What happened
The aircraft experienced a significant mechanical failure involving the left landing gear system while extending the gear during the approach phase of flight. According to the aircraft logbook, a tremendous bang occurred when the gear was extended at 230 knots. This event resulted in the dropping of panels at rows 15 through 7 and masks at row 18. Despite the audible impact and structural displacement, the cockpit indicated three green lights, suggesting normal gear indication. Upon taxiing in, no further abnormalities were noted by the crew.
The investigation
Examination of the failed component revealed that the broken left walking beam was constructed from aluminum. An airworthiness directive for this specific part mandates ultrasonic inspection every 1500 hours to ensure structural integrity. The investigation determined that during a previous inspection, a mechanic incorrectly identified the aluminum beam as steel. Consequently, the required ultrasonic inspection was not performed at the proper time, allowing fatigue or wear to progress unchecked until failure.
Findings
The root cause of the incident was a maintenance error where a mechanic mistook an aluminum walking beam for a steel one. This misidentification led to non-compliance with the mandatory 1500-hour ultrasonic inspection schedule. The manufacturer has taken corrective action, and at the time of the report, all aluminum beams had been replaced.