POLICY FOR UPDATING THE FUEL FACTOR
3.5
Monitored Fuel Factor (%)
3
2.5
2
1.5
1 Sep-01 Oct-01 Nov-01 Dec-01 Feb-02 Mar-02 Apr-02 May-02
2.4 2.5 2.7 2.8 2.9 2.6 2.8 3 2.8
Month
Retained Monitored Fuel Factor
Figure G2 - Step Fuel Factor method
This example assumes a minimum delta of 0.5% is the condition for the update of the fuel factor.
In October 2001, the monitored fuel factor was set to 2.5 %. All the fuel factors (FMS PERF FACTOR, Flight Planning fuel factor…) were updated taking into account this new value.
In the following months, until March 2002, the monitored fuel factor was being evaluated monthly to be compared to the previous one retained (2.5%). None of the monitored fuel factors got above 2.5% + 0.5% = 3.0%, so no update was performed. In April 2002, the monitored fuel factor got equal to 3.0%. The retained fuel factor became 3.0% instead of 2.5%, because the margin was exceeded. In May 2002, the monitored fuel factor got below 3.0% again. No change is made to avoid ups and downs (which cannot be avoided around the step values).
Definitely the advantage of this method is that it is a simple technique, easily controllable. The only point is to define the margin. In common practice, the determination of the fuel factor is scattered and biased. One should ensure the retained margin does not bias the fuel factor too much.
Using this technique, one could also imagine to retain a more conservative fuel factor envelope (i.e. changing the fuel factor as soon as a monitored factor goes above the retained one). In figure G2, we would set the retained fuel factor to 3.0% starting as from October 2001.
Jan-02
Actual Monitored Fuel Factor
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
POLICY FOR UPDATING THE FUEL FACTOR
4.2.3.2. Example 2: Smoothed Fuel Factors The principle of this method is to get rid of the monitored fuel factor variations by smoothing the curve to get a purely monotonous curve. No more margin is then required. This smoothing technique allows to increase the
accuracy of the fuel factor to the decimal. A margin can still be implemented but it can be reduced compared to the previous method.
Generally, this technique is more sophisticated and gives a more accurate trend line.
Figure G3 shows the evolution of the actual monitored fuel factor and the retained one over time. The actual monitored fuel factors were averaged over the last three months, which gives quite acceptable results and trends.
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