What happened
On September 8, 2003, a Boeing 737-500, registration EI-CDE, was performing a scheduled flight from Dublin to Cork. During the approach to Runway 17 at Cork Airport, the flight crew heard a loud bang originating from the landing gear area as the undercarriage was being deployed.
While the aircraft landed normally and there were no injuries to the 19 passengers or 6 crew members, a subsequent inspection at the stand revealed that the left-hand main landing gear trunnion pin had sheared at its head. The aircraft was subsequently towed to the stand, and the damaged components were replaced. Following the incident, the aircraft was ferried to Dublin, where further inspections revealed corrosion on the right-hand trennion pin, which was also replaced before the aircraft returned to service.
The investigation
The investigation focused on the failure of the LH main landing gear trunnion pin (S/N AT067). Laboratory analysis of the fractured part revealed that the break occurred at the transition radius between the shaft and the retract flange. The examination of the fracture surfaces showed three distinct planes of propagation, a morphology consistent with stress corrosion cracking.
Investigators examined the history of the component, noting it had accumulated 23,922 cycles since new and had been overhauled in 1998. While no traces of the corrosion-inhibiting compound JC5A were found on the pin at the time of testing, the investigation considered its previous use as a potential contributing factor. The analysis also identified significant degradation of the chrome plating in the area of the fracture, with the plating thickness found to be approximately 0.013 inches, suggesting potential issues with the quality of the plating process during the 1998 overhaul.
Findings
- The primary cause of the pin fracture was stress corrosion cracking.
- The fracture originated in and near the radius where the shaft meets the flange.
- There was a lack of protective primer or cadmium plating at the site where the crack initiated.
- The chrome plating applied during the 1998 overhaul appeared to be of substandard quality, with several areas where the plating was missing entirely.
- The use of the corrosion inhibitor JC5A in the past may have contributed to the initiation of the corrosion process.