TANK LEVEL SENSING - DESCRIPTION AND OPERATION

** ON A/C NOT FOR ALL

1. General
The Fuel-Level Sensing-System (FLSS) gives:

high level sensing
low level sensing
full level sensing
underfull level sensing
overflow level sensing
temperature sensing
shut-off of fuel recirculation for Integrated Drive Generator (IDG) cooling
center (transfer) tank to wing tank fuel-transfer control.

The Fuel-Level Sensing-System (FLSS) gives:

high level sensing
low level sensing
full level sensing
underfull level sensing
overflow level sensing
temperature sensing
shut-off of fuel recirculation for Integrated Drive Generator (IDG) cooling
wing tank intercell fuel transfer control
center tank fuel pump control.

When the Additional Center Tank(s) (ACT(s)) is (are) installed, the Auxiliary Level Sensing Control Unit (ALSCU) also gives outputs to:
- Identify when an ACT is installed.
- Give system data to the Auxiliary Fuel Management Computer (AFMC) (Ref. AMM D/O 28-51-00-00).
- Monitor the condition of the transfer and vent pipes in the forward ACTs.
- Isolate the forward ACTs if the transfer or vent pipes are damaged.
- Stop refuel to each ACT at high level (Ref. AMM D/O 28-25-00-00).
- Identify when the fuel level in each ACT is at a high or low level.
- Make sure that the high level lights come on at the refuel panel (Ref. AMM D/O 28-25-00-00).
- Control the operation of the ACT transfer pump during the fuel movement when the aircraft is on the ground (Ref. AMM D/O 28-28-00-00).
When the Additional Center Tank(s) (ACT(s)) is (are) installed, the Auxiliary Level Sensing Control Unit (ALSCU) also gives outputs to:
- Identify when an ACT is installed.
- Give system data to the Auxiliary Fuel Management Computer (AFMC) (Ref. AMM D/O 28-51-00-00).
- Monitor the condition of the transfer and vent pipes in the forward ACTs.
- Stop refuel to each ACT at high level (Ref. AMM D/O 28-25-00-00).
- Identify when the fuel level in each ACT is at a high or low level.
- Make sure that the high level lights come on at the refuel panel (Ref. AMM D/O 28-25-00-00).
- Control the operation of the ACT transfer pump during the fuel movement when the aircraft is on the ground (Ref. AMM D/O 28-28-00-00).
The Fuel-Level Sensing-System (FLSS) gives:

high level sensing
low level sensing
full level sensing
underfull level sensing
overflow level sensing
temperature sensing
shut-off of fuel recirculation for Integrated Drive Generator (IDG) cooling
center (transfer) tank to wing tank fuel-transfer control
fuel level data to the ACT transfer control circuit (Ref. AMM D/O 28-28-00-00), if the ACT(s) is/are installed.

The system has level sensors that the Fuel Level Sensing Control Units (FLSCUs) monitor continuously. The sensors are installed at different positions in the fuel tanks to give fuel level data in the range low level to overflow. The FLSCUs supply a voltage to the sensors electrical resistive-element. When the voltage returns to the FLSCUs it is compared with a specified reference to find if the related sensor is 'wet' or 'dry'. The FLSS uses the fuel level data to:

control the operation of the refuel valves during refuel and fuel transfer when the aircraft is on the ground

control the operation of the transfer pumps when the center (transfer) tank is at low level
make sure that the fuel in the wing tanks does not decrease to less than 200 kg (441 lb) below 'full', when the center (transfer) tank contains fuel
give fuel low-level warnings to the Engine/Warning Display (EWD) (Ref. AMM D/O 31-66-00-00).

Make sure that the fuel level in the wing tank(s) does not increase above the FULL mark when in flight. This occurs when the fuel (from the center tank) comes from the engines through the recirculation system (Ref. AMM D/O 28-16-00-00).
Make sure that when the aircraft is on the ground, the temperature of the engine oil is above 90 deg.C (194 deg.F), and the fuel level in the wing tank(s) does not increase above the FULL mark. This occurs when the fuel (from the center tank) comes from the engines through the recirculation system (Ref. AMM D/O 28-16-00-00).
Make sure that the fuel in the wing-tank inner cells does not decrease to less than the lower level when the wing-tank outer cells contain the fuel.
The fuel low-level warnings to the Engine/Warning Display (EWD) (Ref. AMM D/O 31-66-00-00).

control the operation of the center-tank transfer-valve actuators when the center-tank is at low level
make sure that the fuel in the wing tanks does not decrease to less than 200 kg (441 lb) below 'full', when the center (transfer) tank contains fuel
make sure that the fuel in the center-tank does not increase above high level, during automatic fuel transfer from the ACT(s) (if installed)
give fuel low-level warnings to the Engine/Warning Display (EWD) (Ref. 31-66-00).

For the ACT(s) installed, the FLSCU uses the center-tank high-level signals to control the manual mode fuel-movement from the ACT (when the aircraft is on the ground).
For the ACT(s) installed, the ALSCU monitors the level sensors in each ACT. The sensors are installed at different positions in the ACT(s) to give signals for the high and low level conditions. The ALSCU supplies voltage to the resistive element of its related sensors. When the voltage goes back to the ALSCU, it is compared with a specified value to find if a sensor is wet or dry.
The ALSCU uses the high-fuel level signals during refuel to make outputs that:
- Will close each ACT inlet valve as the fuel gets to the high level
- Will cause a specified warning light to come on at the ACT refuel panel (801VU) as each ACT gets to the high level.
The ALSCU uses the low-fuel level signals to make outputs to control the operation of the ACT inlet valves during the movement of the fuel.
The system has temperature sensors that the FLSCUs continuously monitor. The FLSCUs supply a voltage to the sensors electrical resistive-element. When the voltage returns to the FLSCUs it is compared to a specified reference to find if the fuel adjacent to the sensor is "hot". The FLSCUs use the "hot" signal to control the operation of the fuel recirculation for the cooling system (Ref. AMM D/O 28-16-00-00). Logic within the FLSCUs convert the "hot" signal into a control signal for fuel recirculation. This signal is then transmitted to the Full Authority Digital Engine Control (FADEC) (Ref. AMM D/O 73-20-00-00) to control the operation of the Fuel Return Valve (FRV) (Ref. AMM D/O 73-10-00-00).

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2. Component Location

Tank Fuel Level Sensing - Component Location ** ON A/C NOT FOR ALL

Fuel Level Sensing - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with ACT provision) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with forward ACTs installed) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with aft ACTs installed) - Component Location ** ON A/C NOT FOR ALL

ACT High-Level Sensor Selector-Switches 72QJ1,72QJ2 and 72QJ4 ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing - Component Location (with provision for ACTs) ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with ACT provision) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with aft ACTs installed) - Component Location ** ON A/C NOT FOR ALL

ACT High-Level Sensor Selector-Switches 72QJ1,72QJ2 and 72QJ4 ** ON A/C NOT FOR ALL

