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Stage III Jet Engines, Where Are We At Present?  
User currently offlineEcuadorianMD11 From , joined Dec 1969, posts, RR:
Posted (5 years 2 months 3 days 6 hours ago) and read 8540 times:

On aging aircrafts like the 737-200 or the 727 I regularly read "Stage III" printed on the engines, which I believe is related to certain noise dampers installed on the engines; improvements to what (I presume) was once known as "stage II".

Are the above assumptions correct? Are these stages just noise related and does this exist for modern jets as well? For argument's sake, is the Boeing 747-4 and the 777 part of stage V for instance and a 787 stage VI etc etc?
I believe that certain noise levels are now "illegal" in Europe (and USA?), would that be stage II (and less) that became unwelcome?

If this "hush kit thing" has been discussed a zillion times before I apologize, but I couldn't find anything in the search engine as such.

One more additional thing thinking about these 737-200 engines; the diameter of these engines seems so small, how can a relatively large plane get away with such narrow jets?

Hope you airplane gurus out there are prepared to discuss this item just once more to enlighten this fresh / green A.netter........in his stage I.

Cheers,
Ecuadorian MD11

25 replies: All unread, showing first 25:
 
User currently offlineAA737-823 From United States of America, joined Mar 2000, 5722 posts, RR: 11
Reply 1, posted (5 years 2 months 3 days 5 hours ago) and read 8439 times:

The current requirement is that an airliner (and certain/most other aircraft) meet stage III noise levels as defined by the ICAO. Any aircraft that surpasses this level (and many modern Boeings and Airbusses do) either meet projected stage IV requirements, or simply "exceed stage III."

For old JT-8Ds on DC-9s and 727s and 732s, this meat shoving a mixer up the exhaust duct. It mixes fan air with core air, and brings the exhaust scream down to a tumultuous rumble.

As far as the 732 (the plane on which I am a mechanic! Great aircraft...) and small engine diameter, it's really not that the engine is small, it's more that the newer engines are large.
707s, 727s, 737s, DC-8s, and DC-9s, Caravelles, Mercures, and BAC-111s all had engines of roughly the same diameter.
But we discovered a few decades ago that hi bypass is more efficient. So, whereas you had half of the air going through the core section of the engine, and half of it going around through the bypass duct, NOWADAYS you have only 20% of your air actually going through the combustion core, with 80% going through the bypass duct on the CFM-56-7 that powers 737-6/7/8/900 aircraft.

Hope that answers your questions, and welcome to A.net!


User currently offlineTimz From United States of America, joined Sep 1999, 6781 posts, RR: 7
Reply 2, posted (5 years 2 months 2 days 18 hours ago) and read 8248 times:

Also, the engines themselves aren't Stage II or Stage III-- the aircraft is. Its noise is measured at certain distances from the runway, and if the aircraft is lighter it will climb faster and so its takeoff noise measurement will be lower, even tho the engine itself is as noisy as ever.

(But the allowed noise to comply with Stage III also decreases somewhat with aircraft weight.)


User currently offlineEcuadorianMD11 From , joined Dec 1969, posts, RR:
Reply 3, posted (5 years 2 months 2 days 13 hours ago) and read 8177 times:

Thanks AA737-823 & Timz,

that was helpful.

Interesting was that it's the whole aircraft that is stage III, not just the engine.
Is this a worldwide standard?
I was told that 727's are not welcome in certain European countries for instance.

Do you think we're looking at mandatory stage IV requirements in the decades to come?
I guess we will........

Then there's that other matter about the 737-200 engine size.
Basically, having bigger fans (+ air inlets) turns out to be more powerful / economical, right?
But I wonder how running more air into the engine but bypassing the actual combustion chamber can make an engine more economical. Can you develope that point in Layman's terms perhaps?

Also, why is the modern 737 jet engine flat at the bottom?
See pic:

Big version: Width: 1504 Height: 1000 File size: 452kb


A nice pic I think.........with the famous sugar loaf illustrating my question!

