Old Engineer

Here is something perhaps of interest regarding compressor stalls. Note the section that begins at ~ the 6 minute mark. It is the most violent of all stalls in terms of noise and everything else.

YouTube - pt. 2 of 3 Turbofan Jet Engine failure recognition

Old Engineer

I would agree with you on this, partly. However, when that disc broke, it made a BANG just like a test bar failing during tensile testing, only louder.
Note the puncture hole at the top surface of the wing, it corresponds much like the cross-section of the IPT disc. It is hard to say what else came out inside the wing, but I think it isn't the bevel gear, it is too far forward in the engine and it would not appear there is much external damage forward of the HPT rotor.

I see where you get this from but bear in mind that there is no CVR at this time so loud bangs cannot be placed on the timeline. Yes the overheat parameter is timed but "at the same time" is vague in the context of the report and does not necessarily refer to the bangs, I would say.

firstfloor

Yes, that is the counter-argument. It is too bad that the CVR is not available.

Turbine D

I hadn't thought of the test machine, and you can't forget that bang. But with that, you're within 3 or 4 feet. But I agree, it would be much louder. Still, the response of the ear is logrithmic, so it wouldn't sound as loud as it actually is, and the passengers are a little farther away. What I regret is that so often the passengers are not questioned with some respect for the information they might have-- here if those opposite the engine had heard only one bang, that would suggest that it was two nearly simultaneous bangs, but at opposite ends of the engine.

Rising oil temp: Today I discovered an explanation for this not mentioned here, that can occur without a broken pipe. I'll put up a post on this tomorrow.

Old Engineer
 

post nr. 1999 and 2003
I took once again a glance in that FAG internet appearance and those brochures.
I found the - 54 to +120 deg C values you mentioned, and yes, those technical data wouldn´t cover the requirements in an engine at all I assume. So I went on and found this Schaeffler Technologies | Sectors / Key Industries | Product range
Its general description but at least they state there that they can cover a temperature range from -250 to + 500 deg C.

post nr. 2013
What I take from the holes in the wing picture is , a big piece did the damage to the leading edge structure and front spar. Thats the big hole. The other, smaller one, where a smaller piece puntured a wing tank, resembles very well the cross section of that disintigrated disk.
The assumed trajectory of these two pieces I think are drawn in that sketch on p.18 of the ATSB report. These are missing I think, and should be somewhere to right side of the flightpath. The big piece recovered seems to have left the engine in the oposite direction. Amazing how far it was thrown away from the flightpath.

The "bang(s)" are intriguing.

It is possible that what was heard were two separate sources of sound that happened
(close to) simultaneously.

Possibly the IT was 'slumped' (out of shape) and had fractured, but was restrained by the Shaft, although at its virtual limit of integrity.

The Stall may have been initiated by blade loss (stator contact) and reduced N2. With the instantaneous "hybrid" gas path (stall) upsetting the Disc (not to mention the entire engine/pylon), the wheel disintegrated and blew out the case. Two very loud reports, of different cause and sonic "character". One metallic, one gaseous. The pilots may have been (through experience) better able to notice the differing character of each sound, even with "simultaneity". It would explain the final impetus of the Disc failure, especially in light of the Miami machine's retention of disc "integrity" sans "Stall". I don't think it necessary to include overspeed as a component.

Oil Fire certainly would account for added heat, but logically, the heat would be somewhat erratic, and the mode of failure of the wheel is described as 'patent',eg 'in three roughly equal parts'. If true, that would suggest less than a chaotic heat soaking due to fire. A more blended heat soak (evenly distributed) would most likely have been the result of friction at the Drive Arm/Stator (who brought up 'fluid bearing?).

Most important, metallurgically, then, would be a determination of the "Point" of failure, whether Rim (Fire), or Drive Arm. The chronology of the event suggests to me, a 'build' of heat, something I would relate to a slow "Migration" of the Wheel, and a concomitant elevation of Oil temp. It sounds counterintuitive to describe this "failure" as 'gentle' in any way, but a slow build of heat through the structure seems more accurate, given the timeline, than that engendered by something as chaotic as Oil Fire.