FIN	FUNCTIONAL DESIGNATION	PANEL	ZONE	ATA REF
** ON A/C ALL
7QJ	FLSCU-1		121	28-46-34
9QJ	FLSCU-2	92VU	122	28-46-34
15QJ1	SENSOR-LOW LEVEL 750/1500KG,L WNG TK INR FWD CHANNEL B	540AB	540	28-46-15
15QJ2	SENSOR-LOW LEVEL 750/1500KG,R WNG TK INR FWD CHANNEL B	640AB	640	28-46-15
16QJ1	SENSOR-LOW LEVEL (750KG),L WNG TK INR REAR CHANNEL A	540AB	540	28-46-15
16QJ2	SENSOR-LOW LEVEL (750KG),R WNG TK INR REAR CHANNEL A	640AB	640	28-46-15
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17QJ1	SENSOR-HI LEVEL, LEFT WING TANK INNER CHANEL A	540GB	540	28-46-15
17QJ2	SENSOR-HI LEVEL, RIGHT WING TANK INNER CHANEL A	640GB	640	28-46-15
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17QJ1	SENSOR-HI LEVEL, LEFT WING TANK INNER CHANEL A	540NB	540	28-46-15
17QJ2	SENSOR-HI LEVEL, RIGHT WING TANK INNER CHANEL A	640NB	640	28-46-15
** ON A/C ALL
20QJ	SENSOR-HIGH LEVEL, CHANNEL A	147AZ	141	28-46-15
21QJ	SENSOR-LOW LVL, L CTR TK CHANNEL A	147AZ	141	28-46-15
22QJ	SENSOR-LOW LVL, R CTR TK CHANNEL B	148AZ	142	28-46-15
23QJ1	SENSOR-FULL LVL, L WNG TK INR 1 CHANNEL B	540AB	540	28-46-15
23QJ2	SENSOR-FULL LVL, R WNG TK INR 1 CHANNEL B	640AB	640	28-46-15
24QJ1	SENSOR-UNDERFULL, L WNG TK INR 1 CHANNEL B	540AB	540	28-46-15
24QJ2	SENSOR-UNDERFULL, R WNG TK INR 1 CHANNEL B	640AB	640	28-46-15
** ON A/C NOT FOR ALL
25QJ1	SENSOR-UNDERFULL, L WNG TK INR 2 CHANNEL B	540CB	540	28-46-15
25QJ2	SENSOR-UNDERFULL, R WNG TK INR 2 CHANNEL B	640CB	640	28-46-15
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25QJ1	SENSOR-UNDERFULL, L WNG TK INR 2 CHANNEL B	540EB	540	28-46-15
25QJ2	SENSOR-UNDERFULL, R WNG TK INR 2 CHANNEL B	640EB	640	28-46-15
** ON A/C NOT FOR ALL
26QJ1	SENSOR-UNDERFULL, L WNG TK INR 3 CHANNEL B	540GB	540	28-46-15
26QJ2	SENSOR-UNDERFULL, R WNG TK INR 3 CHANNEL B	640GB	640	28-46-15
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26QJ1	SENSOR-UNDERFULL, L WNG TK INR 3 CHANNEL B	540LB	540	28-46-15
26QJ2	SENSOR-UNDERFULL, R WNG TK INR 3 CHANNEL B	640LB	640	28-46-15
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27QJ1	SENSOR-FULL LVL, L WNG TK INR 2 CHANNEL B	540JB	540	28-46-15
27QJ2	SENSOR-FULL LVL, R WNG TK INR 2 CHANNEL B	640JB	640	28-46-15
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27QJ1	SENSOR-FULL LVL, L WNG TK INR 2 CHANNEL B	540PB	540	28-46-15
27QJ2	SENSOR-FULL LVL, R WNG TK INR 2 CHANNEL B	640PB	640	28-46-15
** ON A/C ALL
28QJ1	SENSOR-HIGH LVL, LEFT WING SURGE TK CHANNEL A	540AB	540	28-46-16
28QJ2	SENSOR-HIGH LVL, RIGHT WING SURGE TK CHANNEL A	640AB	640	28-46-16
** ON A/C NOT FOR ALL
29QJ1	SENSOR-TEMP, LEFT WING TK INNER CHANNEL A	540AB	540	28-46-17
29QJ2	SENSOR-TEMP, RIGHT WING TK INNER CHANNEL A	640AB	640	28-46-17
** ON A/C NOT FOR ALL
29QJ1	SENSOR-TEMP, LEFT WING TK INNER CHANNEL A		540	28-46-17
29QJ2	SENSOR-TEMP, RIGHT WING TK INNER CHANNEL A		640	28-46-17
** ON A/C NOT FOR ALL
30QJ1	SENSOR-TEMP, LEFT WING TK OUTER CHANNEL A		540	28-46-17
30QJ2	SENSOR-TEMP, RIGHT WING TK OUTER CHANNEL A		640	28-46-17
** ON A/C ALL
38QJ1	SENSOR-LOW LEVEL, LEFT WING TK INNER CHANNEL A	540AB	540	28-46-19
38QJ2	SENSOR-LOW LEVEL, RIGHT WING TK INNER CHANNEL A	640AB	640	28-46-19
39QJ1	SENSOR-LOW LVL (750KG), L WING TANK CHANNEL A	540AB	540	28-46-15
39QJ2	SENSOR-LOW LVL (750KG), R WING TANK CHANNEL A	640AB	640	28-46-15
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40QJ1	SENSOR-LOW LVL (1500KG), L WING TANK CHANNEL A	540AB	540	28-46-15
40QJ2	SENSOR-LOW LVL (1500KG), R WING TANK CHANNEL A	640AB	640	28-46-15
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47QJ	FLSCU 3	92VU	121	28-46-34
** ON A/C NOT FOR ALL
60QJ	CONTROL UNIT-AUXILIARY LEVEL SENSING (ALSCU)		121	28-46-35
61QJ1	SENSOR-HIGH LEVEL NO1, ACT1		152	28-46-00
62QJ1	SENSOR-HIGH LEVEL NO2, ACT1		152	28-46-00
61QJ2	SENSOR-HIGH LEVEL NO1, ACT2		152	28-46-00
62QJ2	SENSOR-HIGH LEVEL NO2, ACT2		152	28-46-00
72QJ1	SEL SW-ACT1 HIGH LEVEL SENSOR	102VU	122	28-46-00
72QJ2	SEL SW-ACT2 HIGH LEVEL SENSOR	102VU	122	28-46-00
72QJ4	SEL SW-ACT4 HIGH LEVEL SENSOR	102VU	122	28-46-00

** ON A/C NOT FOR ALL

3. System Description

Tank Fuel Level Sensing - Component Location ** ON A/C NOT FOR ALL

Fuel Level Sensing - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with ACT provision) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with forward ACTs installed) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with aft ACTs installed) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing - Component Location (with provision for ACTs) ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with ACT provision) - Component Location ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing (with aft ACTs installed) - Component Location ** ON A/C NOT FOR ALL

A. Fuel-Level Sensors

The tank level sensing system has sensors which are installed in the tanks as follows:

The tank level sensing system has sensors which are installed in the tanks as follows:

The tank level sensing system has sensors which are installed in the tanks as follows:

(1) High Level Sensors:

one in each wing tank 17QJ1 (17QJ2)
one in the center tank 20QJ

close the related refuel valve
cause the related HI LVL light to come on at the refuel panel (Ref. AMM D/O 28-25-00-00).

(2) High Level Sensors:

one in each wing tank 17QJ1 (17QJ2)
one in the center tank 20QJ
two in the ACT, one on each fuel quantity probe 65QT1, 66QT1.
Only one of the high level sensors is connected to FLSCU-3 (Ref. AMM D/O 28-42-00-00).

The high level sensors are installed near the top of each fuel tank. The FLSCUs use signal conditioning to independently monitor the high level sensors. When the high level sensor in a fuel tank becomes wet, the FLSCUs close the related refuel valve.

During automatic forward transfer from the ACT to the center tank, if the center-tank high level sensor becomes wet:

the control circuit closes the ACT fuel transfer valve and the ACT fuel inlet valve.

If the center tank high-level sensor becomes dry, the FLSCU 3 sends a signal to the ACT transfer control circuit (Ref. AMM D/O 28-28-00-00) for the forward transfer of fuel to start.

(3) High Level Sensors:

one in each wing tank 17QJ1 (17QJ2)
one in the center tank 20QJ
two in the ACT1, one on each fuel quantity probe 65QT1, 66QT1.
two in the ACT2, one on each fuel quantity probe 65QT2, 66QT2.
Only one of the high level sensors in each ACT is connected to FLSCU-3 (Ref. AMM D/O 28-42-00-00).

The high level sensors are installed near the top of each fuel tank. The FLSCUs use signal conditioning to independently monitor the high level sensors. When the high level sensor in a fuel tank becomes wet, the FLSCUs close the related refuel valve.

During automatic forward transfer from the ACTs to the center tank, if the center-tank high level sensor becomes wet:

the control circuit closes the ACT fuel transfer valve and the ACT fuel inlet valves.