My best guess is increased ground clearance in case of an unfavorable landing.
The majority of modern jet engines are complety round however, is this just to do with the aircraft's overall aerodynamics or is this because of engine performance as well?

Cheers once again,

Ecuadorian MD11.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (5 years 2 months 2 days 10 hours ago) and read 8156 times:



Quoting EcuadorianMD11 (Reply 3):
Also, why is the modern 737 jet engine flat at the bottom?

The engine is still round, it's the nacelle that's flattened.

Quoting EcuadorianMD11 (Reply 3):
My best guess is increased ground clearance in case of an unfavorable landing.

Increased ground clearance period...even in normal ground attitude the bottom of the nacelle is very very close to the ground.

Quoting EcuadorianMD11 (Reply 3):
The majority of modern jet engines are complety round however, is this just to do with the aircraft's overall aerodynamics or is this because of engine performance as well?

Round is usually aerodynamically best and provides the best inlet geometry, but ground clearance may prevent you from going round on something as low as a 737. It's a trade between clearance, ease-of-maintenance, and performance.

I've heard annecdotally that the "squashed" inlet actually performs better, but I've never been able to verify that, or figure out why that might be.

Tom.


User currently offlineEGNR From United Kingdom, joined Feb 2004, 508 posts, RR: 0
Reply 5, posted (5 years 2 months 1 day 23 hours ago) and read 8082 times:



Quoting Tdscanuck (Reply 4):
Quoting EcuadorianMD11 (Reply 3):
Also, why is the modern 737 jet engine flat at the bottom?

The engine is still round, it's the nacelle that's flattened.

A330s with RR Trents also appear to have a flattened nacelle, although not to the same extent as the 737.


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User currently offlineDescendVia From , joined Dec 1969, posts, RR:
Reply 6, posted (5 years 2 months 1 day 22 hours ago) and read 8049 times:



Quoting EcuadorianMD11 (Thread starter):
I believe that certain noise levels are now "illegal" in Europe (and USA?), would that be stage II (and less) that became unwelcome?

IIRC anything under stage III aircraft are banned from Switzerland. I remember reading something in either GVA or ZRH airport pages that non stage III can get a special wavier but like 48 hour prior permission is required or something like that.


User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 7, posted (5 years 2 months 1 day 21 hours ago) and read 8036 times:



Quoting EGNR (Reply 5):
A330s with RR Trents also appear to have a flattened nacelle

That is so cool, I didn't know the A330's had that as well!



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineKlemmi85 From Germany, joined Mar 2009, 210 posts, RR: 0
Reply 8, posted (5 years 2 months 1 day 1 hour ago) and read 7904 times:

Which aircraft types belong to stage3?

Must be quite some I guess?!


User currently offlineRwessel From United States of America, joined Jan 2007, 2311 posts, RR: 2
Reply 9, posted (5 years 2 months 10 hours ago) and read 7821 times:
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Quoting EcuadorianMD11 (Reply 3):
Then there's that other matter about the 737-200 engine size.
Basically, having bigger fans (+ air inlets) turns out to be more powerful / economical, right?
But I wonder how running more air into the engine but bypassing the actual combustion chamber can make an engine more economical. Can you develope that point in Layman's terms perhaps?

It's basically always more efficient to accelerate a larger mass a smaller amount, than a smaller mass a larger amount, even though the total useful energy this produces might be the same. IOW, the total energy/thrust generated by the engine is basically the mass of air it "processes" times the exit velocity. A turbojet (pure jet) processes a fairly small amount of air, but has a very high exhaust velocity. A high bypass turbofan moves much more air, but has a much lower exhaust velocity.

The ideal exhaust velocity is of the same general magnitude as the velocity of the aircraft. Which is why pure jets are used on supersonic aircraft, fans on fast subsonic aircraft, and propellers (which effectively have a very high bypass ratio) are used on slow subsonic aircraft.

Consider also helicopters, which use engines of roughly 1.5-2 times the size as similar weight conventional aircraft, but generate several times more thrust with the rotor (a really big propeller, after all), at a few knots effective airspeed (in the vertical direction).

