What happened
During landing at Karratha Airport, the right engine thrust reverser on VH-NHA, a Boeing 737, failed to deploy. The failure occurred because an in-service lockout bolt remained installed in the mechanism following maintenance performed in Perth prior to the flight.
The investigation
The investigation focused on why the bolt remained in place and why the error was not detected before departure. It was determined that a maintenance engineer used a bolt located in the cockpit to safely isolate the thrust reverser mechanism during servicing. Because this specific bolt was used instead of the designated maintenance lockout bolt, the standard safety protocols were bypassed. Specifically, the use of this bolt meant that the red warning flag, which would typically alert personnel to the bolt's presence, was not activated. Furthermore, the standard tool check procedure, which is designed to ensure all maintenance equipment is accounted for, was not effectively utilized to identify the missing bolt.
Findings
- The engineer utilized a bolt from the cockpit for the isolation task rather than the specified maintenance lockout bolt.
- The absence of the red warning flag prevented the maintenance engineer, dispatch engineer, and flight crew from being alerted to the installed bolt.
- The tool check process failed to identify that the bolt had not been returned to its proper location.
- There were no warning labels within the cockpit to alert the flight crew to the potential presence of the bolt.
- The failure to remove the lockout bolt resulted in the thrust reverser being deactivated when the aircraft returned to service.