Aviation Investigation Report A98H0003

1.12.10  Examination of Aircraft Structural Components

  1. - General
  2. - Empennage and Wings
  3. - Cabin Outflow Valve Doors
  4. - Cockpit Sliding Clearview Windows  General (STI1-77)

Specific structural components were identified in the main debris field from all extremities of the aircraft, from nose to tail, from left wingtip to right wingtip, and from belly to fin. A performance analysis determined that the aircraft did not accelerate to a speed that could result in aircraft structure separating prior to the time of impact, nor is there any other indication of such structural failure. There was no indication that the fire had burned through the structure, or that any of the structure had separated as a result of fire damage.  Empennage and Wings

Pieces of the aircraft structure over 1.22 m (4 feet) long were examined for damage patterns that might help determine the attitude and configuration of the aircraft at the time of impact. These larger pieces were considered the best for providing the clearest patterns showing the deformation of the structure as a whole.

The deformation and fragmentation damage to the two horizontal stabilizers was relatively symmetrical. However, the right stabilizer displayed slightly more damage, which suggests that the right side contacted the water before the left side. (STI1-78)

No conclusive deformation or fracture patterns were observed in any of the rudder pieces. (STI1-79)

The deformation and fragmentation patterns of the two wings was relatively symmetrical. (STI1-80) Therefore, it was concluded that there were no significant differences in the magnitude and orientation of the impact forces that acted upon them. Close examination of wing skin bending and torsion, stringer damage, shear clip damage, spar damage, rib damage, slat track bending, and engine pylon attachment fitting damage found subtle differences that suggested that the left wing may have sustained greater damage, and that the aircraft may have been yawed nose right at the time of impact. However, the differences were so subtle and involved such a small number of pieces that they were inconclusive. Owing to the lack of information, no determination could be made, based solely on the wing examination, about the angle of bank or pitch angle of the aircraft at the time of impact.  Cabin Outflow Valve Doors (STI1-81)

The cabin outflow valve doors were examined to determine whether they were open or closed at the time of impact. Although there were numerous scratches, gouges, and dents on the surfaces and edges of both doors, none of these marks were continuous across both doors. This suggests that the doors were not closed when these marks were made. Additional marks made by the deformation of the door frame and the bending of the door hinges indicates that both doors were open to some degree at the time of impact.

When opening, the forward door swings outward away from the fuselage into the airstream and the aft door swings inward into the left tunnel area. Light to moderate soot was observed in areas on the exterior skin of the aft door, and light soot was observed along the hinge line of the forward door. The observed soot pattern is consistent with the doors being in an open position for a sufficient length of time to permit the doors to become sooted from the fire effluent.  Cockpit Sliding Clearview Windows (STI1-82)

The MD-11 is equipped with two sliding clearview cockpit windows (one on each side of the cockpit) that are normally closed and latched during flight. These windows can be opened by the pilots when in unpressurized flight; this action is part of the Swissair Smoke/Fumes Removal checklist. The structure around the clearview windows was examined to determine whether they were open, or closed and latched, at the time of impact.

The cockpit clearview windows are plug-type windows that rest against a flange around the perimeter of the sill. Each window can be moved through a crank-and-chain mechanism located on the bulkhead below the windowsill. To open the window, the locking mechanism is first released by the pilot. This preliminary action releases the latches on the aft edge of the window, but does not move the window. The initial turning of the hand crank pulls the aft edge of the window inboard enough to clear the sill, allowing the window to slide aft when the crank is further turned. The window can be unlocked when the aircraft is pressurized, but the outward force on the window is so great that turning the hand crank cannot open the window.

The damage to two of the three latch plates on the first officer's clearview window was aligned with the locked position and the damage had the same shape as the ends of the latches. This is consistent with the latches having been locked at the time of impact. The separation of the third latch plate was consistent with the direction of the damage to the first two. This damage pattern is consistent with the window having been closed and latched, and forced inboard by the impact with the water.

The damage to the three latch plates, as well as the latching mechanism, window, and sill on the captain's window, were consistent with the latches having been unlocked at the time of impact. There were also impact marks along the sill that were consistent with the pitch and location of the fasteners around the perimeter of the window. In most cases, the depth of these gouge marks penetrated both the paint and primer, and indented the metal. This is consistent with the window having been in the closed position at the time of impact.

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