Annex14
What is the meaning of the 400°F you mention in your post Nr. 257 ?
Oil begins to coke at a temperature of 440℉ to 450℉. The 400℉ is what you attempt to design to in the sump oil wetted wall areas to prevent coking as a safety margin, but, also having an idea as to what the maximum soak back temperature would be/could be through a heat transfer analysis program that examines that entire engine area.
In that connection, you mentioned several times the plenum in front of the IPT disk. It is supposed by best knowledge, I assume, to be the space the oil feed tube that finally failed runs through. At the same time it is the supporting structure for the bearing chamber of the IP and HP roller bearings.
Yes, that would be my opinion.
How likely would you consider the possibility to be that, although the engine had that bearing problem, was in repair shop from Sept. 2009 untill Dec. 2009, was boroscoped in June 2010, no one had a look into that plenum chamber and detected that cracked feed tube spilling oil into that space ??
The bearing problem was with a ball bearing cage and race that I assume to be in the fan/IPC area of the engine. To get at that, the engine would have to be torn-down in a modular fashion. It is conceivable the IP Turbine module containing the plenum wasn't looked at from an internal point of view. It is hard to say what happened for sure. It could have been weeping or slightly seeping or not yet leaking at the time of the bearing replacement. The boroscope inspection was for the spline wear, again, in the Fan/IPC/HPC area of the engine.
For a while the cooling air fed into that chamber might have prevented heating beyond SIT of the coke-oil mixture. It was said that this temperature is substantially lower than plain oil SIT.
The cooling air really isn't that cool in the IPT area of the engine. I would suspect sections of the frame, Casing, hot gas path airfoils (including outer and inner bands and the plenum itself would be made of different materials depending on expected temperatures. The plenum material would have to be a medium temperature material capable of being formed (hot forming?) given the very contoured shape fitting in the cavity in that area. It may also have to be a weldable material. In two spool engines, this would be a "turbine mid-frame", something that was finally eliminated as it was such a pain to both make and then subsequently dealing with never ending problems in service.
So when the fire started that way after engine start at SIN would that fire accomplish a burn through??
IMO, the fire was intense enough to burn through the plenum wall, exposing the bore of the IPT disc to temperatures beyond the disc material's load carrying capability. I am sure the bearing structure supported by this frame was adversely affected by all of what was taking place. It is very possible the bearing upset together with the weakened disc (stretching) could cause an uneven spin plane from normal.