When does it burn fuel to carry fuel? [Archive]

enicalyth

1st Sep 2008, 09:39

A certain wide-bodied turbofan is expected to perform as follows at M0.8 ISA over 2000nam with FL350 and FL390 available. Apologies for my slight rounding of figures to protect the innocent.

GW kg; Payload kg; Block Fuel kg; Reserve Fuel kg
137000; 20500; 19700; 5450
141500; 24300; 20300; 5600
(146100; 28200; 20900; 5725)
(150600; 32000; 21400; 5850)

Increasing the Gross Weight by 4500kg from 137000kg increases the burnout by 600kg, adds 150kg to your reserves giving a nett gain to your payload of about 3800kg.

So, crudely speaking if you tried to tanker 4500kg for 2000nam you’d burn off maybe 700kg in doing so. Sorry it is a bit rushed.

I used p55 of Boeing: Commercial Airplanes - Commercial Aviation Services - Flight Operations Support - Airport Technology - 767 Airplane Characteristics for Airport Planning (http://www.boeing.com/commercial/airports/767.htm) to rough out my figures but downsized contingency to 5%. Also, and don't quote me, at 154300kg M0.8 the CL of said 767 is 0.5 at FL350, tsfc is ~ 0.59 and the sar is ~ 93nm/1000kg.

Best Rgds

The "E"

barit1

1st Sep 2008, 12:13

There is a difference if you are "logging hours" in a 152 - if it's local flying, you aren't measuring productivity in terms of miles traveled. You probably fly at a fixed power setting, and the extra weight means your TAS is a percent or two low. Who cares? You log the hours - not the miles.

Transport flying is different: miles count! Extra fuel = extra weight = more power (thrust) to maintain Mach/TAS, thus higher fuel burn, and maybe you can't make the best cruise altitude when heavy. Big difference!

Keygrip

1st Sep 2008, 12:25

Many thanks for the answers so far - I'm more than interested to note that the majority are suggesting the same average figure of 4%.

Back to the original question, however, for the other two or three replies - I asked (in bold italics) when does it make a significant difference.

Minimum 737? Really? Not a Lear Jet? Not an ATP, Shorts 360, Citation? Gulfstream V?

I already said I knew it would burn more even in a "spam can" but when would it make a significant difference.

What the flight time v. miles has to do with, I've no idea.

Just because it's somebody elses fuel in a rental doesn't mean I should consider flying it any different, or less professionaly (does it?)....and no, I don't fly at a fixed power setting. I fly at the power setting required to achieve the performance I desire.

BOAC

1st Sep 2008, 13:54

Keygrip - i'm not really clear what your question is? The extra fuel required to carry 'extra' fuel is ALWAYS a factor in relative terms. Whether it becomes 'significant' depends on your finances. If you can easily absorb the cost of around 4% per hour of the extra fuel you are going to carry, it matters not a jot. In a one-off trip in a 150 it is unlikely to be 'significant'. Carrying 8 gallons extra on every 1 hour flight, and flying 6 hours a day every day.......? That's around 2 extra gallons burnt every day, or getting on for 700 a year. Is that significant?

Keygrip

1st Sep 2008, 17:52

Ah, now the light comes on.

Put *that* way BOAC it makles more sense.

I was assuming that heavy transport was using around 4% extra which could be significant on a long haul flight, but reducing to a thimble full per tank on a Cessna 150 and, as such, was so fractional as to be not worthy of consideration....and that somewhere along the diagonal line between the two aircraft styles, it started to make the bean counters think, "Errr, hang on a minute"

So aircraft X with full tank capacity of 100 gallons, goes on a two hour flight at 10 galls per hour, needs 25 gallons (to include 30 minutes reserve) will have "tankered" 75 gallons around the sky for fun - and will have burned an extra 6 gallons (4% of 75 multiplied by 2 [hours of flight]) in doing so.

So it will land with 69 gallons in the tanks and will have burned a US$30 hole in the profit margin in doing so? (assuming US$5 per gallon of avgas).

If renting the aircraft "dry", I would consider that already significant (certainly on my own budget just now).

Going one step further then, if the hourly fuel burn is reducing (as 4% of less is less than 4% of lots) there must come a mathematical time where it is cheaper (all things being equal) to fly on for another hour or two than to land with lots of spare fuel and "top off" the tanks.

