Background
The aircraft involved was a Boeing 737-236, registration G-BGDI, manufactured in 1980 and powered by two Pratt & Whitney JT8D-15A turbofan engines. The incident occurred on 6 May 1998 in Dusseldorf, Germany. There were no persons on board as the flight was a pre-departure inspection.
Incident Details
During the pre-departure check, a longitudinal crack approximately 11 cm long was found on the left side of the fuselage, below the window belt and reportedly at a station just aft of the main wheel well. The operator's engineering personnel in the United Kingdom were informed and an Engineering Order was raised to carry out an approved temporary repair before a non-revenue flight, empty and unpressurised, was conducted back to the aircraft's maintenance base at Gatwick. The temporary repair included 'stop drilling' the crack and the application of 'speed tape', in accordance with the Boeing Structural Repair Manual (SRM).
Investigation
Upon arrival at Gatwick, it became apparent that there had been some misunderstanding regarding the exact location of the crack. Its actual position was just forward of fuselage Frame No 727B, above Stringer No 16. This placed it in a zone familiar to Boeing 737 structural engineers, as such cracking was the subject of Service Bulletin (SB) No. 737-53-1065, originally issued in 1985 to counter skin cracks, disbonding, and wrinkling between the skin and doubler in this area. The SB (selected by the FAA-sponsored Ageing Fleet 737 Structures Working Group for incorporation on older Boeing 737 aircraft) details a structural modification to be embodied at or before the accumulation of 75,000 flight cycles. It also calls for an inspection of the zone after 16,000 flight cycles, with repeat inspections at 3,000 flight cycle intervals. Inspections can be visual if both internal and external surfaces are examined; if non-destructive test (NDT) techniques are employed, inspection can be confined to external surfaces. Any cracks are repaired in accordance with the SRM, typically using a 'scab patch'. Embodiment of the structural modification constitutes terminating action for the SB.
In the case of G-BGDI, SB 737-53-1065 inspections were arranged to be conducted at every P1 check, which occurred at approximately 2,100 flight cycle intervals. The most recent check was carried out in October 1997, when the aircraft had achieved 35,117 flying hours and 28,040 cycles. The incident occurred when the aircraft had accumulated 36,285 hours and 28,907 cycles. However, it was found that the operator's inspections had consisted of a visual examination of the external surface only.
Findings
The fuselage skin in the area of interest had a doubler bonded onto the inner surface, forming a lattice following the lines of stringers and frames. The crack in the skin had followed the line of the doubler above Stringer 16 in an aft direction, then started to turn upwards, still following the edge of the doubler, immediately ahead of Frame 727B. The doubler had thus fulfilled its function as a 'tear stopper'. At its forward end, the crack was still following the edge of the doubler above the stringer, and had not yet reached the next frame.
The piece of skin containing the upper fracture face was cut out and subjected to metallurgical examination. Flexing of the skin had caused the two halves of the fracture face to rub together, removing microscopic evidence, so a striation count to indicate the number of flight cycles over which the crack had grown was not possible. However, there were multiple fatigue origins over the inside edge of the central 61 mm of the crack. Either side of this relatively slow growth region, the failure had progressed in a shear process with somewhat more rapid crack growth. The general appearance of the fatigue propagation area suggested the crack had been growing over a considerable period, and was probably present when the area had been inspected at the last P1 check. Additionally, the external paint was poorly bonded to the primer and could easily be peeled away, raising the possibility that the paint initially remained intact, thus masking the crack from visual detection in its early stages.
Operator Action
As a result of this incident, the operator intended to conduct a fleet inspection and to employ NDT methods in future periodic inspections, as required by SB 737-53-1065.