What happened
On June 8, 2004, a B737-204ADV, registration EI-CJC, departed Dublin Airport for Aberdeen. Shortly after takeoff, the flight crew discovered that the left-hand main landing gear failed to retract. The pilots notified air traffic control and initiated a return to Dublin. The aircraft landed safely on Runway 28 using a "Flap 40" configuration. Upon arrival at the stand, a technical inspection revealed that the trunnion pin of the left-hand main landing gear had sheared, which had disconnected the undercarriage actuator from the gear.
The investigation
The investigation focused on the history and condition of the failed trunnion pin. The component had been previously overhauled and was part of a fleet-wide rotation. Records indicated that during a prior overhaul, a contractor had identified cracks in the chrome plating on the inside face of the fork, but subsequent inspections by a subcontractor did not flag these specific areas as needing rework. The component was eventually released as overhauled and installed on EI-CJC in early 2003.
Laboratory analysis conducted by the aircraft manufacturer provided critical insights into the failure. The examination of the metal revealed that the chrome plating was of poor quality, exhibiting "chicken wire" cracks and poor adhesion. Furthermore, evidence of grinding damage was found on the inner face of the lug, which had created an overtempered martensite layer beneath the plating.
Findings
- The primary cause of the fracture was Stress Corrosion Cracking (SCC).
- The failure originated in the area where previous crack indications in the chrome plating had been noted.
- The chrome plating exhibited severe through-thickness cracking and poor adhesion characteristics.
- Grinding damage on the inner face of the lug contributed to the structural degradation.
- The fracture of the trunnion pin resulted in the disconnection of the actuator, making gear retraction impossible.