What happened
On 10 May 2013, a CASA 212-200 operated by PT. Nusantara Buana Air was performing a scheduled passenger flight from Sultan Babullah Airport in Ternate to Gebe Airport. While the aircraft was climbing through 6,000 feet, the left propeller assembly detached from the rear hub half. The crew observed the component flying forward from the left side of the aircraft, which caused an immediate yawing motion that the pilot had to correct using the rudder.
Upon inspecting the instruments, the crew noted that the left engine torque had dropped to zero and the oil temperature was rising. After visually confirming the loss of the propeller, the pilots shut down the left engine using the fuel shut-off valve. The aircraft successfully returned to Ternate and landed safely. There were no injuries among the 20 people on board.
The investigation
The investigation focused on the structural failure of the propeller hub and the maintenance history of the aircraft. Examination of the fractured left propeller hub revealed that the failure was caused by fatigue cracks that had originated at the edge of a fastening hole. These cracks had progressed significantly, leaving only about 3/10 of the original area intact to support the load.
Investigators also scrutinized the aircraft's maintenance logs. They discovered a discrepancy in the recording of flight cycles, where data transfer errors had resulted in an undercount of 114 cycles. Furthermore, the propeller log book used by the operator only featured a column for flight hours, lacking a dedicated space for flight cycles. This omission contributed to a misunderstanding of the required inspection intervals.
Findings
- The primary cause of the failure was an undetected fatigue crack in the rear hub half.
- The required Non-Destructive Inspection (NDI) had been missed, as the aircraft had exceeded its scheduled inspection interval by 155 cycles.
- Maintenance personnel had misinterpreted the inspection requirements because the propeller log book did not track flight cycles.
- Errors in transferring data between maintenance logs led to an inaccurate count of the aircraft's total flight cycles.
Safety action
Following the incident, PT. Nusantara Buana Air conducted NDI inspections on all propellers within its CASA 212-200 fleet, finding no further cracks. The operator also issued new engineering information to better control inspection intervals for both hours and cycles. Additionally, the Directorate General of Civil Aviation issued Airworthiness Directive No. 13-06-001 to address the propeller hub failure.