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Airbus A340 - Why A Quad And Not A Trijet?  
User currently offline1337Delta764 From United States of America, joined Oct 2005, 6474 posts, RR: 2
Posted (5 years 4 months 2 weeks 4 days 12 hours ago) and read 6416 times:

I was wondering, does anyone why Airbus chose to go with a quad engine design for the A340 vs. a trijet? I know that Airbus wanted to make the aircraft free from ETOPS restrictions, however, couldn't Airbus have used a trijet design to accomplish that?


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30 replies: All unread, showing first 25:
 
User currently offlineCanadianNorth From Canada, joined Aug 2002, 3389 posts, RR: 9
Reply 1, posted (5 years 4 months 2 weeks 4 days 12 hours ago) and read 6415 times:

I'm going to be guessing here, but I'd say
a) commonality with A330
b) tri-jet as we know it means an engine way up in the tail, which means maintenance activities that would normally only require a flashlight and a screwdriver would suddenly need ladders/trucks/etc.
c) weight of all the fuel lines etc to the tail engine as well as reinforcement of the structure would probably cancel out weight savings of not having a fourth engine.



CanadianNorth



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User currently onlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 2, posted (5 years 4 months 2 weeks 4 days 9 hours ago) and read 6346 times:

This has been discussed ad nauseam in many threads. CanadianNorth has it right.

The real question should be "Why a quad and not a twin?" I guess the answer to that is the fuel savings (at the time) on ultra long routes, as well as the unavailability of an engine powerful enough to make a 777 type aircraft (remember that the 330/340 are almost a decade older).



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLAXDESI From United States of America, joined May 2005, 5086 posts, RR: 48
Reply 3, posted (5 years 4 months 2 weeks 3 days 22 hours ago) and read 6164 times:

Why do the engines have the same thrust on a quad? Can there be an advantage to offering two different thrust levels with smaller set of engines optimized for take-off and the other larger set optimized for cruise?

Would it be possible to operate the aircraft on the larger engines during cruise for potential fuel saving? I am thinking of some autos that use fewer cylinders at higher speed.


User currently offlineSAA380 From South Africa, joined Mar 2008, 213 posts, RR: 0
Reply 4, posted (5 years 4 months 2 weeks 3 days 21 hours ago) and read 6136 times:



Quoting Starlionblue (Reply 2):
The real question should be "Why a quad and not a twin?" I guess the answer to that is the fuel savings (at the time) on ultra long routes, as well as the unavailability of an engine powerful enough to make a 777 type aircraft (remember that the 330/340 are almost a decade older).

Yes, and remember the A332/3 and A342/3 have almost exactly the same fuselages.
They were both developed the same time. I am thinking that they probably had to modify the existing fuselage of the A340 if they wanted to make it a trijet, and differ it from the A330, bringing the production costs up. It also seems unnecessary due to reasons CanadianNorth mentioned.
To me it just seems unnecessary to change an existing fuselage to make it a trijet,
instead of a 4 holer.

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User currently offlineMetroliner From United Kingdom, joined Jan 2007, 1067 posts, RR: 1
Reply 5, posted (5 years 4 months 2 weeks 3 days 15 hours ago) and read 6017 times:

Or they could have just accepted an asymmetrical thrust situation and chucked a third engine on somewhere between #2 and the fuselage...  Wink

Or attempted some crazy asymmetrical design à la Rutan or Blohm und Voss...



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User currently onlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 6, posted (5 years 4 months 2 weeks 3 days 13 hours ago) and read 5995 times:



Quoting LAXDESI (Reply 3):
Why do the engines have the same thrust on a quad? Can there be an advantage to offering two different thrust levels with smaller set of engines optimized for take-off and the other larger set optimized for cruise?

Nah. Too complex. The costs don't make it worth it. The closest I can think of are the thrusting APU concepts that crop up now and again, as well as the 4th engine on later versions of the Trident. Cheaper and simpler to go with same size engines.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMir From United States of America, joined Jan 2004, 21558 posts, RR: 55
Reply 7, posted (5 years 4 months 2 weeks 3 days 12 hours ago) and read 5974 times:



Quoting Starlionblue (Reply 2):
(remember that the 330/340 are almost a decade older).

The difference isn't nearly that big. The 330 was started in 1987, first flying in 1992 - similar dates for the 340. The 777 was started in 1988 and first flew in 1994.

Had Airbus waited a few more years, the engines would have been there for them to make the 340 a twin. But then the airplane would have looked a lot different, so....

I guess we'll never know.

