Journal of Accident Investigation

and forward on the side of the delamination associated with
the lug layers, and on average pointed upward and aft on
the mating sides, consistent with the lug pieces moving
downward relative to the remaining structure. In the portion
of the delamination above the lug-to-skin transition, hackles
generally pointed downward and forward on the outboard
side and upward and aft on the mating side, indicating a shear
direction consistent with fracture that occurred with bending
to the left. River patterns generally coalesced upward and aft,
indicating crack propagation extending upward from the lower
end. Investigators looked for but did not find any evidence of
fatigue, such as striations in the matrix or edge rounding of the
fiber ends on the translaminar fracture surfaces or matrix rollers
or striations on the delamination surfaces.
A schematic summarizing the observed fracture patterns is
shown in figure 11. Results of the microscopic examination
showed that the failure pattern of fracture in tension on the
right side was consistent with an overall bending of the vertical
stabilizer to the left. On the left side, the failure pattern of
tension and bending to the left was consistent with an overall
bending of the vertical stabilizer to the left after the lugs on the
right side fractured.
Overstress tension and
bending to the left
FORWARD
LEFT
RIGHT
MATERIALS EXAMINATION OF THE VERTICAL STABILIZER FROM AMERICAN AIRLINES FLIGHT 87
Overstress tension and
bending to the left
Investigators noted that the only compression translaminar
failure features were present on the vertical stabilizer at the
outboard side of the center aft lug. Typically, composites have
less strength in compression than in tension. However, the
design of the vertical stabilizer was such that the magnitude of
the load needed to fail a lug in tension was less than the load
needed to fail the lug in compression. Furthermore, after the
lugs on the right side failed, the curvature of the panel would
have caused tension loading in the forward and aft lug and
compression in the center lug with continued bending to the
left. Other unknown factors, such as changes in air loading as
the vertical stabilizer deflected after the initial fractures on the
right side, would further influence the failure patterns on the
left side.
Lug Tests
Using aerodynamic loads calculated from information
gathered on flight data recorders, investigators conducted a
comprehensive structural analysis in conjunction with the
materials examination of the vertical stabilizer and rudder to
determine stresses that developed in the structure during the
accident flight. Structural analysis indicated that under accident
Overstress tension and
bending to the left
Overstress tension
Overstress tension Overstress tension
Figure 11. Main lug fracture pattern as summarized in a schematic view of the lower end of the vertical stabilizer. Fracture features on the right side were
consistent with fracture under tensile loads. Fracture features on the left side were consistent with fracture under tensile loads and bending to the left.
Unlabeled arrows in the schematic indicate fracture propagation directions at each lug as determined from translaminar fracture features. Both of the
forward lugs and the right aft lug failed through the bolthole—that is, the lug fractured at the bolt location. Translaminar fracture features indicated that
as these lugs failed, fracture propagated outward from the bolt holes. The two center lugs and the left aft lug failed above the bolthole, in the lug-to-skin
transition area. Translaminar fracture features at the right center lug indicated that as the lug separated from the rest of the vertical stabilizer, fracture
propagated aft to forward. Translaminar fracture features at the left center and left aft lugs indicated that as these lugs separated from the rest of the
vertical stabilizer, fracture propagated forward to aft.
NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1 19
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