What happened
On 19 October 2000, a Beech Super King Air 300, registration RP-C1587, was conducting an acceptance test flight following a major five-year maintenance program at Seletar Airport. Shortly after takeoff, air traffic controllers observed that the left main landing gear had failed to retract. The pilot confirmed that the gear remained in the extended position and was not locked, noting that the landing gear circuit breaker had tripped and a warning light was illuminated.
Despite attempts to reset the circuit breaker and cycle the landing gear, the left main gear remained unlocked. The crew subsequently performed an emergency gear extension and returned to Selet and Airport. During the landing on Runway 21, the left main landing gear collapsed, causing the aircraft to skid off the runway onto the left grass verge. There were no injuries to the five occupants on board, though the aircraft sustained damage to the left wing tip, trailing edge flaps, propeller blades, and the landing gear assembly.
The investigation
Investigators examined the fractured component of the landing gear assembly, specifically the actuator clevis. Technical analysis by the ATSB and SETSCO revealed that the clevis had suffered an intergranular fracture. This type of failure was linked to the material being embrittled due to improper heat-treatment controls during the manufacturing process.
Furthermore, the investigation looked into the installation of the component. Evidence suggested that the clevis head had likely been installed without the necessary number of spacing washers. This omission caused landing loads to be transferred as bending loads through the shank of the component, contributing to the fracture. The investigation also reviewed maintenance practices at the Seletar facility, finding that several job cards had been signed off before the actual work was completed, and that grease nipples on the gear assembly had been obscured by paint.
Findings
- The primary cause of the landing gear collapse was that the gear was not in a locked position.
- The gear failed to lock because the actuator clevis fractured under a bending load.
- The bending load was induced by the insufficient use of spacing washers during the installation of the clevis head.
- The clevis material was found to be embrittled, a result of poor heat-treatment control during its manufacture.
- Maintenance records from a previous modification in Manila lacked sufficient detail to confirm the exact installation process of the washers.
Safety action
- Recommendations were made for the aircraft manufacturer to review the heat-treatment processes for similar components to identify potential hydrogen-embrittlement risks.
- The investigation highlighted the need for maintenance organizations to ensure that all tasks are fully completed before job cards are signed off and that all modification details are accurately recorded.