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QANTAS A380 Uncontained failure.

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QANTAS A380 Uncontained failure.

Old 24th Dec 2010, 17:31
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bearfoil
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QANTAS A380 Uncontained failure.

Continued from previous thread here.

Turbine D

You propose a "false bearing" developing between IPT and LPT NR. You also suggest two possible causes for this contact. Fire is one, and Shaft failure another. I would offer that in the two previous failures, the disc was retained because there was not overspeed. In one case, (Miami) the Disc had separated from the Drive Arm in a "Circumferential Fracture". The second is still under investigation. For the reputed cause of QF32 uncontained failure, RR has offered "Oil Fire". Oil Fire in the Roller Bearing case was due to uncontrolled combustion specifically (Your tenet). There is not evidence of oil fire showing on the LPT module, and the gases involved would have to have overcome the Pressure in the IP/LP cavity, no mean feat. So what is left is heat acting on the Drive Arm from the front face, having escaped two sets of seals, and blowing past Roller bearings that presumably were still serviceable. Sounds unlikely, and that leaves Shaft Failure as the cause of disintegration of the IPT. The Shaft failure that comes to mind is the one predicted as a result of Spline wear/failure by the AD's.

Do you have an opinion on the generation and advance of the "Oil Fire"? How do you read the AD's prior to November 4? Do you consider the Splines as exonerated?
 
Old 24th Dec 2010, 19:55
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Bearfoil

You propose a "false bearing" developing between IPT and LPT NR. You also suggest two possible causes for this contact. Fire is one, and Shaft failure another.
In a design of an engine, fire and shaft failure are important items that must be addressed due to severe resulting consequences. In the Trent 900, I think fire was the culprit, not shaft failure.

I would offer that in the two previous failures, the disc was retained because there was not overspeed. In one case, (Miami) the Disc had separated from the Drive Arm in a "Circumferential Fracture".
I Agree.

For the reputed cause of QF32 uncontained failure, RR has offered "Oil Fire". Oil Fire in the Roller Bearing case was due to uncontrolled combustion specifically (Your tenet).
Not exactly. The fire did not occur inside the roller bearing box, it occurred in the cavity which is part of the frame next to the IPT disc.

From the EASA AD's:

An uncontained engine failure has recently occurred on a Rolls-Royce RB211
Trent 900 involving release of high energy debris and resulting in damage to
the aeroplane. Analysis of the available elements from the incident
investigation shows that an oil fire in the High Pressure / Intermediate Pressure
(HP/IP) structure cavity may have initiated a sequence of events leading to
rupture of the drive arm of the IP Turbine (IPT) disc and subsequent overspeed and burst of that same disc.
The IPT rotor did overspeed on the Trent 900 and there was evidence of molten metal splatter on the recovered portion of the fractured disc.
Again, from a EASA AD introducing a electronic software modification:

Engine Fuel & Control Intermediate Pressure Turbine Overspeed
Protection System Software Introduction
The LPT shaft was protected during all of this event by the presence of the IPC/IPT shaft. Everything went outward, some rearward but not as much as if the IPT rotor blades had born the brunt clashing with the Stage 1 LPT nozzle vane airfoils.

Do you have an opinion on the generation and advance of the "Oil Fire"? How do you read the AD's prior to November 4? Do you consider the Splines as exonerated?
I think everything in the Trent engine was operating normally. Remember, the engine had been inspected for spline wear and must have passed the requirements for continued operation as no corrective action was required. Obviously spline wear at an early stage of engine life is of concern and requires a more permanent corrective other than shortened periodic inspection. I do wonder if there is unusual vibration or harmonics being passed through the system when splines are wearing so rapidly. This vibration, if present, could affect anything in the torque field.
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Old 24th Dec 2010, 22:20
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Turbine D

Have you taken into account the AD amendment which lowered the amount of metal left on the Splines necessary for continued on wing? The original AD requirement was for an off wing strip if any single Spline showed wear of 2mm or more. Later the requirement was relaxed to require an average only of not more than 2mm for all the Splines taken as an aggregate.

Obviously, an average of 2mm wear for all the Splines would allow for some splines to have worn a good deal in excess of the 2mm previously allowed. At certain aspects of Load and Strain, this joint can be bearing several thousand pounds on Two Splines alone. One gets the magnitude of accelerated wear (per cycle) when the damage is expressed in this way. No argument why EASA was demanding borescope of the Splines at every third Landing. Likewise, even with the inspection, seriously rated Thrust and Loading METO.