AirRabbit

1st Sep 2008, 19:39

;)Technically, flameout-at-touchdown (FAT) burns the least amount of fuel. Now – do you want to taxi to the ramp or be towed?:hmm:

The farther you go, the more fuel you’ll burn. The heavier you are, the more fuel you’ll burn. The more often you have to land to refuel, the more fuel you’ll burn. So, the next series of questions should be … How far do you want to go? What payload do you want to carry? Do you want to carry maps and charts? How often will you have to land with the fuel load you chose? Did you eat that extra slice of chocolate cake last night? Of course, once these factors are determined, then it remains to determine what route and what altitude will give you the best tail wind against the cost of getting to that altitude or flying that particular route.:cool:

Oh … one other thing … it may be only my opinion, but I believe that “all things” are never equal … in anything.:=

barit1

1st Sep 2008, 20:24

A topnotch performance engineer at a major European carrier once told me that for every extra tonne carried across the pond, 10% of it would be burned off. Obviously the trip length is a factor.

DesiPilot

2nd Sep 2008, 02:27

Keygrip,

The significance of fuel carried also depends on the difference in the fuel price or the fuel price ratio between departure and destination airports.

If you are flying from KOBE to KAPF in your C150, it is worth taking that extra fuel. As the fuel is a lot cheaper at KOBE it outweighs the extra fuel burned and it is still cheaper to tanker it.

For this very purpose most of the manufacturers publish "Fuel Tankering" tables. By using these tables you can calculate whether its worth carrying that extra fuel.
In my present plane (A320 family) charts are give in FCOM 2.05.70.

Keygrip

2nd Sep 2008, 02:52

That's what I meant by "all things being equal" - I'm looking at only the methematics of the science - not a particular task.

I didn't think I needed to spell out that fuel costs would have to be the same, wind velocities would have to be the same, landing fees....etc etc etc.

glawkshuter

2nd Sep 2008, 15:55

It's not the weight of the fuel, but the cost of it that truly determines tankering...

kijangnim

2nd Sep 2008, 17:14

Greetings,

This is determinated using the K factor or transport factor, equal to delta takeoff/delta landing in terms of weight, having said that the weight IS A determining factor because we order fuel in weight but it is supplied in volumes and the relation is the SG, now lets say that you are going from a cold country (SG around .80) to a warm country (SG around .78) so for each liter of fuel loaded you would loose .02 of weight and if your total Fuel is 40T.....:eek: think about it.
If the SG are equal then the price difference IS the determining factor:ok:.
To recap in all case PRICE DIFFERENCE has to be CORRECTED by SG Difference and turn into a percentage, that percentage has to be less than the extra fuel burned usinf the K factor :ok:

Lemurian

2nd Sep 2008, 17:34

I'm looking at only the methematics of the science - not a particular task.
There is a quick - and surprisingly accurate - way of computing the extra fuel needed :
The ratio TOW / LW is, for all intents and purposes constant for a low percentage increase on the TOW ( in the form of extra fuel, for instance). Let's call it K
K= TOW / LW = (LW+Bo) / LW where Bo is the burn-off
K = 1 +( Bo / LW)
so, for a small delta LW, ðBo = (K-1) x ðLW and that's what we're interested in.
For example, (rounded figures to protect the innocents, again) :
TOW = 260,000 kg
Bo = 74,700 kg
LW = 185,300 kg
So, our K is : 260 /185.3 =1.40
so ðBo = .40 x ð LW
You'll be burning 40 % of your extra fuel.
For info, it's for an air distance of 5350 Nm and an 11.10 hr trip
A 777.
Hopes it helps.
Edit : just discovered Kijangnim let the cat out of the bag : K is the transport factor

mutt

2nd Sep 2008, 19:12

LeMurian, I ran CFPs for the B777 over a 10 hr sector.

Fuel Tankered 27270 kgs
Additional Fuel Burn 7772 Kgs
% of fuel burn increase 28.5%
10 hour sector = 2.85% per hour.

So your 40% over 11 hours appear on the high side for that aircraft!

Mutt

flyr767

2nd Sep 2008, 19:23

Our fuel burn tables for the 757 say that for every 10,000 lbs above 170,000 lbs for the landing weight we have to add this much fuel per hour which varies depending on the cruise altitude. Below the 170,000 figure we can reduce fuel in the same way. Same goes for our 767s... well our whole fleet just the weights are different. :)

Lemurian

2nd Sep 2008, 21:41

So your 40% over 11 hours appear on the high side for that aircraft!
The way you wrote, it looks as if you *tankered* an extra 27,270 Kg of fuel, of which you burned 7,770 kg. Right ?
Remember, the formula I gave is about landed weights. from that point of view, in order to land 27,270 - 7,770 = 19,500 kg, you used 7,770 / 19500 = .398 or 39.8 % of the landed fuel.

