What happened
The aircraft completed its flight without incident after experiencing a failure of the right engine while descending toward the destination. The engine shutdown was not caused by fuel issues or pilot error, but rather by internal mechanical failures within the turbine section.
The investigation
Examination of the failed engine revealed that normally stationary third-stage vane clusters in the low-pressure turbine area were driven by gas loads. This movement occurred because the anti-rotation pins, which are designed to hold these vanes in place, had failed. As the clusters rotated within their channels, they machined themselves through the low-pressure turbine case.
Detailed evidence showed that 23 of the 44 third-stage vane anti-rotation pins ruptured initially. The remaining pins subsequently fractured due to shear overload. The root cause of this rupture was identified as bending stresses on the vane pins resulting from vane twist and material creep in the pin material.
Findings
The investigation highlighted a design vulnerability in the original pin configuration. In response, a design task was initiated in 1986 to develop a more durable pin solution. By May 1987, Pratt & Whitney released a new anti-rotation pin made from INCO 901 non-stepped material (part number 803995) to replace the original PN 599714 TINIDUR pin.
Safety message
The failure mode identified in this incident led to a specific design improvement. The introduction of the INCO 901 material for anti-rotation pins addressed the issues of bending stress and material creep, enhancing the durability of the low-pressure turbine assembly.