Boeing 757-200 N921UW incident near Richmond, Virginia

Probable cause

The installed wing panel clips failed due to fatigue, likely caused by the incorrect installation of the fasteners in response to SB 757-57-0027. Contributing to the incorrect installation was the SB's lack of clarity in the instructions and figures on the correct orientation of the clips and spacers.

NTSB analysis

NTSB materials laboratory examined the wing-side portions of the left wing trailing edge panel clips from the inboard, middle and outboard positions, with fasteners. Analysis indicated that the wing panel clips fractured due to fatigue. The fatigue region of the inboard clip covered the largest area fraction and was smoothest among the three clips, indicating that fatigue initiated first in the inboard clip. No evidence of corrosion or material anomalies was observed at the fatigue origin. The shape of the fatigue crack in this clip is consistent with fatigue crack propagation primarily under out-of-plane (inboard-outboard) bending loads.

Boeing Service Bulletin (SB) 757-57-0027 was issued April 28, 1988, to address fatigue cracking of attach clips at the location of the fracture in the incident airplane clips. Thickness of the middle clip from the incident airplane indicates that the clips were replaced using kit number 012N8546-1 clips. However, the clips were installed on the incorrect side of the vertical stiffener, and the clips were not installed with spacers as required in SB. Installing the clip on the outboard side of the rear spar vertical stiffener instead of the inboard side would be expected to put additional tensile stress on the inboard side of the clip, which is where fatigue initiation was observed. It is likely that this incorrect installed increased stresses on the clips, which would increase the likelihood of failure due to fatigue.

Patterns in the sealant and filled holes on the inboard clip indicate that it was repositioned to a lower position at some time. The middle clip showed sealant filling areas where the white paint was missing, indicating that the middle clip might have been removed and reinstalled at some time as well.

Text from the NTSB. Open the NTSB docket for DCA08IA048 for the full record.