Secondly, K will increase 1/- with higher weights, 2/- with longer flights and 3/- with your cruise schedule (the higher the CI, the higher the trip fuel and the TO / LW ratio ).

Thirdly, that extra fuel takes our computation probably away from the realm of quick computation as we're coming - my estimation - some 22 to 25 % of the LW and with that extra load, the flight profile will be very different (Step climbs...) and the fuel consumption will reflect that.

But my example, for obvious reasons, did not portra an actual flight. Let's just say that betwen CDG and HKG with a 30kt tail wind, the real example came out with an 11 hr 6 mins and a K= 1.347. The airplane was some 36 T below MTOW.

Please, re-do the calc using actual weights.

Regards

glawkshuter

3rd Sep 2008, 05:12

It's funny, but does anyone here besides me have a clue as to thier per gallon cost of fuel at the departure or destination point inorder to determine how much fuel to tanker here or there to figure this out..?? If you don't then it's all moot, you simply carry as little as possible to make the plane climb and fly faster, fly as high as you can to get your fuel consumption as low as possible per trip, go with long range power settings...as airliners can go 20000 hours on an engine....unlike corporate... For corporate there is a point where the engine cost out weighs the fuel cost(not lately)...so in the past we went fast, now we go slower, and really shop fuel....

mutt

3rd Sep 2008, 05:36

FUEL TIME ETA NGM NAM FL COMP TEMP WEIGHTS
POA RPLL 077499 10:00 1600 4944 4898 370 P006 M35 148.77 OWE
ALT RPVM 005876 00:54 1654 0345 0350 M004 025.00 PAYLD
HLD 003100 00:30
RES 006414 01:00 112.37 TO/F
REQ 092889 12:24 ATW . . . . / 286.15 TO/WT
XTR 019489 03:09 208.65 LDGWT
BLST 000000
ETOP 000000 00:00
TOT 112378 15:32

Le Murian, these are the actual flight details. How am i expected to know the landed fuel prior to flight?

glawkshuter, our crews are advised of the fuel cost and the multiplier for carrying it on various distances. This way they have a better understand of the costs associated with tankering.

Outta here for next 6 weeks, so dont know if i will be able to continue this discussion, more the pity as it was interesting :)

Mutt

mutt

3rd Sep 2008, 05:51

Keygrip,

I ran CFPs for a C550, 1.40hr sector.

PLD / FUEL BURN / %
500 / 686 / 4.2%
600 / 693 / 3.6%
700 / 699

So for every 100 additional KGS, its costing me circa 4% to carry. But the amounts of fuel increase are so small that they arent worth worrying about.

So to answer your original question, all aircraft burn more fuel to more fuel to carry fuel. you only need to worry about it when the financial costs are sufficient that they impact on your operation......

Mutt

kijangnim

3rd Sep 2008, 05:56

Greetings,
Lemurian and Mutt are both right :eek:
In fact Lemurian computed the K factor using the following ,the LANDING WEIGHT DID NOT CHANGE because the increased TAKEOFF WEIGHT is PAYLOAD,
MUTT used an increased TAKEOFF WEIGHT due to EXTRAFUEL uplift, so the LANDING WIEGHT is AFFECTED and will be LOWER, which will give a LOWER K factor.
In fact the thing to do is to use MUTT result and compare the fuel price (in percentage) difference (corrected by fuel SG) to the K factor and if fuel price is greater then TAKER FUEL.
The method used by Lemurian is the one to use incase of excess weight to be offloaded. let say that for limitation purposes we have to get rid of XXXX KG , then using K we would split the weight between pay load and fuel , which in turns will save you K % of your payload which is $$$$ :ok:

bookworm

3rd Sep 2008, 06:59

Interesting question. I like Lemurian's approach of using Breguet's range formula to assert that take-off weight / landing weight is effectively constant. However I'm not sure it really gets at the heart of Keygrip's question as to why this is an issue on the 777 but not the C152. Let me look at the problem from a different angle.

The key parameter is the % increase in power required per % increase in weight, which I'll call alpha. Mathematically that's

(dP/P) = alpha * (dW/W)

Knowing that, and assuming that fuel consumption is proportional to power required:

Extra fuel consumption rate / Normal fuel consumption rate = alpha * Extra fuel loaded / Weight

Rearranging that:

Extra fuel consumption rate / Extra fuel loaded = alpha * Normal fuel consumption rate / Weight

The left hand side is just the proportion of the extra fuel burned per hour. It depends on (Normal fuel consumption rate / Weight), so it's more (per hour) for a short flight than a long one, but I don't think that goes far towards explaining why it's significant for a 777 but not a C152.