Now to me, I "get" the "Oil Fire". I understand the willingness of some to claim the new (post uncontained event) AD absolves the Operators of application of sanctions re: the original (and still in force) AD's. To my knowledge the Splines AD's are still in force. If this is so, the manufacturer may be performing retro fits of the internals that caused the Splines to wear so quickly, and replacing "Oil Pipes" at the same time. I am unfamiliar of the Regulatory process that allows an operator/manufacturer to ignore prior and "unrelated" AD's simply because they are honoring new ones?? Can you help with this??

Add: The vibration problem is well known, yet seems to have escaped comment here, will you be the one to elaborate?
Also, the drive gear for the gearbox, Fuel Pump is fixed to the IP Shaft, is there possibility of some loss of metal when the IP Shaft migrates aft?

I take your point re: the Blades/Vanes miss and would like to point out that the friction/clash w/o blade clash would occur at the Drive Arm Return "corner" verse the Stator. If aft migration of the Wheel/Drive Arm was gradual, one can entertain the heat that must have been generated. Lost (molten) metal from the Stator/Drive Arm interface would explain the "splattering" molten metal on the Aft face of the Disc. It is this metal/metal contact I believe the AD references as possibly causing uncontained failure with damage to airframe and humans on the ground. It is for this reason among others that I believe the engine failure was caused not by Fire (forward of the IPT) but heat and damage Aft of it. I don't believe there was time for the Oil Fire to have soaked the IPT from one side, when the Drive Arm failure is not only demonstrable here, but also in two prior events.

Last edited by bearfoil; 24th Dec 2010 at 22:38.
 
Old 25th Dec 2010, 05:52
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A possible source for the ball bearings that resist thrust in this part of the engine is New Hampshire Ball Bearing, HiTech Division. Airbus notes their product as being in the A380. The bearings "Inch Series, Thin Section - Radial and Gothic Arch" are available in 1/2" bore steps up to 10" bore. I have to check Airbus' other bearing suppliers for the A380 before I can be more certain, assuming I can get equally good data (experience teaches this will not be true with every supplier). Their link is at nhbb.com and p29 in the catalog (p30 in the pdf) is wanted, in case of trouble with the link:

http://nhbb.com/files/catalog_pages/HiTech-29-30.pdf

I'm not sure these are big enough for this job, as 13754 lbs thrust is the highest capacity listed, using chrome steel (52100). You can find special products on the site, which mentions that spent oil pickup extensions can be placed on the inner races, and that puller grooves can be put in the outer races, dogs to prevent rotation in the outer housing, etc. NHBB makes custom bearings also; the largest cataloged standard bearings are 1/2" cross-section with 1/4" balls.

One thing to note is that thrust capacity increases with diameter, and is hard to come by. Taken with the fact that machinery is usually designed with only two bearings per shaft, and a flexible coupling is used with more than two, we immediately know where the center bearing is on the LP shaft. It has to be on the larger forward shaft; on, adjacent or near to the boss housing the interior splines. These splines have to accomodate angular deflection between the two sections of the shaft, more at some times than other times.

I have been wondering for some time if the helical twist to the splines is also put to use providing a clamping force for the inner race of this bearing. Given that thrust resistance is so hard to come by in this bearing, I have no doubt that the primary purpose of the twist is to transmit the tension load, created by the aft gas forces on the LPT blades, directly forward to the fan. This reduces the force on this bearing.

Parts of this reasoning can be applied to the IP shaft, which also has a set of splines.

What I am thinking is that if we have an oil fire, we have to look for a source of ignition. Talk about oil flash point (tech board) is all well and good, but the fire point is another, where a pool of oil under an ignited flash will burn without self-extinguishing. That's a bit higher, but the auto-ignition point is substantially higher--404 degrees C in the recommended Mobil Jet Oil II (as shipped from the factory, a highly-inhibited ester synthetic). There seems to be some discussion as to a change in oil in some of the QF32 engines.

Where was the source of energy to ignite this oil fire? Even an explosive mixture of gasoline fumes will not be ignited by just any spark. I think even 15 mA circuit contacts are permitted now in such atmospheres. I used to use a bowl of gasoline to quench the stream of sparks coming off my grinding wheel (the same leaded gas I washed my hands in). Well, definitely don't try this at home .