Quoting EcuadorianMD11 (Reply 3):
Also, why is the modern 737 jet engine flat at the bottom?

To expand on Tom's answer a bit, the 737 was designed (just) before the era of high bypass turbofans, and simply the landing gear is too short. This required squashing the nacelles to get them to fit under the wing. Lengthening the landing gear is difficult since that impacts a lot of primary structure in that area - the two wheels are already about as close together as is possible when retracted, so you'd have to move the attachment points outward, which would not only mess up the wing structure, but generate significantly higher stresses because the gear would now have a longer lever arm. A longer gear leg also generates more side forces in a landing, and a bunch of other stuff. Basically it would take a lot of work to lengthen the gear.

Other possibilities include some sort of collapsing main gear leg, which might let you keep the same attachment points, but still leave you with new stresses from the longer leg during landing, but that comes at an obvious cost in complexity and weight. Or perhaps mounting the gear lower, but that would require an oddly shaped leg and a big fairing around the mounting.

None of that is impossible, just hard to fit into the existing structure.

You'll also notice that in addition to squashing the nacelle, the 737 also mounts it rather closer to the wing than most aircraft.

Basically Boeing has (so far) not seen fit to do that work, presumably because the net gain would be modest relative to the cost (IOW, the squashed and high nacelles are good enough). OTOH, there is much talk of engines with higher bypass ratios and bigger fans (the P&W Geared Turbofan - GTF, for example), that might convince Boeing to do the work.

Airbus, OTOH, designed the A320 after the hi-bypass era started, and basically designed in much more ground clearance (including a rather longer gear leg).


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 10, posted (5 years 2 months 7 hours ago) and read 7792 times:



Quoting EcuadorianMD11 (Thread starter):
One more additional thing thinking about these 737-200 engines; the diameter of these engines seems so small, how can a relatively large plane get away with such narrow jets?

The JT8Ds are Low bypass,theoritically 1:1 ratio.
Thrust provided on the later versions can go upto 16,000-16,500 lbs of thrust.

Quoting EcuadorianMD11 (Reply 3):
Also, why is the modern 737 jet engine flat at the bottom?

Ground clearence on the CFM56-3 series,as the MLG on the classics were common.I thought the CFM56-7 series was different due to the higher MLG.
regds
MEL.



Think of the brighter side!
User currently offlineTristarsteve From Sweden, joined Nov 2005, 3977 posts, RR: 34
Reply 11, posted (5 years 2 months 4 hours ago) and read 7777 times:



Quoting EGNR (Reply 5):
A330s with RR Trents also appear to have a flattened nacelle

Yes. I had never noticed that before. Just went out and looked at a SAS aircraft, and the bottom of the nacelle is flat, by about two inches


User currently offlineTheSonntag From Germany, joined Jun 2005, 3516 posts, RR: 29
Reply 12, posted (5 years 1 month 4 weeks 1 day 18 hours ago) and read 7709 times:

The noise from low bypass engines is created by the shockwaves from the hot exhaust, which is leaving the engine very fast, creating thrust.

With other words, relatively few air is accelerated a lot, this creates a lot of noise. On modern high bypass engines much air is accelerated much less, which is quieter.

This only works well for subsonic engines, supersonic engines need a low (or no) bypass ratio for several reasons. Thats the reason why Concorde had to use a turbojet, and why most military aircraft are so loud.


User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 13, posted (5 years 1 month 4 weeks 1 day 13 hours ago) and read 7675 times:



Quoting EGNR (Reply 5):
A330s with RR Trents also appear to have a flattened nacelle,



Quoting AirframeAS (Reply 7):
That is so cool, I didn't know the A330's had that as well!

They don't. It's an illusion caused by the squared bulges of the case at 4 and 8 o'clock. Superimpose a coin over the actual inlet itself and you'll see it's clearly a circle, or cover the bulges and you'll get the same effect.

