Airbus Fuel Accuracy

Operational Liaison Meeting FBW aircraft 

Fuel accuracy 

Content 

 

Introduction



What systems to determine Fuel quantity in the aircraft?







Fuel Quantity Indicator (FQI) System



Manual Magnetic Indicators (MMI)

Why Fuel quantity discrepancies: 

On ground?



In flight?

Conclusion

Content 

 

Introduction



What systems to determine Fuel quantity in the aircraft?







Fuel Quantity Indicator (FQI) System



Manual Magnetic Indicators (MMI)

Why Fuel quantity discrepancies: 

On ground?



In flight?

Conclusion

FQI System description

 



The FQI system is a computerized system that measure: 

the total fuel mass



the total fuel volume



fuel quantity in each tank

Fuel Volume and mass are determined using  

Fuel probes (capacitance is proportional to fuel level) densitometers





FQI System description

Example of Probes location in an A320 aircraft

 

FQI System description

Fuel Quantities are displayed on the ECAM A320

A330

 

FQI System description

Fuel Quantities are displayed on the ECAM A320 FQI is inaccurate



Manual magnetic Indicators



MMIs are installed in each wing tanks.



Each tank’s fuel quantity on ground can be determined: 

reading the MMI dipstick number 



reading the aircraft attitude (grid square letter and number)



using the MMI table

1

A320 MMI

Wing Tanks MMI Table (Liter 



Content 



Introduction



What systems to determine Fuel quantity in the aircraft?







Fuel Quantity Indicator (FQI) System



Manual Magnetic Indicators (MMI)

Why Fuel quantity discrepancies: 

On ground?



In flight?

Conclusion

Why Fuel Quantity discrepancies on ground? 

 



At the end of refueling, discrepancies reported between: 

uplifted quantity measured by the bowsers



uplifted quantity measured using FQI

Those discrepancies may be due to: 

Bowser quantity and density error 



APU fuel used



FQI accuracy

 

Why Fuel Quantity discrepancies on ground? 

Bowser quantity and density tolerances: 

Volume tolerance: generally less than 0.5% of fuel uplifted



Density tolerance (both temperature accuracy and density reading error): 0.25% of fuel uplifted

 Total



bowser tolerance: 0.75% of fuel uplifted

APU fuel used: 

To be taken into account when comparing FOB pre-refuel and FOB post-refuel



depends on external condition and generator load

 APU

fuel consumption: between 100 and 150Kg/h



Why Fuel Quantity discrepancies on ground? 



The FQI is a gauging system.



Bias and tolerances can affect FQI system accuracy: 

tank manufacturing tolerances



FQI computer inaccuracies



tolerances in density determination



water that causes FQI over-reads



wing deflection

 

FQI accuracy on ground 

Single Aisle Aircraft

FOB 0 to 0.5% Max FOB 0.5%Max FOB to Max FOB



OVERREAD 0 1% FOB

UNDERREAD - (0.5% Max FOB +0.5% FOB)

Long Range Aircraft:

FOB Below 2% Max FOB 2% Max FOB to Max FOB

OVERREAD Decrease linearly to 0 +(0.5% Max FOB +0.5% FOB)

UNDERREAD Decrease linearly to -1% Max FOB - (0.5% Max FOB +0.5% FOB)

 

FQI accuracy on ground 

Single Aisle Aircraft

FOB 0 to 0.5% Max FOB 0.5%Max FOB to Max FOB



OVERREAD 0 1% FOB

UNDERREAD - (0.5% Max FOB +0.5% FOB)

Long Range Aircraft:

FOB Below 2% Max FOB 2% Max FOB to Max FOB

OVERREAD Decrease linearly to 0 +(0.5% Max FOB +0.5% FOB)

UNDERREAD Decrease linearly to -1% Max FOB - (0.5% Max FOB +0.5% FOB)

FAR/JAR 25.1337: Each fuel quantity indicator shall be calibrated to read “zero” during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply

Why Fuel Quantity discrepancies on ground? 



Single Aisle FQI Accuracy (GROUND)

MAX FOB= 19T

0.25

0.2

0.15

0.1

  s   e   n   n 0.05   o    T    E    C    N 0    A    R    E    L    O    T -0.05    I    Q    F -0.1

-0.15

-0.2

-0.25 0

2

4

6

8

10

Actual FOB Tonnes

12

14

16

18

20



Why Fuel Quantity discrepancies on ground?  LONG RANGE FQI ACCURACY (GROUND) MAX FOB=113T

1.5

1   s   e   n   n 0.5   o    T    E    C    N 0    A    R    E    L    O    T -0.5    I    Q    F

-1

-1.5 0

20

40

60

Actual FOB Tonne s

80

100

120



Why Fuel Quantity discrepancies on ground? 

Max discrepancy = Max [Fuel load(per bowser)- Fuel added on ECAM (FQI end-FQI start)]= [±Bowser dev ± FQI dev(end) ± FQI dev(start) +APU fuel (refuel)]

Why Fuel Quantity discrepancies on ground? 

