Richmond, VA, USA
N921UW
Boeing 757-200
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.
On March 22, 2008, at about 9:00 am EDT, a USAirways Boeing 757-200, flight number 1250, registration N921UW, lost a left upper wing trailing edge panel during cruise flight at FL270. Initial reports were that the flight crew had experienced "light chop" at the time. The flight was enroute from Orlando International Airport, Orlando, Florida (MCO) to Philadelphia International Airport, Philadelphia, Pennsylvania (PHL). The panel struck the window of passenger row 19ABC and cracked only the outer portion of the window. Pressurization was not lost. The flight crew continued on to PHL. Upon approach to PHL the flight crew informed ATC that they could not increase their airspeed due to possible airframe damage. The flight landed uneventfully. There were 174 passengers and 6 crew members on board with no reported injuries. The passenger windows are two pane design, with outer pane carrying normal cabin pressure loads and the 'middle' pane to carry fail-safe pressure in the event of loss of outer pane during flight. It would appear that the middle pane safely carried fail-safe load in this event. At the time of the event Boeing had no records of a depressurization event associated with failures of any of the 737/747/757/767/777 model passenger windows, but there have been outer pane cracking or departure events. The 757 passenger windows are certified for pressure only with no regulatory or Boeing requirements for impact. The outer panes are made of stretched acrylic material, which is durable for day-to-day service, but cracks can be initiated when subjected to impact with sharp object or an object with significant energy such as the panel in this incident. Boeing subject matter expert estimated that the weight of the fixed trailing edge panel, part number 113N1611-9, and hardware that separated with the panel was approximately 35 pounds. The NTSB has reviewed the data extracted from aircraft's flight data recorder (FDR), and in conjunction with statements from the flight crew, the Safety Board found that there was no substantial change to the aircraft's handling characteristics after the panel separated from the wing. Because of this, the NTSB has classified this event as an incident. Since the incident on March 22, US Airways reported to the Safety Board that it had inspected the wing panels on all of its 757s and found problems with wing panel fasteners on several other aircraft, which were since repaired and returned to service. After problems with the 757 wing panel fastening system were identified in the late 1980s, the Federal Aviation Administration issued an Airworthiness Directive (AD) requiring operators of 757s to install a redesigned fastening system. Eastern Airlines, which ceased operations in 1991, operated the 757 involved in this incident at the time the AD was issued in 1991, and had installed the redesigned system. It was these redesigned fasteners that failed on flight 1250. NTSB issued three recommendations to address the potential for improperly installed fasteners and spacers.
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.
Source: NTSB Aviation Accident Database
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