I use to think it was flat too till I read a book (Giant Jetliners) that pointed out the optical illusion.  Smile


User currently offlineEcuadorianMD11 From , joined Dec 1969, posts, RR:
Reply 14, posted (5 years 1 month 3 weeks 6 days 14 hours ago) and read 7558 times:

Many thanks to Rwessel, Hawk21M and the Sonntag,

I understand a bit better now.

Often wondered why military jets don´t have a large fan and why they are SO darn loud indeed!
As a young boy I had the pleasure of standing very (illegally) close to the runway when 2 NF-5´s took off, thanks to my Dad working at Eindhoven airport..........and still enjoy high whistling in my auditory system at times! The friendly wave from the pilot just before take off I will never forget though, definitely worth losing an ear or two.

So, if this high mass of air theory proves to be so efficient, we could see B777 size fans on A318 size planes, correct? If this is practicle anyways.............

The background about the flattened nacelle is interesting too.
I suppose if Boeing gets away with it, why not?
The 737 seems to live on forever, still giving Airbus headaches, so if it´s not broke, why fix it?
Perhaps in future designs they need to give her higher legs though..........depending on how efficient the competition becomes I suppose!

Hats off to the A330 picture of EGNR! Very sharp indeed!
The A330 never stands out at a busy airport, there are bigger and more eye catching planes out there I suppose. But this picture clearly shows her true beauty!!


Cheers people!

Ecuadorian MD11


User currently offlineRwessel From United States of America, joined Jan 2007, 2311 posts, RR: 2
Reply 15, posted (5 years 1 month 3 weeks 6 days 9 hours ago) and read 7532 times:
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Quoting EcuadorianMD11 (Reply 14):
So, if this high mass of air theory proves to be so efficient, we could see B777 size fans on A318 size planes, correct? If this is practicle anyways.............

You eventually reach a practical limit in that the engine weighs more and more, but the increase in efficiency slows way down. You also have additional drag from the bigger engine to reduce efficiency. An even bigger issue is that with conventional designs, the fan is driven at the same speed as the turbine it's connected to. Already this results in a significant compromise - the fan is turning too fast and the turbine too slow for optimal efficiency. Increasing the diameter of the fan just makes that worse. Something like the GTF addresses that issue, although I suspect the other factors will leave us with mostly modest net growth in fan size (but still some), but significant increases in bypass ratio as the core gets smaller and extracts more of the available power for the fan.

OTOH, GE-90 sized fans on a 737-sized bird have already happened. The GE36 unducted fan (aka UDF or Propfan), flew on an MD-80 and 727 in the eighties (at least on one side). The GE36's fan was 137 inches in diameter, compared to the GE90-115's 128 inch fan (and 135 inches overall).


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User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 16, posted (5 years 1 month 2 weeks 5 days 8 hours ago) and read 7185 times:



Quoting Rwessel (Reply 15):
You eventually reach a practical limit in that the engine weighs more and more,

I'd think that the thermal limitations on the turbine are more important. If you could increase the bypass ratio by just adding more LPT stages, I'm sure we would be doing so.


User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 17, posted (5 years 1 month 2 weeks 4 days 17 hours ago) and read 7113 times:



Quoting FLY2HMO (Reply 13):
They don't. It's an illusion caused by the squared bulges of the case at 4 and 8 o'clock.

Then you could say the same on the 737NG series aircraft?  confused 

Just saw a Hainan Airlines A330 in SEA. I saw the nearly flat inlet when it was at the gate. So it is close enough.



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 18, posted (5 years 1 month 2 weeks 4 days 16 hours ago) and read 7097 times:



Quoting AirframeAS (Reply 17):
Then you could say the same on the 737NG series aircraft?

I don't think so, if I understand the argument correctly. The 737NG inlet (the shape the air is flowing through, as opposed to the nacelle) is actually squashed. The A330 inlet is actually round, with bulges on the nacelle. The nacelle bulges make the inlet look distorted, but it's actually round.

Tom.


User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 19, posted (5 years 1 month 2 weeks 4 days 14 hours ago) and read 7088 times:



Quoting Tdscanuck (Reply 18):

Speaking of the inlet only, not the rest of the engine nacelles.



