jsgoldbe

Jet fuel prices vary across the US, so I imagine they vary around the world. How do I know? I was listening to Ch 9 on ORD-SFO a couple of months ago and the captain was giving a short lecture on how much fuel we were using, etc. He explained that we had on board more fuel than than we needed, because we were tankering the fuel due to fuel costs being less in ORD than SFO. The extra weight was hurting our fuel economy slighly, but the loss was justified by the price differential. Interesting stuff!

N227UA

This is not a simple math that we can just figure out by having some algebra.

Since SFO/LAX-SYD flight takes off with almost full fuel capacity, the plane should consume relatively much pound per minute. As it flies, the fuel reduces, then less pound per minute or so.

It should be able to be calculated with some fancy programs for engineering, and I was wondering if any of you have it.

I just came back from SYD to SFO, and now sitting at the RCC in T3. We flew exactly over Nadi and Honolulu, and I wondered why UA doesn't make a fule stop at HNL.

I believe a fule stop at HNL can save some money. BTW, Sydney was wonderful. At least, much better than hell Los Angeles.

indo79

Please explain how a fuel stop can save money? Why were nonstops pioneered in the first place? Taking off and landing would mean not cruising at the most efficient speed and altitude.

I would be most interested to hear your theory/explanation

Lurker

And now I think we are getting to the heart of the matter. Surely, take off and climb use a massive amount of fuel compared to cruising (at ever increasing altitudes) across the Pacific ocean?

Therefore, an average fuel burn calculation (gallons or litres or pounds per mile) - as has been used here - is not the same for a long transpacific flight as it is for a short hop.

However, even if I am right, we are quibbling over small amounts when this difference is divided between the 300+ px we agree are paying the surcharge.

Lurker

SEA_Tigger

Would you like to sit on the ground in HNL for 120 minutes to land, taxi, fuel, taxi, and depart again if you didn't have to?

I sure wouldn't, even if I was in F.

This why the 777-200LR sells, even in the face of the A380 - being able to go from just about anywhere to anywhere without needing to "top off the tanks" along the way.

JS

Yes, a fuel stop might save money. I don't have the needed details for an actual SYD-SFO flight on a 747-400, but let's use this as an example:

Amount of fuel burned for a trip of M miles is M * exp(M/10000) + 500

The added 500 pounds is the overhead for climbing to cruising altitude (virtually nothing for landing). The exp(M/10000) describes the increasing average rate of fuel burn over the flight during cruise for longer distances, and the M* that is that average fuel burn per hour times the distance.

some examples:

100 mile trip
601 pounds fuel
6 pounds/mile

1,000 mile trip
1,605 pounds fuel
1.61 pounds/mile

2,500 mile trip
3,710 pounds fuel
1.48 pounds/mile

5,000 mile trip
8,743 pounds fuel
1.75 pounds/mile

8,000 mile trip
18,304 pounds fuel
2.29 pounds/mile

This sounds pretty reasonable, given the poor fuel economy of a 100 mile flight (imagine a 747-400 going just 100 miles), and the increasingly poor fuel economy on long flights that are getting so long, you burn much of the fuel just to carry that fuel.

Now, compare the 5,000 mile flight with two 2,500 mile flights. 8,743 pounds of fuel for the non-stop flight, or 7,420 pounds of fuel for two 2,500 mile flights. Hmmm... some savings there.

It's just an example, and it may be the case that the characteristics of the 747-400 are such that a fuel stop would not save fuel unless the trip is over 12,000 miles (since you would carry almost nothing but fuel), but I really have no idea.

JS

Not really, considering that's 30 minutes or so compared to another 15 hours of cruise. If you make a fuel stop, you make two climbs, but each climb is faster (or at lower fuel burn rates, same thing basically). It adds up to more fuel for climb but not quite double.

It doesn't matter if the economics are against you (and in large enough $$$ amounts). People just loved crossing the Atlantic in 3 hours rather than 8, but the economics of the Concorde didn't make up for the convenience.

If you gave people the option of paying $1,000 for a SFO-SYD trip with a stop in HNL or $1,200 for a non-stop flight, there will be people stopping in HNL, including me!

indo79

JS

Thanks for your explanation and I can see eye to eye with most of the points except the above. You mention that the rate of fuel burn increases over the flight. But when a 747 reaches its most efficient cruising speed and altitude, shoudn't fuel burn remain constant or perhaps even decrease?

Maybe I do not know airplanes as much as I should so I thank you in advance for your enlightenment.

N227UA

Nice explanation, JS! I would also have a stop at HNL. 7500mi was a too long flight for me. SFO-SYD is the probably the longest flight among that I've ever taken. It'll give me some extra miles too.

Speaking of climb, yes planes do consume ton of fuel. However think about descend as well.

N227UA

Try Flight Simulator to see fuel consumption.

JS

Fuel burn is related to the plane's weight (more weight ==> more drag ==> more fuel needed to create the thrust needed to offset drag). So, when flying with more fuel, you have increase the burn rate.

Yes, the burn rate will decrease as the fuel load decreases, but as the total flight distance increases, the average fuel load, and thus average fuel burn, increases. The average fuel load is roughly half the initial load, which obviously is greater the longer the flight.

JS

Descent is pretty cheap by comparison. Don't forget that climbing is basically just turning chemical energy into potential energy, which is then released on the descent. It's still a net loss because the engines need to be running during descent when it's theoretically unnecessary (aside from go-arounds, air conditioning, frivolities like that )

indo79

last question I promise

so with that said, the plane is heavily loaded with fuel at take off and becomes lighter as the flight stretches..so still, as they burn fuel, the plane becomes lighter and there will be less drag so rate of fuel burn should decrease?

Thanks

JS

That is correct. Rate of fuel burn in the last two hours of flight is exactly the same whether the flight was only two hours long (excluding climb), or you left SYD 13 hours ago. Rate of fuel burn in the previous two hours of flight is a little more, and so on, and when you add it all up and average it over the course of the flight, the average fuel burn during cruise on the two hour flight is less than the average fuel burn on the 15 hour flight.

CTPremEx

Agreed, but the 744 rule of thumb I stated as 24,000lb fuel per hour takes into account the very high consumption at takeoff power/climb, and the low burn at light weights in the last couple of hours plus descent. This works remarkably well for 6+ hours trips.

But using an actual flightplan, I see 314,875 lbs of fuel for a 13:20 flight - which is darned close to the rule of thumb figure of 324,000lbs for 13.5 hours.