Whether the Stub Pipe was born wrong or wore through, its failure appears gradual, and conforms to worries announced in the ADs. Though a three shaft powerplant, any insufficiency of bearing performance at one location, translates immediately to the other bearings. The vibration record (both power and timing) is truly mesmerizing, and tells a tale.

The photo of the recovered disc segment (1/3 sector) strongly suggests it is NOT a rim-originated failure, but rather starting at the bore and extremely rapidly progressing outboard. Such is characteristic of an overspeed burst, and the metal splatter from the 'false bearing' is supportive of this.

The root cause of the rotor burst is the fact that after the IPT disc decoupled from the IPC, the IPT blades remained more-or-less intact in the disc, and continued to convert N3 gas stream energy into torque to accelerate the disc to destruction. Note this sequence is different from the RB211 failure on QF74, 31 Aug., where the blades were destroyed/removed either by blade root failure or by clashing with the LPT nozzle ring.

Don't get me wrong; Oil fire is nasty - you DO NOT want that blowtorch playing on bearings and shafting. But it should not precipitate an overspeed to the point of a rotor burst!

Annex14's

assembly of engine parameters is interesting. I am put off by the 'zeroes'. I think neither N1 nor N2 siezed, so is the reading of '0' RPM a hard number, or one generated by the EEC after loss of the actual signals from the engine? N1 and N2 are sensed at the respective Shaft via phonic wheel. Four each, two pair, each pair sending its output to either channel A or channel B of the EEC. N3 is derived from the generator affixed to the gearbox connected via driveshaft to the Ring gear on the HPC Shaft. If the shaft speeds are disabled, the EEC computes "generalized" numbers to send to the cockpit for display. So I am reluctant to assign events to the 'timeline' from the list.

I will also suggest a rationale for the 'double boom'.

1) The A380 is a very large aircraft
2) Sound travels faster through metal than through air
3) The first boom heard in the cockpit could have been propagated by the aircraft structure, and the second by airborne sound wave. The airborne wave might arrive 50-100 milliseconds after the first. This is easily discernible by the ear.

bearfoil

I agree with you, one bang was gaseous in nature - the stall, the other mechanical in nature, the disc rupturing. If you assume the following: The disc was free on the shaft, it was being subjected to higher temperature conditions than what it was designed to handle, it was rotating at some speed perhaps not approaching what would normally be burst speed (a degraded burst speed margin due to temperature), and the disc all of a sudden is subjected to a huge shock wave of energy (the stall), it might have been enough to push it "off the cliff" so to speak. Rapid P30 collapse - prelude to a stall, engine overheat - a stall - disc failure within ~ 3 seconds. Too bad the CVR of this wasn't available.

When you think about that disc carrying over 100 turbine blades at high rpm's, the load could approach 1,500,000 pounds or more.

It sure would be nice to have seen a photo of the recovered portion of the disc depicting the forward face (we only see the rear face). I say this because we could perhaps see if it is discolored resulting from a fire. There is a hint of this in the photo of the disc fracture surface, but it may only be a shadowing effect from the camera angle and lighting arrangement. Also, the recovered piece doesn't show any evidence of a rim failure.

If the Disc developed a 'circumferential fracture' of the Drive Arm, a la Miami, it very well could have separated from the remnants of the Arm to remain in the case while rotating independently and progressively more elliptically, scrubbing off its blades whilst gaining rapid deviations in planar rotation. At some point, the energy is greater than the thin case's ability to contain it, and it divides itself into the requisite three piece 'suite', to exit. If the Disc separated, but was contained in some elliptical path within the case, it becomes clear the generation of "fold back" of the Arm remnants on the Wheel's failure signature.

At 7000 RPM, sufficient energy is available to accomplish the Burst, an overspeed is not necessarily required.

