This article was just posted on a Google military aviation group. I thought
some of ya'll might like to read it.
Scott Wilson

Robins team believes last snag cleared in F-15 groundings
19-12-2007
By Gene Rector -
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 very significant amount
of support," the colonel stressed. The team may be able to enjoy Christmas
if nothing else emerges. "We pushed extremely hard over the weekend to issue
what we believe will be the last (inspection order)," Niemantsverdriet
pointed out. "They deserve kudos for all the hours they have put in that
have brought us to where we are today. They've done a great job."