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On what basis is the weight unit of the FQIS (Fuel Quantity Indicating System) selected by airlines of various countries? Does it depend on the local country for using imperial or metric units?

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Short answer

There is no mandatory unit, manufacturers are not constrained, customers use a fuel unit consistent with the unit used for other mass data, like gross mass, either kg or lbm.

Fuel mass information is computed from volume sensed by tank probes and fuel density.


Applicable regulation

For US and EU countries and large aircraft:

  • US: CFR 25.1337.b Fuel quantity indicator says

    There must be means to indicate to the flight crewmembers, the quantity, in gallons or equivalent units, of usable fuel in each tank during flight

  • EASA countries, Certification Specifications, CS 25.1337(b):

    There must be means to indicate to the flight-crew members, the quantity, in litres, (gallons), or equivalent units [...].

Manufacturer and customer choices

From a manufacturer standpoint:

  • This is defined at country level, actually at regulator level.
  • For FAA and EASA regulated countries, as highlighted by @Bianfable, the actual physical quantity to measure is not specified, it can be mass or volume.
  • For FAA and EASA regulated countries, there are no mandatory units, imperial or international.

This means manufacturers have no constraint on the unit.

What can do airlines:

  • Use an international mass unit, kg or tonne, since energy is dependent on fuel mass. This is likely the choice for most countries around the world where SI is used for business. This unit will be consistent with other mass data in kg:

    enter image description here

    A330 ECAM, all masses in kg, source

  • Use an avoirdupois mass unit, pound-mass (lbm) or long ton, in countries where avoirdupois units are used for business.

    enter image description here

    A320 ECAM, all masses in lbm, source

  • Use a units of volume, liter (l) or gallon (gal) as suggested in regulations. This is an unlikely choice for transport aviation as mass is meaningful both for mass/weight and balance calculation and and range determination. So there is little interest in displaying a volume which would need to be converted into mass by the crew.

Converting from volume to mass

Raw data is a volume sensed by capacitive or ultrasonic probes in the tanks. When mass is shown to the crew, it's just a conversion from volume to mass:

  • This conversion must take into account fuel density. Fuel density for a given type of fuel varies with temperature.
  • Simpler conversion systems use a standard fuel density of 0.8 kg/l corrected by actual fuel temperature when available.
  • There is an option to use the actual density measured by an additional densitometer.

As tanks have complex shapes, e.g. to match wing available space, and probes sense a fuel level, volume evaluation is difficult:

The relationship between height and volume depends on the pitch angle ’θ’ and roll angle ‘φ’ of the tank. At present aircraft such as Airbus 320 use look up tables to link height to volume for a set of pitch and roll angles [...] In order to accurately determine the quantity of fuel contained in these tanks large amount of measured data is continuously available as the output of the sensors and interpreted.

For more on this conversion topic, see How is fuel mass measured in airliners?


Mixing units

As mentioned in @busdriver answer, using different units is a source of confusion, and has led to fuel loading errors in the past, with the much talked-about dual engine failure of Air Canada 143:

  • Canada had just switched from avoirdupois to SI
  • Newly delivered Boeing 767 had their fuel system set in kg.
  • AC143 crew ordered fuel in kg.
  • The refueller computed a volume with a coefficient valid for lbm to l conversion, as it was common at the time.
  • Half the quantity of fuel was loaded.

The FQIS was not functioning, the error went unnoticed by the crew. Pilots were able to land the Boeing 767 after they ran out of fuel, gliding on a distance of 120 NM.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Jamiec
    Nov 4, 2021 at 17:14
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In modern airliners this is configuration option that the operator can choose. I don't think there is a requirement on which unit to use, but it would make sense to use the unit that is intuitive to the operators pilots and other staff. The famous case of mixing kgs and lbs is of course the Gimli Glider.

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