View Full Version : Long haul fuel considerations

14th Sep 2005, 04:12
Quote from a recent 'Professional pilot' article.

While not a quality issue, longhaul operators should be aware of differences in refining methods around the world that could affect range capability. AIG Dir Aviation Franklin Davis says that his Bombardier Global Express has reduced full tank range when coming out of Asia, as specific gravity of the local fuel differs from what it is stateside. "fuel is refined differently around the world. There are different types of Jet A. Volume, based on the viscosity of the fuel can vary, and you might find specific gravity differences of 5-10%."

Quote from 'Business and Commercial Aviation' article.

Given a manual volume shutoff system(float valves), the fuel system on an airplane shuts off at the same liquid level all the time. If you could look in the wing of a full Global Express, indicating 14,700 pounds of warm fuel or a fuel refined with a lower specific gravity, there would be just as much liquid in there as if it shut off by full volume, and indicated 15,000 pounds in a cooler climate or somewhere where the refined densities are a little higher.
Jet engine efficiencies depend on temperature change from intake. This less dense fuel while still refined within the specifications of Jet-A, is more burnable than more dense kerosenes. The less dense Jet-A's produce more heat per volume unit and as a result require less units to do the same job. Also, since you actually can weigh many hundreds or even thousands, of pounds less with less-dense jet-A than Kerosene at any given time in the trip, you are able to achieve the same cruise parameters with less energy. I have consistently flown longer, with more fuel remaining at the end of the trip, when I started with full tanks of less-dense fuel.
The point of all this is that if you have a flight plan that indicates a doable trip, a lower indication of fuel when the tanks are full volume wise should not affect that flight plan.

Do these two articles contradict each other?
Is this something that is taken into consideration by anybody out there?

14th Sep 2005, 14:52
Thermodynamically, it is BTU/lb. that matters. Suppliers will give you an ESTIMATE of their BTU/lb (typically 18600) but it often varies.

When Boeing or whoever sets out to make a record-breaking long-range flight, they first determine whether they are weight-limited or volume-limited. Based on this, they may select a high-density, lower BTU/lb. fuel like Jet A (for the volume-limited case), or a low-density, high BTU/lb fuel like the old JP-4 (for the weight-limited case). (I think high-latitude operators used Jet B, for lower slush point and higher volatility)

If volume-limited, they may also pre-cool the fuel to pack more pounds on board.

Gary Lager
14th Sep 2005, 14:56
I always thought it was the mass of the fuel per hour which gave all the relevant performance data (TSFC, Thrust etc) - so as long as you had the right number of kilograms on board you would make your planned range/reserve predictions.

Less dense fuel means you need more litres for the same fuel mass - something we always check (using actual, rather than standard SG) as part of pre-departure preparation.

I'm not a long-haul operator so am unaware of any situations where tanks were full (are they ever full in a B747 or A340?) but did not contain the required fuel weight.

Jet engine efficiencies depend on temperature change from intake

Yes, but surely the difference in engine efficiency between using fuel at 30C and using fuel at 0C would be negligible, considering the temperatures involved in combustion?

I have consistently flown longer, with more fuel remaining at the end of the trip, when I started with full tanks of less-dense fuel.

Maybe because AUW was significantly less than planned? (planned with 'heavier' fuel, that is) Again, I have little experience of operating at max range with full tanks (wouldn't be able to carry many pax!) so not sure how this is manifested in the bizjet world, which presumably is the thrust of the article.

How does the author of the second article account for in-flight cooling (to higher densities) of his fuel? Would that make a difference, according to that argument?

16th Sep 2005, 09:16
AS in the 'olden' days of the DC8/B707,for more Range use weight
So people would request kerosene/Jet A for more BTU's per tank than JP4,which could withstand colder regimes.
More BTUs means more Heat for less fluid equals further Range
As has been said above..
cheers(up the D'arcy):ok:

Old Smokey
18th Sep 2005, 01:09

Automotive and aviation fuels, as we know them, are those in the Hydro-Carbon Methane series, rangeing from gaseous Methane, CH4, at the bottom, through to Hexadecane at the 'top end' of heavy fuels used for aviation kerosene. Automotive fuels usually comprise Septane and Octane, and immediately after, the range from Nonane to Hexadecane is used for aviation turbine fuel.

Volatility is high, and flash point is low, at the lower end of the series, and, as we progress through the series, volatility decreases, and flash point increases. (If you want to smoke, and give me the choice of being in a room full of Methane or Octane vapour, against a room awash with Hexadecane, I'll go for the Hexadecane room every time).

If we examine the range of fuel used for aviation turbine aircraft, we look at the range beginning with Nonane, Decane.....through to Hexadecane. (Nonane is very close to Octane, the highest cut used for automotive fuels).

It is absolutely true that "less dense fuel while still refined within the specifications of Jet-A, is more burnable than more dense kerosenes". The problem is that "more burneable" (lighter) fuels have less calorific value than denser fuels, and calorific value is what you need to produce thrust. Burneability is NOT directly related to calorific value. If you were to use a lighter grade of aviation kerosene, you would need a commensurately larger volume to achieve the same task as would a heavier fuel. All fuel tanks are eventually volume limited, and a full tank of light fuel will carry you a lesser distance than a heavier fuel.

Consider this, if I need 7500 Kg (16535 Lb) of fuel for a particular journey, with a 2000 Kg (4409 Lb) reserve, I will need to load 9500 Kg (20944 Lb) of fuel. If I load fuel of SG 0.75, I must carry 12667 Litres (3346 US Gal) of fuel, and have 2667 Litres (705 US Gal) remaining after the flight. If I load fuel of SG 0.85 for the same flight, I must carry 11177 Litres (2953 US Gal) of fuel, and have 2353 Litres (622 US Gal) remaining after the flight.

It does not surprise me then, that, if you load a lighter fuel, you will have a greater VOLUME at the end of the flight, the weight will, of course, be the same as if you'd taken the heavier fuel.

Even this is something of an over-simplification. It considers only that lighter fuel have less calorific value per unit of volume than do heavier fuels. Heavier fuels also have greater calorific value per unit of mass than do lighter fuels. This is not usually considered, as in the normal ranges of the mixes used for turbine fuel, calorific value per unit of mass does not vary too greatly. If, however, we did allow for the difference in calorific value per unit of mass, you would need to load less volume and less mass of a heavier fuel, and have less mass and less volume at the end of the flight compared to the lighter fuels.


Old Smokey

18th Sep 2005, 03:03
Heavier fuels also have greater calorific value per unit of mass than do heavier fuels.

Right On!! :ugh:

Old Smokey
18th Sep 2005, 03:32
Thank you barit1, typographical error suitably corrected.:ok:

Trivia thought for the day - 1 Kg of Aviation Kerosene contains more energy than 1 Kg of Dynamite.

It's the rate of release of the energy that makes the latter more impressive than the former.


Old Smokey