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineEcuadorianMD11 From , joined Dec 1969, posts, RR:
Reply 20, posted (5 years 2 weeks 2 days 16 hours ago) and read 6579 times:



Quoting EGNR (Reply 5):
A330s with RR Trents also appear to have a flattened nacelle, although not to the same extent as the 737.

Did the ill fated AF flight have (had) these same engines?

Ecuadorian MD11


User currently offline747fan From United States of America, joined Jun 2007, 1185 posts, RR: 1
Reply 21, posted (5 years 2 weeks 1 day 3 hours ago) and read 6421 times:



Quoting EcuadorianMD11 (Reply 20):

AF's A330's all have GE CF6's.


User currently offlinePhollingsworth From United Kingdom, joined Mar 2004, 825 posts, RR: 5
Reply 22, posted (5 years 1 week 6 days 6 hours ago) and read 6282 times:



Quoting TheSonntag (Reply 12):
The noise from low bypass engines is created by the shockwaves from the hot exhaust, which is leaving the engine very fast, creating thrust.

On military and other supersonic aircraft this can be the case; however, you need a converging-diverging nozzle to have supersonic exhaust. The old subsonic civil aircraft with turbojets and low-bypass turbofans had only converging nozzles. Therefore, the flow could only be sonic. Now you might have residual shocks off of the turbine blades (I am not a aeromechanical turbine person). Most of the jet noise is caused by the shear layer between the jet flow and the ambient air, it scales with something like the 7th power of velocity.

Quoting TheSonntag (Reply 12):
With other words, relatively few air is accelerated a lot, this creates a lot of noise. On modern high bypass engines much air is accelerated much less, which is quieter.

Yes, this is a function of the fact that you have moved away from the purer Brayton/Joule cycle to the hybrid cycle. Just moving air slower would kill your efficiency for a true turbojet.

Quoting Thegeek (Reply 16):
I'd think that the thermal limitations on the turbine are more important. If you could increase the bypass ratio by just adding more LPT stages, I'm sure we would be doing so.

Couple of things happen here. The real problem is getting core power densities high enough. You can add more stages to the LPT fairly easily; however each successive stage must be larger and heavier to extract the work from the ever lower pressure flow. This means that they also have higher tip speeds. If you want to increase core power densities you need to increase OPR, which means that you are likely to hit thermal limits on the HPT or the HPC. Many current designs are not limited by HPT temp but by HPC temp.

Quoting AA737-823 (Reply 1):
The current requirement is that an airliner (and certain/most other aircraft) meet stage III noise levels as defined by the ICAO. Any aircraft that surpasses this level (and many modern Boeings and Airbusses do) either meet projected stage IV requirements, or simply "exceed stage III."

The current noise policy is contained in Chapter IV of Annex 16 of the ICAO convention. In this US, where the noise regulations, in 14 CFR Part 36, are termed Stages (1-4) the ICAO policy is included by reference. This was not the case for Stage III, which was Chapter III with a few minor changes. The noise Chapters are certification limits only, they do not affect in production or in service aircraft. However, ICAO separately adopted a policy concerning the phase out of aircraft that would not meet the Chapter III requirements if they were certified in the late 1990s. This policy was designed for and adopted in the "Western" world. The US, EU, and others put the policy into their regulations. This is where all of the hushkitted Stage II aircraft come from. Note, due to the high charges for marginal Chapter III level aircraft in Europe, most airlines did not hushkit at the economics were poor, in the US where you only had to meet the rule and there were no further benefits hushkitting made more sense.

The A380 is a Chapter IV aircraft. There may be a few variants of other aircraft that were certified after Ch IV effective date, but as a rule all of the other aircraft being sold today are only Ch III compliant. However, when Ch IV went into effect all in production aircraft had a sufficient margin (10 EPNLdB cumulative) that they could have been certified to meet Ch IV.

Quoting Timz (Reply 2):
(But the allowed noise to comply with Stage III also decreases somewhat with aircraft weight.)

