Great job by the Robbins Team, I’ve worked with this fine team on numerous occasions when I was on active duty. They due a great job and are a major asset for the Air Force.
Robins team believes last snag cleared in F-15 groundings
BY: Gene Rector, Macon Telegraph (Ga.)12/19/2007
ROBINS AIR FORCE BASE -- 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.Robins engineers have issued eight inspection orders to flying units since early November -the latest at midnight Dec. 15 - identifying areas to be inspected and the techniques to be used.
"Based on our review and interaction with Boeing and the accident board, we believe we have captured and mitigated all the risks," Niemantsverdriet indicated. "We're just at the point of getting the airplanes flying again, although that decision will be made by Air Combat Command and other using command."
McDonnell-Douglas, the original manufacturer of the F-15, was purchased by Boeing some years ago.
A thinning of the longeron at a key stress point - possibly due to a manufacturing defect - may be the root cause of the mishap and the cracks found in the eight aircraft."More than likely it is a manufacturing issue and we have pulled all the Boeing material discrepancy reports," the group commander said. "So far, we have not been able to isolate it to a particular production run or series. The cracks show up in aircraft as old as 1978 and as new as 1985."
Recurring checks had not called for a review of the area in the past."It was a 25,000 to 100,000-hour part," the colonel stressed. "So it was not included in our depot and phase inspections. It was designed to significantly outlast the aircraft."
However, the Robins team has learned that a number of factors create additional stress on the component - a splice joint in the two-piece longeron with different material thicknesses coming together, reduced width proceeding from the joint, angle variations and changes in the canopy sill.
"So a large amount of activity occurs in that single spot," Niemantsverdriet acknowledged. "It's like a creek that narrows down, making the water flow faster."
The inspections call for a thorough review for cracks in the longeron and a check for prescribed thickness. "We're looking at a good portion of the longeron - about 30 inches - and we're checking thickness at about 90 different locations. For an aircraft to be released to fly, there must be no cracks and the thickness must meet production tolerances," he said.If thickness issues crop up on crack-free aircraft, Boeing - at the request of Robins - will conduct fatigue life analysis to see if the aircraft can be returned to flight.
Replacement longerons will be manufactured at Robins. Niemantsverdriet said a production order has been placed for 15 with an anticipated 120-day lead time."We believe we can accelerate that," he said, "although one of the limiting factors will be availability of material."The Robins commander had high praise for his engineering team, the non-destructive inspection lab on base and technicians on the center's maintenance line who have assisted with validating and verifying inspection orders before they were released to the flying units."My hat's off to our engineering team and the people who have given us a
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