TD

Did you ever look at that A330 (Singapore) "double bang" emer. return?

It seemed a bit similar to Qantas UF, then disappeared?

bear

While the corrective action relative to the oil fire (faulty pipe manufacture, etc.) is entirely appropriate, I hope that R-R doesn't stop there.

The fact that the T900 engine cannot survive a IP shaft disconnect is really disturbing to the turbomachinery folks I see often. I give you the RB211/744 failure QF74, 31 Aug 2010, KSFO - the shaft broke, the IPT oversped, but it shed its blades before reaching burst speed. An uncontained failure (escaped turbine blade fragments), but with considerably less airframe damage.

A couple of aggravating circumstances here

It may not have been just a pure overspeed failure. Given the coloration of the disk there may have been abnormally hot temperatures at the bore weakening it further than the shed speed of the blades.

The uniqueness of the IP module area may have prevented the massive tangling between blade airfoils and stator vane airfoils.

nevertheless the first priority corrective action should be against the massive oil leak that caused the shaft attachment failure in the first place.

Yes, a sump fire is a nasty situation, but there appears to be a six-second interval between the shaft separation (evidenced by N2 IPC spooldown) and the IPT burst. Given the gas stream energy still available from N3, that unloaded IPT disc must have reached a very high speed.

I perceive a lack of "what if" planning. The detached disk drifted aft until it contacted some static structure (mid frame) and established what designers call a "false bearing", which had low enough friction to permit the disc to continue accelerating. There were no static nozzle guide vanes to clash with turbine blades, thus destroying the driving torque; the blades remained attached as the disk kept accelerating; and the next part to fail was THE DISK ITSELF.

So at least two opportunities to survive the sump fire were lost IN THE DESIGN PROCESS.

Edit: On second thought, I wish to change the word "survive" in the last sentence to "mitigate".

She's on her way back home...

:)

--Bill

pardon me for asking this..

but.. I really need to know what action(s) have been taken in light of this engine failure?

From what I know, the problems only affects those A380 with RRoyce Engines correct?

I'm due to travel with Singapore Airlines soon.. transatlantic flight to LA, USA..
I believe all SQ's A380's are fitted with RRoyce engines..

This kinda make me wary..

I have the option of flying with another airline.. they use B777-300ER for route to LA,USA..

I am rather confused now... go with SQ a380 or shall i just go with B777-300ER :)

any comments appreciated! Thx

Kucing

I wouldn't worry if I were you. This particular problem will have been examined to the Nth degree.

Turbine disc failures are very rare and have happened to most (if not all) large engines, not just Rolls Royce. It is even more unlikely to happen again now after all the work that has been done.

Just take the most convenient flight and enjoy it!

To be quite clear about QF32 and similar failures:

In ANY multispool engine, IF the high-pressure spool keeps running after a shaft separation of (one of) the lower-pressure spools, that low-pressure turbine will be driven to an overspeed condition.

Thereupon, one of perhaps three things will occur:

1) The turbine blades will, by design or accident, suffer a blade root failure. This creates medium-energy shrapnel, but also removes the driving torque from the free disc, so it can coast to a stop without further failure. eg QF74, SFO 744, RB211 engines.

2) If 1) doesn't happen, the free disc assembly will be driven aft where the blades will contact static parts such as a downstream nozzle guide vane ring; this will beat up the airfoils to the point that the disc stops accelerating. I have seen CF6 failures of this type.

3) if 2) doesn't happen, then the disc may continue accelerating to the point that it bursts. I give you QF32, Trent 900 - and also perhaps the T1000 test bench failure a few weeks before QF74.

Turbomachinery designers must take shaft failure into account in the design process. Some may do a better job than others.

barit1,

Good post!:ok:

I completely agree with what you said, it's all in the design...

Regards,

TD

Than there is the problem of too much oil leakage and resulting fire overheating the disk and it's attachment shaft to the point of burst before it even has a chance to move aft.

Thus the fix is to minimize a short term oil leak of this magnitude.

Obviously a lesson for all manufacturers in their quality control efforts.

lomapaseo:Yeah, that will address this particular failure sequence (overheated disc flange).

But decades of experience says there are potentially other causes for shaft separation, and the fundamental issue is the lack of any provision in the Trent to destroy the runaway turbine airfoils prior to disc burst.

I will grant that the six-second interval between N2 compressor spooldown and N2 turbine burst might yield promise for a software fix >> namely, fuel cutoff if sensed N2 is too far out of whack.

Barit1

I doubt that the regulators will agree with that

Do any of the manufacturers altually demonstrate a shaft separation at takeoof conditions, or do they just point at some design feature that might have worked in the past?

As in all accidents there are lessons to be learned by all

lomapaseo,

What you do is understand this is a possibility, and you design so that if the power drive arm would fracture releasing the turbine wheel from the shaft, it would not reach an overspeed condition where the disc would come apart. Although this was done on previous Trent engines, it wasn't done on this engine. It wasn't a lesson learned, it was a lesson forgotten, simple as that. You don't have to demonstrate it, you just use experience and good common sense to prevent it.