The limits allow for more noise as the aircraft get heavier, up to a point where there is a cap. For the approach noise there is also an increase in allowable noise as you go from two, to three, to four engines. This is compared to QC which is based on the absolute noise, (Flyover+Sideline)/2 for departure and Approach-9 for arrival.


User currently offlineEcuadorianMD11 From , joined Dec 1969, posts, RR:
Reply 23, posted (5 years 1 week 5 days 17 hours ago) and read 6214 times:



Quoting Phollingsworth (Reply 22):
The noise Chapters are certification limits only, they do not affect in production or in service aircraft.

I´m not sure I get this, forgive the ignorance!
It´s a bit nosiy here......can´t think straight perhaps!

But how doesn´t this affect in production or in service aircrafts please?

Sorry to be no(i)sy!

Ecuadorian MD11.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (5 years 1 week 5 days 11 hours ago) and read 6184 times:



Quoting Phollingsworth (Reply 22):
Now you might have residual shocks off of the turbine blades (I am not a aeromechanical turbine person)

You shouldn't. The fastest point in the turbine path will usually be the inlet guide vanes, and they'll be chocked for the vast majority of real operating conditions (M=1). Since the gas is expanding, cooling, and slowing all the way through the turbine from there, you shouldn't get shock on any of the turbine blades.

There were experiments with supersonic compressors (pressure increases across a shockwave, so it's not a terrible idea) a while back, but you pay too much efficiency penalty in the shockwave to make it work out.

Tom.


User currently offlinePhollingsworth From United Kingdom, joined Mar 2004, 825 posts, RR: 5
Reply 25, posted (5 years 1 week 5 days 6 hours ago) and read 6158 times:



Quoting EcuadorianMD11 (Reply 23):
I´m not sure I get this, forgive the ignorance!
It´s a bit nosiy here......can´t think straight perhaps!

But how doesn´t this affect in production or in service aircrafts please?

No problem, I probably should have been more explicit. A new Chapter/Stage, e.g. Chapter IV, only affects aircraft certified after its effective date. In other words aircraft that were already certified/in production don't have to meet the new regulation. So the 777 which was certified in 1995 is a Chapter/Stage III aircraft. The A380 is a Chapter IV aircraft, because it was certified after Chapter IV came into effect. Now, as I mentioned the Chapter IV rules are fairly simple the new limits are Chapter III -10dB cumulative, a minimum of -2 dB on any two certification points, and no louder than the Chapter III limit on the loudest certification point. Now the 777 is a Chapter III aircraft, but it actually would meet the Chapter IV limits if it had to be re-certified (so would every other western commercial transport in production today).

There are three types of policies/regulations that come out of the ICAO/CAEP process:
1. Certification - applies only to aircraft certified after the effective date
2. Production - applies to aircraft produced after the effective date
3. Operation - applies to aircraft in operation after the effective date

Almost all of the ICAO policies are certification policies. Some exceptions are the Chapter II phaseout, which effectively requires operational aircraft to meet the Chapter III limits (though they did not have to be fully re-certified as Chapter III), and the ICAO CAEP/2 emissions rule which is a production rule.

Quoting Tdscanuck (Reply 24):
You shouldn't. The fastest point in the turbine path will usually be the inlet guide vanes, and they'll be chocked for the vast majority of real operating conditions (M=1). Since the gas is expanding, cooling, and slowing all the way through the turbine from there, you shouldn't get shock on any of the turbine blades.

There were experiments with supersonic compressors (pressure increases across a shockwave, so it's not a terrible idea) a while back, but you pay too much efficiency penalty in the shockwave to make it work out.

I didn't think so, but didn't want to overstep my knowledge. I knew about the supersonic compressor tests, and in certain conditions some modern aircraft have compressor tips speeds that are slightly above local Mach 1, though most high-bypass fans have tip speeds above Mach one at high thrust conditions. There are cases where you are willing to accept the losses for the extra thrust, but it isn't the most efficient place to operate as you generally want to minimize P0 losses, that and T0 increases in the compression mechanisms (obviously a T0 increase in the combustor is a good idea).


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