If the center tank high-level sensor becomes dry, the FLSCU 3 sends a signal to the ACT transfer control circuit (Ref. AMM D/O 28-28-00-00) for the forward transfer of fuel to start.

B. Fuel-Level Sensors (with ACTs installed)

The tank level sensing-system has sensors which are installed in the tanks as follows:

(1) High level sensors:

One in each wing tank 17QJ1 (17QJ2) and 28QJ1 (28QJ2)
One in center tank 20QJ
Two in AFT ACT1, active sensor 61QJ1 and spare sensor 62QJ1
Two in AFT ACT2, active sensor 61QJ2 and spare sensor 62QJ2
Two in ACT FWD, active sensor 61QJ4 and spare sensor 62QJ4.

NOTE: For the New Engine Option (NEO) aircraft that has an ACT installed in the forward cargo compartment, the term ACT FWD can also be written as ACT 4.1.

NOTE: Only one ACT high-level sensor in each ACT is in operation at a time.

The ACT high-level sensor that is in operation is selected with the related ACT high-level-sensor selector-switches (72QJ1, 72QJ2 and 72QJ4) on panel 102VU.
The high level sensors are installed near the top of the tanks that follow:

The center tank
The left wing tank, near rib 13
The right wing tank, near rib 13
The left wing tank, near rib 25
The right wing tank, near rib 25
AFT ACT1, AFT ACT2 and the ACT FWD.

The FLSCUs use the condition of the signal to independently monitor the high level sensors in the wing and center tanks. When the high level sensor in each tank becomes wet during refuel, the FLSCUs will give outputs to:

Close the related refuel valve
Cause the related HI LVL light to come on at the refuel panel (Ref. AMM D/O 28-25-00-00).

During the forward fuel movement from the ACT (manual mode on the ground), when the center tank high-level-sensor becomes wet, FLSCU1 will stop the operation of the ACT transfer pump (1QH).
The Auxiliary Level Sensing Control Unit (ALSCU) uses the signal condition to independently monitor the active high-level sensor in each ACT during refuel. When a high level sensor becomes wet, the ALSCU will give outputs to:

The Auxiliary Fuel Management Computer (AFMC) (Ref. AMM D/O 28-28-00-00)
Close the related ACT inlet valve
Cause the related HI LVL light to come on at the refuel panel.

(2) High level sensors:

One in each wing tank 17QJ1 (17QJ2) and 28QJ1 (28QJ2)
One in center tank 20QJ
Two in AFT ACT1, active sensor 61QJ1 and spare sensor 62QJ1
Two in AFT ACT2, active sensor 61QJ2 and spare sensor 62QJ2

NOTE: Only one ACT high-level sensor in each ACT is in operation at a time.

The ACT high-level sensor that is in operation is selected with the related ACT high-level-sensor selector-switches (72QJ1 and 72QJ2) on panel 102VU.
The high level sensors are installed near the top of the tanks that follow:

The center tank
The left wing tank, near rib 13
The right wing tank, near rib 13
The left wing tank, near rib 25
The right wing tank, near rib 25
AFT ACT1 and AFT ACT2.

The FLSCUs use the condition of the signal to independently monitor the high level sensors in the wing and the center tanks. When the high level sensor in each tank becomes wet during refuel, the FLSCUs will give outputs to:

Close the related refuel valve
Cause the related HI LVL light to come on at the refuel panel (Ref. AMM D/O 28-25-00-00).

The Auxiliary Fuel Management Computer (AFMC) (Ref. AMM D/O 28-28-00-00)
Close the related ACT inlet valve
Cause the related HI LVL light to come on at the refuel panel.

(3) Low Level Sensors:

two in each wing tank for 1500 kg (3307 lb) fuel levels, 15QJ1 (15QJ2) and 40QJ1 (40QJ2)
two in each wing tank for 750 kg (1653 lb) fuel levels, 16QJ1 (16QJ2) and 39QJ1 (39QJ2)
two in the center (transfer) tank 21QJ and 22QJ.

The low level sensors in each wing tank are installed in pairs as follows:

1500 kg (3307 lb) low level sensors. When the two sensors 15QJ1(15QJ2) and 40QJ1(40QJ2) are dry the FLSCU2(FLSCU1) causes a Fuel On Board MEMO to be supplied by the ECAM.
750 kg (1653 lb) low level sensors. When the two sensors 16QJ1(16QJ2) and 39QJ1(39QJ2) are dry the FLSCU2(FLSCU1) sends a fuel low level left (right) signal to the ECAM. When the fuel low level left (right) signal has been there for 30 seconds, the ECAM supplies a fuel low level left (right) warning.

(4) Low Level Sensors:

three in each wing tank for 750 kg (1653 lb) fuel levels, 15QJ1 (15QJ2), 16QJ1 (16QJ2) and 39QJ1 (39QJ2)
two in the center tank for 130 kg (287 lb) fuel levels 21QJ and 22QJ.

The low level sensors in the LH wing tank are installed as follows:

750 kg (1653 lb) low level sensors.
When the two sensors 16QJ1 and 39QJ1 are dry, the FLSCU2 sends a fuel low level left signal to the ECAM.

The low level sensors in the RH wing tank are installed as follows:

750 kg (1653 lb) low level sensors.
When the two sensors 15QJ2 and 39QJ2 are dry, the FLSCU1 sends a fuel low level right signal to the ECAM. When the fuel low level left (right) signal has been there for 30 seconds the ECAM supplies a fuel low level left (right) warning.

The two low level sensors in the center tank are installed near to the lowest points on the LH and RH side of the tank. The low level sensors control the automatic operation of the center tank fuel pumps. When one of the sensors has been dry for five minutes the FLSCUs send a stop signal to the related fuel pump (Ref. AMM D/O 28-21-00-00).

(5) Low Level Sensors:

two in each wing tank for 1500 kg (3307 lb) fuel levels, 15QJ1 (15QJ2) and 40QJ1 (40QJ2)
two in each wing tank for 750 kg (1653 lb) fuel levels, 16QJ1 (16QJ2) and 39QJ1 (39QJ2)
two in the center (transfer) tank 21QJ and 22QJ
two in the ACT1 (if installed), one on each fuel quantity probe 65QT1, 66QT1 (Ref. AMM D/O 28-42-00-00)
two in the ACT2 (if installed), one on each fuel quantity probe 65QT2, 66QT2 (Ref. AMM D/O 28-42-00-00).

The low level sensors in each wing tank are installed in pairs as follows:

1500 kg (3307 lb) low level sensors. When both the sensors 15QJ1(15QJ2) and 40QJ1(40QJ2) are dry the FLSCUs send a fuel low level warning to the ECAM.
750 kg (1653 lb) low level sensors. When both the sensors 16QJ1(16QJ2) and 39QJ1(39QJ2) are dry the FLSCUs send a fuel low level warning to the ECAM.

The two low level sensors in the center (transfer) tank are installed near to the lowest points on the LH and RH side of the tank. The low level sensors control the automatic transfer of fuel from the center (transfer) tank to their related wing tank. When one of the sensors has been dry for five minutes, a close signal is sent to the related control valve (Ref. AMM D/O 28-26-00-00).
The two low level sensors in the ACT(s) are installed near to the bottom, on the LH and the RH side of the tank(s).
When the ACT2 low-level sensors become dry, the FLSCU 3 sends a signal to the ACT transfer-control circuit to stop the forward transfer of fuel from ACT2 to ACT1.
When the ACT1 low-level sensors become dry, the FLSCU 3 sends a signal to the ACT transfer-control circuit to stop the forward transfer of fuel.

(6) Low level sensors:

There are four in each wing tank for 750 kg (1653 lb) fuel levels 15QJ1 (15QJ2), 16QJ1 (16QJ2), 38QJ1 (38QJ2) and 39QJ1 (39QJ2)
There are two in the center tank for 130 kg (287 lb) fuel levels 21QJ and 22QJ.

When the ACT(s) is (are) installed, there are:

One low level sensor in each ACT (three in total) that are part of the related fuel-quantity probes 70QT, 72QT and 76QT (Ref. AMM D/O 28-42-00-00).

