4.2. Some precautions
The following illustrates the way to change the fuel factors throughout aircraft life. The result of cruise performance analysis gives the fuel factor as a function of time. Figure G1 shows an example of monitored fuel factor versus time.
Figure G1 - Example of Monitored Fuel Factor degradation with time
4.2.1. Monitored fuel factor trend line
The monitored fuel factor is established with a certain accuracy level as already explained at the beginning of this brochure. The determination method is a statistical one. For each month, the monitored fuel factor displays a certain scatter. This induces that the trend of the fuel factor is not purely monotonous. The monitored fuel factor can decrease from one month to another, while common sense may make one wonder how aircraft performance can increase with time.
This state of affairs imposes to be careful when changing the fuel factor. Indeed, changing it based on the monitored fuel factor over the preceding month will make the fuel factor go up and down by a few decimals. Some techniques are possible to get around these ups and downs. Some examples are given in paragraphs follows.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
POLICY FOR UPDATING THE FUEL FACTOR
4.2.2. Update frequency
Figure G1 shows something interesting about the evolution of the measured fuel factor. Indeed, over 6 months, the monitored fuel factor went up by 0.5%.
As of a consequence, checking the evolution of the monitored fuel factor is useless when it is performed too often. Most of the airlines check fuel factors once a month, which ensures noticeable and acceptable variations.
This rule applies for fuel factors determination. Aircraft performance monitoring with the APM program may also be used to monitor the aircraft and engine condition. In that case, the frequency must be adapted in order not to smooth the variations of the different and to hide some indicators.
4.2.3. Two examples of trigger condition for updating the fuel factors
The two examples explained below illustrate the way the decision to change the fuel factor is made in two different airlines. This procedure depends on the amount of conservatism the airline is ready to add to fuel fact determination of the fuel factor.
Indeed, changing fuel factors too early will increase predicted aircraft fuel consumption on computerized flight planning, leading to possibly carry more fuel than required. Airbus has not yet performed any check concerning the possible impact and is ready to discuss this item with any airline interested in the subject. Yet, the uncertainty on the monitored fuel factor is such that this does not affect the operations in a large extent.
4.2.3.1. Example 1: Step Fuel Factors The principle of this method is to retain approximate values for monitored fuel factors. The fuel factor is changed when a difference of more than a given percentage is noticed between the new figure and the last one retained. In other
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