D-AIGV Arriving into the Motor City from Frankfurt - 06 NOV 2024

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SherryJug@reddit

The only widebody to use the CFM56!

Fun fact: The A34O-300 is actually quite competitive in regards to fuel burn. According to various sources, it has better fuel burn per passenger mile than similar sized aircraft including the A330-300 and 777-200ER. (Though this is of course not the case when compared with more modern aircraft like the 787, A350 or A330-900neo).

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Bjoerska@reddit

Do you still have the source? I would like to read it!
Thx!

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SherryJug@reddit

From Airbus : "The A340-300 provides the best economy in its class, reaching a 5 per cent lower fuel per trip compared to competing aircraft."

Some dude stating it is official LH data on Airliners.net (trust me, dude): "Aircraft, Fuel Consumption [litres / 100 PKM] A330-300 4.10 A340-300 4.02 A340-600 4.21 B747-400 4.27"

Nothing incredibly trustworthy, especially because it's so difficult to define and compare real fuel burn anyway (Wikipedia indicates higher fuelburn for the A343, but their source is long gone).

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HokieAero@reddit

I wonder what conditions and assumptions went into this comparison. Why would the A340-300 have better fuel consumption per pax-km than an A340-600? That's strange.

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SherryJug@reddit

Oh, the A343 undoubtedly has better fuel consumption (probably much better) than the A346.

The 346 is a big gas guzzler and went out of favor relatively quickly because of it.

There are several key factors for this, all of them are related to the kind of engines they use and their MTOW.

The CFM56 used by the 340-300 are optimally sized for fuel burn, that is, they are pretty much the smallest, least powerful engines they could put on the 340-300 while attaining just enough thrust to certify it. Being so small compared to the much larger Trent 553 and 556, the CFM56 just adds less drag and weight, and brings the extra advantage that they work at higher power settings the majority of the time, which depending on the design of the specific engine might be more efficient.

Idle and taxiing fuel burn also has an effect on the overall fuel burn. The CFM56, being a small narrowbody engine, burns relatively little fuel during these phases, especially compared to much bigger typical widebody engines.

But ultimately, it has a lot to do with MTOW. While the 300 variant has an MTOW of around 276.5 tons, the 500 and 600 variants are up to 100 tons heavier at 380 tons! This is an increase of around 30%, while they certainly do not carry 30% more passengers. Part of it is just the added weight from the much heavier engines. It also doesn't help that the engines are quite oversized, in line with other 4-engine widebodies of the era (despite the increase in MTOW being around 30%, the thrust of the Trent 553 and 556 is almost 80% more than that of the CFM56! You could argue then that those engines are quite overpowered for the aircraft).

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HokieAero@reddit

Perhaps - but the numbers quoted are not outright fuel burned but per PKM, which is per passenger-km. So on a per seat basis, the stretch is almost always better than the shorter fuselage variant.

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SherryJug@reddit

No, per passenger kilometer is just per seat kilometer / load factor (a.k.a. occupancy rate).

So unless there are dramatically more seats vacant on average in the larger variants, which is probably not the case, the fuel burn per seat is still lower on the A340-300.

I don't really understand where you get the idea that the larger variant should be more efficient. This might be true for aircraft in which both variants use the same or very similar engines, but in this case the smaller variant has much lighter, more efficiently sized engines and a much lower MTOW (the difference in MTOW is much larger than the difference in passenger capacity, which straightforwardly leads to a better fuel burn per seat)

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HokieAero@reddit

I disagree. When an airline does a comparison like this, they do it with full pax+bags (or possibly a chosen load factor). Have to compare apples to apples. And in my experience fuel burn per seat always favors the stretched variant. That's why it seems strange that the A340-300 is quoted better numbers than the A340-600. And the A340-300 having better fuel burn than the A330-300 seems really wonky.

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SherryJug@reddit

Well, it's not that difficult to play a bit with the variables in the flight mechanics equilibrium equations to see why the case of the A340-300 is indeed different than most other aircraft models.

But thanks for your input and have a nice day!

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IsetfireIzetfire@reddit

This makes me wonder, would a A340Neo with LEAP engines be viable? Like, I mean would there be a scenario that an airline would choose it over a 359 or 78X other than etops?

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HokieAero@reddit

Possibly - but only if an airline really really wants the safety of a 4-engine aircraft and is willing to pay the extra costs.

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Wernher_VonKerman@reddit

I heard the opposite, but that it was still more efficient than the MD-11.

However, it makes sense if you consider the smaller engines are probably operating close to their individual maximum thermodynamic efficiency.

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EtwasSonderbar@reddit

That's what happens when you equip an airliner with hairdryers rather than turbofans.

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tjis_christ@reddit

Where did you take this picture from, I am interested in going up there but I haven’t heard of any good spots?

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Delta95Heavy@reddit (OP)

East side of DTW at the intersection of Middlebelt Rd. and Goddard. There’s an abandoned gas station on the corner (long since gone now, nothing there but the driveway and grass).

Offers a great view of the approach end of runway 21L with great lighting during the morning and early afternoon arrivals. Later in the day it’s backlit.

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