A Possible Mode of Failure?
I need to reassure any RR engine men who might have time to read PPRUNE that the following is pure conjecture… but it is based on some historic experience.
I find it interesting that the photographs, firstly of a piece of disk resting in the back of somebody’s van (Post 42), and secondly of another piece that presumably has been found after the appeal by the authorities in Indonesia (Post 588), show a disk that has lost its turbine blades. I don’t think that they are the same piece, but I am open to correction.
The question that arises is: when did the turbine blades separate from the disk? Was that separation after the disk had departed from the engine, or did the blades (or possibly, some of the blades) come out of the disk whilst the disk was more or less still in the engine?
It is extremely hard to imagine that the fir tree roots would let go under ‘normal’ circumstances. However, if the disk had been subjected to a high over temperature, possibly as a result of an oil fire, then its metallurgical properties might be compromised such that it softened, or maybe ‘crept’, and this resulted in the fir trees no longer holding the blades. If some of the blades departed sooner than their neighbours, then there would be a severe imbalance at 12,000 rpm or so, and this might precipitate the departure of the disk, taking various other components with it as it left.
The AD refers to “wear on the abutment faces of the splines on the IP shaft rigid coupling”. This interface “provides the means of controlling the turbine axial setting and wear through of the splines would permit the IP turbine to move rearwards” If this movement resulted in damage to the bearing chamber oil seals, and there was an oil leak, then it is possible that an oil fire might ensue. A stoichiometric mix of oil and the hot air around the turbine face could give a very nice torch of flame that might have played on the disk, and led to its unzipping.
Any movement resulting from the IP coupling wear presumably is measurable; or possibly there only has to be a small amount of movement for there to be observable excessive wear on one of the bearing chamber seals. Measuring the movement, or looking for such seal wear, probably is relatively easy if one knows where to go though the boroscope holes, and possibly this is why the SQ and LH engines, and (most of), the remaining QF engines, have been cleared for continued operation relatively quickly.
I again emphasise that the postulated failure mechanism is pure conjecture on my part, and I have no inside track on the investigation.