Quoting LAXDESI (Reply 3):
Why do the engines have the same thrust on a quad? Can there be an advantage to offering two different thrust levels with smaller set of engines optimized for take-off and the other larger set optimized for cruise?

First of all, having the same engines makes maintenance a lot simpler. There might be some benefits, but in the end, the added complexity kills it. Second of all, you don't want to have the wrong sort of engine fail at any point in the flight.

-Mir



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User currently offlineLAXDESI From United States of America, joined May 2005, 5086 posts, RR: 48
Reply 8, posted (5 years 4 months 2 weeks 3 days 11 hours ago) and read 5957 times:



Quoting Mir (Reply 7):
First of all, having the same engines makes maintenance a lot simpler. There might be some benefits, but in the end, the added complexity kills it.

As I understand, the total weight of quad engines, all else being equal, to produce a given amount of thrust is greater than that of twin engines producing the same thrust.

But are the bigger twin engines less fuel efficient in cruise? By shutting off the smaller set of quad engines during cruise, wouldn't one achieve lower fuel burn? If yes, could it be enough to offset the additional complexity/cost of two different types of engines on a quad?


User currently onlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 9, posted (5 years 4 months 2 weeks 3 days 8 hours ago) and read 5922 times:



Quoting Mir (Reply 7):

Quoting Starlionblue (Reply 2):
(remember that the 330/340 are almost a decade older).

The difference isn't nearly that big. The 330 was started in 1987, first flying in 1992 - similar dates for the 340. The 777 was started in 1988 and first flew in 1994.

Point taken. One reason for the quad may be that Airbus wanted to straddle a wider market segment with the 330/340 than Boeing with the 777. And at the time the quad for long haul made sense, with lower fuel consumption and fewer ETOPS issues.

Agreed that if they had waited a few more years, there probably only would have been a quad.

There is also the fact that the 330 and 340 aren't really two separate aircraft. More like 1½ families than 2. So the development cost was nowhere near what would have been required for two families.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (5 years 4 months 2 weeks 3 days 7 hours ago) and read 5908 times:



Quoting LAXDESI (Reply 3):
Would it be possible to operate the aircraft on the larger engines during cruise for potential fuel saving? I am thinking of some autos that use fewer cylinders at higher speed.

It's possible, but the market strongly suggests that the incremental fuel savings wouldn't cover the extra maintenance, spares, and development costs.

Quoting LAXDESI (Reply 8):
As I understand, the total weight of quad engines, all else being equal, to produce a given amount of thrust is greater than that of twin engines producing the same thrust.

True.

Quoting LAXDESI (Reply 8):
But are the bigger twin engines less fuel efficient in cruise? By shutting off the smaller set of quad engines during cruise, wouldn't one achieve lower fuel burn? If yes, could it be enough to offset the additional complexity/cost of two different types of engines on a quad?

This is something of a loaded question. Engine efficiency improves with pressure ratio, which goes up with thrust. So running a fewer number of engines harder will generally improve efficiency. However...larger engines have an inherent design advantage because they can hold tighter relative clearances in the gas path. So it's entirely possibly that a very large engine running at, say, 50% thrust might have higher efficiency than a smaller engine running at 65% thrust.

There's also the issue of technology leaps...engine generations gain a lot in efficiency, so that may completely overshadow their comparison to a smaller but older competitor.

Tom.


User currently offlineRheinwaldner From Switzerland, joined Jan 2008, 2222 posts, RR: 5
Reply 11, posted (5 years 4 months 2 weeks 3 days 4 hours ago) and read 5872 times:



Quoting LAXDESI (Reply 8):
As I understand, the total weight of quad engines, all else being equal, to produce a given amount of thrust is greater than that of twin engines producing the same thrust.

That is true in reality but to me it is far from clear why it MUST be that way. If weight and thrust would scale up proportionally it would mean that this statement would not be true. IMO it should be possible to have a more or less constant engine-weight/thrust ratio at different thrust classes.

The key advantage for the quad is in this:

- Basic assumption: all other things are equal (given in case of A343 and A333)
- Requirement which is the driver: taking the twin design to longer ranges
- From this follows the need to increase the MTOW
- From this follows the need to have more take off thrust
- And here comes the key: the minimum take off thrust must be available even in case of one-engine-off: now the quad shines. The excess thrust that the quad needs to cope with the engine-off requirement is 33%. The excess thrust that the twin needs is 100%.