What about this factor "alpha"? Well that will depend on how much of the drag is lift- (and therefore weight-) dependent. That depends very strongly on the speed you fly at.
At minimum drag speed, half the drag is weight-dependent induced drag. As you increase the speed, weight-dependent induced drag becomes a much lower proportion, and % extra power required per % extra weight is much lower.

If I take a drag polar that looks like (^2 is "squared")

drag = W^2/v^2 + v^2

then I get

alpha = 2/(1 + v^4)

where v is the speed relative to minimum drag speed. That's a really strong dependence on v. At minimum drag speed alpha is 1. At 1.4 times minimum drag speed, alpha has fallen from 1 to 0.4. At 1.7 times minimum drag speed, alpha has fallen to 0.2.

I don't fly big jets but I think that's where the difference lies between them and light aircraft.

My light twin burns about 85 lb/hr at 3400 lb, so my proportion of extra fuel burned per hour is alpha * 2.5%. But that's cruising at about 50% above minimum drag speed, so alpha is about 0.3, and it costs me less than 1% per hour to tanker fuel.

If I'm interpreting the Boeing figures I've got here for a 757, and cost index 0 is a reasonable indication of minimum drag speed, even cost index 70 suggests a cruise speed just 1.07 faster. At today's lower cost indices, that might be even 1.05 times minimum drag speed, and so alpha is going to be pretty close to 1 (say 0.9). I appreciate my naive drag polar might not be accurate at Mach 0.8, but the point stands that you're flying at a speed where weight dependent drag is a much greater proportion.

So do get back to Keygrip's question, it starts to matter to a significant extent when you start flying at speeds closer to minimum drag where weight-dependent drag is a significant proportion of total drag. That tends to apply to fuel-conscious ops in big jets, and less to the always-cruise-at-75%-power C152s.

SNS3Guppy

3rd Sep 2008, 08:05

We use 5% of the load increase per hour. If we intend to tanker 50,000 lbs of fuel, at the end of 10 hours we will have consumed 50% of that above the planned burn without it. In other words, if we'd have planned to use 180,000 lbs of fuel for the flight, we'll have burned 205,000 lbs instead...tankering the extra fuel cost us half of the tankered fuel over a 10 hour flight over and above the fuel burn we'd have had without the extra fuel.

We do look closely at the cost of the fuel at departure and destination and that plays a role in determining what we can tanker. However, our chief criteria for determining how much fuel is the required fuel burn vs. the payload; we certaily won't carry extra fuel at the expense of revenue payload just to tanker cheaper fuel...because we're burn and waste much of the extra fuel. The payload pays the fuel costs...so unless the price difference is grand, or the circumstances of the flight can justify the significant cost increase of tankering the fuel (for safety), we generally don't tanker.

glawkshuter

3rd Sep 2008, 08:08

So Guppy...based on what you paid for fuel at your destination, where's the cut off in price per gallon that will determine if you tanker down or back?

BOAC

3rd Sep 2008, 08:53

I expect Keygrip is REALLY pleased he asked this question:)

We see you are back, ssg, and still 'shuting the glawk' too. If you cannot work that out from SN3's post......................:confused:

barit1

3rd Sep 2008, 13:04

Early 80s, DL had a number of KLAS flights - and the fuel price there was 20-30% higher than KLAX. So they substituted a DC-8 on some of the flights simply to tanker in fuel to KLAS for their 727's, DC-9's etc. They didn't like to do so, but for a brief time the numbers made sense. :eek:

glawkshuter

3rd Sep 2008, 15:31

Well color me confused by all the complicated math.....Met a Continental pilot who told me that everyone is flying around in his airline with min reserves as has been directed by management....For me, I will try to save a buck but not compromise safety doing it...

SNS3Guppy

3rd Sep 2008, 18:05

So Guppy...based on what you paid for fuel at your destination, where's the cut off in price per gallon that will determine if you tanker down or back?

There isn't a cutoff point, as it doesn't work that way.

glawkshuter

3rd Sep 2008, 18:39

Guppy...must be very nice not to have to worry about the actual price of fuel when considering whether to tanker or not.
In my world the price per gallon determines the actual fuel cost of the trip...but maybe you have a oil refinery at your airport...

