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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.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 15 -
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 enroute
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.
Contingency Fuel - A320-214
0
100
200
300
400
500
600
700
800
900
1000
500 1000 1500 2000 2500 3000
Sector Distance - nm
Fuel - kg
Minimum - 5 min Hold at 1500ft
15 min Hold at 1500ft
20 min trip fuel
3% trip fuel
5% trip fuel
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 16 -
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”.
To minimize the alternate fuel, the alternate airports should be chosen as
near as possible to the destination.
Both the JAA and FAA do not require the alternate fuel reserve in certain
cases, depending on meteorological conditions and the suitability of the airport.
More details are given in “Getting to Grips with Aircraft Performance”.
Another part of the reserves is the extra fuel, which is at the Captain’s
discretion.
There are many reasons why this extra fuel is necessary. It could be due to
uncertain weather conditions or availability of alternate and destination airfields,
leading to a probability of re-routing. However it is often due to lack of confidence
in the flight planning and the natural desire to increase reserves.
This is the one area where a significant impact can be made through
accurate flight planning. With this in place, the aircrew will see that the flight
plans fuel burns are being achieved in practice. They will realize that the planned
reserves are adequate and that there is no need for more.
Contingency Fuel - A340-642
0
1000
2000
3000
4000
5000
6000
7000
2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
Sector Distance - nm
Fuel - kg
Minimum - 5 min Hold at 1500ft
15 min Hold at 1500ft
20 min trip fuel
5% trip fuel
3% trip fuel
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 17 -
4.4 TAXIING
Good estimate of taxi times are required. Actual times need to be
monitored and standard estimates changed as necessary. Jet engine performance
is optimised for flight conditions, but all aircraft spend considerable time on the
ground taxiing from the terminal out to the runway and back. This time has
increased due to airport congestion, and increased airport size. This all leads to a
waste of precious time and fuel.
Only using one engine for taxiing twin-engine aircraft, or two engines for
four-engine aircraft can give benefits in fuel burn. Such procedures need to be
considered carefully, and operators have to define their field of application.
　
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