CliveL
The problem was identified in the early 1970s, but on the CF6-6 engine. The LPT on this engine is different, different airfoils and disks. It is a 5 stage turbine, whereas the CF6-50 engine has a 4 stage LPT.
The CF6 LPT S3 disk resonance response to HP rotor unbalance was first identified in the GE CF6-6 engine, which shares the CF6-45/-50 type certificate. The CF6-6 experienced four uncontained LPT S3 disk forward spacer arm separations between 1975 and 1978 due to HP rotor unbalance.
As a result, GE redesigned the CF6-6 LPT S3 disk so that an HP rotor unbalance condition would not excite the LPT S3 disk and result in disk failure. The CF6-50 engine has experienced 12 instances of LPT S3 disk forward spacer arm cracking since 1973. Eight of the cracked CF6-50 disk forward spacer arms were discovered during shop-level inspections when LPTs were disassembled for unrelated reasons, such as engine model conversion or the replacement of life-limited parts. In the remaining cases, all of which are cited above, disk cracks progressed to failure, leading to in-service uncontained engine failures.
The rotor unbalance condition leading to the excitation of the stage 3 disk and the subsequent fatigue cracking of the forward arm is not sensed by the vibration detectors that are used. You have to remember these engines are between 30 to approaching 40 years old. Servicing of these engines is somewhat unclear as to procedures used. For instance, it is required to not only balance and test individual rotor stages, but the completed rotor assemblies. It is known this was not happening. So another FAA AD was issued this year requiring a complete vibratory assessment of the core engine to assure at specific set engine cycles to assure undetected vibration does not cause future disk failures.
A similar FAA AD has been issued for JT8-D engines that are also aged and are widely distributed throughout the world.
Hope you find this informative.
Turbine D