One thing that seems worth a look is the effect of the unrestricted ops for oil temp up in the 190s-C, as compared to the more common 160-165-C in some other jet turbines. I'd say the 160-165 considers a limit to viscosity reduction to maintain adequate bearing lubrication and cooler running, although it may consider use (military?) of the Dow non-varnishing synthetic (glycol). The higher temp in the 190s seems to push the 3% evaporation limit in 6.5 hrs (a regulation?), here at 204-C for the supplied oil, on the cutting edge upstream of the measurement. Oil also has the advantage of being someone else's oil, and maybe not the oil type shipped in the engine. Just observations. I could not locate any lubrication recommendations on the NHBB site, except possibly in referenced industry standards (typically costly to obtain now).
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Old 25th Dec 2010, 10:09
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TurbineD

Thanks for the comprehensive explanation, I am always open to learn more!

I think what all our efforts to clarify the sequence of events hampers is the shortage of reliable information and pictures. A closeup picture of the rear - roller bearing chamber and drive arm residues, or a similar picture from the central ball bearing chamber and LP / IP / HP shaft situation may probably give us the information we now more or less are guessing at.

I also think that the findings of ATSB, stating break of an oil tube and oilfire near the IPT disk are doubtless correct. They have had the pieces in their hands, we have only seen pictures.

Yet, the question must be allowed to ask: what, where and when started the oil fire ? Was it really the pimary cause - the oil fire - or secondary to the extend that it had no influence on the drive arms break up ??
I have a problem to become convinced that the oil fire did it, seen the total time elapsed since take off, the obvious scarce space in front, looking FTA, of the IPT disk and underneath towards the drive arm and the total mass of that disk. Therecovered part - about 1/3 was an estimate - weights about 70 Kg, times 3 makes a total of roughly 210 - 220 Kg or 450 - 500 lbs. I cant see how that big chunk of metal can be heated beyond desighn limits under the circumstances prevailing.

One other hint that might point to a different cause I have found checking through these graphs added to the ATSB preliminary report, Fig. A2 and Fig. A3. The latter is the one that shows the recording og oil related data. There is that faint drop in pressure - total of about 5 Psi - starting at the same time the oil temperature raise. Oil pressure remains the entire final second above minimum oil pressure requirement - taken from the Certification Form, it even hikes momentarily before disk break up, drops, comes back before final drop. Oil temperature raises steady and falls after break up. The steady rise of oil temperature indicates to my understanding an constant heating in progress.
Meanwhile and untill several seconds after breakup the measured oil quantity remains level.
My conclusion, the final break of the repeatedly mentioned oil tube occured either seconds before, at or immediatly after the breakup. Oil will have become ignited by the hot parts of the engine or the hot engine gases beeing blown into that cavity on break up.
Could be that I am wrong with this entire theory but I thought there a lot indications that I cant be that far off.
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Old 25th Dec 2010, 17:15
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Old Engineer

What I am thinking is that if we have an oil fire, we have to look for a source of ignition. Talk about oil flash point (tech board) is all well and good, but the fire point is another, where a pool of oil under an ignited flash will burn without self-extinguishing. That's a bit higher, but the auto-ignition point is substantially higher--404 degrees C in the recommended Mobil Jet Oil II (as shipped from the factory, a highly-inhibited ester synthetic). There seems to be some discussion as to a change in oil in some of the QF32 engines.
I think the fire occurred in the chamber that is part of the frame located just forward of the IPT rotor. In a two spool engine, this frame would be best described as a turbine mid-frame. It is a very troublesome and complex component because of its location and the temperature gradients (stresses) that occur on acceleration/deceleration and resulting fatigue. On early two spool, high by-pass engines, this mid-frame was present, but on newer engines it has been eliminated by moving the bearings and bearing structures forward towards the compressor rear frame.

If you look at the Trent 900 cutaway, the frame (a multi-piece fabrication) consists of an outer ring (casing), airfoil like struts in the hot gas path (could be radial or could be tangential) connected to a highly formed and shaped thin wall 360 plenum chamber which supports the bearing system at the inside diameter of the frame. I would suspect each of the major components are of different materials selected for temperature, strength and formability considerations. I would believe the interior of the the struts and plenum are cooled from air drawn off the compressor to cool the oil lines that pass through the struts and plenum which feed the bearings and drain oil from the sump. Although not shown in the cut-away, I think the stub pipe in question is located in this plenum. This is a very hot area of the engine.

The gas path temperature entering this frame would be somewhere around 1800℉ (982C) or higher. I say this because the IPT rotor blades, which are solid (non-aircooled) are made of a single crystal high temperature superalloy.