This all means that a quad achieves a higher MTOW at a lower total installed thrust than a twin (the twin would have the double of the minimally required take off thrust). This principle is universally valid. You could take any twin, leave anything like it is, and make a quad out of it (ok, the structure and aerodynamics must support the higher MTOW). The result would be a plane that sacrifices some efficiency but gains range.


User currently offlinePlaneWasted From Sweden, joined Jan 2008, 516 posts, RR: 0
Reply 12, posted (5 years 4 months 2 weeks 2 days 22 hours ago) and read 5815 times:

Wasn't the A340 supposed to use some kind of new engine technology, that never worked out? Maybe the engines wasn't available with more thrust.

User currently offlineLAXDESI From United States of America, joined May 2005, 5086 posts, RR: 48
Reply 13, posted (5 years 4 months 2 weeks 2 days 21 hours ago) and read 5803 times:



Quoting Rheinwaldner (Reply 11):
Quoting LAXDESI (Reply 8):
As I understand, the total weight of quad engines, all else being equal, to produce a given amount of thrust is greater than that of twin engines producing the same thrust.

That is true in reality but to me it is far from clear why it MUST be that way. If weight and thrust would scale up proportionally it would mean that this statement would not be true. IMO it should be possible to have a more or less constant engine-weight/thrust ratio at different thrust classes.

It seems that there is economy of scale, at least for current engines/technology, when it comes to thrust/engine weight.

Quoting Rheinwaldner (Reply 11):
And here comes the key: the minimum take off thrust must be available even in case of one-engine-off: now the quad shines. The excess thrust that the quad needs to cope with the engine-off requirement is 33%. The excess thrust that the twin needs is 100%.

Looks like Tom has an explanation:

Quoting Tdscanuck (Reply 10):
Engine efficiency improves with pressure ratio, which goes up with thrust. So running a fewer number of engines harder will generally improve efficiency. However...larger engines have an inherent design advantage because they can hold tighter relative clearances in the gas path. So it's entirely possibly that a very large engine running at, say, 50% thrust might have higher efficiency than a smaller engine running at 65% thrust.



User currently onlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 14, posted (5 years 4 months 2 weeks 2 days 14 hours ago) and read 5685 times:



Quoting PlaneWasted (Reply 12):
Wasn't the A340 supposed to use some kind of new engine technology, that never worked out? Maybe the engines wasn't available with more thrust.

Yes, the P&W "Super Fan".



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (5 years 4 months 2 weeks 2 days 12 hours ago) and read 5669 times:



Quoting Rheinwaldner (Reply 11):
That is true in reality but to me it is far from clear why it MUST be that way. If weight and thrust would scale up proportionally it would mean that this statement would not be true. IMO it should be possible to have a more or less constant engine-weight/thrust ratio at different thrust classes.

Because weight and thrust don't scale proportionally. A GE90-115B is 3.5 times heavier than a CFM56-7B but produces 4.2 times the thrust, for example. The basic reason that it's not constant for all thrusts is that several components don't scale with thrust...a lot of your infrastructure (e.g. EEC, bleed air control, etc.) doesn't inherently scale with thrust and has a minimum practical size. Things like the fan case are sized much more by fan rotational energy (function of material, fan speed, number of blades, etc.) than by thrust. Big engines also get an inherent efficiency gain from tighter clearances, therefore can extract more power with smaller (lighter) components.

Quoting PlaneWasted (Reply 12):
Wasn't the A340 supposed to use some kind of new engine technology, that never worked out? Maybe the engines wasn't available with more thrust.



Quoting Starlionblue (Reply 14):
Yes, the P&W "Super Fan".

I thought it was IAE, but it was definitely called the SuperFan. The long-lost ancestor to the GTF.

Tom.


User currently offlineLAXDESI From United States of America, joined May 2005, 5086 posts, RR: 48
Reply 16, posted (5 years 4 months 2 weeks 2 days 11 hours ago) and read 5658 times:



Quoting Tdscanuck (Reply 15):
Because weight and thrust don't scale proportionally. A GE90-115B is 3.5 times heavier than a CFM56-7B but produces 4.2 times the thrust, for example.

Isn't GE90-115B engine superior technology relative to CFM56-7B? I think your point about thrust/weight ratio rising with bigger engine would still stand for a given technology.


User currently onlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 17, posted (5 years 4 months 2 weeks 2 days 11 hours ago) and read 5655 times:



Quoting Tdscanuck (Reply 15):
Quoting Starlionblue (Reply 14):
Yes, the P&W "Super Fan".

