35Year Pilot
I think your processes are valid but am curious why the IPT blades were not "cookie cutted" away unpowering the IPT when the IPT migrated rearwards.
Good question... My thoughts on this would be that the rotor sped up as soon as the power drive arm fractured at the 580 bolts, given the N3 speed at 98% and then it moved aft, with the blades contacting the smooth leading edge of the Stage 1 LPT nozzle inner band forward overhang. Now when this happened, there may have been enough force to shear the blades off, just above the blade platform, depending on some features I will discuss below. There was no contact with the Stage 1 nozzle vane airfoils as they are "recessed" too far aft. Obviously, this is not what you want to have happen from a design point of view to slow the rotor down. You would like blade on vane contact. In the failed engine, it is hard to tell some things relative to the blades. If you look at the recovered portion of the disc photo in the ASTB report, Figure 14, Page 21, you can see distinctly two blade dovetails in the corresponding disc slots (maybe a third, not sure). All the others are missing completely. The thing I wonder about is the fastening method used to keep the blades in the disc, forward and aft. I am pretty sure the mechanism to keep the blades from moving aft is a tang that protrudes downward at the blade dovetail (leading edge side and part of the blade casting) that keeps it from moving aft. The blades are inserted in the disc from the forward side during assembly. To keep the blades from coming out of the disc forward, a retainer clip is inserted in the space under the dovetail of the blade (you can see the open area in the photo) from the aft side of the disc, and it is then bent upward both forward and aft sides, thus locking both blade and clip in place. However, there is no evidence of any clips in the Figure 14 photo. Another important feature that I can't tell is if the blades are shrouded at the tip or un-shrouded. If they are un-shrouded, the blades (some) could be pushed out of the disc slots (whole blades) when contacting the nozzle inner band with some being fractured off as the retainers would fail from the stress/heat. If they are shrouded, I would think all blades would fracture off. We just don't see enough and I am not familiar enough with the Trent 900 blades to know for sure. But, either way, I think it was too late as the rotor was already speeding towards burst point before any contact of the stator behind it. Then there is the possibility the disc/rotor was wobbling as it moved back (uneven contact).
For your info, the two bangs were separated by about 1/2 second, so the two bangs were from very different processes. I like your thoughts of an HPC stall (boom) followed by the disc burst (boom). A time plot P30 would be of great help.
I would agree. If the cockpit voice data recorder could have saved the sounds, it could be overlaid with the rest of the engine data to determine the exact points of the bangs in the failure sequence. Now it will be a guess.
Regards and thanks for your comments,