Fuel Management

[Mike Tee]

Hi all,
just wondering if anyone with some technical knowledge can help me understand the Fuel Management in Jet Transport Aircraft.
For example when a pilot boards an aircraft for a 3 hour flight how does he know,
A. How much usable fuel is remaining in the tanks from the previous flight.

B. At what point does the pilot know how much fuel he needs to uplift prior to departure.

And lastly, how accurate are the Aircraft's cockpit fuel gauges.
Thanks.

[Tu.114]

The fuel required to be on board for an individual flight is calculated during flight preparation. Then, via one way or another, the fueller is called to fill the aircraft to this required amount; this is usually not done by requiring an amount of liters to be added, but by specifying a target fuel amount to be on board.

Then, the fueller adjusts the fuelling panel to show the required amount, hooks up his hose and applies pressure. The aircraft systems then will themselves shut off the flow when the required amount has been reached, at which point the fueller will remove the hose, close the panel and receptacle and write the receipt.

So the amount to be added is usually not really of interest to the flight crew but a function of fuel remaining and the required block fuel and taken care of by the fueller.

One excemption applies: if the remaining fuel on board is only a little less than the required block fuel, one might have to observe minimum uplift amounts (often 1000 liters or a ton). This is generally not so much of a problem on larger aircraft, but on fuel misers like the DH8 or ATR, it will need to be minded by the flight crew.

So to answer Your questions:

ad A.: either by asking dispatch about what the previous crew called in with or by checking the gauges after powering up the aircraft.

ad B.: After flight planning, it is known how much fuel is required to be on board.

ad C: Rather accurate. Their accuracy (and the honesty of the fuelling staff) is checked after each refuelling by comparing the fuel gauge indications after refuelling vs. the fuel on board before refuelling plus the mass of the fuel indicated on the fuelling receipt. Tolerances are type dependent; if they are exceeded, usually a dipstick (or dripstick) reading will be performed by maintenance.

[Denti]

The aircraft systems then will themselves shut off the flow when the required amount has been reached,

Some aircraft can do that, quite a lot others cannot do it. For example the 737 usually cannot do it (might be a customer option, haven't seen it yet though). In that case the flightcrew tells the fueller how much fuel they need and he fills until the gauges read the required amount and then shuts off the valves manually.

We do know how much fuel remained from the previous flight, for one it is automatically noted on the electronic journey log (rounded to the next 100kg) and the flight crew normally leaves a note that tell us the amount rounded to the next 10kg. After fuelling is finished, a plausability calculation is done (electronic journey log in the ACARS part of the FMC again). Remaining fuel, uplifted fuel converted from liters to kilogram (using the fueller provided density) is added and calculated fuel on board is compared to actual fuel on board. If the difference is bigger than 3% or 500kg (whatever is more) we have to call maintenance to do a dripstick test of the fuel quantity on board. Fuel tank capacity is shown with an a precision of around 1%.

Fuel required is presented on the operational flight plan which is prepared by the OPS department which also prepares some fuel usage statistics about past flights on the same city pair and the crew decides based on all available information, especially weather, time of day and destination (traffic density) but also fuel price indexing if they need any extra fuel and how much they will take.

So, all in all:

A. is covered by information on the journey log and usually info from the previous crew.

B. We could calculate, but usually we just check what amount of total fuel in tanks we need and do a rough crosscheck if its less than 800kg/1000l (minimum uplift) and use that total amount.

C. 1% (on our 737).

[Mike Tee]

Thanks guys,
took a while to get my head around all of that but I now do understand.
If I may ask another question with regard to the "dipping of tanks", would this involve measurements being taken at each separate tank, ie wing tanks, central tank etc or is the level measured at one central "collector" tank.
Thanks for your patience.

[Tu.114]

Generally, multi-engined aircraft have no central collector tank feeding all engines but each engine is fed by its own dedicated tank(s), feeding the individual engines collector bay, from where fuel is then routed to the engine itself. The collector bay is kept completely full when the engine is running (and sufficient fuel is available) and will only gradually deplete after shutdown with the fuel seeping back to other tank sections.

So dipping only the collector bays will not reliably show the amount of fuel on board; in addition to them, at least one stick further out in the wing will need to be pulled. On many types, the fuller the tank, the further outboard You need to pull the sticks: the wings have some anhedral that will make the fuel flow towards the wing root by gravity. Should the individual type have other tanks e. g. in the fuselage or on the wing tips, those will of course be checked as well.

Also, on some types the bank and pitch of the aircraft standing on a non-level apron will have to be minded when dipping the tanks. On the Fokker 70/100 for example, there is a spirit level installed that displays one bubble floating over a two-dimensional grid; the position of the bubble determines which table to enter with the dipstick readings (a call to maintenance is typically required to get the values).

This procedure is generally not exactly popular, as it usually requires having a stair brought to the aircraft, finding a screwdriver to pull the sticks and having the readings (type dependent) converted into fuel mass via a table often only available to maintenance. Some delay is usually inevitable by then. But fortunately, it is a rare occurrence. In my experience, at maximum once every few years.

[llondel]

I assume everyone is really careful nowadays not to confuse pounds and kilograms...

[Mike Tee]

Thanks again guys, all clued up now !!.

[Denti]

I assume everyone is really careful nowadays not to confuse pounds and kilograms...

I would hope so. From my experience in europe everything is done in kilograms, although fuellers give receipts based on litres. As fuelling is done with reference to the aircraft gages it is quite simple as those are in kilograms as well, both those on the wing as well as those in the flightdeck.

[Out Of Trim]

US Carriers still use pounds even in Europe, and Olympic used to operate a fleet of 737s which still used pounds for fuel calculations, but most use kilograms these days.