Now if the stub pipe ruptures as they say it did, the oil pours out into the plenum chamber, no doubt causing a change in cavity pressure and perhaps overcoming the cooling air being provided. If the pressure becomes great enough, a failure could result in the plenum itself or the connection of it to the the airfoil struts resulting in a flash fire right next to the IPT rotor disc.
The heat generated increases the temperature of the disc itself, and it wants to stretch starting at the bore giving way to the failure of the power drive arm at the shaft flange 580 bolt holes resulting in the capability to begin to overspeed and move rearward.

That is my theory on how the fire ignition started.
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Old 25th Dec 2010, 18:04
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Bearfoil
Have you taken into account the AD amendment which lowered the amount of metal left on the Splines necessary for continued on wing? The original AD requirement was for an off wing strip if any single Spline showed wear of 2mm or more. Later the requirement was relaxed to require an average only of not more than 2mm for all the Splines taken as an aggregate.
The way I would read this is that, through the inspections and measurement process of the original AD, it was determined that the wear of the splines was somewhat consistent spline to spline rather than being a skewed situation where some wore much greater than others resulting in the amended AD.

To my knowledge the Splines AD's are still in force. If this is so, the manufacturer may be performing retro fits of the internals that caused the Splines to wear so quickly, and replacing "Oil Pipes" at the same time. I am unfamiliar of the Regulatory process that allows an operator/manufacturer to ignore prior and "unrelated" AD's simply because they are honoring new ones?? Can you help with this??
I would believe the AD for spline inspection is a continued requirement until such time a corrective measure is proposed by Rolls Royce and approved by EASA. Then the current spline AD would be replaced with a new one (or revised one) that would instruct the operators to incorporate the change/improvement by a certain time or specific engine cycle completion after the last spline wear inspection. The two AD's are completely separate from one another and accomplishing one (check for oil leaks) does not relieve responsibility to perform the other (spline wear inspection and measurement). I am sure this doesn't make any of the airline operators happy at the moment as it is disruptive to scheduling, etc.
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Old 26th Dec 2010, 08:18
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Old Engineer

May be this link is of help for your search for the bearings used in Trent 900
Schaeffler Technologies *|*Sectors / Key Industries *|*Publications
Look for the .pdf file "Takeoff to the Future and there on page 13. There they state that all the engine bearings used in the Trent 900 as well as the GP 7200 are their brand.
Hope that helps
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Old 26th Dec 2010, 11:54
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Thanks Turbine D for your analysis, to me it makes sense.

You seem to know your way around a jet turbine (guess your name stands for turbine designer, is 70 correct though or are you still active?).

The fact that RR did not have guide vane noses who could stop the IPT is interessting, saftey sacrificed on the efficiency altar?

Re bearings, FAG is one of the renowned bearing companies in the world together with SKF, Timken et al.
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Old 26th Dec 2010, 14:28
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The direct link to the PDF file is

http://www.schaeffler.com/remotemedi...9110_de_en.pdf
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Old 26th Dec 2010, 14:50
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what, where and when started the oil fire ? Was it really the pimary cause - the oil fire - or secondary to the extend that it had no influence on the drive arms break up ??
Check the silhouette cross section of the recovered disk piece with the cross section published above.
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Old 26th Dec 2010, 15:05
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Turbine D

I note your take on the AD amendment to allow an averaging of wear for this powerplant's Splines. I don't fully get your conclusion. If the 5mm wear limit was not superseded, the engines would have been "less available" to the operator. An allowance for averaging seems clearly meant to allow any single Spline with maximum wear to continue on wing, whilst its mates had time to "Catch up". I cannot read this amendment another way. If wear had presented as consistent spline/spline, no change in AD would have been considered?? If not consistent, an average allows the weak link to degrade further??
 
Old 26th Dec 2010, 15:10
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A poster has directed me to assess the chronologies reported in the ATSB prelim. Report. Specifically the N values as they wander, synchronised with the Oil System reads. It is quite revealing. The time line would dispute quite clearly a cause of failure as some Oil System anomaly. Take a look. Check Oil: Pressure, Temperature, and Quantity. It is quite interesting.
 
