The addition of one tonne to the landing weight, means an addition of K tonnes to the take-off weight.
For example, if K=1.3 and 1300 kg fuel is added at the departure, 1000 kg of this fuel amount will remain at the destination. So carrying one tonne of fuel costs 300 kg fuel more.
 
The extra-cost of the loaded fuel at departure is
Fuel weight x departure fuel price (.TOW x Pd  = .LW x K x Pd)
 
The cost saving of the transported fuel is
Transported fuel x arrival price               (.LW x Pa)
 
The cost due to a possible increase in flight time is
Flight time increase x cost per hour (.T x Ch) 
 
It is profitable to carry extra fuel if the cost saving exceeds the extra fuel loaded cost plus the extra time cost.
  (.LW x Pa) >(.LW x K x Pd) + (.T x Ch)
 
That is to say:
  .LW ( Pa - K x Pd) - (.T x Ch)  >   0
 
 
 
Therefore, if .T=0, it is profitable to carry extra fuel if the arrival fuel price to departure fuel price ratio is higher than the transport coefficient K.
  Pa  >  K  
  Pd
Thus carrying extra fuel may be of value when a fuel price differential exists between two airports. Graphs in the FCOM assist in determining the optimum fuel quantity to be carried as a function of initial take-off weight (without additional fuel), stage length, cruise flight level and fuel price ratio. The following graph is an example for an A320.
 

 
 
However the needs for accurate fuel planning is necessary to avoid arriving at the destination airport overweight. This could result in the economic benefit being eroded or negated due to extra hold time or circuits.
4.6  AUXILIARY POWER UNIT

 The Auxiliary Power Unit (A.P.U.) is a self-contained unit which makes the aircraft independent of external pneumatic and electrical power supply and environmental conditioning.
A.P.U. fuel consumption obviously represents very little in comparison with the amount of fuel for the whole aircraft mission. Nevertheless, operators have to be aware that adopting specific procedures on ramp operations can help save fuel and money.
 On the ground, A.P.U. fuel consumption depends on the A.P.U. type load and the ambient conditions. The minimum is when the APU is in the RTL(ready to load) condition. As additional loads, such as Electrical Loads(EL) and Environmental Conditioning System (ECS), are connected, the fuel consumption increases as shown in the following table (ISA, SL conditions).
 
Aircraft Type
 APU Model
 RTL
 RTL
Max EL
 Min ECS
　
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