oil - coking - vibration
Turbine D
What is the meaning of the 400°F you mention in your post Nr. 257 ? It is too low, as far as I know, for self ignition temperature of oil. Is it than a border temperature for coking ?
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.
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 ??
If the engine was checked including that section - what I would expect to happen with such a major repair - and there was no oil, it means the leak developed later. If however, this section wasnīt checked at all - means neither in the repair shop nor on the boroscopic inspection - the oil leak and spil could have started quite a long time before there was that oilfire.
The ATSB investigators have come to the conclusion there was an oil fire - so for us thats fact. Question is, when started it and what damage did it ?
If - worst case - the oil leak existed undetected for a longer period of time it appears likely that over the many hours of usage quite a bunch of oil coking has happened. I would expect to have the lower part of that plenum chamber substantially filled with a swamp of coke and oil. 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.
So when the fire started that way after engine start at SIN would that fire accomplish a burn through?? or is the material the plenum is made of heat resistant. I think it has to in that hot region of the engine.
So if in fact no burn through occured, just weakening of the lower half of that supporting structure, what consequences would that have on the precise fixture of the bearing chamber ?? Is my assumption correct that the chamber triggered by vibration may have started some kind of motion that made the disk tumble ??
DERG
In your post Nr. 262 you mention now twist, vibration and kinetic energy. So if I understand that post correct, you point to the same causes that I mentioned in my former post about shaft failure with specific engine / propeller installations.
But is that something to consider ??
What I have thought also about several times is balance. I know balancing the plain disk is not the art, mounting several tenth or hundred of vanes on that disk and still keep it in balance is the tricky part.
Probably everyone will say: no impossible ! But yet I would like to ask is it technically possible to balance a compound of pieces so secure that it will run without any imbalance at all possible rpmīs ??? If not ?? If only balanced to bearable amounts of vibration for cruise power ?? Thatīs the power setting longest used on a flight and appears more important than anything else.
Remember lomopaseo said that all kind of vibration and harmonics are common items in engine development and use that normally are covered by computer programs. That should suggest that those sketched possibilities are not existing. Correct ???
I hope this becomes another trigger to push the efforts on this circle closer to the truth behind the engine failure.
A said before, everyone would be wiser if the pictures of the inner parts of the engine would be released. But I am sure ATSB will come along sooner or later with a more profound report than a prelim ever can be.