So it becomes ever more telling; are the reads on the list accurate?

The 'Rigid Coupling' is accused of failure (ATSB), however, it may have failed in the respect of allowing the AftShaft to wander aft, not in losing contact, other than axial, and that sufficient to keep the shaft operating as a unit. (AD's) IMO.

bearfoil
 

As I mentioned in my messages before the list was posted through help of mm43 all I did, have the Graphs published in that ATSB report printed enlarged, put them side by side and read of the data shown. It must be clear to everyone that this is not a very sophisticated evaluation method. My intention was, with the data at hand to make a more logical approach to the facts at hand.
So zero in my list means zero in the ATSB graphs. These might be in original prints 2 or 3 time as large and much more precise. What none of us knows How perfect calibration of the recording media was. As is well known, many little drifts off the ideal line can add to a major error.
Hope this explains a bit the content of my list.

Turbine D
 

Coincidence of thoughts. Me too have had a look at this picture showing the fracture surface. Upon enlarging it I thought to see the "shade" to the left to be some kind of an airbubble plastic wrap.
Sorry if my wording is sometimes not very correct, but have to translate it from my mothertong.

Annex14

I get it now. The zeroes seemed arbitrary, for obvious reasons. I am not very good at computer work, and until I remembered the EEC's role in cockpit reporting management, I was stuck. It's all relative. Thanks again, I think you've opened up a fruitful harvest of data.

Annex14

Look at the disc fracture face photo, in the upper left of the disc, just below the dovetail serrations you will note a dark area which is the forward face of the disc. It is this that I wonder about compared to the lighter metallic surface of the rear surface of the disc that is more clearly evident. Is it dark because of the effect of fire/temperature? Or is it dark because of the lighting shadow?

Sometimes these surfaces can tell you somethings relative to conditions that were happening at the time.

Turbine D
 

I did check and also amplified those pictures in the ATSB report to 800 x the original size, beyond pixelling starts and there is too much noise.
I must admit that I cannot say wether what one sees in the area you mentioned but also at another one in the lower left base is a dark deposit or just shade. Unfortunately it looks like the picture was taken just off axis, slightly right of the center of the cross section. So either is possible.

However, I also checked for pictures of the disk piece in earlier posts. Check the post nrs. 42; 84 ; 587 ; 680 - they show several aspects that has to be considered. In post 42 is a picture where police officers carry that piece by hand and using some questionable dirty pieces of cloth for that. Post nr. 84 shows that part probably in a private car trunck before or at delivery to the police. Although these pictures show a non professional handling of that important part some kind of deposit remained untouched to some extend.

Post nr. 587 shows that same part on a small lory, also only the rear side of the disk. What is remarkable there are still some darker areas visible may be some kind of deposit ???
Finally there is that false colour picture in post nr. 680. It too shows some darker areas, even in false colour.

What I also have realized is a difference in surface colour of those early pictures compared to the "official" one in the ATSB report. Explanation for that - either the different colours come from repeated treatment, handling and reproduction of the original pictures or the disk was cleaned, hopefully after samples of any foreighn material on the surfaces was secured,
A pitty, I have to say it again, that there are no better and more detailed pictures at hand.

Could the second bang have been the disc segment hitting the wing?

No, you'd be hearing a "Thump". Nowhere near the noise of the Burst. And you'd need to be seated close by. The Cockpit did not hear it. The wing is built remarkably lightweight, and even the Spar is no match for the Disc. Knife, Butter....imo. The good news is that the shape of the debris will then be remarkably unfazed by passage thru of the Nickel Alloy Steel; note the cookie cutter deficit, as if the skin were nonexistent.

Disc/Color/Surface.

