Getting to Grips with Aircraft Weight and Balance(74)

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reference fuel vector that is leaned on operational flight and landing operational limits.
As actual fuel vector may be different from the reference one (due to a different fuel density),
flight and landing operational allowances have to include an additional margin covering the zero
fuel limit design.
To do so, maximum forward and aft index variation at critical points (because they are the points
used in the zero fuel limit design) between the reference fuel vector and the extreme fuel vectors
have to be included into the fuel allowance.

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C. BALANCE CHART DESIGN

b) Inaccuracy on the fuel weight
On the balance chart the fuel influence is taken into account as a delta index per total fuel quantity which is the sum of the delta indexes per individual tanks. For each tank the fuel quantity is known with a certain accuracy. This inaccuracy is linked to the Fuel Quantity Indicator (FQI) system inaccuracy whatever the flight phase an inaccuracy needs to be taken into account.
This difference between estimated and real fuel weight per tank will generate an inaccuracy on the aircraft CG determination. This inaccuracy is named fuel weight inaccuracy.
. Fuel weight inaccuracy
The real fuel weight is Wfuel real = Wfuel estimated ±.Wfuel For each tank, the inaccuracy is ±1% of the tank capacity, plus ±1% of the actual fuel quantity in the tank. .Wfuel tank n = ±1% Total_weighttank n ± 1% Actual_weighttank n
Note : if one tank is empty the .Wfuel tank is considered nil.
Note : These figures are conservative ones (refer to Aircraft Maintenance Manual section 28-42-00).

Each tank quantity is independent from the other tanks‘ one.

Such a weight deviation per tank has an impact on the actual fuel vector shape, some points of
the vector being shifted forward or aft.
Some points are more critical than others: those points corresponding to a fuel quantity for which
±.Wfuel may cause the aircraft CG be shifted out of the certified limits.
There are called critical points.

Critical points are defined phase per phase, one being critical for the forward certified limit and
another for the aft certified limit.
They are determined leaning the fuel vector on the aircraft certified limits, indeed throughout the
whole fuel vector the aircraft CG must remain in the limits.

WEIGHT AND
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WEIGHT AND
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C. BALANCE CHART DESIGN C. BALANCE CHART DESIGN

 

WEIGHT AND
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C. BALANCE CHART DESIGN

Fuel Weight inaccuracy (Efuel weight) is determined for the fuel quantity corresponding to the critical point. And it is calculated for each individual tank. Efuel weight = ±.Wfuel x (H-arm.Wfuel - H-armlimit)
Then the total inaccuracy is determined considering each fuel tank weight inaccuracy independent from the other ones.
　
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