Magnetic Indicators and Fuel Temperature

Two measurement systems that need no spotlight until the day they decide a dispatch: the manual magnetic indicators (MMI) — unpowered dipsticks built into the wing skin that can prove the fuel load when the gauges cannot — and the fuel temperature measurement that feeds the LO TEMP / HI TEMP alerting with one deliberate blind spot.

1. MMI — measuring fuel with a magnet and gravity

The MMI (also called dripless sticks) are graduated rods housed in the lower wing surface. The principle:

        inside the tank
   ┌───────────────────────┐
   │   ~~~~~ fuel ~~~~~    │
   │   ┌────────┐          │
   │   │ FLOAT  │ ← rides the fuel surface,
   │   │ (ring  │   carries a magnet
   │   │ magnet)│          │
   │   └───╔════╗──────────│
   └───────║rod ║──────────┘
           ║    ║ ← graduated stick, unlocked and
           ╚════╝   lowered from below the wing;
            ▼▼▼     its top magnet couples to the float
       read the scale at the skin line

A quarter-turn unlocks the stick; it slides down until its magnetic head couples with the float ring riding the fuel surface; the graduation read at the wing-skin datum gives the fuel height at that station. Height is not yet mass: the reading is converted through tables, corrected for aircraft attitude (pitch/roll from the ground check) and fuel density — the same physics the FQI handles electronically (fuel quantity indication), done by hand.

What makes the MMI worth its weight:

totally unpowered — works on a dark aircraft;
independent of the FQI chain — shares no probe, computer or wiring with the gauges, so it can arbitrate when the FQI is in doubt (the FUEL OVERREAD aftermath, suspect refuel figures, FQI-degraded dispatch);
dispatchable in bulk: some operators' MEL allows MMIs inoperative as a low-category item — they are a verification tool, not a flight instrument; the operational cost appears only in the specific MEL items that require an MMI check as a dispatch condition (several FQI-degraded relief items do exactly that).

2. Fuel temperature — four probes and a missing one

Fuel temperature is measured in the LH outer tank, both inner tanks, and the trim tank — displayed on the SD FUEL page per tank. The RH outer tank carries no temperature probe.

[!warning]- Why the RH outer goes unmeasured The outer tanks are thermally the most exposed (thin wing, tip position — they cool first) and thermally symmetric: left and right see the same air at the same speed. One probe in the LH outer covers both wings' worst case. The asymmetry is a deliberate economy, not an oversight — but it means a right-outer temperature is inferred, never read. (Rationale is integrative synthesis; the probe fit is per the FCOM indication description.)

How the readings are used:

cold side: the LO TEMP alert thresholds sit per tank (inner −37 °C / outer −40 °C / trim −40 °C at the first level), against the fuel's freezing point — the full cold-fuel strategy (transfer the cold fuel to warmer tanks, raise TAT) is in fuel temperature faults;
hot side: the inner tanks are the watch items on the ground (IDG cooling return dumps heat there — engine feed); the HI TEMP alert logic keys on the inners;
trim probe and dispatch: the trim-tank temperature indication rides FCMC2's excitation half — with FCMC2 inoperative, the trim temperature is considered lost (FCMS computers), one reason the LIM-FUEL JET B/JP4 rule (trim tank empty and isolated) and cold-fuel procedures get stricter on a degraded aircraft.

3. Where the two systems meet

Both are truth sources of last resort: the MMI for quantity, the probe set for temperature. A useful habit treats them as a pair — when the computed picture (FQI mass, predicted fuel temperature) disagrees with the physical one (stick reading, indicated tank temperature), the physical source wins until proven otherwise. The procedures encode the same priority: OVERREAD trusts the conservation arithmetic and level sensors over the gauges; LO TEMP trusts the indicated tank temperature over any forecast.

Self-test

[!note]- Q1. How does an MMI reading become a fuel mass? The stick gives fuel height by magnetic coupling to a float; tables convert height to volume corrected for measured aircraft attitude; density (from the fuel slip / measurement) converts volume to mass — the FQI's computation, done manually.

[!note]- Q2. What two independences make the MMI valuable? No power needed, and no shared components with the FQI chain — it can arbitrate an FQI dispute even on an unpowered aircraft.

[!note]- Q3. Which tanks have temperature probes, and which does not? LH outer, both inners, trim. The RH outer has none — left-outer data covers the symmetric worst case.

[!note]- Q4. Why do the inner tanks matter most for high fuel temperature? The IDG cooling return dumps engine-generator heat into the inner tanks — on the ground with little burn-off, that is where temperature climbs first.

[!note]- Q5. The FQI says 60 t, the sticks say 55 t. Which governs? The physical source until the disagreement is resolved — MMIs exist precisely to arbitrate the gauges; a 5 t disagreement is a no-go item for investigation, not averaging.

Key takeaways

Point	Value
MMI principle	float-magnet coupled stick; height → tables (attitude + density) → mass
MMI independence	unpowered, separate from FQI; arbitration tool
MMI dispatch	inoperative MMIs low-category — but some FQI relief items require an MMI check
Temperature probes	LH outer, both inners, trim; RH outer none (symmetry economy)
Hot watch	inner tanks (IDG return); cold watch per-tank LO TEMP thresholds
Priority habit	physical source over computed picture when they disagree

References

AMM 28-42/28-44 (MMI construction and use); FCOM DSC-28-10-130 (temperature probe fit and indication).
FCOM LIM-FUEL (temperature limits context); PRO-ABN-FUEL (LO TEMP / HI TEMP usage of the probes).
Some operators' MEL (MMI items; FQI relief items requiring stick checks).
The RH-outer rationale and the arbitration habit are integrative synthesis.

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.