Lyman,
My purpose from the outset is to frame the evidence within the data provided in the AD as enforced. If some mechanism failed the connection other than high heat, or overspeed, let's agree it is not present in the analysis? I think the metallic spatter is the product of friction stir at the arm/engine frame, as predicted by EASA.
Lets go through some facts again.
From the latest ATSB Interim-factual Report:
"The investigation has found that the intermediate pressure (IP) turbine disc failed as a result of an overspeed condition, liberating sections of the IP turbine disc that then penetrated the engine case and wing structure. The disc failure was initiated by a manufacturing defect in an oil feed pipe that resulted in a wall thickness reduction in an area that is machined to receive a coarse filter. That section of the oil feed pipe sustained a fatigue crack during engine operations that lead to an internal engine oil fire that weakened the IP turbine disc. In turn, a circumferential fracture was induced around the disc, allowing it to separate from the IP turbine shaft. The unrestrained disc accelerated to critical burst speed. This lead to the No 2 engine failure and subsequent significant penetration damage to the airframe structure and systems."
Your quotes directed to barit1:
You cite the N2 loss of rpm, I think that was a result of metal/metal from shaft, case, as predicted. I would say the shaft slowed whilst joined to the Turbine, and EEC poured extra fuel in to compensate for the reduction in power applied to the melting metal in the core.
These are informed from the AD, directly. How can N2 unwind for seconds, as a result of separation of IPT? Again, the IPT without the Drive Arm connection is a sizable herd of horses loose in the barrel. The Orbit would have to remain precisely free of obstruction for the Wheel to accelerate.
The way a three spool turbine engine works is this:
1. The low pressure turbine (LPT) drives the fan and low pressure compressor.
2. The intermediate turbine (IP) drives the intermediate compressor.
3. The high pressure turbine (HPT) drives the high pressure compressor.
So, if the IP turbine disconnects from the shaft resulting from a circumferential fracture of the power drive arm, it is no longer driving the intermediate compressor. Therefore, N2, that is measured at the intermediate compressor begins to decrease, in this case from 94.5% to 93.2% at UTC 0201:00. A 100% speed for the IP turbine rotor means it was rotating at 8,300 RPMs. At 94.5% speed it is rotating at 7844 RPMs. Burst speed would be approximately 10,375 RPMs, assuming a 25% margin above maximum operating RPMs. It didn't have to speed up (overspeed) very far to get to the burst point. The rate of speed up would be determined by the expanding air passing through the IP turbine blades received from the HPT blades. It was probably less than 7 seconds from normal operation to burst.
The Drive Arm/IPT resemble a Bell on a bearing at the small end. If the connection is lost, the mass is not only free to wander, chaotically, but the rim of the Bell is quite distant from the bearing attach, any imbalance would instantaneously throw the entire asembly into an extreme eccentric.
No it doesn't. It only resembles that way on the disc after the failure. Prior to failure, it is a bent downward plate (towards the engine centerline), parallel to the disc face, a circumferential feature that attaches to the end of the shaft by means of a series of bolts that pass through bolt holes in the power drive arm.
If the connection is lost, the mass is not only free to wander, chaotically, but the rim of the Bell is quite distant from the bearing attach, any imbalance would instantaneously throw the entire asembly into an extreme eccentric.
The mass you suggest, the IP turbine rotor disc and blades attached is free to very rapidly accelerate until such time the disc bore stretches which is indicative of pending failure and then moves back. At that point, IMO, the only contact would be near the base of the turbine blades embedded in the disc slots contacting the stage 1 LPT nozzle. That rubbing would create metal splatter observed on the aft surfaces of the disc, similar to what happens during an inertia welding process, an initial false bearing without adequate pressure to slow the rotation of the disc. It was not chaotic at all until the burst occurred. Again, IMO, eccentric motion had little effect on what was about to happen.