 

EXEMPLE: (A320-200) 

ACTUAL FOB before refueling: 3T



ACTUAL FOB after refueling: 18T



Max TOTAL Bowser inaccuracy :



APU Fuel used for 30minutes



Max FQI deviation*

115kg 50kg 220kg  ______ 

TOTAL: 

385kg

The FQI deviation is only significant when FQI Over/ Under-readings before and after refueling are very different



Why Fuel Quantity discrepancies on groun Single Aisle FQI Accuracy (GROUND)

MAX FOB= 19T

0.25

0.2

0.15

0.1

  s   e   n   n 0.05   o    T    E    C    N 0    A    R    E    L    O    T -0.05    I    Q    F -0.1

-0.15

MAX deviat durin refuel



Why Fuel Quantity discrepancies on ground?  LONG RANGE FQI ACCURACY (GROUND) MAX FOB=113T

1.5

1   s   e   n   n 0.5   o    T    E    C    N 0    A    R    E    L    O    T -0.5    I    Q    F

-1

-1.5 0

20

40

60

Actual FOB Tonne s

80

100

120



Why Fuel Quantity discrepancies on ground? 



What about MMI use?



MMI accuracy: +/-5% FOB



Why Fuel Quantity discrepancies on ground?  Single Aisle FQI Accuracy (GROUND)

MAX FOB= 19T

1.5

1

  s   e 0.5   n   n   o    T    E    C    N    A    R 0    E    L    O    T    I    M    M    /    I    Q    F -0.5

FQI MMI

-1

-1.5 0

2

4

6

8

10

Actual FOB T onnes

12

14

16

18

20



Why Fuel Quantity discrepancies on ground?  LONG RANGE FQI ACCURACY (FLIGHT) MAX FOB=113T

8

6

4   s   e   n   n 2   o    T    E    C    N    A    R 0    E    L 0    O    T    I    M -2    M    /    I    Q    F

FQI 20

40

60

-4

-6

-8

Actual FOB Tonnes

80

100

120

MMI



Why Fuel Quantity discrepancies on ground? 



What about MMI use?



MMI accuracy: +/-5% FOB Do

not use MMI to check FQI system

Only

use MMI when FQI is inoperative

Content 

 

Introduction



What systems to determine Fuel quantity in the aircraft?







Fuel Quantity Indicator (FQI) System



Manual Magnetic Indicators (MMI)

Why Fuel quantity discrepancies: 

On ground?



In flight?

Conclusion



Why Fuel Quantity discrepancies in flight? 



Why Fuel Quantity discrepancies in flight? 

Why Fuel Quantity discrepancies in flight? 





When performing this check, discrepancies come from: 

BLOCK FUEL accuracy



FU accuracy



FOB accuracy

FU accuracy 





FU is the integration of the Fuel Flow over time



The accuracy of the Fuel Flow meter depends on: 

Fuel flow rate



Temperature conditions

But, keep in mind that the FU indication does not take into account APU fuel consumption

FU accuracy 





FU accuracy: 

Estimated to be better than +/-1.5%



For normal Cruise conditions ~+/-1%



FU accuracy on long Range Aircraft  Maximum F.USED Deviation 0.7 0.6 0.5 0.4 0.3    s    e 0.2    n    n    o 0.1    t    n 0    o    i    t -0.1    a    i    v-0.2    e    D-0.3 -0.4 -0.5 -0.6 -0.7 0

10

20

30

40

50

60

70

80

90

100

110

FOB tonnes F.US ED 31 tonnes

F.USED 67 tonnes

F.USED 103 tonnes

 

FQI accuracy in flight 

Single Aisle Aircraft

FOB 0 to 0.5% Max FOB 0.5%Max FOB to Max FOB



OVERREAD 0 1% FOB

UNDERREAD - (1% Max FOB +0.5% FOB)

Long Range Aircraft:

FOB Below 2% Max FOB

OVERREAD Decrease linearly to 0

2% Max FOB to Max FOB

+(0.3% Max FOB +0.95%FOB)

UNDERREAD Decrease linearly to –0.64% Max FOB - (0.3% Max FOB +0.95% FOB)



Why Fuel Quantity discrepancies in flight?  Single Aisle FQI Accuracy (Flight) MAX FOB= 19T

0.3

0.2

0.1   s   e   n   n   o    T 0    E    C    N    A    R    E    L -0.1    O    T    I    Q    F

-0.2

-0.3

-0.4 0

2

4

6

8

10

Actual FOB Tonnes

12

14

16

18

20



Why Fuel Quantity discrepancies in flight?  LONG RANGE FQI ACCURACY (FLIGHT) MAX FOB=113T

2

1.5

1

  s   e   n 0.5   n   o    T    E    C    N 0    A    R 0    E    L    O    T    I -0.5    Q    F

20

40

60

-1

-1.5

-2

Actual FOB Tonnes

80

100

120

Why Fuel Quantity discrepancies in flight? 

 

The maximum theoretical Combined FOB and FU deviation in indication is possible when: 

At departure the FQI system has the maximum deviation (positive or negative)



At time X, the FQI system has the maximum deviation, but with the opposite sign to the FQI at departure, and the F. USED deviation moves in the same sense

Why Fuel Quantity discrepancies in flight? 

 

EXEMPLES: 

A340 aircraft which takes off with 113 Tons on board. Fuel check with 10 Tons on board:

 MAX

Theoretical Combined FOB and FU deviation:

2.554 Tons 

A340 aircraft which takes off with 41 Tons on board. Fuel check with 10 Tons on board:

 MAX

Theoretical Combined FOB and FU deviation:

1.749 Tons 

Comparing FOB departure-FOB+FU is a good practice !