What happened
During a high-power ground test of the left-hand General Electric CF6-80C2 engine, a Boeing 767 experienced an uncontained failure of the stage 1 high pressure turbine (HPT) disk. The test was being conducted to troubleshoot a pilot report indicating that the engine was unable to produce sufficient climb thrust.
The investigation
Investigators examined fragments of the failed HPT stage 1 disk and discovered that the failure was caused by an intergranular fatigue crack. This crack originated from a small depression located on the blade slot bottom aft corner radius. Additionally, two other intergranular fatigue cracks were identified, which also originated from small depressions on other blade slot bottom aft corner radii. Such intergranular fatigue cracks are typically associated with stresses that exceed the material's structural capabilities.
Findings
While General Electric had issued service bulletins for inspections of CF6-80A and -80C2 HPT stage 1 disks, and the FAA had mandated these via airworthiness directives, these instructions did not initially establish a specific compliance schedule. When the compliance schedule was later revised, it permitted disks with significantly higher cycles since new to remain in service, even though those disks had higher-than-expected failure risks compared to previously identified cracked disks.