Lyman,
I am trying to build on what is (was) known at the time. The AD warned specifically of aftward axial transit of a shaft, causing damage to the a/c, and those on the ground (sic). The wear, specific to the AD, was located at the splines forward of the IPT (at the other end of its shaft), and reporting of the initilal exam of the engne contents stated "Rigid Couling Failure".
We have been through this many times. This engine had been inspected and passed the spline wear requirements before being placed back in service. The rigid coupling of the IP shaft, located in the compressor section of the engine, did not fail. The IP shaft did not move rearward. The initiation of the failure event started with an oil leak in a compartment just forward of the IP Turbine disk. A fire started as this area is hot. The fire caused the IP turbine disk to begin to heat above temperature capability of the disk alloy and rotational stresses being encountered at the time. Now to understand what happened next, you have to understand the metallurgy of superalloys used in disks.
The best disk superalloys have an operating temperature of between 1200℉ and 1400℉. These alloys have good ductility and creep rupture capabilities within operating temperature ranges. If the temperature is exceeded, creep and in the case of a spinning disk, outward growth (stretching) will occur fairly rapidly. As the stages of creep progress, the final stage progresses to failure very rapidly. So lets apply this to the IP disk in question.
The IP disk is attached to the turbine end of the IP shaft by a series of circumferential bolts which secure the power drive arm of the IP disk to the shaft. The designers determine the thickness of the power drive arm, the web thickness of the disk and the disk bore mass based on anticipated stress levels and temperatures during engine operations throughout the flight envelope plus a safety margin to preclude disk burst. So what happens when an oil fire develops in the compartment just forward but adjacent to the IP disk? The disk begins to overheat from the bore to the disk web with the overheating commencing from the forward side of the disk. As the temperature begins to exceed the superalloy capability, the disk begins to stretch somewhat unevenly from front to rear, but radially. But, the power drive arm is firmly attached to the end of the shaft. As the stress limits begin to be exceeded, the power drive arm fails, releasing the disk and the disk is free to rotate with no control over rotational speed. As the rotational speed increases with no impediment to slow it, it bursts. That, I believe, is what happened on this engine.
Regards,
TD