When does it burn fuel to carry fuel?

[Keygrip]

Granted, heavy metal transport aircraft - all the current reminders about tankering fuel or carrying an extra ton or so for the wife and kids results in comments of "it burns extra fuel to carry that fuel" - but when does that start to a significant extent?

Does it burn enough fuel to take into consideration when flying a Cessna 150 with full tanks on a one hour flight? Should that 150 go with 1½ hours fuel quantity (all things being equal with regard to known quantities/crap gauges etc).?

When does the fuel burn become significant? What size of aircraft?

[Old Fella]

Tankering fuel

If my fading memory serves me correctly, the figure of 6% of extra fuel carried per hour is the cost on the -200 & -300 B747. Loading an extra 5000 kgs above planned fuel and carrying it for 4 hours would result in an additional 1100 kgs over planned fuel being burnt. (5000 - 6% = 4700 - 6% = 4418 - 6% = 4152 - 6% = 3903 Kgs of original extra 5000 Kgs remaining). Stand to be corrected if my memory has failed me.

[stilton]

Keygrip,

Even on a light aircraft carrying more fuel than necessary will cause you to burn more fuel.However the amount is so small as to not be significant.

I would carry as much as makes you feel comfortable (weight limitation and cg considerations taken into account) up to and including full tanks.

[TopBunk]

I think 3.5 - 4.0% per hour more useful as a guide nowadays.

So SIN-LHR = 13 hours = between 46 - 52% of 'extra' fuel loaded will have been burnt on arrival. So if you to have an extra 30 minutes holding fuel on arrival, you have to load about an hours fuel on departure! This equals to loading about 10,000ks extra to end up with 5,000kgs at destination.

Back to the original query on the Cessna, I would suggest that so long as you remain within CofG and MTOW/MLW calculations, the extra burn will be barely noticeable [4% of say 3 gallons = 0.12 gallons or 1 pint).

[rubik101]

For hundreds of posts regarding 'extra fuel' simply type that into the search fuunction.

[ACMS]

Simple:- Every time, all the time. Anything that adds weight to the Aircraft will cause it to burn more fuel. My lunch or that extra fuel I added for mum and the kids.

Anything bigger than a 737, then consider it.

Anything smaller, don't bother.

From the CX manuals:-- The penalty for carrying additional fuel is between 2.5% and 4.5% per hour of flight, dependant on aircraft type.

Just as topbunk said above.

[enicalyth]

an example

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 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]

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]

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]

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]

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]

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

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.

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

[barit1]

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]

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]

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]

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

[kijangnim]

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..... think about it.
If the SG are equal then the price difference IS the determining factor.
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

[Lemurian]

A quickyish way

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]

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]

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.