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1337Delta764
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Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 12:50 am

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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CanadianNorth
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 12:55 am

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.



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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 4:15 am

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).
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 3:11 pm

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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 4:35 pm



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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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 10:33 pm

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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RE: Airbus A340 - Why A Quad And Not A Trijet?

Wed Apr 08, 2009 11:55 pm



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.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 1:16 am



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.

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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 2:37 am



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?
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 5:34 am



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.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 6:16 am



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.

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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 9:26 am



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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 3:24 pm

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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 3:54 pm



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.

 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Thu Apr 09, 2009 11:17 pm



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".
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Fri Apr 10, 2009 1:00 am



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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Fri Apr 10, 2009 2:16 am



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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Fri Apr 10, 2009 2:27 am



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 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Fri Apr 10, 2009 3:51 am



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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sat Apr 11, 2009 4:44 am



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.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sat Apr 11, 2009 5:57 am



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.

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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sat Apr 11, 2009 9:11 pm



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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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sat Apr 11, 2009 11:20 pm



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.

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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sun Apr 12, 2009 12:30 am



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.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sun Apr 12, 2009 5:31 am



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.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sun Apr 12, 2009 11:55 am



Quoting SAA380 (Reply 4):
Yes, and remember the A332/3 and A342/3 have almost exactly the same fuselages.

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 body (sans the center gear) but the wing, the A330 has the plumbing for 4 engines structurally... you can't do this if one was a tri-jet... and the costs would be enormous...

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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sun Apr 12, 2009 12:20 pm



Quoting DocLightning (Reply 24):

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

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 engines the argument for more than two engines disappears, and the argument for less than two will never overcome the redundancy requirement.

Quoting Mandala499 (Reply 25):
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 body (sans the center gear) but the wing, the A330 has the plumbing for 4 engines structurally... you can't do this if one was a tri-jet... and the costs would be enormous...

Let's face it; a trijet would be a different airplane. Airbus wanted to offer a twin and a quad with essentially the same airframe, and they did it.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Sun Apr 12, 2009 6:12 pm



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.



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?

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 into big windmills when shut down, and windmilling engines create a lot of drag. Even if you somehow created a system to feather the fan blade (which would probably weigh a lot as well) you're still hauling the dead weight from 2 engines that aren't doing anything useful during cruise.
 
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Mon Apr 13, 2009 4:43 am



Quoting SEPilot (Reply 26):

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 engines the argument for more than two engines disappears, and the argument for less than two will never overcome the redundancy requirement.

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 is astronomical. The human cost of a crash is astronomical. Nobody, not even the most miserly bean counter, wants to risk it.
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Mon Apr 13, 2009 1:30 pm



Quoting DocLightning (Reply 28):
Until such a time as no engine failures have occurred in centuries, there will be no single-engine commercial aircraft.

That's why I said "never."
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RE: Airbus A340 - Why A Quad And Not A Trijet?

Fri Apr 17, 2009 12:40 am



Quoting DocLightning (Reply 24):
Every Boeing aircraft since the 747 has been a twin. This is for a good reason.

Quads still have certain performance benefitrs at hot and high airports and where terrain clearance after an engine failure at or soon after takeoff is an issue. I believe the engine failure issue is also a factor on certain routings that overfly areas like the Himalayas with mountains up to 29,000 ft.

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