The low level sensors in the left wing tank are installed as follows:

750 kg (1653 lb) low level sensors:
When the two sensors 16QJ1 and 39QJ1 are dry, FLSCU2 sends a fuel low-level left-signal to the Electronic Centralized Aircraft Monitoring (ECAM).

The low level sensors in the right wing tank are installed as follows:

750 kg (1653 lb) low level sensors:
When the two sensors 15QJ2 and 39QJ2 are dry, FLSCU1 sends a fuel low-level right-signal to the ECAM. When the fuel low-level left (right)-signal is there for 30 seconds, the ECAM supplies a fuel low-level left (right)-warning.

The two low level sensors in the center tank are installed near to the lowest points on the left and right sides of the tank. The low level sensors control the automatic operation of the center-tank fuel pumps. When one of the sensors is dry for five minutes, the FLSCUs send a stop signal to the related fuel pump (Ref. AMM D/O 28-21-00-00).
When the ACT(s) is (are) installed, the ALSCU monitors the condition of each low level sensor. If the sensor becomes dry, the ALSCU gives an output to the AFMC. This uses the input to control the related ACT inlet valve during the auto mode fuel-transfer (Ref. AMM D/O 28-28-00-00).

(7) Low level sensors:

There are four in each wing tank for 750 kg (1653 lb) fuel levels 15QJ1 (15QJ2), 16QJ1 (16QJ2), 38QJ1 (38QJ2) and 39QJ1 (39QJ2)
There are two in the center tank for 130 kg (287 lb) fuel levels 21QJ and 22QJ.

When the ACT(s) is (are) installed, there are:

One low level sensor in each ACT (two in total) that are part of the related fuel-quantity probes 70QT and 72QT (Ref. AMM D/O 28-42-00-00).

The low level sensors in the left wing tank are installed as follows:

750 kg (1653 lb) low level sensors:
When the two sensors 16QJ1 and 39QJ1 are dry, FLSCU2 sends a fuel low-level left-signal to the Electronic Centralized Aircraft Monitoring (ECAM).

The low level sensors in the right wing tank are installed as follows:

750 kg (1653 lb) low level sensors:
When the two sensors 15QJ2 and 39QJ2 are dry, FLSCU1 sends a fuel low-level right-signal to the ECAM. When the fuel low-level left (right)-signal is there for 30 seconds, the ECAM supplies a fuel low-level left (right)-warning.

(8) Full and Underfull Sensors:

two full sensors in each wing tank 23QJ1 (23QJ2) and 27QJ1 (27QJ2)
three underfull sensors in each wing tank 24QJ1 (24QJ2), 25QJ1 (25QJ2) and 26QJ1 (26QJ2).

The full and underfull level sensors are installed in the wing tanks. The FLSCUs use the full and the underfull level sensor data to control the automatic operation of the center tank fuel pumps. This controls the wing tank fuel level.
Data from the full level sensor is used to make sure the fuel level (in the wing tank) does not increase above the full mark.
Data from the underfull level sensor is used to make sure that the fuel level (in the wing tank) does not decrease to less than 500 kg (1102 lb) below the full mark (with fuel in the center tank).

(9) Full and Underfull Sensors:

Two full sensors in each wing tank 23QJ1 (23QJ2) and 27QJ1 (27QJ2)
Three underfull sensors in each wing tank 24QJ1 (24QJ2), 25QJ1 (25QJ2) and 26QJ1 (26QJ2).

The full and underfull level sensors are in the wing tanks. The FLSCUs use the full and the underfull level sensor data to control the automatic operation of the control valves.
When, during automatic fuel transfer, the full level sensors are wet, the control valves are set to off.
When, during automatic fuel transfer, the full and the related underfull level sensors are dry, the control valves are set to on.
The difference between the full and the underfull level sensors is approximately 200 kg (441 lb).

(10) Overflow Sensors:

one in each wing surge tank 28QJ1(28QJ2).

The overflow sensor is installed in the wing surge tank, immediately below the overflow point of the tank. Small quantities of fuel from the tank venting system (Ref. AMM D/O 28-12-00-00) can go into the surge tank (and return to the fuel tank) but do not touch the overflow sensor. Thus the usual operation of the tank venting system does not cause the overflow sensors to give an incorrect indication.
If an overflow sensor becomes wet the FLSCUs will send a signal to:

the annunciator light test and dimming system (Ref. AMM D/O 33-14-00-00). This system will signal the related L XFR/R XFR TK pump pushbutton switch (5QL, 6QL) (Ref. AMM D/O 28-26-00-00) FAULT annunciator to come on
the ECAM Engine/Warning Display (EWD) to show the warning FUEL - L(R) WING TK OVERFLOW
the FADEC to close the FRV (and thus stop the recirculation system (Ref. AMM D/O 28-16-00-00)).

(11) Overflow Sensors:

one in each wing surge tank 28QJ1(28QJ2).

The overflow sensor is installed in the wing surge tank, immediately below the overflow point of the tank. Small quantities of fuel from the tank venting system (Ref. AMM D/O 28-12-00-00) can go into the surge tank (and return to the fuel tank) but do not touch the overflow sensor. Thus the usual operation of the tank venting system does not cause the overflow sensors to give a wet indication.
If an overflow sensor becomes wet the FLSCUs will send a signal to the FADEC to close the FRV (and thus stop the recirculation system (Ref. AMM D/O 28-16-00-00)).

(12) Overflow Sensors:

one in each wing surge tank 28QJ1(28QJ2).

the ECAM Engine/Warning Display (EWD) to show the warning FUEL - L(R) WING TK OVERFLOW
the FADEC to close the FRV (and thus stop the recirculation system (Ref. AMM D/O 28-16-00-00)).

(13) IDG Shut-off Sensors:

one in each wing tank 38QJ1(38QJ2).

If an IDG shut-off sensor becomes dry (and the fuel quantity is less than 280 kg (617 lb)) the FLSCUs will send a signal to the FADEC to close the FRV (and thus stop the recirculation system (Ref. AMM D/O 28-16-00-00)).

The FLSCUs continuously monitor all the sensors. The FLSCUs use the data from the fuel sensors to:

control other parts of the fuel system
send fuel low level warnings to the EWD.

C. Fuel Level Sensing Control Units (FLSCUs)
The FLSCUs (7QJ and 9QJ) are in the FWD electronics rack 92VU, in the aircraft avionics bay.
The FLSCUs use signal conditioning to correctly find the condition of the level and temperature sensors.
Each FLSCU sends an electrical current to its related sensors, the voltage of which is set. When the electrical current comes back to an FLSCU from a sensor the voltage is measured. The FLSCUs compare the measured voltage to a set value to find if:

the sensor is wet or dry (for the level sensors)
the adjacent fuel is hot (for the temperature sensors).

The voltages that go back to the FLSCUs from groups of low level, full and underfull sensors go through groups of logic gates. The logic gates in the FLSCU control fuel transfer from the center tank to the wing tanks and IDG re-circulation.

The FLSCUs (7QJ and 9QJ) are installed in the FWD electronics rack 92VU, in the aircraft avionics bay.
The FLSCUs use signal conditioning to correctly find the condition of the level and temperature sensors.
Each FLSCU sends an electrical current to its related sensors, the voltage of which is set. When the electrical current goes back to a FLSCU from a sensor the voltage is measured. The FLSCUs compare the measured voltage to a set value to find if:

the sensor is wet or dry (for the level sensors)
the adjacent fuel is hot (for the temperature sensors).

The voltages that go back to the FLSCUs from groups of low level, full and underfull sensors go through groups of logic gates. The logic gates in the FLSCU control fuel transfer from the center tank to wing tanks and IDG re-circulation.

The FLSCUs (7QJ, 9QJ and 47QJ) are in the FWD electronics rack 92VU, in the aircraft avionics bay.
The FLSCUs use signal conditioning to correctly find the condition of the level and temperature sensors.
Each FLSCU sends an electrical current to its related sensors, the voltage of which is set. When the electrical current comes back to an FLSCU from a sensor the voltage is measured. The FLSCUs compare the measured voltage to a set value to find if:

the sensor is wet or dry (for the level sensors)
the adjacent fuel is hot (for the temperature sensors).

