Turbine D - DERG
I've been following with great interest the discussion in this thread relative to the several failure scenarios that have been set forth re: Qantas T900 uncontained IPT failure. Back in the conversation (#'s 221 and 223) you briefly brought up the topic of oil and possible coking problems during heat soaking. There was a brief suggestion that inclusion of an external oil circulator unit would have been a sensible part of the overall design for longevity.
Could you discuss from your engineering standpoints, the possible or probable effects of short and long term coking on the actual performance of the hot section bearing(s) in this engine? I've been wondering if it's possible that bearing performance from coking could have degraded to the point where it may have overheated, chattered, and or vibrated itself to the extent that it either partially or even totally disintegrated, whilst also impacting the stub pipe. What I'm asking is, is it possible the bearing itself was causal in the sequence of events ending in the IPT overspeed? Maybe I missed someone discussing this very event in an earlier part of the thread?
Failures of this type are not unknown, of course, in automotive and aircraft turbochargers. The simple solution has always been to, one way or another, make sure the shaft bearings are bathed in circulating oil after shut down. Even old dirty mineral oil is ok to stop most coking, just so long as it keeps coming. If this solution is found necessary for T900 bearings I'd almost bet that it could be done for far less penalty than a 50kg bolt-on accessory.
Thanks everybody for all your great contributions to this thread. What a wonderful resource this forum is.