Turbine D wrote:
I am not dismissing the spline wear as a non-issue here, it could be contributing in some way.
My thoughts are the moment are that the spline wear may be being used as the canary in the mine, if in this case it was not the actual cause of the event.
Someone (saying this may help) posted a calendar-format brochure "Modern Gas Turbine" by RR in 2000. I can't find the reference, and because it was like a tri-fold publicity handout I didn't then realize its usefulness. Later I noticed it had individual cabinet drawings of the individual 8 modules of the RB211-type turbine, at different scales to suit the printer's layout (only an inconvenience).
What I noticed is that the LPT module #8 is furtherest aft and carries both reaction bearing for the LP shaft and the anchorage for the drag link that transfers the engine thrust (by compression of the link) to the pylon. The reaction bearing is on the axial centerline of the engine, but the drag link reaction is offset above it at the top outer perimeter of the case. The drag link imparts the entire 72000 lb thrust of the engine to the airframe. This arrangement will cause a bending moment in the turbine case that is most severe at TO or in climb thrust, as the case may be, once per cycle. Not always max, of course.
It is possible this bending is either elastic with rebound, or partly plastic due to high temp of the turbine case, with ever increasing permanent curvature of the case. That the inspection cycles of the splines are ever fewer cycles with increasing damage, a non-linear rate of damage is suggested. This leads me to suspect the latter-- ever increasing permanent curvature.
Such curvature can be relatively small, and yet cause unanticipated wear if the spline/gear teeth are precisely made-- which likely they are. I will explain why in a follow-on post. Vibration will also result and increase from the splines from this cause.
LCF and HCF:
That had not occurred to me, in regard to the spline wear. But of course if thermal stresses are the cause of LCF in this event, it would not be unusual for those to be the greatest stresses in certain structures, or here certain parts of the structure-- certain frames, as you say. It is so normal in structural design to think of the load stresses only (meaning non-thermal stresses) that the thermal stresses can be either overlooked, or just considered only for the major load-bearing elements of the particular structure. And fatigue is (or was in my day) so poorly understood, that after 50 years our understanding still continues to increase, usually the hard way.
Mostly a good measure is a structural layout that accommodates thermal expansion. A turbine has axial symmetry and is ideally unconstrained in longitudinal expansion. But the temperature rises are not uniform in all parts, particularly in sudden transient high loading, which can be awkward. The axial symmetry is not without small deviations, and the longitudinal expansion is eccentricly restrained by the thrust link. It is possible that bearing alignments in the frames are being degraded, with effects on the splines.
That doesn't mean, as you say, that the splines were the proximate cause in this event. However, I still don't see any ironclad proof that an oil fire preceded the breakup. Nothing to disprove it either. As
bearfoil said (IIRC) conveniently nothing about oil fire can be contested either way, based on the data released.
OE