After a tedious, highly detailed process much like crime scene investigation, Robins Air Force Base engineers believe they have cleared the last technical hurdle affecting the nation's F-15 fleet.

Older models of the supersonic fighter have been grounded three times - the latest happening Dec. 4 - following the Nov. 2 crash of a Missouri Air National Guard F-15C.

The last two stand downs have affected only the F-15 A through D models, about 450 aircraft. The newer, heavier and more robust F-15Es - 224 aircraft - were returned to flight Nov. 11.

The investigation has focused on upper cockpit longerons near the canopy area. To this point, cracks have been found in eight aircraft - all F-15Cs. Officials zeroed in on that area after eye witnesses to the crash said the jet appeared to separate immediately behind the cockpit during normal training maneuvers. The pilot ejected with only minor injuries.

Col. Stephen Niemantsverdriet, 880th Aircraft Sustainment Group commander at Robins, said cracks have been found in both the right and left upper cockpit longerons of the eight aircraft. He said 97 percent of the fleet has been inspected. Longerons are metal rails that run horizontally and hold the fuselage together.

The Warner Robins Air Logistics Center at Robins is the worldwide manager of the Air Force's F-15 fleet, providing resupply, periodic overhaul and engineering oversight for the premier, but aging air superiority weapon system.

The F-15 first entered the Air Force inventory in 1975. The latest model - the F-15E - joined the force beginning in the late 1980s. The fleet averages 25 years of service.

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