Event Description
A Boeing 767 was undergoing a high-power test of its left-hand General Electric CF6-80C2 engine to troubleshoot a reported inability to achieve climb thrust. During the test, the engine experienced an uncontained failure of the high pressure turbine (HPT) stage 1 disk.
Examination Findings
Examination of the disk fragments revealed that the disk failed from an intergranular fatigue crack. This crack originated from a small depression located on the bottom aft corner radius of a blade slot. Additionally, two other intergranular fatigue cracks were found, also originating from small depressions on the bottom aft corner radii of other blade slots. Intergranular fatigue cracks are typically associated with very high stresses exceeding the material's capability.
Previous Directives and Recommendations
General Electric issued service bulletins (SB) requiring inspections of CF6-80A and -80C2 HPT stage 1 disks, which the Federal Aviation Administration mandated with airworthiness directives (AD). However, the initial SBs and AD did not establish a compliance schedule. When subsequently revised to include a compliance schedule, the schedule permitted disks with significantly higher cycles since new—compared to those that had previously failed or were found cracked—to remain in service. On August 26, 2006, the Safety Board issued safety recommendations A-06-60 through A-06-64 to address these deficiencies.