D. Fuel Level Sensing Control Units (FLSCUs) (ACT/s installed)

When the ACT(s) is (are) installed, FLSCU1 controls the ACT transfer pump (Ref. AMM D/O 28-28-00-00). During manual transfer on the ground FLSCU1 will set the transfer pump to the off position when the center tank gets to the high level.

E. Auxiliary Level Sense Control Unit (ALSCU)
The ALSCU (60QJ) is installed in FWD electronics rack 92VU in the aircraft avionics bay. When ACTs are not installed and the ALSCU is not energized then all its outputs are set to open circuit.
When the ACT(s) is (are) installed, the ALSCU sends an electrical current to each related low level sensor. It also sends an electrical current to the high level sensors selected at the ACT high-level sensor-selector-switches (72QJ1, 72QJ2 and 72QJ4). The electrical current that is received by the ALSCU is compared with a set value to find if the sensor is wet or dry.
The ALSCU uses the high level status to:

Put the high level lights on at the refuel panel (Ref. AMM D/O 28-25-00-00) if necessary
Close the applicable ACT inlet valve during auto mode refuel.

The ALSCU uses the low level condition to give outputs to the AFMC:

To auto fuel-transfer control (Ref. AMM D/O 28-28-00-00)
To show when faults occur during the auto fuel-transfer sequence in the cockpit. (Ref. AMM D/O 28-28-00-00).

The ALSCU (60QJ) is installed in FWD electronics rack 92VU in the aircraft avionics bay. When the ACTs are not installed and the ALSCU is not energized then all its outputs are set to open circuit.
When the ACT(s) is (are) installed, the ALSCU sends an electrical current to each related low level sensor. It also sends an electrical current to the high level sensors selected at the ACT high-level sensor-selector-switches (72QJ1 and 72QJ2). The electrical current that is received by the ALSCU is compared with a set value to find if the sensor is wet or dry.
The ALSCU uses the high level status to:

Put the high level lights on at the refuel panel (Ref. AMM D/O 28-25-00-00)
Close the applicable ACT inlet valve during auto mode refuel.

The ALSCU uses the low level condition to give outputs to the AFMC:

To auto fuel-transfer control (Ref. AMM D/O 28-28-00-00)
To show when faults occur during the auto fuel-transfer sequence in the cockpit. (Ref. AMM D/O 28-28-00-00).

F. ACT High-Level Sensor Selector-Switches (with ACTs installed)
The ACT high-level sensor-selector-switches (72QJ1, 72QJ2 and 72QJ4) are installed in panel 102VU in the aircraft avionics bay. Each switch has a "NORM" position and a "SPARE" position. The switches select which ACT high-level sensor is to be used in each ACT. You can put the ACT high-level sensor to the grounded position if the sensor is not selected.
The ACT high-level sensor-selector-switches (72QJ1 and 72QJ2) are installed in panel 102VU in the aircraft avionics bay. Each switch has a "NORM" position and a "SPARE" position. The switches select which ACT high-level sensor is to be used in each ACT. You can put the ACT high-level sensor to the grounded position if the sensor is not selected.

G. Temperature Sensors

The tank level sensing system has one temperature sensor, 29QJ1 (29QJ2), installed in each wing tank. It is sensitive to temperatures of more than 52.5 deg.C (126.5 deg.F).

The tank level sensing system has two temperature sensors, 29QJ1 (29QJ2) and 30QJ1 (30QJ2), installed in each wing tank. The temperature sensor 29QJ1 (29QJ2) is in the wing tank inner cell and is sensitive to temperatures of more than 52.5 deg.C (126.5 deg.F). The temperature sensor 30QJ1 (30QJ2) is in the wing tank outer cell and is sensitive to temperatures of more than 55 deg.C (131 deg.F).
Each temperature sensor is near the lowest part of the tank. This makes sure that the temperature sensor is in the fuel for most of the time.
Each temperature sensor is near the lowest part of the related fuel cell. This makes sure that the temperature sensor is in the fuel for most of the time.

H. Dummy Load Resistors

(1) Center-Tank High-Level Dummy-Load Resistor (6QJ)
The FLSCU 1 and the FLSCU 2 are the same. But only the FLSCU 1 supplies center-tank high-level sensing (through the high level sensor 20QJ). The equivalent FLSCU 2 center-tank high-level sensing channel has the dummy-load resistor (6QJ).
The dummy-load resistor makes sure that the center-tank high-level sensing channel in each FLSCU is used.

(2) Wing-Tank Temperature Dummy-Load Resistors (41QJ, 42QJ)
The FLSCU 1 and the FLSCU 2 each have two temperature sensing channels. One temperature sensor 29QJ1(29QJ2) is connected to each of the FLSCUs. Thus one temperature sensing channel in each of the FLSCUs is not used. This channel has the wing-tank temperature dummy-load resistor 41QJ(42QJ) connected to it
The dummy-load resistors make sure that the temperature sensing channel in each FLSCU is used.

** ON A/C NOT FOR ALL

4. Power Supply

High Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Overflow Level, IDG Shut-off and Temperature Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Full Level and Underfull Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Low Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

High Level Sensing - Electrical Schematic (with ACT1 and ACT2 installed) ** ON A/C NOT FOR ALL

Overflow Level, IDG Shut-off and Temperature Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Full Level and Underfull Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Low Level Sensing - Electrical Schematic (with provision for 2 ACTs) ** ON A/C NOT FOR ALL

Power Supplies - High Level Fuel Control ** ON A/C NOT FOR ALL

Power Supplies - High Level Sensing, ACT1 and ACT2 ** ON A/C NOT FOR ALL

Power Supplies - High Level Sensing, ACT 4.1 ** ON A/C NOT FOR ALL

Power Supplies - Low Level Sensing, ACTs ** ON A/C NOT FOR ALL

Power Supplies - Center Tank Pump and Recirculation Control ** ON A/C NOT FOR ALL

Power Supplies - IDG Cooling Control ** ON A/C NOT FOR ALL

Power Supplies - High Level Fuel Control ** ON A/C NOT FOR ALL

Power Supplies - High Level Sensing, ACT1 and ACT2 ** ON A/C NOT FOR ALL

Power Supplies - Low Level Sensing, ACTs ** ON A/C NOT FOR ALL

Power Supplies - Center Tank Pump and Recirculation Control ** ON A/C NOT FOR ALL

Power Supplies - IDG Cooling Control ** ON A/C NOT FOR ALL

** ON A/C NOT FOR ALL

5. Interfaces

Tank Fuel Level Sensing - Interfaces ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing - Interfaces (with ACT(s) installed) ** ON A/C NOT FOR ALL

Tank Fuel Level Sensing - Interfaces (with ACT installed) ** ON A/C NOT FOR ALL

** ON A/C NOT FOR ALL

6. Component Description

A. Level Sensor
[15QJ1] [15QJ2] [16QJ1] [16QJ2] [17QJ1] [17QJ2] [20QJ] [21QJ] [22QJ] [23QJ1] [23QJ2] [24QJ1] [24QJ2] [25QJ1] [25QJ2] [26QJ1] [26QJ2] [27QJ1] [27QJ2] [28QJ1] [28QJ2] [38QJ1] [38QJ2] [39QJ1] [39QJ2] [40QJ1] [40QJ2]
[15QJ1] [15QJ2] [16QJ1] [16QJ2] [17QJ1] [17QJ2] [20QJ] [21QJ] [22QJ] [23QJ1] [23QJ2] [24QJ1] [24QJ2] [25QJ1] [25QJ2] [26QJ1] [26QJ2] [27QJ1] [27QJ2] [28QJ1] [28QJ2] [38QJ1] [38QJ2] [39QJ1] [39QJ2]

B. Level Sensor (ACTs installed)
15QJ1(15QJ2), 16QJ1(16QJ2), 17QJ1(17QJ2), 20QJ, 21QJ, 22QJ, 23QJ1(23QJ2), 24QJ1(24QJ2), 25QJ1(25QJ2), 26QJ1(26QJ2), 27QJ1(27QJ2), 28QJ1(28QJ2), 38QJ1(38QJ2), 39QJ1(39QJ2), 61QJ1, 62QJ1, 61QJ2, 62QJ2, 61QJ4 and 62QJ4.
15QJ1 (15QJ2), 16QJ1 (16QJ2), 17QJ1 (17QJ2), 20QJ, 21QJ, 22QJ, 23QJ1 (23QJ2), 24QJ1 (24QJ2), 25QJ1 (25QJ2), 26QJ1 (26QJ2), 27QJ1 (27QJ2), 28QJ1 (28QJ2), 38QJ1 (38QJ2), 39QJ1 (39QJ2), 61QJ1, 62QJ1, 61QJ2 and 62QJ2.