The Discs are highly polished prior to install, the final rub is Colloidal Silica. Similar to jeweler's rouge. Now this Wheel had seen 200plus cycles, and heat debris and friction, Gas components can work their mischief on the shine. In the ATSB picture, my guess would be that the Disc had been cleaned and inspected, but the image is prior to destructive sampling of the metal. The fracture at Drive Arm appears to be "Puddled/Smeared", as if autonomously and randomly 'forged' prior to parting the Shaft remnants of Drive Arm. It wouldn't be a wild guess that the Disc had experienced a circumferential fracture. Note the Drive Arm 'fracture portion of the image'. It is beaten around, and it lacks sharp fracture delineation that the Disc separations show. So lost from the Shaft through circular fracture, a small hesitancy while describing ever more elliptical orbits of the Drive Arm (with consequent 'folding over'), then separation into three large pieces, and immediate exit. The remnants of Blades are suggestive of a gradual loss (tenths of a second?), and an inconsistent failure mode, the blades/roots which are missing may be more indicative of a 'vibratory/centrifugal/non planar' loss.

Turbine D, old engineer, annex14, mm43, first floor.......all

http://www.atsb.gov.au/media/2891297/vh-oqa-fig8.jpg

Take a big look at the severed torque links, and a peek inside the IPLP cave. Does one see the remnant of NGV1LP? Or is that a remnant of the Intermediate Turbine? Also beneath the centrifugal Oil Breather, see the burned Stator ring hanging down?? displaced Much?? Without the Torque tubes (which look like they snapped in tension!!) the engine can swing to its heart's content, not good in Yaw, and one step away from losing the powerplant off the pylon. Thanks to the suspension cleats, no worries!!

Chu Chu
I agree with bearfoil. It would appear the disc separated into 4 pieces, the largest being the 135º section recovered. It must have exited the engine downward and to the left based on its recovery point. the other three pieces exited to the right as defined in the ASTB report's diagram.

The recovered piece weighed almost 70 Kg, 150 Lbs. So the total weight of the disc alone, minus the turbine blades approached 400 Lbs. or more.

It was probably rotating at a speed of 7,000+ RPM at the time of failure.

The sound of breakage and the energy released is so immense it would overcome any sound of the pieces going through the wing.

Also, watch the video on compressor stalls and the noise created by the explosion of the gasses going forward and aft, that was in one of my previous posts, the site is given below.

YouTube - pt. 2 of 3 Turbofan Jet Engine failure recognition

bearfoil

It looks like a Stage 1 LPT nozzle vane, hanging down, to me.

You can also look at Figures 8 and 13 to see more things.

In Figure 8, you are looking at the Stage 1 LPT blades, the stator in front of this is gone.

In Figure 13, you can see the gas path vanes that are part of the frame the plenum joins to. Also, you can see the peeled back casing that was above the IPT rotor.

The main engine mounts did a good job retaining the engine on the pylon.

Turbine D

NGV1LP- Nozzle guide Vanes, #1, Low Pressure Turbine (my shorthand, sorry).

The Stage one Guide vanes and Stator are attached to the Intermediate Pressure case with the IPT, No? One wouldn't expect to see it with the LPT Module on strip, would one?

Note the #1 wheel, LP. The damage (missing blades) is the same as on the front view of the LP Module (why wouldn't it be?).

I agree the Stator hangs down, did you see the outer Vanes Platform? It has separated completely from the IT case (or the case blew away from it). It also is bent as it enters the cavity. One also gets to see the Oil vent from the HPIP bearing case (no sign of heat damage), the oil lines to and from the Oil Breather centrifuge, and the mount for the EEC. The EEC has been removed from the Fan Case, note the 15 leads wrapped in plastic bags and the Four (anti vibration) mount towers. Bleeds are visible, as well as the air dam on the pylon.

bearfoil

Now there is resolution-- 14 megapixels times 3 layers for the colors, I think. Is there a similar camera copy for the report showing the LP shaft that might show the condition of the splines? Not that that would mean anything after two hours of windmilling in an engine casing that could hardly have remained straight...

Am I looking at the inside of one of those large ball bearing thrust bearings as well? My looking around tells me that the entire assembly may be as large as 3-3/4" square (two rings and the included spherical balls), plus parts of the turbine made integral with the rings. I'd want to do some scaling off the pixels relative to the fan diameter, with a little perspective shortening thrown in, if I can locate a line parallel to the shaft-- well, the dolly should be, shouldn't it?