SNS3Guppy

3rd Sep 2008, 20:12

When one can save a few pennies or even twenty five percent on the cost of a gallon of fuel, that sounds great...until one burns half of the tankered fuel just to move it from A to B. In that case, the effective cost of the fuel you have remaining...that is, the fuel that wasn't wasted getting it from A to B...has just doubled. Save a dollar or a euro or a pound on a gallon of fuel...but have half of the tankered fuel remaining...and you haven't saved that dollar...you've doubled the cost on the fuel instead. Why? You just paid a little less, but got half as much. That's not economy.

Further, the airplane climbs slower, uses more runway, lands heavier, has a lower climb gradient, lands faster, may require higher power settings and thus experiences greater engine wear, yada, yada, yada.

You tanker 10,000 lbs, a purchase of 1,500 gallons. You do this because the destination has fuel at 4.00 a gallon, whereas your departure has it for 3.00 a gallon. This sounds good; you'll save 1,500 dollars on the cost of the fuel. You make a five hour flight, consuming five percent of the surplus tankered fuel, per hour. The surplus tankered fuel is 10,000 lbs, meaning you consume 25% of the tankered fuel during that five hour flight (5% per hour for five hours).

Fuel at departure would be 4,500 dollars, whereas one would have to pay 6,500 dollars at the destination for the same amount of tankered fuel. Unfortunately, because 25% of that fuel is burned on the way to the destination (fuel burn has gone up due to carrying the extra load), you have only 75% of that fuel remaining...1,125 gallons of tankered fuel. Dividing the total cost of the tankered fuel you bought, 4,500 dollars, by the amount of fuel you have remaining (1,125 gallons), you find that for the fuel you've got, you really ended up paying four dollars after all. You've saved nothing, but instead dragged ten thousand pounds of fuel across the country or internationally, for nothing. That's ten thousand pounds of revenue paying payload that one could have carried instead...or at least 75% of those ten thousand pounds...7,500 lbs of payload that would have paid and been profit.

Break even point? It's not nearly so simple as looking only at the cost of fuel at departure and destination. One needs to consider numerous other factors, including the purpose of the flight, the payload available, the conditions of flight, etc, when making such a decision. Very often tankering provides no additional benifit economically.

We do plan fuel stops where the difference in cost justifies the stop, but in that case we're tankering nothing; we carry the proper amount to reach the tech stop, then take on the proper amount to reach the destination. Even then, merely making the stop for the sake of fuel cost is more complicated than the raw cost of the fuel. Time lost when the aircraft could be on another trip making money, additional trip time and time on airframe and engines, landing and engine thermal cycles, etc, all must be factored into the justification in making the stop. Most of the time, a tech stop just for the cost of fuel isn't justifiable.

Where a tech stop must be made due to the length of the trip (which for us is sometimes the case), we'll break up two six or seven hour legs with a fuel stop selected for it's convenience to the route, it's minimal turn around times, and yes, the cost of the fuel.

You see, in the real world, it's not quite as simple as your world.

glawkshuter

3rd Sep 2008, 21:15

.. the calculated formulas are for a captain to decide if taking on extra fuel for safety is worth the 'x' amount of cost to the company by carrying the extra fuel.

Lemurian

3rd Sep 2008, 23:21

I'm quite disappointed in you :
One of the uses of the transport factor is really about profitability of tankering.
Let's go back to the initial def : K = ðTOW / ðLW,
which means in layman's terms that for each landed ton of transported fuel, one needs to uplift K tons of extra fuel.
The price of that landed fuel will be Po * ðTOW = Po * ðLW * K...... ("o" for origin ).
Let's call Pd the fuel price at destination : the transported fuel, ðLW, would have been paid Pd * ðLW, had one not transported any fuel.
So, tankering will be profitable if :
Pd * ðLW > Po * ðLW * K, in other words if the cost of the extra fuel at departure is lower than the cost of the transported ( meaning "landed") fuel at destination.
Simplifying the above equation :
Tankering is profitable if : Pd/Po > K

The beauty of it is that it works, whatever the type of aircraft, whatever the distance....be it a 744 or a Cessna 150.
It allows to have a ball park figure while waiting for one's dispatch to finalise the computations.
Beware, though : for greater quantities of extra fuel on long sectors, one could alter significantly the flight profile through delayed steps... and find that one would spend some extra time in the air, adding the effects of DOC to the economics of the flight.

