What happened
On 6 May 1998, during a pre-departure inspection in Dusseldorf, Germany, a longitudinal crack measuring approximately 11 cm was discovered on the left side of the fuselage of a Boeing 737-236, registration G-BGDI. The crack was located below the window belt, near the main wheel well.
Following the discovery, the operator's engineering team in the United Kingdom issued an order to perform a temporary repair. This involved stop-drilling the crack and applying speed tape, as permitted by the Boeing Structural Repair Manual. The aircraft was then flown on an unpressurised, empty non-revenue flight to its maintenance base at Gatwick.
Upon arrival at Gatwick, further inspection revealed that the crack's position was slightly different than initially reported, situated just forward of fuselage Frame No 727B, above Stringer No 16.
The investigation
The investigation examined the structural integrity of the area and the effectiveness of previous maintenance inspections. The specific zone of the crack was subject to a Boeing Service Bulletin (SB 737-53-1065), which was designed to address skin cracking and disbonding issues. This bulletin requires inspections after 16,000 flight cycles, with subsequent checks every 3,000 cycles.
While the aircraft's most recent P1 check in October 1997 had been completed, investigators found that the operator had only performed a visual examination of the external surface. Metallurgical analysis of the fractured skin was limited because the movement of the metal during the incident had rubbed away microscopic evidence, preventing a precise striation count to determine the exact growth duration. However, the investigation established that the crack had multiple fatigue origins and had likely been growing for a significant period.
Findings
- The crack had progressed along the edge of an internal doubler, which acted as a tear stopper.
- The operator's inspections had been limited to the external surface only, rather than examining both internal and external surfaces as required for visual inspections.
- Poor adhesion between the external paint and the primer may have masked the crack during earlier inspections.
- The crack was likely present during the previous P1 maintenance check.