Anyway, the outer race of that bearing looks kinked as well. Could we call that bearing damage? I've found a reason to put bearing damage on the list of usual suspects, even if it had a fully functional oil supply. Tomorrow. Good work, bearfoil, in finding that picture. Does the raw file include any camera data such as lens focal length and size of its sensor chip (or camera model)?

http://www.atsb.gov.au/media/2891300/vh-oqa-fig14.jpg

A follow up on my post above re: the Drive Arm/Wheel fraction.

Notice the fracture area at the central arrow. It is beaten while quite hot, imo. Also, the surface of the Drive Arm area appears that it spent some time within the remaining (shaft) area of the Drive Arm, or the inner Stator ring. The failure furrow is worn smooth in this area, while remaining crisp further out. Also, the outer 'layer' of metal is peened off by the inner Arm. The metal spatter appears to be bits of molten Drive Arm, supporting a theory of intense heat from kinetic or Fire products. However, that begs the necessity of the Fire being in the IPLP cavity, not in the area in front of the Wheel (bearing case). In fig 8 we can see what is left of the Stator at its center closest the IPT. Out of view is the area of contact. The Oil quantity hadn't depleted, although it did read once at 11 quarts, but returned to twelve to remain there.

So what of the stub pipe? At ~5/8 inch and a pressure of ~65psi, one would think a serious loss of Oil, but the quantity read doesn't show that. The Temperature of the Oil is elevated, and that shows some tramp heat, but perhaps not a Fire at the area of scavenge??

I am trying to follow the arguement so far. It looks like this, Oil leakage, oil fire, heating of drive arm leading to failure (first bang?), turbine wheel overspeed because of disconnect from load (but not instrumented) leading to disintegration (second bang?).

firstfloor

I think at 12 quarts, the Powerplant may have been 'leaking' chronically. With the tank at 12, the system had to account for 16 quarts. As to Oil fire, the quantity remained constant prior, through, and after "fire"*. If the IT separated from the Drive Arm, it would certainly overspeed independently of the Shaft**; I think the Shaft maintained its integrity, but slipped at the Thrust area coupling, (axially).

I lean toward an engine Stall with interruption of N2 gas, the first bang, and having blown through the IPLP, and upset the IT to disintegrate, it exited the case, second bang. I kind of think the noise was approximately simultaneous, but due to the character of the reports, each was 'heard'. This is to say the Drive Arm failure caused the N2 loss, which caused the Stall, which caused the Burst, something like that.

*we cannot forget the "Stub Pipe" here. Supposedly it was pouring Oil into the HPIP Plenum, but the reads of Annex14's don't support this.

**but not for long, at least not in some orderly manner, almost immediately would blades start to depart, as they contacted the Stator Guide Vanes. The Wheel would orbit its Shaft with a larger bore radius than it started with, causing severe out of balance issues (see vib HP %RPM). The focus of stage one vibration would be the bearing space shared with HP, I feel that is why the vib was highest at the HPT.

bearfoil:An engine that will not tolerate (mechanically) a high-energy compressor stall WILL NOT withstand all the test regimen in development and certification testing. Whatever failings the Trent 900 might have, I find your thesis to be very far-fetched.

Turbine discs are very stout. The 9/11 crashes demonstrated that; of all eight engines that were destroyed that day, all the turbine discs were pretty much intact. To envision a compressor stall inducing a disc burst is "round the bend".

barit1

I think you misunderstand my post. The IT failed at the Drive Arm, This left the Disc rotating with its full energy in an out of balance fashion. This is not the first time an IPT has fractured fully from its Drive Arm. The idea is that the Disc would burst and exit with or without the Stall. Forget the Stall. The moment the disc parted its Arm, the clock was running. As the Turbine lost its blades having migrated into the Stator, N2 diminished. At perhaps five hundred pounds and 7500 RPM, the Burst was on its way. A Stall makes the bang, one of two. I list the Stall as the first bang. It could have easily been the second loud report. No need to link the Stall with the failure,[I] if indeed there was a Stall. If there was, it isn't round the bend to think it may have tipped the Disc, aggravating and perhaps speeding the Burst along?

