Zero Fuel limit determination

3.9.1. Definition of the Zero Fuel Limit

On the balance chart it is necessary to check the Takeoff CG position against the Takeoff operational limit, this ensures that the aircraft CG position is within the certified takeoff limit.

Nevertheless this check is not sufficient to ensure that the aircraft CG remains within the in flight and landing limits during the whole flight.

This second check cannot be made easily by checking the landing CG position and the different flight CG positions because of the difficulty to estimate these values.

To perform this check an additional limit has been introduced : the Zero Fuel Limit.

The Zero Fuel limit is determined to ensure that during the whole flight and at landing the aircraft CG remains within the limits.

3.9.2. Zero Fuel limit computation method

Two different methods are used to determine the Zero Fuel limit, depending on procedure applied on the balance chart.

a) Zero fuel limit takeoff protected :

− IF Zero fuel CG is within the Zero fuel limit

− And IF takeoff weight and landing weight are compliant with the operational takeoff and landing maximum weights.

THEN Takeoff, Landing and In Flight CG are within the allowed limits using the whole flight.

With this type of limit, assuming that the takeoff and landing weight limitations are checked on the AHM516 (Load Message) the only check to be performed on the balance chart is to ensure that the Zero Fuel CG position is within the Zero Fuel limit.

b) Zero fuel limit not takeoff protected :

− IF Zero fuel CG is within the Zero fuel limit

− IF Takeoff CG is within the Takeoff operational limit

− And IF takeoff weight and landing weight are compliant with the operational takeoff and landing maximum weights.

THEN Takeoff, Landing and In Flight CG are within the allowed limits using the whole flight.

With this type of limit, assuming that the takeoff and landing weight limitations are checked on the AHM516 (Load Message) the check to be performed on the balance chart is to ensure that the Zero Fuel and Takeoff CG position are within the Zero Fuel and Takeoff operational limits.

c) Computation method :

For each type of Zero Fuel limit the calculation principle is the same, it consists in :

− Determining the possible aircraft CG position at Takeoff, Landing and Flight provided the Zero Fuel CG position is known, by analyzing the fuel vectors applicable to each flight phase. (refer to Fuel System Chapter).

− Using the fuel vectors, determining all the allowed Zero Fuel CG positions so that all possible takeoff (for takeoff protected limit), landing and in flight CGs remains within their operational limits from no fuel to maximum fuel quantity.

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The computation is based on the operational limits previously determined and on the possible CG positions at takeoff, landing and in flight for a given Zero Fuel CG position, these positions are given by the different fuel vectors.

For A320 family aircraft the takeoff, in flight and landing fuel vectors are identical.

a) Zero fuel limit takeoff protected :

Each flight phase fuel vector is leaned against the corresponding operational limit.

In the case of A320 aircraft the fuel vector is leaned against the more inside operational limit.

This operation is repeated for Zero Fuel weights between minimum aircraft weight and Maximum Certified Zero Fuel Weight.

For this example the final result is the pink area above.

Note : This area is determined for all fuel quantities between 0 to maximum fuel quantity.

But if the fuel quantity is lower than maximum fuel quantity the resulting Zero Fuel limit would be larger than the one above. So some operators using the takeoff protected Zero Fuel limit may chose to have several limits presented depending on the takeoff fuel quantity.

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b) Zero fuel limit not takeoff protected :

Landing and flight phase fuel vector are leaned against the corresponding operational limit. In the case of A320 aircraft the fuel vector is leaned against the more inside operational limit between landing and flight.

The takeoff limit is not used as the takeoff CG position is supposed to be checked within the operator procedure.

On the forward limit, the final part of the vector, which corresponds to the center tank refueling or consumption, is not taken into account (it goes out of the operational limits on the graph). This optimization is made to enlarge as much as possible the Zero Fuel limit. It is possible because the takeoff CG position is checked and because the fuel burn in the center tank shifts the aircraft CG aft. So it ensures that if the takeoff CG is within of the takeoff operational limit then the other CG positions will remain in the limits also during the whole flight.

This operation is repeated for Zero Fuel weights between minimum aircraft weight and Maximum Certified Zero Fuel Weight

For this example the final result is the pink area above.

Note : In this case the Zero Fuel limit is optimized and there is no need to have different limits for different fuel quantities.

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