What happened
On October 14, 2022, a Cessna 402C, registration HK-3065, was performing a scheduled commercial flight from Bahía Solano to Quibdó, Colombia. Approximately 25 minutes into the flight, while cruising at 5,500 feet, the crew heard a loud noise originating from the left engine. The engine parameters, including RPM, oil pressure, and oil temperature, suddenly dropped, leading the crew to believe the engine had suffered a sudden stoppage.
While the crew was executing emergency procedures, the propeller and spinner detached from the left engine. The crew declared an emergency with Quibdę Control Tower, which activated the aerodrome's emergency response plan. Despite the loss of the propeller, the crew managed a controlled landing at El Caraño Aerodrome. All five occupants (two pilots and three passengers) were unharmed, and the aircraft sustained no additional damage upon landing.
The investigation
The GRIAA investigation focused on the mechanical failure of the engine components and the maintenance history of the aircraft. Investigators examined the engine flange and the remaining mounting hardware, finding that of the six studs intended to secure the propeller, only four remained attached to the engine.
Technical analysis revealed that the propeller mounting bolts had experienced fatigue and material failure. The investigation also scrutinized the maintenance records from three months prior, when the left engine and propeller had been installed. It was discovered that the Quality Control inspector had been performing the same manual installation tasks as the maintenance technician, rather than acting in a supervisory capacity. This lack of separation between execution and inspection meant that the improper torque applied to the propeller bolts went undetected.
Findings
- The primary cause of the incident was the total separation of the left engine propeller due to high vibrations that fractured the mounting bolts and tore other bolts from the propeller hub.
- The failure originated from incorrect torque applied to the propeller mounting components during the last routine inspection.
- Maintenance oversight was a critical factor, as the Quality Control inspector performed installation tasks instead of conducting an independent inspection.
- The operator lacked specific procedures for verifying the technical competence of personnel and lacked robust inspection protocols, such as Required Inspection Items (RII), within their General Maintenance Manual.
- The maintenance manual lacked effective procedures for ensuring the functional integrity of the aircraft following the removal and installation of major engine components.