ULH flights burn much more fuel
 

As much as 20% more according to a simple comment I read.

Is it really that much?

Example....Perth-London versus Perth-Dubai-London.

I think BOAC would agree, I mean 6 fuel stops down to Joburg is obviously the right thing to do. Non-stop would be a ridiculous idea.

Not ULH, but for illustration: a quick look into integrated planning tables for A333 suggests 11% difference in favour of 2x 4000 NM instead of 8000. (how niche is 8k with 333 I do not know)

There is a rule of thumb that says ~13% penalty for 'tankering' fuel - I'd assume ULH would be similar or perhaps a bit less.

Hang on a minute, you're saying longer flights use more fuel ?

There was a thread hereabouts, and from memory the optimum stage distance was about 4,000 miles.

3% per hour for the tankered fuel, in this case the tankered fuel would be fuel for 4000-8000 nms, so lets say 8 hours x3 = 24% of the fuel.

umm... I believe he's saying that longer flights use more fuel per mile.... but then again what do I know.

Related news: Shipping the kerosene needed for the second half of the flight, to the midway point, is more efficiently done by truck or boat than by airplane. Or so I've been told.

But you also have to add in the fuel for terminal operations, taxi, and climbout form the stopover point.

Unless you have detailed fuel plans for each scenario under identical conditions, I don't think you can generalize.

This one ?

http://www.pprune.org/tech-log/14857...g-formula.html

It's exponential, There's no fixed %.

The longer the flight the higher the average percentage.

Yes.

Depending on the efficiency of the airframe, there will be a point where adding more fuel has mathematically virtually no effect on range. We don’t (can’t) operate in this area but the curve is beginning to steepen in the latter half of ULH sectors, so the increase in total fuel burn to go an extra hour at the end is considerably more than you might expect assuming linear behaviour.

I haven't done the sums, but it seems likely that there is a point where adding more fuel actually reduces the range. That assumes sufficient fuel tank capacity, of course.

I'd be fascinated to see any calculations that show that.

Well, if you load an infinite amount of fuel the airplane becomes infinitely heavy. So you need infinite thrust just to get it to move. And for that you need an infinite amount of fuel - all the fuel you loaded will be gone the moment you attempt to start taxiing;-)

The devil is, of course, in the details, and this doesn't take into account taxi, climb, terminal ops, etc. but....

If for a given airplane the cost of tankering fuel is x% per hour (say f'rinstance 3%), meaning that it takes 3 lbs per hour incremental burn to carry 100 incremental lbs of fuel, then a hard theoretical limit of flight duration is going to be 1.0/x hours (say f'rinstance 33 hours).

Fuel your aircraft for 33 hours endurance. Add 1,000 lbs of fuel, and, at the end of 33 hours you'll still be bingo fuel because you will have burned the whole 1,000 lbs carrying the extra fuel.

Mumble, mumble, double integrals, diffe-q, mumble mumble this is, as is the "x% per hour for tankering" rule of thumb, an oversimplification.

Will you ever get to the point that adding fuel reduces your range? I dunno, the only experience I have with Calculus these days is that recurring dream in which you show up for the final exam, only to realize that you didn't go to any of the lectures, didn't read the book, are stark naked, and forgot to bring a pencil....

Conversation years ago with a mathematically-inclined 12 year old who was enthusiastically learning the concepts of infinity:

Q: "If you had a pie, and an infinite number of people lined up, and each person in line, when it was his turn, took half the remaining pie, when would the pie be gone?"

A: "When the whole line had had their turn!"

No, as a simple thought experiment will show.

Hopefully I can leave that to the interested reader.

Tankering fuel means arriving with the extra weight. That is not the same as arriving having burned it.

You need to add the fuel to the start of the flight not the end.

However, every aircraft will have a sweet design spot and simply adding more fuel tanks and reducing traffic for the same weight will of course use more fuel per passenger mile.

Funnily enough I was talking about this the other day. On some (extremely rough) calculations on the DXB-AKL vs DXB-SYD-AKL

DXB-AKL- 231T
DXB-SYD-AKL 225T

Like I said, rough calculations, but the 6T of fuel saved is roughly 30 minutes of flying.

Perhaps someone could give more accurate figures