The explosive disintegration of the Wheel is seen. What on Earth does 9/11 have to do with this?? The Disc is 'very stout' therefore it withstands the forces that fail it, ipso facto?

OK - I have little doubt there was a stall somewhere in the whole sequence, but it wasn't a necessary part of the mechanical failure, but rather it was incidental.

Consider also that the drive arm is much less massive than the disc bore, and was directly in the "blowtorch" zone; the bore would take a lot longer to heat up to the point of failing at 7000-8000 rpm. Thus I submit that it was indeed an overspeed that failed the disc, and that requires the IPT blades to remain more-or-less intact after the drive arm separation. (They could readily disappear when passing through the case etc.)

My reference to the 9/11 engines was simply to point out that normal discs running at normal temperature & rpm do not fail even when horribly abused.

But all this is subject to revision based on more evidence.

Overspeed is the mechanism described in AD 0262 -

Quote:

---

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.

---

Other complications have no standing per rule of Occam's razor.

firstfloor

I agree with your post.

Fire - increasing temperature of IPT disc in the bore area - fracture of drive arm - increase of rotor speed - disc rupture - the end.

In the Trent 772 Edelweiss failure, a similar scenario: Fire - fracture of drive arm - disc does not rupture - the end.

The differences are: The IPT rotor in the Trent was probably turning at lower rpm's (7000 maximum permissible) verses the Trent 900 (8300 maximum permissible).

Temperature at the R850 holes in the Trent 700 disc arm reached 1832℉, a combination of friction and fire according to the NSTB report.

The first contact of the IPT rotor in the Trent 700 would have been at or near the tip of the turbine blades as they contacted the Stage 1 LPT nozzle slowing (breaking effect) the rotational speed of the disc. The Stage 1 LPT nozzle tilts forward at the OD. In my opinion, this feature saved the day in preventing increasing overspeed and disc burst.

The first contact of the IPT rotor in the Trent 900 would have been at the drive arm interface near to the engine centerline creating melting and potential further rotational speed, no breaking effect there. The Stage 1 LPT nozzle does not tilt forward and the airfoils are recessed because of a large forward ID overhang design.

Key factors: Hoop stress at the disc bore - radial stresses in the disc web - temperatures at the bore (keep less than ~500℉) - at the rim (keep less than ~900℉), both depending on the capability of the disc material.

Turbine D

Glad to see that this thread finally ended up in the tech section where it belongs, especially since it's only speculation and not news worthy.

Your post above has come the closest to describing the key factors and differences in the two incidents.

However, once again I would urge you to forget about the significance of friction once the failure scenario has started. At the speeds that turbo machinery runs the interface conditions in a contact environment is nothing but molten metal.

The energy is all in the gas stream, so if you want to corral this energy you have got to get rid of the driving airfoils within the time it takes to blow the flame out the tailpipe. If the airfoils remain for even seconds, it's probably too late.

I've been following this interesting discussion from the beginning on both threads, and other than dropping in the odd graphic have just watched the flow and ebb. However, when looking at a pic taken after the #2 had been demounted and lashed to the trolley, I noted a stray pipe with a rather distressed look to what is now an open end. What would have been carried in this line? I assume this damage was caused by the disc burst.

http://countjustonce.com/QF32/qf32-eng-sec.png

Also, I believe that references have been made to the FDR graphics as published by the ATSB in their Interim Report. Those 5 graphs in their full size (1369 x 976 pixels) can be downloaded as follows:-