Mutt,
From your CFP, I only have the final burn-off of 77,499 kg.
The final figures of TOW = 286.15 T ; LW = 208.65 T give a K= 1.371
Of the extra fuel = 19.500 T you'd keep at destination 19.500 / 1.371 = 14.223 T, thus using 5.277 kg extra burn-off to land 14.223 t at destination.

dlk111

4th Sep 2008, 01:32

The MD80 that I fly used about 78 pounds of fuel for each 1000 pounds tankered on a typical trip from Seattle to Oakland. So about 7.8% was used up.The fuel price in Seattle was better than in Oakland. On a trip from Seattle to Anchorage the burn was about 125 pounds or 12.5% used up.
The price of jet is so bad now that my last trip to San Jose the entire return fuel was tankered! I left Seattle with 32000 pounds of fuel!

kijangnim

4th Sep 2008, 14:04

Greetings
Lemurian....Laisses tomber :ok:

glawkshuter

4th Sep 2008, 15:55

Maybe what needs to be specified, some of the reasons to tanker fuel:1: Total Trip cost reduction. Self Explanatory. By purchasing and carrying as much of the cheap fuel, you reduce having to use the expensive fuel. Over the total trip, the cost is less. This assumes you know the price per gallon at departure and destination airports and can actualy control the amount of fuel purchased. 2: Safety: Taking that extra fuel to eliminate putting a plane in the water, having to turn around midway, Island fuel reserves come to mind...the more fuel you have the more time you have. And more I am sure...But for the purposes of this thread, the question seems to be, when does it start costing money to carry extra fuel around....Since it's obvious that carrying one once over minimal legaly required reserves cost money to carry to the destination, the question remains for those agonizing over carrying tankered fuel: How much safety are you willing to give up, inorder to save a buck for the company?

Lookforshooter

5th Sep 2008, 03:07

I happen to agree with Glawkshuter...it's one thing to sit there and figure out the cost of tankering so as to be brow beaten into not taking any excess fuel thus making the flights just a little unsafer(Airlines) vs simply shopping fuel, discounts, and tankering fuel to make the trip cost much less.

SNS3Guppy

5th Sep 2008, 17:10

Of course you agree with the nimrod, you dunce. You're him...reposting under a new name after you managed to get banned (again)...posting to agree with yourself.

Truly pathetic.

Basil

6th Sep 2008, 10:34

Lemurian & kijangnim,

I'd never heard of the K factor so I stand educated. *
We usually left tankering decisions to you management chaps anyway :)

As far as extra burn is concerned we kept it simple:
3% x hour for extra loaded.
4% x hour for extra planned arrival.
and thanked our lucky stars we didn't work for Ryanair.

( * Edited to say except for K in calculating SAT from TAT)

kijangnim

6th Sep 2008, 12:17

Greetings,
Basil, thanks for your remarks, management takes these decisions simply because they have zee information i.e., the fuel price, nevertheless using the K factor (transport factor) is higly beneficial for tankering, for ofloading and so on :ok: HAPPY LANDING

Lemurian

6th Sep 2008, 23:14

Hello, Basil
The "K factor" for each flight is on the CFP, therefore available to the Flight Deck crew.
As I cannot make public the documents of my airline, I went on the Net to find some text on the subject and Lo! and Behold! there is one, from the Airbus series of "Getting to grips with..." I found in a Smart Cockpit !
You probably know all the fuel conservation techniques it talks about but the chapter 4.5 deals with "fuel for transportation".
Have a look :
Getting to Grips With Fuel Economy (http://www.smartcockpit.com/pdf/flightops/aerodynamics/21)
Click on the screen icon to load.

Best regards.

Basil

7th Sep 2008, 09:28

Lemurian,
Thanks for the URL. How did I get through a career without that website?
Better not try to imagine what Lemurian and Hugh Dibley of BA & Airbus Industrie would say ;)
So I guess the K formula is an elegant way of doing the calculation instead of piecemeal, as I'd have done before my moment of enlightenment.

Lemurian

8th Sep 2008, 08:37

Yes, that website is priceless.
As for my aviation culture, a first officer taught me yesterday how to use the sidestick. I thought that in order to turn, I only needed the rudder pedals... ;)

regards.

Basil

8th Sep 2008, 19:17

Just had a go at sidestick in a 'bus sim a few weeks ago.
Some interesting control laws and responses for a simple lad who's always had a bit of string leading to the wiggly bits :)

mutt

11th Sep 2008, 16:00

LeMurian

The Airbus transport factor deals only with the additional fuel required to carry fuel, it doesnt account for the cost of carrying that fuel in terms of extra stress on the aircraft/brakes/thrust reversers/engines, therefore its not ideal when you are trying to make a financial assessment on tankering fuel....

Mutt

BarbiesBoyfriend

11th Sep 2008, 21:32

Surely the KEY aspect here is not so much HOW MUCH extra you take
but more HOW LONG you carry it for!

In other words for a short haul pilot like moi......