I thought it was IAE, but it was definitely called the SuperFan. The long-lost ancestor to the GTF

My bad. It was IAE.  embarrassed 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 18, posted (5 years 4 months 2 weeks 2 days 9 hours ago) and read 5627 times:



Quoting LAXDESI (Reply 16):
Isn't GE90-115B engine superior technology relative to CFM56-7B?

Kind of a tough call...the latest incarnation of the CFM56-7B (the "Tech Insertion" engine) is only about two years old, and that included some pretty new stuff like 3D airfoils, I believe. I was trying to think of a true apples-to-apples comparison (two new engines with widely different thrusts from the same manufacturer at the same time) but I couldn't think of any.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19513 posts, RR: 58
Reply 19, posted (5 years 4 months 2 weeks 1 day 8 hours ago) and read 5452 times:



Quoting Mir (Reply 7):

The difference isn't nearly that big. The 330 was started in 1987, first flying in 1992 - similar dates for the 340. The 777 was started in 1988 and first flew in 1994.

You also have to remember that in 1987 ETOPS was a rather outlandish idea. For a long time, Airbus marketed the A340 vs. the 777 based on a perceived safety benefit.

Airbus is very good at making planes, but other than the A300, they don't seem to be quite as good at thinking outside the box as Boeing.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (5 years 4 months 2 weeks 1 day 7 hours ago) and read 5440 times:



Quoting DocLightning (Reply 19):
Airbus is very good at making planes, but other than the A300, they don't seem to be quite as good at thinking outside the box as Boeing.

I'm not sure that's fair to Airbus...putting FBW on the A320 was way outside the box for the time.

Tom.


User currently offlineKappel From Suriname, joined Jul 2005, 3533 posts, RR: 17
Reply 21, posted (5 years 4 months 2 weeks 16 hours ago) and read 5353 times:



Quoting Tdscanuck (Reply 20):
I'm not sure that's fair to Airbus...putting FBW on the A320 was way outside the box for the time.

Indeed. They also led Boeing in the use of composites for a long time.



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User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (5 years 4 months 2 weeks 14 hours ago) and read 5326 times:



Quoting Kappel (Reply 21):
They also led Boeing in the use of composites for a long time.

Primary structure composites, yes. Composites in general, no. The original 747 had composite variable camber Krueger flaps before the A300 even existed.

Tom.


User currently offlineTheSonntag From Germany, joined Jun 2005, 3564 posts, RR: 29
Reply 23, posted (5 years 4 months 2 weeks 13 hours ago) and read 5318 times:



Quoting Tdscanuck (Reply 20):

I'm not sure that's fair to Airbus...putting FBW on the A320 was way outside the box for the time.

Add to this that Airbus actually was the first company to develop a common design philosphy for all of its airplanes. A320, A330, A340, A380 and A350 all have the same basic cockpit design. This will be the case for Boeing in the future, as the B777 and B787 will share a lot, but it is nowhere as much the same yet. There are still a lot of differences between a B747, 737, 767 and 777.

While this commonality should not be overrated, it was quite revolutionary nevertheless.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19513 posts, RR: 58
Reply 24, posted (5 years 4 months 2 weeks 8 hours ago) and read 5276 times:



Quoting Rheinwaldner (Reply 11):


This all means that a quad achieves a higher MTOW at a lower total installed thrust than a twin (the twin would have the double of the minimally required take off thrust). This principle is universally valid. You could take any twin, leave anything like it is, and make a quad out of it (ok, the structure and aerodynamics must support the higher MTOW). The result would be a plane that sacrifices some efficiency but gains range.

I give you the 77L. A twin with some serious range. And far more efficient than its A345 competitor.

I give you the 77W. A twin with some serious capacity. And far more efficient than its A346 competitor.

All in all, twins keep on winning these contests.

If the A380 or 747 could be made a twin, they would have done so.

Every Boeing aircraft since the 747 has been a twin. This is for a good reason.


25 Mandala499 : The same wing aswell... Airbus wanted to minimize the development cost out for the 2 types, so they really maximized commonality here... Not only the
26 SEPilot : And I suspect all future ones will be as well. Not only that, all future Airbuses will be twins as well. With the power and reliability of modern jet
27 FLY2HMO : Aside from what everybody else has pointed out, there's another thing that you have to take into account: drag. Those two smaller engines would turn
28 DocLightning : Until such a time as no engine failures have occurred in centuries, there will be no single-engine commercial aircraft. The monetary cost of a crash
29 SEPilot : That's why I said "never."
30 Viscount724 : Quads still have certain performance benefitrs at hot and high airports and where terrain clearance after an engine failure at or soon after takeoff
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