Fuel Quantity Indication

The FOB figure on the EWD looks like a measurement. It is a computation: capacitance probes give fuel height, tank geometry turns height into volume, a densitometer turns volume into mass — with corrections for attitude, acceleration and fuel properties along the way. This article explains the chain, the accuracy contract (and how it loosens with attitude), why FOB does not equal the sum of the tanks, the degradation ladder (amber dashes → XX), and the separate level-sensing system that stays trustworthy when the FQI lies.

1. The measuring chain

"Each FQI probe has a capacitance value that changes in proportion to the depth of fuel in the related tank. The FCMS uses the probe capacitance values and the fuel permittivity to find the volume of usable fuel in that tank. The FQI densitometer 41QT1(41QT2) sends signals to the FCMS which are in proportion to the density of the fuel. The FCMS uses the fuel volume calculation together with the density data to find the fuel mass (fuel quantity)."

  capacitance probes ──► fuel height at each probe
        + compensator (fuel permittivity reference)
        + tank geometry in FCMC memory
        + ADIRU accelerations → effective attitude
                                   │
                                   ▼
                              fuel VOLUME
        + densitometer (actual fuel density)
                                   ▼
                              fuel MASS ──► EWD FOB / SD tanks / 990VU

Probes are distributed through every tank (driven half by each FCMC — the dispatch consequence of that split is in FCMS computers); the compensator probes sit permanently submerged and track the fuel's permittivity so the height measurement self-calibrates to the fuel actually loaded; the densitometers (one per FCMC) close the volume-to-mass step. Temperature and property changes are absorbed by exactly these two correctors:

"Changes in the temperature and the properties of a fuel cause changes in its permittivity and density. When this occurs, the compensators (for permittivity) and the densitometers (for density) make adjustments to their signals."

Attitude is the third corrector: the FCMS knows each probe's position and the tank's shape, takes ADIRU acceleration data, and computes the effective attitude to read a sloshing, banked, accelerating tank correctly. If ADIRU data is lost, defaults apply — in flight, effective pitch +2.0°, roll 0°; on the ground, standard attitude.

2. The accuracy contract — tightest on the ground, loosest inverted

And at the bottom of the tanks, the errors are biased the safe way:

"There is no overread when: ‐ the indicated tank quantity is less than 1% of the tank/cell capacity ‐ the indicated total quantity is less than 2% of total-capacity."

Near empty, the gauge may under-read but will not flatter you. Note the THS condition in the ground row — the stabiliser's angle is part of the quantity contract, the same geometry that limits maximum refuel capacity (refuelling).

3. Why FOB ≠ the sum of the tanks

Fuel lives in pipes as well as tanks, and the FQI accounts for it in three different ways:

So immediately after refuelling, FOB = Σ tanks + 60 L + 252 L. When the lines drain into a tank, the amounts reappear in that tank's figure and FOB = Σ tanks again. The FCMS declares the gallery drained when one inner lo-level sensor is dry and any inlet valve (inner / centre / outer) is open. A flight crew reconciling FOB against tank totals to the last kilogram is chasing plumbing, not an error.

4. The degradation ladder

"If the FQI data is between 2 and 4 times less accurate than usual: ‐ it causes the target Center of Gravity (CG) to be moved forward 1.5% Mean Aerodynamic Chord (MAC) ‐ the tank indication shows =. If the fuel quantity indication data is more than 4 times less accurate than usual: ‐ the indication for that tank shows XX ‐ it stops automatic CG control and sends a signal to the crew to start a forward fuel-transfer."

Degraded data buys conservatism; failed data ends the optimisation. The same grading appears on the refuel panel: during refuel/defuel a degraded or failed tank simply shows no indication in its 990VU window.

5. The 990VU indicator — the master's voice only

During refuel/defuel the panel's FQI indicator repeats the quantities — with a data-source rule worth knowing:

"The FQI indicator only accepts the data from the FCMC that has control of the fuel system (master FCMC). If the master FCMC has no data (or incorrect data), then the FQI indicator does not operate its LED display."

A dark refuel panel display points first at the master FCMC, not the panel. Display resolution: tank windows to 10 kg, ACTUAL and PRESELECTED to 100 kg. And one ergonomic detail: the preselect switch changes slowly for the first 4 seconds, then speeds up — fine adjustment first, coarse second.

6. Level sensing — the independent witness

Separate from the capacitance FQI, discrete level sensors report wet/dry at fixed points: high-level (refuel cut-off), low-level (the LO LVL alerts), and surge-tank overflow. They share neither probes nor computation with the quantity system — which is exactly why the FUEL OVERREAD procedure can declare the quantity indication unreliable and still state "FUEL LO LVL ALERTS REMAIN RELIABLE" (quantity and level faults). One system tells you how much; the other tells you whether you've crossed a line. They fail independently, and the procedures lean on that independence.

Self-test

[!note]- Q1. Walk the chain from probe to the FOB figure. Probe capacitance → fuel height; + compensator permittivity + tank geometry + ADIRU effective attitude → volume; + densitometer density → mass; → EWD/SD/990VU.

[!note]- Q2. Why is FOB larger than the sum of the tank figures right after refuelling? The trim pipe's 60 usable litres and the gallery's 252 litres are counted in FOB but belong to no tank; they rejoin a tank's figure when the lines drain.

[!note]- Q3. What does a tank showing amber dashes cost you? Showing XX? Dashes (2–4× degraded): CG target moves forward 1.5 % — fuel-burn penalty, function preserved. XX (>4×): automatic CG control stops; manual forward transfer.

[!note]- Q4. The refuel panel's display is dark. First suspect? The master FCMC — the 990VU indicator only accepts the master's data and goes dark without it.

[!note]- Q5. The crew has declared FQI unreliable. What may they still trust, and why? The low-level (and other level-sensing) alerts — discrete wet/dry sensors fully independent of the capacitance quantity chain.

Key takeaways

References

• AMM 28-42-00 Description and Operation §7 (chain, accuracy tables, adjustments, 990VU rules, degradation).
• FCOM DSC-28-10-130 (indication, pilot level); PRO-ABN FUEL OVERREAD (LO LVL reliability statement).
• AMM 28-46-00 (level sensing).
• The independence framing in Section 6 is integrative synthesis of the cited systems.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.