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Fuel management on a plane with a complex fuel system - how is it done?

Cobalt wrote:

Probably a reference to fuel systems where the return line from the engine goes to only one and the same tank, regardless of which tank is selected.

Most modern low wing UL/LSA are like that, and an EFI engine with a very substantial return flow (much more than the engine use) doesn’t exactly help. Some have separate return lines to each wing tank, selected by the fuel selector. This is IMO a bit stupid as well, since it will increase the amount of unusable fuel. Start using fuel on the tank that does not have a return flow is very stupid of course.

The G3X, as well as the similar Garmin GPS, has a warning popping up regularly: “Switch fuel tank”. Annoying at times, but all in all rather helpful.

The elephant is the circulation
ENVA ENOP ENMO, Norway

I would take a little more unusable fuel over the risk of dumping a lot overboard because of a mistake, because once overboard it is definitely not usable.

Biggin Hill

1. Peter, what about the fuel tank sensors? I mean if run from the correct tanks first (the ones which are used to return fuel to) to free some space and then continue using them in the right order, would the tank sensor be granular enough to say if the tank full/not really full/half full/…?
2. Can you install flow meter on the return lines?

EGTR

I have no idea about multiple fuel tanks having level sensors which is party why I asked the Q (although gauges tend to be too inaccurate to be really useful) but I am told that engine installations where there is a return to the tank, one has two flow transducers and the indicator subtracts them.

Administrator
Shoreham EGKA, United Kingdom

DA42 has fairly simple-to-use system although it has four separate tanks and fuel level sensors are only in main/wing tanks (DA62 has sensors in all four tanks). Fuel is always supplied from wing tanks and surplus is returned back for heating purposes. Auxiliary tanks are equipped with pumps which transfer fuel to main tanks. Fuel totalizer is connected to FADEC and shows total amount of fuel consumed since last manual input of quantity on board and doesn’t have any feedback from fuel sensors. In addition there are cross-feed levers enabling engines to be cross-fed from the opposite tanks.

Operating is very simple – you try to enter correct amount to totalizer (usually by fueling to the top or being precise on top-up – totalizer is pretty precise), follow main tank gauges and then transfer from auxiliary tanks in one point in time (usually when below 18 gallons in mains) until you dry auxiliary tanks.

Last Edited by Emir at 14 Feb 11:02
LDZA LDVA, Croatia

In the F-16 there are no fuel flow transmitters other than those implicitly in the engine. There are only fuel level gauges, or rather fuel weight gauges, and a lot of them. A fuel level gauge and a fuel mass gauge is essentially the same thing physically, but using weight instead of level causes less sensitivity to fuel type and grade, it’s more accurate. Fuel transfer is done by pressurized air and external tanks are always emptied first as a general rule, but individually. For the internal tanks, the fuel is transferred continuously and automatically giving optimal weight distribution, when the external tanks are empty. If I remember correctly that is. Civilian jets use pumps, and lots of them, a more complex system, but they also use fuel weight, not fuel flow as parameter.

I remember in my teens riding mopeds/light motorcycles. Fuel management was to run until it stopped, then switch to “reserve”, which was simply taking fuel from a lower part of the tank. Fool proof and always worked, unless you forgot you already had switched to reserve The Saab Safir has an identical system, as the only aircraft I know, it works perfectly. But the Safir also has a fuel gauge that actually works very well, and a check list. Running out of fuel, or mismanaging fluel in that aircraft, you have to be pretty brain dead.

The elephant is the circulation
ENVA ENOP ENMO, Norway

. A fuel level gauge and a fuel mass gauge is essentially the same thing physically, but using weight instead of level causes less sensitivity to fuel type and grade, it’s more accurate.

A “fuel mass gauge” is the combined output of a fuel level sensor and a densitometer.

T28
Switzerland

Isn’t mass v volume just a difference in the instrument markings?

One cannot measure mass in the fuel tank (because the pressure on a pressure sensor at the bottom of it is constant regardless of temperature) so the only way to display real mass would be by measuring the level of the liquid surface, and adjusting for its temperature. Hardly worth doing given the poor accuracy of any level gauge.

Mass flow could be done on a totaliser, however. A turbine sensor measures volume flow so to count true mass it would need temperature correction. One previous thread. I am fairly sure nobody in GA does that. There are direct mass flow sensors but they are expensive.

But does mass matter anyway, with piston engines especially? In terms of chemistry, it is mass flow rate of fuel being burnt which produces the power, so if you adjust for best-SFC carefully, 100kg of cold fuel should deliver the same range as 100kg of warm fuel. But the latter will take up more room in the tanks. Avgas expands 0.1%/degC so for a record breaking flight you want to fill up from a bowser which has been standing outside in -20C; you will get 3% more range than filling from an underground tank at +10C

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Isn’t mass v volume just a difference in the instrument markings?

You’ve answered that yourself… It is not due to temperature affecting density.

If I may speculate as to why light aircraft normally use volume and not mass is that temperature differences during operation are not that great. It’s different if you fly in the tropopause with OAT of -56°C.

ESKC (Uppsala/Sundbro), Sweden

T28 wrote:

A “fuel mass gauge” is the combined output of a fuel level sensor and a densitometer.

Not necessarily. The gauges used in the F-16 (and most jets of any kind for that matter) measures capacitance or resistance (don’t remember). This is dependent on the mass inside the level of where it is measured. As long as the mass is constant, the level can vary, due to density differences for instance, it makes no difference. Low density will cause a higher level, but the same resistance, or nearly so. Besides, the density doesn’t vary that much, and it easy to put a temp meter in there and correct it. The engine only cares about the mass flow in any case, and for the aircraft, only the mass is of importance. Each tank has several of them to get a good reading.

Anyway, no matter how you look at it, both mass and volume are derived values from what is actually measured (resistance or capacitance in this case), but it’s more closely related to mass than volume/level.

Last Edited by LeSving at 14 Feb 14:25
The elephant is the circulation
ENVA ENOP ENMO, Norway
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