Annex14
Assisted by what?? Look at Fig. 13 of the ATSB report, enlarge it a bit and check the visible rear side of the nozzle guide vanes of the IP turbine. Its there well visible, just some shiny or dark marking, probable contact on break up ? and the gas channels at - as I consider it - normal performance colouration. There is no Oil, there is no oil soot, there is just plain metal !! Wasn´t that the side where that feroscious -blowtorch like - oilfire had to have its devastating work done ??
Hard to believe !!
You are correct, that is the nozzle vane that sits behind the HPT rotor, it is a hot gas path at that point and the vanes are air-cooled. They are no doubt coated to prevent oxidation/sulfidation and the brownish color is the deposit left by the fuel that has been burned in the combustor. There would be no soot from oil burning it would be completely consumed because of the gas path temperature (~ 1800℉). The vanes are not the problem, it is the plenum chamber directly under these vanes that you can not see that is the problem. As you would move down that plenum towards engine centerline, the HPT/HPC rear roller bear sits, supported by the end of the plenum.Just slightly rearward sits the IPT/IPC rear roller bearing. If there was an oil fire in this plenum, it could cause it to rupture exposing the front face of the disc near the bore. The plenum is (or should be) under positive pressure from the cooling air that has passed through the nozzle vanes. the vanes didn't see the oil at all.
Okay if friction is no player in the game and also no obvious marks about a "fluid bearing" neither in the bore nor at the circumferential crack in the drive arm, and also no mark of contact at least on the recovered part of the disk, what is left as explanation for the disk failure?? Overspeed/ over-stress and a plain "ductile fracture" as explained in the ATSB report ??? Here the engineers are demanded !! Its just a conclusion !
In one of the press releases or media briefings by the ASTB, they talked about the disc and the fact that it showed both melting and molten metal splatter on the rear surface of the disc. I think I mentioned that the light grey area next to the bore was just that. When something is spinning at 7-8K rpms, the metal spreads out into a film (usually a combination of metal and metallic oxide). Also, look at the recovered nozzle vane photo in the ASTB report. They note a coating on the surface of the airfoils they intended to examine. That to could be a molten metal deposit film as well. I believe the disc broke free from the bolt holes when the power drive arm broke at that location. Once free, it was able to rotate to whatever speed the HPT gas flow could drive it and remember the N3 was 98% at its peak. At this point it would be stretching. They will be able to determine this by dimensional measurements taken on this recovered section. So it moved back contacting the Stg. 1 LPT nozzle, not blade to vane but blade to the inner nozzle band forward overhang. Look at the vane photo again, the inner forward overhang is gone, not there. The other possibility is what Bearfoil proposes. No matter which, the disc over-sped. It all happened in a couple of seconds. Mostly everything went out including the Stg. 1 nozzle vanes, very little went back through the LPT which is what you would expect if the disc didn't burst.