If I take half a tonne extra- and my sector time is 65 mins- I'll have pretty much all of my 500kg extra when I get near the airfield-which I can then use to deal with whatever it was that I took it for.

Or land with it unburnt! And damn little of it wasted!

( I know it's a different arguament if you carry it for 13 hours- most of it is gone when you arrive- so why take it?)

For short haul- I'm unconvinced that carrying a bit extra is very wasteful.

btw I was on a UK flight that diverted due to not having 15 mins holding fuel 10 days ago. Wonder what that cost?:rolleyes:

mutt

12th Sep 2008, 05:00

BarbiesBoyfriend,

If you burn 4% of your 500 kgs per flight, with x flights per day x 365 days per year, the numbers start to add up. So you cant look at one flight and say that its not worth it!

The longest sector that we tanker is 10 hours, we use 40% of the loaded tanker fuel to get it there. But with the cost differential between the two airports, its an excellent way to make money.

Mutt

BarbiesBoyfriend

12th Sep 2008, 09:05

Mutt

My FO is fat. I have to carry him 4 flights a day, 635 days a year. It all adds up you know.

Maybe we should put the crews on a diet and just employ size 8 hosties.:ok:.

Everything 'adds up'.

20kg of gas wasted (by tankering 500kg for an hour) is a great trade off for the peace of mind (and the range of available options) that it provides.

20kg FFS.

Now I've thought about it a bit more, thanks to you & your 4% advice, and realised how little is wasted, I shall continue with my present practice and worry about it even less than before- if that's possible!:D

SNS3Guppy

12th Sep 2008, 16:36

Surely the KEY aspect here is not so much HOW MUCH extra you take
but more HOW LONG you carry it for!

It's both, actually. Carry more overage, you'll burn more of it getting it from A to B, regardless of the leg length. If the overburn on tankered fuel is 5%, then you'll burn 500 lbs of that for a 1 hour flight....if you're carrying 20,000 lbs extra, then of course you're burning 1,000 lbs of that to tanker it. So yes, it's both the legth of time you're carrying it, and the amount. Speaking in general terms and rules of thumb, of course. As stated before, we normally plan on 5% of the tankered load as additional burn, per hour...that adds up. For us to tanker just 20,000 lbs, we'd use up half of that on a ten hour leg, meaning the tankered fuel just cost us twice as much...better be a substantial price difference to justify that.

On a short leg, tankering really depends on what we need to meet our landing weights...our landing weight is 200,000 lbs less than our takeoff weight...so generally tankering fuel isn't an option in the first place for us for a short trip.

Then, looking at the requirements at the landing field, if we need to increase our brake setting and experience higher brake wear as a result, we may easily have eaten the cost of the tankered fuel or any savings thereof, before we even leave the runway. Add to that increased turn around times due to hotter brakes due to the heavier landing weight (depending on runway, of course)...we may have just shot ourselves in the foot by losing time that could have been spent flying revenue freight, instead of letting our brakes cool.

It's a tradeoff, but I don't think it's nearly so simple as saying a short leg justifies tankering the load.

Lemurian

12th Sep 2008, 18:05

The Airbus transport factor deals only with the additional fuel required to carry fuel, it doesnt account for the cost of carrying that fuel in terms of extra stress on the aircraft/brakes/thrust reversers/engines, therefore its not ideal when you are trying to make a financial assessment on tankering fuel....

First : Airbus did not invent the *transport factor*. It came from one of these guys who used to ply the sky in the sixties and it's a broad simplification of the Breguet range equation.
In all fairness, you are right but in real terms, we are talking of relatively low increases on the aircraft weight.
Add the fact that tankering over more than three sectors - short haul - is not recommended on most airlines, we are really talking about peanuts.
Example :
Landing 4000 kg of extra fuel, lets say that our landing weight increased from 56 to 60 T.
The landing speeds have one up from 125 kt - 64.3 m/s² - to 130 kt -66.9 m/s².
The kinetic energy upon landing therefore has gone from 115.8 MJ to 134.8 MJ, an increase of 16% either taken by braking energy or brake/reverse energy.
I really don't see a dramatic increase of operational constraints here, as seen by SNS3Guppy.
Tankering on a long haul sector, as I demonstarted is only worth the trouble if the fuel prices are drastically different.
One of the more challenging - and interesting - aspects of our job is that we make decicions based on reason and experience. Going to LHR at six a.m wth the computed fuel seem rather idiotic as a lengthy hold is very likely...How much extra fuel for holding then ? Depends whether you know the ATC controller or you're a pessimist. Or just experienced enough on the sector...
I personally have seen on occasions people who took off 30 min behind me having to divert while I made it after a 50 minute racetrack pettern over BIG...