QF32-FDR-1
QF32-FDR-2
QF32-FDR-3
QF32-FDR-4
QF32-FDR-5

Reference to the engines #2 and #3 N2% scale in image #2 will reveal that the #2 readout has been offset vertically and the scale associated with both engines is corrupt. I suspect the correct scale is that above for N3%.

mm43

Somebody on here may know for sure, but it looks like a structural member to go between the mount points on the engine. Wasp waist engines have a tendancy to distort their cases and rub compressor blade tips without this kind of load carry device.

mm43

Not a pipe, in the conduit sense, it is a Torque Link. Both these struts were severed by the exiting HE Debris. See the mates above, at the pylon cleat. These features prevent yaw of the engine. They snub each attempt in tension/compression, as a pair.

The separation of the Disc from the Drive Arm would allow freespool, surely, but not without instantaneous consequences, imo. The Rim of the Disc is aligned with, and short of, the Platform for attachment of Guide Vanes on the Stator. The rim would rub the Platform, but the loose bore of the broken Wheel permits "Elliptical" rotation, something that would shear the Blades of the IT off immediately. The fact is, as the Drive Arm is contacting the Stator, the rim is contacting the Platform. This contact may have happened prior to the Circumferential fracture at the Drive Arm. There is no way of knowing at this point the exact progression of the fail. I do believe that since the Blades would shear instantaneously in either case, overspeed did not progress far, if at all. This makes one think of the consequence of instant unloading of the IPLP barrel, and forward. Who needs a Stall?? The debris and damage shows remarkable scarcity aft of the Stator. Most everything appears to have been blown out the gaps in the case in a forward direction.

lomapaseo

Thanks for your post. Your comments regarding the airfoils is well taken. The Trent 900 is an extremely high performing engine that produces higher pressures, temperatures, rotational speeds and thrust, higher stresses go with the package. In this new generation of engines, the turbine airfoils are highly loaded, 3-D designed for optimum aerodynamic performance and there are less of them to save weight meaning they have to work harder to extract more energy in the same axial distance span. I hadn't given this a thought until you brought the subject up, indeed they have to go away fast!

lomapaseo, bearfoil;

Thanks for the feedback. A strut it is, and I have made the connection.

The ball thrust bearings - some background
 

My comments are based on, or inferred from, information which I believe was developed at Stonehouse. It was stated that it was believed to have possible application to a contemplated aircraft designated the A3XX which could carry 450-500 people, among others. It appears to have originated with a technique used in fighter aircraft. Note is made of the fact the fighter and commercial aircraft differ substantially in the frequency of engine inspections, at least those involving tear-down of the engine.

I comment as follows:

The steadily rising oil temps-- from about 170-C to somewhere in the 190s, and at a comparatively leisurely pace (20-some seconds here before everything went pear-shaped), are compatible with a bearing running hot under very high thrust load, due to the inability to maintain an oil film under these conditions. This occurs even though the oil supply is completely normal (normal flow rate, and properly reaching the bearing(s), and being cooled in the normal fashion), in commercial A/C.

In the military precedent, the fighter A/C could totally lose oil flow to bearings due to violent turns, etc. A solution had be sought to this problem that would enable the bearing(s) to survive 30 seconds undamaged during oil starvation.

The inability to maintain an oil film can rapidly lead to spalled races and bearing seizure in plain metallic bearings, even those with several different types of surface hardening. Perhaps by accident or lucky chance, it was found that a coating called AP (advanced phosphate) tended to act as a solid lubricant (or perhaps an oil film would be adsorbed onto its surface?), and was not itself damaged in the process... at least not up to a point. (If there is such a point, it is not in the information I have, nor is a full explanation why the AP coating worked as it did.)

A model of a ball thrust bearing was made, roughly 1" x 1" in section including both races. This is thus about a quarter-scale model of what I tentatively believe to be the size bearing used in the OF32 #2 engine. The test bearing, with the AP coating on both races and the caged spherical balls, was test run for about 6 minutes with the appropriate high thrust loading. The inner and outer races rose to rather different temperatures, but by inspection of the graphs, 200-C is a fair estimate of the average of the two temperatures. This could be said to occur over the initial 40 seconds. The text said the AP coating tended to higher bearing temperatures, but that stabilization at about 200-C was typical. On reduction of the high thrust load back to more normal, the temperature fell without any bearing surface damage detectible.

