter.

"The whole fleet was already flying on flight restrictions due to metal fatigue," said Thompson, noting that a fleet-wide grounding is extremely rare, especially for a fighter.

"In this case, the planes that are grounded are supposed to be America's top-of-the line air superiority plane," Thompson added. "This is not like grounding some cargo plane. These are the sinews of our global air dominance."

Despite fears over the plane's safety, it remained unclear whether all F-15s were on the ground or would stay there.

© Copyright 2007 Globe Newspaper Company.