mutt

12th Sep 2008, 20:45

but in real terms, we are talking of relatively low increases on the aircraft weight..... Not us, we are talking about tankering return sector fuel for a 6 hour flight.... So the additional stresses on the aircraft are a factor..... :)

Mutt

Lemurian

12th Sep 2008, 23:48

A bit extreme, I see... ;)

Lookforshooter

13th Sep 2008, 00:37

More in line with Mutt's response, I look at departure and destination fuel, and tanker acordingly to get the trip cost(down and back) to the minimal cost). Always hated to leave a jet parked overnight fuel of fuel, tires tend to set,(thump thump thump on taxi the next day)wet wings have a higher preponderance to leak...not to mention overall higher landing weights, longer rollouts, and more brake and tire wear...

SNS3Guppy

13th Sep 2008, 03:14

I look at departure and destination fuel, and tanker acordingly to get the trip cost(down and back) to the minimal cost). Always hated to leave a jet parked overnight fuel of fuel, tires tend to set,(thump thump thump on taxi the next day)wet wings have a higher preponderance to leak...not to mention overall higher landing weights, longer rollouts, and more brake and tire wear...

Why are you tankering fuel? You fly a microsoft flight simulator.

"Preponderance to leak?" Your computer leaks? Wings with fuel tend to leak, do they? Do you simulated airplanes have holes in the wings?

OverRun

13th Sep 2008, 10:27

I was curious about the cost to the airport if aircraft are tankering fuel. I tried two scenarios. One was a small airport, with 737-800 tankering 2.5 hours fuel – say 6 tonnes. The second was a large airport with a mix of 777, 767 and 737. The 777 was tankering in 30 tonnes, the 767 was tankering 20 tonnes, and the 737 was tankering 6 tonnes.

The design of airport pavements (runways etc) typically assumes that the aircraft arrives lighter than it departs: i.e. it fuels up at the airport. Thus the arrivals are at light weights and the departures are at heavy weights. Since the damaging effect of a load is a function of [weight to the power 4], a heavier aircraft is much much more damaging than a lighter aircraft. If everyone is tankering, then effectively you are doubling the number of heavy flights and cutting the lighter flights to zero.

For both scenarios, the design life of the pavement was cut from 20 years to 12-13 years. If the airport is charging a landing charge per tonne, then simplistically the charge should rise by about 50%. Of course, the cost of pavement deterioration and replacement is only a small percentage of the costs that the landing charge covers, so the true increase should be less.

However since most airports today are run either as shopping centres or industrial parks, I would be surprised if many airports even knew about what happens on the runway.

SNS3Guppy

13th Sep 2008, 15:43

If one bases the arrival and landing weight of each aircraft on it's maximum landing weight, then the matter is irrelevant...it's not going to be any heavier if it's tankering fuel, if the stress analysis is based on each operation at it's maximum weight. "Heavy" and "light" are relative. We generally always depart at maximum takeoff weight, and land at or around maximum landing weight...what percentage of the fuel load is tankered, then, is irrelevant to the considerations of runway or taxiway or apron life.

bookworm

13th Sep 2008, 16:32

For both scenarios, the design life of the pavement was cut from 20 years to 12-13 years. If the airport is charging a landing charge per tonne, then simplistically the charge should rise by about 50%.

Almost every airport charges per ton of maximum take-off weight. By your logic, shouldn't we be getting a refund for operating lighter than maximum? ;)

barit1

14th Sep 2008, 14:18

I once heard of a carrier that ordered its 747s with all the structural upgrades for highest MTOW (820k# at the time), but with smaller brakes that capped MTOW at 785k# IIRC.

The strategy was the lesser MTOW would reduce landing fees, because the higher weight was not necessary for their current routes.

But the flexibility for future brake and MTOW upgrade was good to have in the hip pocket. :ok:

SNS3Guppy

14th Sep 2008, 18:05

I suppose you could do that. We have a few airplanes that are flexible stage II/stage III airplanes for a similiar reason (go Stage II where one can still do it, for the greater weight), and the stage III limits are flap limits for weight and noise. Accordingly, we take the higher speeds and higher brake wear under Stage III with a flap 25 restriction instead of flap 30, in order to meet the noise abatement and carry the greater load...but on some could revert to stage II if legal and able. It's really a paperwork shuffle by the FE.

If the route doesn't require the extra weight...I suppose it would work. I'd rather have the bigger brakes...but then I just drive, and don't pay the bills.