(1) Description

Level or Temperature Sensor (Example) ** ON A/C NOT FOR ALL

Wing and Center Tanks Level or Temperature Sensor ** ON A/C NOT FOR ALL

The level sensor is a probe with a triangular mounting flange. A thermistor is installed in the probe. Holes in the probe let the fuel flow close to the thermistor.
The level-sensor has captive-screw connections that connect to ring-lug terminals. The sensor contains a thermistor bead and a fast-blow fuse to prevent dangerous conditions in the fuel-tank. The sensor assembly is installed on a wing rib.

(2) Description (ACTs installed)

Wing and Center Tanks Level or Temperature Sensor ** ON A/C NOT FOR ALL

ACT High Level Sensor ** ON A/C NOT FOR ALL

The level sensor used in the wing and center tanks has a moulded body with a triangular mounting flange. The sensor is installed on a rib that goes through the wing.
The level sensors has captive-screw connections that connect to ring-lug terminals. The sensors contains a thermistor bead and a fast blow fuse. The holes that go through the body let the fuel flow close to the thermistor. The fuse will prevent possible dangerous electric currents in the aircraft wiring.

The two high level sensors in each ACT are the same (only one sensor is active). The low level sensor in each ACT is part of the FQI probe (Ref. AMM D/O 28-42-00-00). The high level sensors are installed in the top of the ACT.
The sensor has a circular body that includes an electrical connector, a triangular mounting flange and a circular probe that extends into the ACT. The probe and the mating face of the flange include a groove. Each groove holds an O ring seal to make sure that fuel leakage does not occur.
The body holds a circuit board that includes a number of thermistors, resistors and diodes. One of the thermistors is installed in the probe.

(3) Operation
When an electrical current goes through the thermistor, its temperature increases. The electrical resistance of the thermistor changes with its temperature. When the sensor is in the fuel, the temperature increase is less than when the sensor is in the air. The FLSCU compares the current value from the sensor to a specified value to find if the related sensor is wet or dry.

(4) Operation (ACTs installed)
When an electrical current goes through the thermistor, its temperature increases. This change in temperature causes the electrical resistance of the thermistor to change. When the sensor is in the fuel, the temperature increase is less than when the sensor is in the air. The related FLSCU or ALSCU compares the current that goes back from the sensor to a specified value to find if the sensor is wet or dry.

C. Temperature Sensor(s)
29QJ1(29QJ2)
29QJ1(29QJ2), 30QJ1(30QJ2)

(1) Description

Level or Temperature Sensor (Example) ** ON A/C NOT FOR ALL

Wing and Center Tanks Level or Temperature Sensor ** ON A/C NOT FOR ALL

The temperature sensor is a molded probe with a triangular mounting flange. A platinum-wire resistor is installed in the probe. Holes in the probe let the fuel flow close to the resistor.
The temperature sensor has captive-screw connections that connect to ring-lug terminals. The sensor contains a fast-blow fuse. The fuse will prevent possible dangerous electric currents in the aircraft wiring. The sensor assembly is installed on a wing rib.
The resistance value of the sensor is 1000 ohms at 0 deg. C.

(2) Operation
The electrical resistance of the temperature sensor is in relation to the temperature of the adjacent fuel. When the temperature of the fuel, as calculated by the FLSCU, is at a specified level the FLSCU stops the recirculation system (Ref. AMM D/O 28-16-00-00).

D. Fuel Level Sensing Control Unit (FLSCU)
(7QJ,9QJ)

Fuel Level Sensing Control Unit - Detail ** ON A/C NOT FOR ALL

The FLSCU has:

two circuit card assemblies
a chassis
a front plate
an electronic module assembly.

The circuit cards are located in the chassis by integral rails. The front plate closes one end of the chassis, the electronic control module closes the other end. The electronic module assembly has a mother board assembly and a rear plate. The mother board assembly has two connectors that connect to the circuit card assemblies. The rear plate has a connector that has an upper and a lower part, the connector is the units interface with its rack in the avionics bay. The mother board assembly controls all the inputs and outputs to and from the circuit card assemblies and the unit.
The rear plate has three keying posts to make sure that the unit is installed correctly in the rack. Holes in the top and the bottom of the chassis let air flow through the unit to cool the internal components.
The FLSCU has these functions:

a current supply, kept to a limit of safe values, for each sensor
a detection circuit to find when a sensing thermistor is in fuel
a switch comparator which has relay-contact output-signals and logic output-signals
a fault monitoring circuit that continuously monitors for an open circuit or short circuit sensor (or sensor line) (and moves the output relay to the fail safe state)
Built In Test Equipment (BITE), controlled and monitored by the FQI computer, to do a test of the fuel system sensors.

E. Fuel Level Sensing Control Unit (FLSCU) (7QJ,9QJ and 47QJ)

Fuel Level Sensing Control Unit - Detail ** ON A/C NOT FOR ALL

The FLSCU has:

two circuit card assemblies
a chassis
a front plate
an electronic module assembly.

The circuit cards are located in the chassis by integral rails. The front plate closes one end of the chassis, the electronic control module closes the other end. The electronic module assembly has a mother board assembly and a rear plate. The mother board assembly has two connectors that connect to the circuit card assemblies. The rear plate has a connector that has an upper and a lower part, the connector is the units interface with the avionics bay rack. The mother board assembly controls all the inputs and outputs to and from the circuit card assemblies and the unit.
The rear plate has three keying posts to make sure that the unit is installed correctly in its position in the avionics bay rack. Holes in the top and the bottom of the chassis let air flow through the unit to cool the internal components.
The FLSCU has these functions:

a current supply, kept to a limit of safe values, for each sensor
a detection circuit to find when a sensing thermistor is in fuel
a switch comparator which has relay-contact output-signals and logic output-signals
a fault monitoring circuit that continuously monitors for an open circuit or short circuit sensor (or sensor line) (and moves the output relay to the fail safe state)
Built In Test Equipment (BITE), controlled and monitored by the FQI computer, to do a test of the fuel system sensors.

F. Auxiliary Level Sense Control Unit (ALSCU) (60QJ)

The ALSCU has:

A chassis
A front plate
A mother board assembly
A low level sensor-card
A high level sensor-card
A connector/protection board.

The chassis has integral rails in which the circuit cards are installed. The front plate closes one end of the chassis, and the other end is closed by the electronic module.
The electronic module includes:
- A circuit protection board
- A mother board.
The rear plate has three keying posts to make sure that the unit is installed correctly in the avionics rack. Holes in the top and the bottom of the chassis lets the air flow through the unit to decrease the temperature of the internal components.
The rear plate has a connector that has a top and a bottom part.
The connector is the units interface with the avionics rack in the avionics bay.
The circuit protection board has an EMI filter and gives protection during a lightening strike. The mother board includes ARINC 600 connector and supplies the interface between the level sense cards. The mother board assembly controls all the inputs and outputs to and from the circuit card assemblies and the unit.
The ALSCU has Built In Test Equipment (BITE) to monitor its own operation and that of its related high and low level sensors.
If an ACT is not installed, all the ALSCU outputs are set to open circuit.
The ALSCU can:

Identify when an ACT is installed.
Close the FWD ACT isolation-valve, if the FWD ACT is not installed.
Monitor the high level sensors of those ACT that are installed during refuel.
Close the related ACT inlet valves during auto refuel when the fuel gets to the high level.
Cause the HI LVL indicators at refuel/defuel panel 801VU to come on
Change the condition of outputs to the AFMC when the fuel in each ACT gets to the high level.
Monitor the low level sensors.
Change the status of outputs sent to the AFMC when the fuel in each ACT gets to the low level.
Identify when a fault occurs in the high or low-level sensor circuits and give the output to the AFMC.
Change the high-level sensor discrete-outputs when a high-level test is done.
Monitor the FWD ACT vent and refuel/transfer-pipe break detection-wires and close the FWD ACT isolation-valve, the FWD ACT vent-isolation-valve and the FWD-ACT fuel inlet-valve, if the pipe is damaged.
Calculate the ACT transfer sequence (this is used internally and also sent to the AFMC).
Find an ACT transfer fault with the status of the low level sensors.

