Annex14
Thanks for the Trent 900 cutaway. Although it is only a sketch as the Trent 500 cutaway was, it gives some indications of changes that were made, particularly in the area of the IPT rotor and Stage 1 LPT nozzle that has been puzzling me.
First, I am pretty convinced that the fire caused by oil leakage (broken feed line) occurred in this area and was the instigator of the subsequent events leading to catastrophic failure of the components in this area of the engine.
In the Trent 500 engine series, there have been two failures in this same area. One failure has been identified as to coking instigating a fire causing the IPT rotor to fail at the disc rear drive arm 580 bolt holes. The second failure (Qantas B-747 out of SFO) is still under investigation by the ASTB, no cause yet given. However, in both cases the failures were uncontained. In both cases the IPT rotor moved rearward wiping out the Stage 1 LPT nozzle ring and the casing holding the nozzle ring as well as eliminating all the IPT rotor blades. In both cases, the IPT disc did not rupture but there was considerable damage to the LPT nozzles and blades as quite a bit of the debris went through the turbine. So why did the IPT rotor disc not rupture on these engines, but did on the Qantas A-380? All three aircraft were in a climb out mode, with the A-380 at an initial stage, the other two at 20,000+ feet altitude. In making the assumption that all three experienced a fire at or around the IPT rotor, the answer may be some changes made in the Trent 900 engine verses what is present in the Trent 500 engine. In particular, note the gap between the IPT rotor blades and the leading edge of the airfoils of Stage 1 LPT nozzle on the Trent 900. Compare it to the gap present on the Trent 500.
In the design of rotor/stator interfaces in this area of the engine, two important considerations must be addressed, fire and shaft breakage. If the rotor is released due to one or the other, the design should assure that the airfoils come in contact first as a breaking mechanism to prevent disc overspeed and subsequent possibility of a disc burst. The reason for this is a term called "False Bearings". If the disc or a portion thereof contacts first, there is sufficient energy present to melt the contact surfaces and momentarily make a low friction bearing long enough to allow overspeed to occur. This liquid metal bearing is referred to as the "false bearing". Often, the LPT nozzle airfoils have their leading edges bowed outward to assure first contact with the IPT rotor blades should a disconnect occur.
Because of the closeness of these airfoils in the Trent 500, blade to nozzle contact prevented IPT disc overspeed. But in the Trent 900, note the Stage 1 LPT nozzle airfoils are more rearward precluding contact and breaking effect before overspeed occurs.
So my thought is the oil fire and temperatures in the cavities "softened the IPT rotor disc, causing the rear drive arm to fail, the rotor moved aft, but the blades didn't contact the nozzle airfoils first causing a disc overspeed condition and subsequent burst.
Turbine D