airsurfer

I am curious about the amount of fuel a passenger (i.e. per passenger and per km / nm) km costs with a modern long-range jet (A380 / 777 / 787 / A350) compared with the first generation wide bodies like 747-100, DC10-30 or L-1011 or even narrow bodies like 707 or DC8.

superangrypenguin

You mean CASM?

You can look them up for the airline that you're interested in. There are a LOT of factors that are not related to fuel.

Efrem

Here's a factoid for you:

Before Boeing designed the 787, they went to airlines with a proposal called the Sonic Cruiser. Its selling point, relative to other aircraft then in the air, was "15 percent faster, burns the same amount of fuel." The airlines said "If you can do that, you can also do 'same speed, burns 20 percent less fuel.' Now go do it."

That's why we have the 787.

sbm12

http://www.airliners.net/aviation-fo...d.main/3647392
http://www.boeing.com/commercial/aer...ConsSeries.pdf

Just a couple I found quickly searching online.

invisible

Wondering why nobody yet have proposed idea of having speed cut by half if it also means burning half of fuel.

Oh wait, Solar Impulse's average speed is 28mph. Here is future of aviation.

Kevin AA

A 737 can't fly at half of cruise speed unless it flies at a much lower altitude, and then your fuel savings are gone.

invisible

No, seriously, just a thought experiment - do you think if there would be a technology cutting half of fuel price at the expense of increasing travel time by the same amount - will it have market opportunity?

Imagine that SFO-JFK flight would take 10h but twice less fuel.

eigenvector

I doubt it. As you decrease speed, you also increase other costs. Your capital costs go up because now you need twice as many planes (assuming your total passenger miles stay the same, you need twice as many planes if each plane goes half the speed). Your operational costs go up because your crews are working twice as many hours (they actually go up by even more than 2x because you will need 3 or 4 pilots rather than 2). Now that your SFO-JFK is a long haul flight, you need lie-flat seating in business class and full meal service in economy.

invisible

So when/where exactly savings from fuel become insignificant in light of other cost increase?

In other words, is the current configuration is the most optimal from cost perspective?

CPRich

With current aircraft design, flying either faster or slower than the optimum cruising speed burns more fuel in aggregate. Aero drag if you're going faster, increased drag from increased lift needed from the wing if you're going slower (parasitic and induced drag).

If you're open to replacing all of the current planes with new designs, turbofans and other technology can be more fuel efficient, but they will fly more slowly and have the associated costs/impacts noted above. Or we can go back to turbo props.

It's obviously much more complicated, but that's a start (but I do still have my aero, gas dynamics, thermo, etc., textbooks if you'd like to dig in )

Proudelitist

I think a HUGE factor, even in the same aircraft, was the development of the FMS. Prior to this, crews would calculate burn manually. The FMS calculates all that now, plus accounting for V speeds, climb angle, wind factors etc. In short, the computer did it better. And on the more modern planes it also make constant subtle adjustments to the engines to maintain an almost surgical level of efficiency.

Add to that the more efficient engines we have now, and the lighter materials used and it is clear that there is a huge difference from decades past.

YVR Cockroach

The economical speed issue applies not just to planes. On my recent container ship voyage, I talked to the ship engineers who told me that the ship built around, 2010 (one of 8-9 pretty identical sister ships), has one of the biggest ship engines (rated by horsepower) in the world, ever. Newer ships, ordered by the same line just a few years later (2012-2013) have engines about 10-15% less HP but carry substantially more (17-18,000 TEU vs ~11,300 TEU). The ship I was travelling at was cruising across the Pacific at 21 knots. That's the top speed of the newer ships. Oil prices at time of design and ordering probably has a lot of correlation to maximum and economic cruise speed design.

As for speed of aircraft, they can be designed to fly more economically and slower, but at a cost (capita and other operational costs as mentioned in another post but also cost of competition vs. other airlines). The 757 and 767 were both designed during one of the oil shocks (early/mid '70s) and have a slightly-slower optimal cruise speed than an earlier design, the 747 (designed in the early '60s when fuel was cheap), or even the 727 (even earlier design when fuel prices were even lower).

Efrem

And to cars, as well. Most cars get the best steady-speed gas mileage at whatever speed let the engine turn over at just above the point where torque drops off substantially in its highest gear.

Today's factoid: Many people think their cars get better mileage at higher speeds. They confuse the fuel-saving effect of driving on a highway with modest grades, no sharp turns, and no cross streets or other reasons to stop or slow down versus driving on city streets with all the interruptions of doing that. They know they get better mileage at 75 than at 40, but they don't understand that it isn't because the car inherently gets better mileage at 75 than at 40. It doesn't. (If you are one of the many who fool themselves this way, sorry to burst your bubble. Feel free to forget this post and keep on deluding yourself that your car gets its best mileage at 75, if thinking that makes you happy.)

Sheikh Yerbooty

Eh, no.

The FMS is a tool that tells the autopilot and auto throttle what to do; climb, descent and turn. It has a very large built-in nav database, enabling precision navigation between fix points, rather than flying radar vectors or VOR bearings. It receives input from nav radios, an inertial navigation systems and, increasingly frequently, a GPS receiver which may be augmented. The auto-throttle basically operate in 'take'off', 'climb', 'cruise' and 'descent' modes, or slight variations thereof. All are, however, more or less fixed settings where the main variable is weight of the aircraft. Whether that thrust is set automatically or manually is immaterial; the engines will produce the same thrust and burn the same fuel. It's beyond argument that auto throttles makes a pilots life easier, and with constant adjustments it is able to outwit a human. But not to the degree you are implying, not even by a long shot.

And whilst the FMS does 'calculate burn', it would do so regardless of which engines you hooked it up to. Moreover, in the greater scheme of things a well-drilled F/E could whisk up fuel numbers to match any FMS within a couple hundred pounds. And that is, as they say, close enough for government work.

No, the biggest difference between 1970 and now are construction techniques (making stuff lighter, stronger and simpler), engine technology (burning less for the same oomph) and aerodynamics (less draggy, optimised lift, FBW). Of those, engine developments have produced the biggest gains.

ft101

Your capital costs are depreciated over the life of the plane. This lifetime would double with the cycle rate being halved, so no change overall.