Incident Overview
During a flight, the main cargo door opened, and the crew returned the aircraft for a normal landing. No further details about the aircraft or operator were provided in the investigation.
Circuit Breaker Findings
Two circuit breakers were required to be pulled prior to takeoff. After the incident, one of these circuit breakers was found still engaged.
Conformity Inspection
A conformity inspection of the cargo door installation identified several areas of nonconformity. The door locks were found to have strength below the required specifications. Additionally, the door locks lacked paint, which is used for visual identification. A bundle of wires was found damaged; these wires were part of the system that indicates door closing and locking.
Warning Light System
The investigation revealed that it was possible for the cargo door to be not completely closed while the door warning light indicated that it was fully locked. This discrepancy could lead to an incorrect indication of door security.
Conclusion
The incident involved an in-flight opening of the main cargo door, with the aircraft landing normally. The investigation identified a circuit breaker issue, nonconformities in door lock strength and visual identification, damaged wiring, and a potential failure of the warning light system to accurately reflect the door's locked status.