The ALSCU has:

A Chassis
A Front plate
A Mother board assembly
A Low level sensor-card
A High level sensor-card
A Connector/protection board.

The chassis has integral rails in which the circuit cards are installed. The front plate closes one end of the chassis and the other end is closed by the electronic module.
The electronic module includes:
- A circuit protection board
- A mother board.
The rear plate has three keying posts to make sure that the unit is installed correctly in the avionics rack. Holes in the top and the bottom of the chassis lets the air flow through the unit to decrease the temperature of the internal components.
The rear plate has a connector that has a top and a bottom part.
The connector is the units interface with the avionics rack in the avionics bay.
The circuit protection board has an EMI filter and gives protection during a lightening strike. The mother board includes ARINC 600 connector and supplies the interface between the level sense cards. The mother board assembly controls all the inputs and the outputs, to and from the circuit card assemblies and the unit.
The ALSCU has Built In Test Equipment (BITE) to monitor its own operation and that of its related high and low level sensors.
If an ACT is not installed, all the ALSCU outputs are set to open circuit.
The ALSCU can:

Identify when an ACT is installed.
Monitor the high level sensors of those ACT that are installed during refuel.
Close the related ACT inlet valves during auto refuel when the fuel gets to the high level.
Cause the HI LVL indicators at ACT refuel/defuel control-panel 801VU to come on
Change the condition of outputs to the AFMC when the fuel in each ACT gets to the high level.
Monitor the low level sensors.
Change the status of outputs sent to the AFMC when the fuel in each ACT gets to the low level.
Identify when a fault occurs in the high or low-level sensor circuits and give the output to the AFMC.
Change the high-level sensor discrete-outputs when a high-level test is done.
Calculate the ACT transfer sequence (this is used internally and also sent to the AFMC).
Find an ACT transfer fault with the status of the low level sensors.

** ON A/C NOT FOR ALL

7. Operation/Control and Indicating
The tank level sensing system operates automatically. When a sensor is in the fuel it is wet and when it is not in the fuel it is dry. When a sensor becomes wet or dry it changes its signal to the FLSCU.
The tank-level sensing system operates automatically. When a sensor is in the fuel, it is wet and when it is not in the fuel, it is dry. When a sensor in the wing or center tank becomes wet or dry, it changes its signal to the FLSCU. When a sensor in the ACT(s) becomes wet or dry, it changes its signal to the ALSCU.

A. High Level Sensing

High Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

The high level sensors, 17QJ1(17QJ2) and 20QJ, control the high fuel level in each tank during refuel or a fuel transfer. When the high level sensor becomes wet, the FLSCU closes the related inlet valve. During a refuel, the FLSCU also causes the applicable HI LVL light (3QJ, 4QJ and 5QJ) to come on at the refuel/defuel control panel 800VU.

High Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

High level sensors 17QJ1(17QJ2) and 20QJ control the high fuel level in each tank during refuel or fuel transfer. When the high level sensor becomes wet, the FLSCU closes the related inlet valve. During a refuel procedure, the FLSCU also causes the applicable HI LVL light (3QJ, 4QJ and 5QJ) to come on at refuel/defuel control-panel 800VU.
The high level sensors in the ACT (if installed) will control the maximum fuel level in the ACT during auto refuel. When a high level sensor becomes wet, the ALSCU closes the inlet valve. It causes the HI LVL light 49QJ to come on at refuel/defuel control-panel 801VU at the same time.
During manual mode transfer of fuel from the ACT (if installed) (on the ground), the FLSCU will stop the ACT transfer pump when the center tank contents get to the high level.

B. High Level Sensing

High Level Sensing - Electrical Schematic (with ACT1 and ACT2 installed) ** ON A/C NOT FOR ALL

High level signals from the FLSCUs control the operation of the refuel valves during refuel and ground fuel transfers. The center tank high level signal from the FLSCU 1, is sent to the ACT transfer-control circuit (Ref. AMM D/O 28-28-00-00).
During a refuel, if the ACT(s) are installed, the FLSCU also operates the applicable HI LVL light (3QJ, 4QJ, 5QJ, 49QJ and 58QJ) on the refuel/defuel control panel 800VU and 801VU.

C. High Level Sensing

Power Supplies - High Level Sensing, ACT1 and ACT2 ** ON A/C NOT FOR ALL

High level sensors 17QJ1(17QJ2) and 20QJ control the high fuel level in the wing and center tanks during refuel or fuel transfer. When the high level sensor becomes wet, the FLSCU closes the related inlet valve. During a refuel procedure, the FLSCU also causes the applicable HI LVL light (3QJ, 4QJ and 5QJ) to come on at refuel/defuel control-panel 800VU.
The high level sensors 61QJ1 or 62QJ1, 61QJ2 or 62QJ2, 61QJ4 or 62QJ4 control the maximum fuel level in their related ACT during auto refuel. When a high level sensor becomes wet, the ALSCU closes the applicable inlet valve. It causes the related HI LVL light to come on at refuel/defuel control-panel 801VU at the same time.
During manual mode transfer of fuel from the ACT (on the ground), the FLSCU will stop the ACT transfer pump when the center tank contents get to the high level.

Power Supplies - High Level Sensing, ACT1 and ACT2 ** ON A/C NOT FOR ALL

High level-sensors 17QJ1 (17QJ2) and 20QJ controls the high fuel level in the wing and the center tanks during refuel or fuel transfer. When the high level sensor becomes wet, the FLSCU closes the related inlet valve. During a refuel procedure, the FLSCU also causes the applicable HI LVL light (3QJ, 4QJ and 5QJ) to come on at refuel/defuel control-panel 800VU.
High level-sensors 61QJ1 or 62QJ1 and 61QJ2 or 62QJ2 controls the maximum fuel level in their related ACT during auto refuel. When a high level sensor becomes wet, the ALSCU closes the applicable inlet valve. It causes the related HI LVL light to come on at refuel/defuel control-panel 801VU at the same time.
During manual mode transfer of fuel from the ACT (on the ground), the FLSCU will stop the ACT transfer pump when the center tank contents get to the high level.

D. Lo-level Sensing

Low Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

When the center-tank low-level-sensor (21QJ,22QJ) is dry for more than 5 minutes, the related center-tank pump control valve will close. If, during the 5 minutes the sensor becomes wet, for a minimum of 11 seconds, the center-tank pump control valve will stay open. The control valve will stay open until the sensor is continuously dry for five more minutes.

Low Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

When the center-tank low-level-sensor 21QJ, 22QJ is dry for more than five minutes, the related center-tank pump will stop. If, during the five minutes, the sensor becomes wet, for a minimum of 11 seconds, the center-tank pump will continue to operate. Each center-tank pump will operate until the sensor continues to be dry for five more minutes.

