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2.2 Aircraft Structural and Systems Integrity
Most of the wreckage was found and dredged from an area of about 60 m x 80 m in the
river bed. The distribution of the wreckage found in the river was documented by a sonar
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picture and during dredging. The relatively small impact area indicated that the aircraft
impacted at a relatively steep angle.
Although approximately 73% of the aircraft by weight was recovered, only a small
amount of the wreckage were identifiable and in a condition useful for the investigation.
The inspection of the recovered wreckage suggests that the aircraft was structurally intact
until the outer sections of the horizontal stabilizers, elevators and sections of the rudder
separated from the aircraft.
The examination of the fracture surfaces and wreckage fragments showed no evidence of
pre-existing structural defects, fire or explosion in flight.
The examination of the recovered passenger oxygen generators revealed no evidence of
activation from which it is concluded that the aircraft did not experience depressurization
in flight.
One of the investigations is to determine the cause of the separation of parts of the
empennage, as described in the following paragraphs.
2.2.1 Horizontal Stabilizers and Elevators
The recovered sections of the leading edge of the horizontal stabilizer found on land
showed no surface impact marks. This suggests that the separation of the horizontal
stabilizer sections was not initiated by a foreign object impact.
The damage on the horizontal stabilizer box structure showed characteristics of the
occurrence of high vertical and torsional reverse loading. In addition, examination of the
multiple small sections of front spar, leading edge, rear spar lower chord, trailing edge
rib, and interspar ribs and skin (which were found on land) showed characteristics
consistent with numerous cycles of reverse loading.
The parts of the horizontal stabilizer center section (“Texas Star”) were recovered in the
river and were found broken due to impact, see Figure 11. In the recovered parts no
evidence was found which could be attributed to fatigue failure.
The elevator hinges and surrounding area at elevator stations 213 and 176 contained
evidence of elevator over-travel in the up and down directions and twisting of the
bearings or hinge plates. Deformation of the right hand elevator control tab rods showed
that they were bent in the downward direction. Both control rods for the left-hand
elevator displayed multiple helical breaks that indicate torsional loading.
2.2.2 Vertical Stabilizer and Rudder
All recovered parts of the vertical stabilizer box were found at the crash site. All of the
vertical stabilizer spar terminal fittings and the fin trailing edge hinge fittings were also
found at the crash site. This indicated that the vertical stabilizer structure was intact on
impact.
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All of the rudder hinge fittings were complete except for the hinge fitting of the rudder
station 194, which was missing the aft portion of the attachment. The rudder upper hinge
showed evidence of over travels in the left and right directions. Other hinges exhibited
twisting damage consistent with excessive rudder movement in the clockwise direction
(looking forward). However, because the hinge fittings were found at the crash site, it was
difficult to determine whether the twisting damage was incurred during flight or during
impact.
An estimated 50% by area of the rudder skin honeycomb pieces were found scattered on
land. The top balance weight was found also on land. Small pieces of the rudder spar (still
attached to the hinge fittings) were recovered from the crash site, as were parts of the
mid-spar balance weight. This indicated that the rudder disintegrated partially in the air.
2.3 Break Up of the Empennage
Some parts of the empennage that separated in flight were found on land. Taking into
consideration the relative positions of these empennage parts and their location with
respect to the crash site, BASI and NTSB conducted trajectory studies (Section 1.16.4) to
determine the trajectory of the aircraft and to estimate the altitude when the break-up
could have occurred. Furthermore, to explain the reason for the empennage break-up,
NTSB and Boeing conducted the flutter study as mentioned in Section 1.16.5.
2.3.1 Results of Trajectory Studies
The studies were based on the theory that the trajectory of a separated mass will be
determined by its initial conditions at the moment of separation, taking into consideration
speed, heading, and altitude, as well as its aerodynamic characteristics. Corrections to
allow for wind speeds and directions were applied.
　
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本文鏈接地址：NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(22)