getting to grips with fuel economy(9)

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Within JAR OPS, there are several definitions of Contingency fuel, depending on diversion airfields, fuel consumption monitoring, etc. Full details can be found in “Getting to Grips with Aircraft Performance”, but briefly the fuel is the greater of two quantities:
 
1. 5 minutes hold fuel at 1500 feet above destination at ISA

2. One of the following quantities:

 
 5% of trip fuel,
  3% of trip fuel with an available en route alternate airport 
 15 minutes hold fuel at 1500 feet above the destination at ISA 
 20 minutes trip fuel, based upon trip fuel consumption.
 
The last 3 options require airworthiness approval and the last 2 options require fuel consumption monitoring with fuel based on results. What we can conclude is that depending on the flight distance, there is a lowest contingency fuel. 
 
 
The following graphs show the different contingency fuel quantities for different distances for an A320.
 
The graphs for other members of the A320 family are similar and indicate that below about 500nm, the contingency fuel is set by the minimum 5-minute hold value. Above about 1000nm, contingency fuel can be reduced to 3% of trip fuel if there is an en-route alternate available. If not, reductions can be made above about 2000nm by using the 15-minute destination hold option, which always requires less fuel than the 20 minute trip fuel option.

 
The graphs for the other aircraft show different characteristics because of their longer-range capability.
 
The A340-600 picture, on the following page, indicates that with no en-route alternate the 15-minute destination hold requirement enables the contingency fuel to be reduced above 2150nm. An en-route alternate will give more benefit until 3500nm, beyond which the 15-minute destination hold minimises the contingency fuel requirement. The A340-500 is similar.
 
The A300, A310, A330 and other A340’s have slightly different critical distances as follows: 
 
 5%  trip fuel/15-minute hold 1700 to 1900nm. 
 3%  trip fuel/15-minute hold   2800 to 3200nm
 
However these will also vary with weight, winds, temperature, etc so 
the limiting reserve should always be checked. Each aircraft type will show critical sector distances beyond which a change in contingency policy will yield benefits.
 
 

One further method of reducing the contingency fuel is by using a Decision Point or Redispatch Procedure. This involves the selection of a decision point where the aircraft can either continue to the destination as the remaining fuel is sufficient, or it can reach a suitable proximate diversion airport. More details are given in “Getting to Grips with Aircraft Performance”.
　
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