Perhaps it can be said that there is a trade-off here in the weight of more oil cooling capacity. But maybe this type of AP coating is just going to run at about 200-C regardless (Edit: but only on failure to form the normal oil film). I don't want to get too far beyond my information; I have sought more recent information, but without success at the moment. I do know that such a bearing (or bearings, there are 5) was used in an advanced turbofan engine built by BMW Rolls-Royce AeroEngines.

At the moment, and I always stand to be corrected, I incline toward a bearing problem initiating a bearing overheat problem. Then obviously this will lead to hot lubricant in all the bearings of that engine, and they will all contribute to the overheating of the oil. Well, of course, the unrestricted certification oil temperature of the RR RB211-972 is up in the 190s (194-C? 197-C?).

The warning upon which there was time to take action was the rise and rate of rise of the oil temperature beyond 170-C. But this rise apparently is not communicated to the FD, just to maintenance. I cannot emphasize too strongly that we do not need any oil fire to have this happen. Yes, I've read the official report. I see a lot of clean metal in the area of the missing turbine casing-- a lot of that is clean as a whistle, including one large piece that had rotary symmetry sundered by a vicious, meandering tear. I also see a large, clean ball thrust bearing, with a buckled outer race.

I agree with barit1 that it is unlikely that a compressor stall was the start of this. But drag from a failing bearing may have disturbed the relation among the 3 compressors and 3 turbines leading to a stall. Then the forces involved in this stall may have made the 30-second delay, perhaps ordinarily enough to not incur disc disintegration while seeking stabilization of the engine, a wrong choice in the software. This is now changed in software.

A lot of questions arise out of this information. Did engine #2 have bearings with the AP coating? Or a later coating accomplishing the same purpose? Did vibration and impact from excessive spline wear damage this coating? There is certainly a heavy watch on spline wear. I leave more questions to later.

old engineer

I take note of your theory of film loss, and this covers heating of the engine oil. Not enough is being made of high vibration imo, for this is a more damaging way to lose lubricating performance. Though the linear aspect of Ball bearing thrust attenuation here is not usual, I note your confidence in it. A spherical bearing in thrust as here, has two points of contact fore/aft, two per raceway (Four total). Under the load of IPT aft force, a vibration works to squeeze a film to zero, and the phosphating you discuss is left not to lubricate, but armor the Race and the Ball against significant "chatter"? It is not out of the realm of possibilities to suggest that the number of surfaces diminishes as vibration (and imposters, a resonant) can move to unload a number of balls, leaving fewer still to take the load. The Main bearing was reworked due to spalling, what was the remedy? More frequent inspections. Rolls may have been too casual with some results from the test bed prior to introducing this Engine. Too late with the Rumour, but better than never.

old engineer
I don't think anyone suggested the compressor stall started the chain of events leading to the engine failure. It was primarily a discussion of the two "bangs" heard and reported by the flight deck crew. The reality of it is though, when the compressed airflow of the IP compressor (N2) significantly dropped (no IPT drive), the HP (N3) compressor eventually choked and stalled, especially if it was in a increasing rpm mode to create thrust as demanded by fuel flow to the combustor. This was the situation as I see it. A HP compressor stall was inevitable.
When you look at the views of the engine presented in the ASTB report, I think everything you see is from areas to the rear of the combustor casing. You can still clearly see the complete combustor case with the fuel inputs and electrical igniters, the wires shielded with a reddish insulation, as the engine is being removed from the pylon. I don't believe there was any destruction of the compressors or rupture of the compressor casings (IP or HP), that would release thrust ball bearings. I have seen compressor failures on engines, and the results are really not pretty, even more catastrophic in appearance than on this engine.