Power Supplies - Low Level Sensing, ACTs ** ON A/C NOT FOR ALL

E. Lo-level Sensing

Low Level Sensing - Electrical Schematic (with provision for 2 ACTs) ** ON A/C NOT FOR ALL

When the center-tank low-level sensor (21QJ,22QJ) is dry for more than 5 minutes, the related center-tank pump control valve will close. If, during the 5 minutes the sensor becomes wet (for a minimum of 11 seconds) the center-tank pump control valve stays open. The control valve will stay open until the sensor is continuously dry for five more minutes.
If the ACT(s) are installed, when the ACT low level sensors become dry the FLSCU 3 sends signals to the ACT transfer-control circuit (Ref. AMM D/O 28-28-00-00) for forward fuel transfer to stop.

F. Lo-level Sensing
When both low-level sensors 15QJ1 (15QJ2) and 40QJ1 (40QJ2) in one wing are dry a "FOB BELOW 3T" memo is shown on the ECAM display unit. When both low-level sensors 16QJ1 (16QJ2) and 39QJ1 (39QJ2) in one wing are dry for 30 seconds continuously, a 750 kg (1653 lb) LO LEVEL warning is shown on the ECAM display unit.
When the LH wing low-level sensors 39QJ1 and 16QJ1 are dry for 30 seconds continuously, a 750 kg (1653 lb) LO LEVEL warning message is shown on the ECAM display unit.
When the RH wing low-level sensors 15QJ2 and 39QJ2 are dry for 30 seconds continuously, a 750 kg (1653 lb) LO LEVEL warning message is shown on the ECAM display unit.
The low level sensor (Ref. AMM D/O 28-42-00-00) in each ACT is used during auto fuel transfer to the center tank. When a low level sensor becomes dry, the ALSCU changes the input to the AFMC to show the correct condition.
When the low level sensor 38QJ1 (38QJ2) is dry the FLSCU signals the fuel return valve to close. This stops the fuel recirculation system (Ref. AMM D/O 28-16-00-00), to prevent an increase in the amount of unusable fuel in the wing-tank.

G. Full and Underfull Level Sensing

Full Level and Underfull Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

The full level sensors, 23QJ1 (23QJ2), 27QJ1 (27QJ2), and the underfull level sensors, 24QJ1 (24QJ2), 25QJ1 (25QJ2) and 26QJ1 (26QJ2), are used to control the automatic operation of the fuel transfer to the wing from the center-tank jet-pumps. When one of the full level and one of the related underfull level sensors become dry, the FLSCU signals the center-tank jet-pump control-valve to open. When both full-level sensors become wet again the FLSCU signals the control valve to close.

Full Level and Underfull Level Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

The full level sensors, 23QJ1 (23QJ2), 27QJ1 (27QJ2) and the underfull level sensors 24QJ1 (24QJ2), 25QJ1 (25QJ2) and 26QJ1 (26QJ2) are used to control the automatic operation of the center tank fuel pumps. When any one of the full level and any one of the related underfull level sensors become dry, the FLSCU signals the related center tank fuel pump to operate. When both of the full-level sensors become wet again the FLSCU signals the related center tank fuel pump to stop.
The difference between the full and the underfull level-sensors is equal to a fuel quantity of 500 kg (1102 lb).
The difference between the full and the underfull level-sensors is equal to a fuel quantity of 200 kg (441 lb).

H. Overflow Level Sensing

Overflow Level, IDG Shut-off and Temperature Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Power Supplies - IDG Cooling Control ** ON A/C NOT FOR ALL

If fuel from the wing tank overflows into the vent surge tank, the FLSCU signals:

the Full Authority Digital Engine Control (FADEC) (Ref. AMM D/O 73-20-00-00) to close the fuel return valve.
the annunciator light test and dimming system (Ref. AMM D/O 33-14-00-00). This system will signal the related CTR TK XFR pump pushbutton switch (5QL, 6QL) (Ref. AMM D/O 28-26-00-00) FAULT annunciator to come on
the ECAM Engine/Warning Display (EWD) to show the warning FUEL - L(R) WING TK OVERFLOW

the Full Authority Digital Engine Control (FADEC) (Ref. AMM D/O 73-20-00-00) to close the fuel return valve.

the Full Authority Digital Engine Control (FADEC) (Ref. AMM D/O 73-20-00-00) to close the fuel return valve
the ECAM Engine/Warning Display (EWD) to show the warning FUEL - L(R) WING TK OVERFLOW.

I. Temperature Sensing

Overflow Level, IDG Shut-off and Temperature Sensing - Electrical Schematic ** ON A/C NOT FOR ALL

Power Supplies - IDG Cooling Control ** ON A/C NOT FOR ALL

The system has temperature sensors, the electrical resistance of which changes with the temperature of the adjacent fuel. The FLSCU measures the value of the current from the sensor and compares it to a specified value.
In flight, the FLSCU signals the EIU (Ref. AMM D/O 73-20-00-00) to close the FRV when the temperature of the fuel in the wing is more than 52.5 deg.C (126.5 deg.F). This keeps the fuel temperature in limits.

NOTE: On the ground, the signal from the FLSCU to close the FRV is overridden by the EIU. If the temperature of the fuel is high, the FWC (on the ECAM display) will show warnings (Ref. AMM D/O 28-40-00-00).

In flight, the FLSCU signals the EIU (Ref. AMM D/O 73-20-00-00) to close the FRV when:

the temperature of the fuel in the wing tank inner cell is more than 52.5 deg.C (126.5 deg.F)
the temperature of the fuel in the wing tank outer cell is more than 55 deg.C (131 deg.F).
This keeps the fuel temperature in limits.

NOTE: On the ground, the signal from the FLSCU to close the FRV may be overridden by the EIU (depending on engine type) . If the temperature of the fuel is high, the FWC (on the ECAM display) will show warnings (Ref. AMM D/O 28-40-00-00).

In flight or on the ground, the FLSCU signals the EIU (Ref. AMM D/O 73-20-00-00) to close the FRV when the temperature of the fuel in the wing is more than 52.5 deg.C (126.5 deg.F).
This keeps the fuel temperature in limits.

** ON A/C NOT FOR ALL

8. Test
The high-level sensors and their circuits are tested from the refuel/defuel panel 800VU, with the TEST switch (2QJ). The same switch also has a filament test facility. The Fuel Quantity Indicating System (FQIS) (Ref. AMM D/O 28-42-00-00) computer controlled Built-In Test Equipment (BITE) has test facilities for all the sensors and their circuits. When BITE is operated, with a failure-monitoring-signal, the Centralized Fault Display System (CFDS) (Ref. AMM D/O 31-30-00-00) will show and identify a:

defective sensor and/or
defective control-unit.

The high-level sensors and their circuits are tested from the refuel/defuel panel 800VU, with the TEST switch (2QJ). The same switch also has a filament test facility. The Fuel Quantity Indicating System (FQIS) (Ref. AMM D/O 28-42-00-00) computer controlled Built-In Test Equipment (BITE) has test facilities for FLSCU 1 and 2, all their sensors and circuits. When BITE is operated, with a failure-monitoring-signal, the Centralized Fault Display System (CFDS) (Ref. AMM D/O 31-30-00-00) will show and identify a:

defective sensor and/or
defective control-unit.
There are no BITE test facilities for the FLSCU 3 ACT level sensing circuits.

The test switch (2QJ) (on the panel 800VU) lets you test:

The lights and LEDs at the refuel/defuel panel
The high level sensors.

This toggle switch can be set to the LTS, the center off or the HI LVL positions.
When you set the switch to the LTS position:

The high level indicators on panel 800VU and panel 801VU come on.
The fuel quantity indicator (6QT) displays will show all 8s.
The fuel quantity preselector (5QT) displays will show all 8s.

When you set the switch to the HI LVL position:

The high level indicators that were off come on.
The high level indicators that were on go dim.

High level indicators that do not change condition shows a fault with the related sensor.

The Fuel Quantity Indicating System (FQIS) (Ref. AMM D/O 28-42-00-00) computer controlled Built-In Test Equipment (BITE) has test facilities for all the sensors and their circuits. When BITE is operated with a failure monitoring signal, the Centralized Fault Display System (CFDS) (Ref. AMM D/O 31-30-00-00) will give a message to identify:

A defective sensor in the wing or center tanks
A defective control unit.

[Rev.10 from 2021] 2026.04.02 07:01:10 UTC