TD

So if you've never seen it in a model series it's good common sense

but once it happens it's an oops

Good design practice requires a failure modes / criticality analysis (FMECA), and a good analysis will consider historical known failure types. Not just those required by regulators, but ones that (for example) could taint the reputation of the product or the company.

For example, a modern propeller design incorporated features that lost sight of lessons learned over 6 or 7 decades. As a result, a regional airliner augured in 20 years ago. Neither the designers nor the FAA had retained the corporate wisdom to prevent the design error. :ouch:

barit1

The LP shaft by observation incurred no visible injury from the burst of the IPT. The HP shaft, nested withn the Drive Arm "Bell" is not seen. The third shaft of three, The Ishaft, terminates in a cylinder, the "Bell". Would you have any comment as to the vulnerability of this "Bell" to harmonic vibrations produced by the nested pair, and possible "whip" of this assembly, due wear of splines more forward of the IPT case? Considering the vibratory result can be multiplied through the length of the IP/HP shaft pair as it progresses aft, and in a super heated environment, would mechanical disruption play an important part in the burst (not to diminish the effects of "oil fire").

N speeds are not available at the time (per design) to effect a fuel cut. Instead, when N2/N1 values are rejected, the ECM recieives an NCD re: N3, and a value is providied that is 'theorized' by the Engine computer. (as I understand it). This allows the HP to soldier on, at least for six seconds?

regards

Lyman:

By use of the term "shaft failure" I mean the functional disconnect of the IPC from the IPT, regardless of the specific location of the break. The IPT drive arm is one component of the shafting. You can theorize all day long about vibration, shaft harmonics, etc. but these are merely possible logical routes to the ultimate shaft disconnect.

Good design practice assumes that the shaft will, despite all your efforts, suffer a failure someday; and it's the designer's job to mitigate the failure.

And yes, I know that R-R N2 sensing is at the compressor end, so there's no hard data from the IPT itself if a disconnect happens. But the observed N2 spooldown is non-characteristic when the other rotors are still up to speed, and should be interpreted as the signature of a shaft failure, IMHO.

I am trying to build on what is (was) known at the time. The AD warned specifically of aftward axial transit of a shaft, causing damage to the a/c, and those on the ground (sic). The wear, specific to the AD, was located at the splines forward of the IPT (at the other end of its shaft), and reporting of the initilal exam of the engne contents stated "Rigid Couling Failure". Rigid coupling failure was reported as the cause, not the result. The "Burst" was an artifact of the failure of the Bell, (Drive Arm), not the cause of the uncontained failure. Now it's possible to chiicken/egg this, but I am aware of the Rigid couple demise as the cause of the problem, and the couple disintegrated due, What? I don't think the couple would fail from heat, nor vibration alone. I also sense that the consensus here is that the wheel failed "first". i think that is wrong, from a read of the report. It is also inconsistent from the previous failure, (Miami), where the wheel failed from migration, not oil fire. imo.


For instance, couldn't the N2 spool down have been caused by the contact with the web? The actual friction and resistance slowing the shaft and puddling the "false bearing"? This while the shaft/turbine maintained its integrity, until the rigid coupling lost the Wheel?

your thoughts?

Lyman,

We have been through this many times. This engine had been inspected and passed the spline wear requirements before being placed back in service. The rigid coupling of the IP shaft, located in the compressor section of the engine, did not fail. The IP shaft did not move rearward. The initiation of the failure event started with an oil leak in a compartment just forward of the IP Turbine disk. A fire started as this area is hot. The fire caused the IP turbine disk to begin to heat above temperature capability of the disk alloy and rotational stresses being encountered at the time. Now to understand what happened next, you have to understand the metallurgy of superalloys used in disks.

The best disk superalloys have an operating temperature of between 1200℉ and 1400℉. These alloys have good ductility and creep rupture capabilities within operating temperature ranges. If the temperature is exceeded, creep and in the case of a spinning disk, outward growth (stretching) will occur fairly rapidly. As the stages of creep progress, the final stage progresses to failure very rapidly. So lets apply this to the IP disk in question.

The IP disk is attached to the turbine end of the IP shaft by a series of circumferential bolts which secure the power drive arm of the IP disk to the shaft. The designers determine the thickness of the power drive arm, the web thickness of the disk and the disk bore mass based on anticipated stress levels and temperatures during engine operations throughout the flight envelope plus a safety margin to preclude disk burst. So what happens when an oil fire develops in the compartment just forward but adjacent to the IP disk? The disk begins to overheat from the bore to the disk web with the overheating commencing from the forward side of the disk. As the temperature begins to exceed the superalloy capability, the disk begins to stretch somewhat unevenly from front to rear, but radially. But, the power drive arm is firmly attached to the end of the shaft. As the stress limits begin to be exceeded, the power drive arm fails, releasing the disk and the disk is free to rotate with no control over rotational speed. As the rotational speed increases with no impediment to slow it, it bursts. That, I believe, is what happened on this engine.

Regards,

TD

While I don't have details at hand, Lyman may be referring to the RB211 failure (QF74, SFO 8/2010) in which turbine shaft splines were wiped.