Old 26th Dec 2010, 15:32
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lomapaseo

Also, check the ATSB Preliminary Report, A0-2010-089, Page 21, the photo of the disc cross-section, the fracture surface. Additionally, note the wording on Page 21 and 22 as to the description of the disc failure itself.
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Old 26th Dec 2010, 15:56
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Take note of the "Oil Tube" image. Enlarge and notice what appears marked as "Misaligned" Counterbore. The arrows point to surfaces that have worn due to vibration and constant relative movement. The missing metal is clearly NOT machined away, evidenced by the round "coving" at the intrusion on to the affected wall. The missing "Rim" material appears to have broken off cleanly, rather than fractured irregularly as the other evidence demonstrates. This would indicate manufacturing problems, but also argue AGAINST an 'off line' end mill. I would propose that this oil line failed primarily due vibration, in situ, and not as the result of some coarse and glaring QA blunder. Replacing this tube to "solve" the Burst issue is therefore unsupportable.

The TRENT is a marvelous Machine. It is ground breaking in its design, and promises many years of loyal and dependable service. Any new machine is probationary, singling out the TRENT for abuse due its "teething problems" is unwarranted. However, shielding these problems from end users is unconscionable.

The lack of disclosure due Qantas, and the apparent complicity of Lufthansa Mx, is in imminent danger of tarnishing the Firm's Reputation. The TRENT remains a marvel, and being dishonest relative to its weaknesses is dangerous.

The FIRM, NOT THE MACHINE.

 
Old 26th Dec 2010, 16:22
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The time line would dispute quite clearly a cause of failure as some Oil System anomaly. Take a look. Check Oil: Pressure, Temperature, and Quantity. It is quite interesting.
I am not sure I understand your point here. If you take the chart that has all the oil data on it and then look at the other chart that gives the time line of engine parameters N2%, N3%, etc., the time of the start of oil temperature rise and drop off of oil pressure (0200.20) isn't on the time line of the engine parameter chart which starts at 0200.48. So the oil temperature rise and pressure drop started before the abnormal N2%, N3% fluctuations which began just prior 0201.00. This would indicate to me that something was happening in the oil system prior to the beginning of the "event".

I don't know the inter-workings of the oil system, or where the oil temperature measurements are coming from on the Trent 900 engine. Unless you know that, you can't say for certain oil wasn't in play leading to the engine failure.
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Old 26th Dec 2010, 16:34
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bearfoil - but the rest of the world , including those who have seen and handled the part in question seem to take the view that it was mis-machined. These people will include RR engineers, EASA people, Aussie investigators and the AAIB. FAA BEA and Airbus will have also had a piece of the action by now. Do you think some awful conspiracy is in place involving all these organisations?

In this part of the world end mills are rarely used to machine bores. Bores are drilled, reamed or bored, all of which processes provide the minor circumferential scoring seen in the bore. Wear frettage would be much more irregular in appearance.
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Old 26th Dec 2010, 17:08
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Keep an open mind, as there is no technical text accompanying the picture, by no mistake, producing judgments such as yours.

The "Misalignment" damage shows a coving at the break with the wall of the pipe. This is not the product of a drill, or bit, or jet. A drill has sharp facets at the terminus of the drilling face, not a deficit that allows for "rounding" in the piece. Notice the "lines" on the surface of the bore. These are obviously not a product of tooling, as they are irregular and non concentric, as well as being out of sync with eachother.

The rim that is missing shows a "shadow" where the pieces departed, and does suggest a problem on manufacture, but shows evidence not of misalignment, but the opposite, a concentric mill, not an elliptical one.

The most important telltale is at the rim itself, the jagged trough of missing metal that includes a particularly ugly remnant of damage, most probably due to profound vibration. This is evidence of the mate to this pipe having a loose and compromised fit, where vibration over time led to degradation of this area.

It is apparent that there was a separation under stress of this joint, at least to me, and may have signalled the initial drop in Oil Pressure.

There are no accompanying comments with this picture, leaving the door open to conjecture. I have seen damage of the sort shown in fittings of this type. From experience, I stand by my comments. Discuss??

Turbine D........... I have some understanding of the 972 oil system architecture, if you'd like, perhaps I could answer some questions?
 
Old 26th Dec 2010, 19:12
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From the report AO-2010-089:

"Misaligned stub pipe counter-boring is understood to be related to the manufacturing process."
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Old 26th Dec 2010, 19:25
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Let's see......Manufacturing defects are understood to have been related to failure......

Fatigue failure of the Stub Pipe...is related to failure.

Fatigue failure of the IT is related to disintegration..........

Fatigue.......Failure........Oil Fire.......IPT Burst.........Deck Chairs.........Titanic.....



 

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