PICTURES: Manufacturing defects caused cracks that downed USAF
F-15
By Graham Warwick
Fatigue cracks started by manufacturing defects in a fuselage
longeron caused the in-flight break-up of a Boeing F-15C Eagle on 2
November 2007, the US Air Force accident investigation has
concluded.
Longeron cracks have been found in another nine F-15Cs, and
manufacturing defects that could potentially cause fatigue cracking
have been detected in a further 182 F-15A-Ds. All of these aircraft
remain grounded.
Examination of the wreckage of the crashed F-15 revealed the right
upper longeron, a critical load-carrying component in the forward
fuselage, failed because of a fatigue crack that formed where the
metal was thinner than specified in the blueprint.
Instead of being the specified 0.090-0.110in (2.3-2.8mm) thick, the
flat top, or web, of the aluminium longeron that failed was as thin
as 0.039in less than a millimetre - in the area when the fatigue
crack formed.
The thinning was caused when the longeron was machined by McDonnell
Douglas during production of the aircraft, which was delivered in
1982. Similar manufacturing defects - undercuts, ridges or surface
roughness that could potentially cause stress concentrations have
been detected in upper longerons in 40% of fleet.
This was by far the worst thinning of a web discovered by the
fleet-wide inspections that followed the crash, says Maj Gen Thomas
Owen, commander of Warner Robins Air Logistics Center, the USAFs
F-15 depot.
Undercuts during machining thinned the web below blueprint
specification © US Air Force
The accident aircraft had accumulated 5,700 flight hours, says
Owen, or 11,000 equivalent spectrum hours taking in account the
additional fatigue stress imposed by high-G manoeuvring. But the
longeron was projected to last 31,000h, well past the life of the
aircraft, he says.
In the 2 November in-flight break-up, the cockpit separated at
canted fuselage station 377, where the forward and aft sections of
the two-piece upper longerons are spliced. The fatigue crack had
formed just forward of the splice, in an undercut produced when a
sloping transition was machined in the underside of the
longeron.
Investigators say the crack started on the underside of the
longeron, working its way upwards and outwards through the
over-thin web until it reached the outer posts, which carry the
bulk of the forward fuselage loads.
Fatigue crack spread through the web, and weakened longeron
fractured © US Air Force
Weakened by the crack, the posts fractured as the aircraft pulled
almost 7.8G in an air-combat manoeuvre. When the upper right
longeron failed, the other three upper and lower longerons could
not carry the loads and the forward fuselage separated.
Owen says the USAF has ultrasonically inspected 84 specific
locations on each upper longeron on every F-15A-D. Thats 168 data
points per aircraft, and if any one is under blueprint, then the
aircraft remains grounded.
Acknowledging its conservative approach, the USAF says each
grounded aircraft will be individually analysed to determine if it
can be returned to flight with recurrent inspections or flight
restrictions, or whether it must be repaired or retired.
Detailed analysis is under way at the Air Force Research Laboratory
to determine the rate at which the fatigue cracks spread, so that
requirements for recurrent inspection of affected longerons can be
established.
Examples of longeron cracks found in other F-15s © US Air
Force
While the USAF has not decided which, if any, aircraft will be
repairs, Warner Robins is manufacturing an initial batch of 15
longerons at a cost of $15,000. Owen says replacement of the
longeron in an aircraft will cost $250,000.
Owen also says the USAF will also conduct detailed inspections of
other primary structure in a sample of the F-15 fleet to determine
whether similar manufacturing defects exist in other critical areas
of the aircraft.
DATE:11/01/08
SOURCE:Flightglobal.com
