What happened
On 29 January 2019, a Boeing 737-800, registration EI-GXG, departed from Frankfurt-Hahn Airport on a scheduled passenger flight. During the initial climb, the flight crew initiated the landing gear retraction sequence. As the gear moved, the crew heard an unusual noise, followed by a noticeable leftward roll of the aircraft and the activation of landing gear discrepancy lights.
After attempting to stabilize the aircraft and completing the required checklists, the crew decided to return to Frankfurt-Hlan. Although the crew requested the presence of emergency services as a precaution, they did not declare an emergency. The aircraft landed without further incident; however, upon reaching the gate, the crew was notified that a piece of metal had been found on the runway. Inspection revealed that the upper surface of the left wing had sustained substantial damage.
The investigation
The investigation, conducted by the AAIU in coordination with the German BFU, focused on the separation of a pivot pin from the left-hand main landing gear walking beam hanger. Investigators found that the pivot pin and a tang washer had detached and were located in the wheel well, while the necessary castellated nut and cotter pin were missing from the scene.
Analysis of the aircraft manufacturer's production records revealed that while the installation of the hardware was recorded as complete and within torque specifications, the actual method used by mechanics involved applying torque to the head of the pin rather than the nut due to limited access. Furthermore, examination of the tang washer showed no wear on the side that should have been in contact with the nut, suggesting the nut was not properly seated.
Findings
- The primary cause of the incident was the migration of the pivot pin from the walking beam hanger assembly.
- The castellated nut and cotter pin intended to secure the pin were either not installed or were incorrectly installed during the manufacturing process.
- Production line inspections failed to detect the improper installation of the securing hardware.
- The pivot pin was not held in a secure position during the assembly phase.