Weight / Volume of the fuel and metering system.

[Fri13]

Just came up to my mind that as kerosine weights just 0.8kg per liter and I remember reading that the Su-25 etc calculates the fuel in kilos instead in liters, the question came up to my mind.

 

How does aircraft meter the weight of the fuel? It would be easy to understand how it is metered in volume but the weight is little question. Or do they just meter it in volume and then report it in kilos?

 

What is directly my question is what mechanism is used to meter the amount of fuel and how does it work. It is clear that it is easy to do when aircraft is parked on ground or flying steady and on level. But how about when pulling G's etc?

[fltsimbuff]

I suppose it would depend on the plane... The F-15E for example:

 

"The fuel quantity indicating system provides readouts in pounds of all usable internal, external and CFT fuel quantities. Fuel quantities are measured by floating type fuel level sensors hence erroneous fuel indications are possible during and immediately after hard maneuvering resulting from fuel slosh in the tanks."

 

Source: http://www.f-15e.info/joomla/technology/fuel-system/93-fuel-quantity-indicator

[cichlidfan]

I don't think it depends on the aircraft. I can't imagine any other practical method than to measure volume, and then, if desired, translate it to weight.

[fltsimbuff]

I don't think it depends on the aircraft. I can't imagine any other practical method than to measure volume, and then, if desired, translate it to weight.

 

I read the question as asking how the volume is measured rather than how that volume is converted to weight (which is pretty straight-forward). Maybe I misread.

[cichlidfan]

We actually addressed different parts of the post, now that I read it again. ;)

 

EDIT: I would assume that most, if not all aircraft, have inaccurate fuel readings while maneuvering. In general, it probably isn't an issue, because if you are performing abrupt or high g maneuvers, you probably aren't worried about your fuel at that moment.

Edited September 29, 2015 by cichlidfan

[Sierra99]

I don't think it depends on the aircraft. I can't imagine any other practical method than to measure volume, and then, if desired, translate it to weight.

 

No...Fuel is measured by Weight, not volume. By volume, a gallon of fuel can change significantly with temperature changes.

By weight, a pound of fuel always has the same amount of thermal energy in it. Depending on where the fuel is refined and what additives it has, you can get anywhere from 6.2 to 6.9 lbs per gallon.

 

Grist for the mill:

 

http://www.dtic.mil/get-tr-doc/pdf?AD=ADA554221

 

Sierra.

[sobek]

No...Fuel is measured by Weight, not volume.

 

And how is this accomplished, inside an airplane?

[chaos]

And how is this accomplished, inside an airplane?

 

Every day the current 'specific gravity' of the fuel is calculated (depends on temp.). The pilot requests a certain amount in kilos/pounds for 'the mission'. The fueling department can then calculate the amount of litres/gallons to be brought onboard.

 

Same thing happens inside an aircraft (the bigger ones anyway... not your uncle's Cessna). The probes in the tanks measure volume and by conversion present it as kilos/pounds in the cockpit.

Edited September 30, 2015 by chaos

[mvsgas]

Each aircraft may have different ways to determine volume, weight , etc. or several at the same time.

Some examples:

A capacitance-type Fuel Quantity Measuring Subsystem (FQMS) is used to provide an indication of the quantity and location of fuel in the aircraft.

 

Four thermistor-type sensor assemblies are located in the reservoirs to provide air ejection and fuel low indications.

 

In addition, IVSC A1 and B1 each contains an identical Fuel Interface Modules (FIM) that provide drive and control voltages, currents, and signal conditioning to operate the height, density, and velocity sensors, and interface to IVSC data and power busses. The sensors provide fuel height (25 sensors), Velocity Of Sound (VOS) (5 sensors), fuel level (4 DC capacitive tank units), and fuel density (2 densitometers) data to the FMS. Each module controls and reports approximately 50 percent of the total fuel interface information. Fuel probes, ultrasonic and capacitive, are arranged throughout the aircraft in an interleaved scheme allowing either FIM to independently report fuel quantity from any major tank.

Edited September 30, 2015 by mvsgas

[Retu81]

Every day the current 'specific gravity' of the fuel is calculated (depends on temp.). The pilot requests a certain amount in kilos/pounds for 'the mission'. The fueling department can then calculate the amount of litres/gallons to be brought onboard.

 

Same thing happens inside an aircraft (the bigger ones anyway... not your uncle's Cessna). The probes in the tanks measure volume and by conversion present it as kilos/pounds in the cockpit.

 

Sounds pretty complex. What we did with Hawks was just check the required amount in kilos on the cockpit fuel gauge and then multiply that with 1.3 to get the required amount in litres. Of course if one wanted to be really accurate, then one could have just climbed in the cockpit, turn on the battery and watch the fuel gauge during refuelling. Though that would have been against the regulations and no one did that, of course. :music_whistling:

[sobek]

The probes in the tanks measure volume and by conversion present it as kilos/pounds in the cockpit.

 

This is exactly what i was getting at. Fuel measurement inside vehicles is done based on fill level, not some weighing measurement technique.

Edited September 30, 2015 by sobek

[chaos]

. Though that would have been against the regulations and no one did that, of course. :music_whistling:

 

Noone breaks the rules in aviation... it's a rule! :D