QANTAS A380 Uncontained failure.

[Lyman]

TurbineD

As always, thanks for your patience and expertise. A quote from you....

You will note that if the stage 2 HPT disc (similar position to the IPT disc) became liberated for one reason or another, if it would move back, the turbine blades would clash with the stage 1 LPT nozzle slowing the disc speed. It is an important design feature to inhibit runaway disc speed.

In either engine, the Power Turbine, (IPT in the TRENT, and HPT2 in G/P) send the gas path into the (stationery) nozzle of the engine's LP (turbine) cavity. In the G/P, the vanes are elongated and occupy the plane directly behind the blades of the Power Wheel. The Trent's Vanes appear shorter in length, and are arranged at some distance aft of the preceding rim of the STATOR. This means, as you say, that blade clash will not occur instantly, and the effacement in aft drift occurs with two smooth surfaces, rather than Blade/Blade.

I am speculating as to design. To me, it (TRENT9) represents a sequential approach to Disc runaway. The Wheel is slowed, and borne temporarily at Blade roots/Stator platform, (your "On the recovered LPT vane segment, the inner band is gone. Also, the turbine blade retainers on the recovered portion of the IPT disc are all missing.") So you have relatively one smooth surface contacting another relatively smooth surface. This wipes the retainers, and the blades loosen in the IPT. As the blades begin to depart the wheel, the IPT captures less of the power of the gas path, and the blades shred, to fill the cavity with shrapnel. This shrapnel, I see as a benefit to further loss of blades and resultant loss of rpm. As the IPT travels further back, the Platform degrades into more robust areas of the Platform, and eventually the Blades/Vanes efface.

The IPT, off arm, does not have a solid support at the bearing box, though it does have concentric structures post fracture. This allows an eccentric braking action, though the vibration and noise must be extreme.

At the last, the IPT foils (if any are left) scrub the Vanes of the LPT nozzle, removing the last of them.

As you point out above, blade/blade is the G/P approach to arrestment, RR similar, but with a primary contact at blade roots, Stator platform. Again, I believe that the desire is to slow the wheel, but I would add that equally important is to defeat the gas path mechanically, a fuel cut is not possible in this time sequence.

So I can only say that the approach is different in the 900, but to me represents a step ahead of that taken by GE/PRATT.

I do not discount the overspeed, but I note that you agree the design limit for separation of the wheel into three parts may have happened at lower than maximum rpm for integrity. Yes?

Thanks again