3. A PERF FACTOR FOR EACH AIRCRAFT?
For the sake of simplification, it may be tempting to try and determine fuel factors, applicable to all tail numbers. Indeed, doing so will avoid multiple calculations for a specific aircraft and would allow to use the same flight planning basic information for the whole fleet. Yet, this will certainly penalize most of the fleet. Indeed, the different tails of an airline are not delivered at the same date. The different aircraft may be allocated on different routes, accumulating different cycles and flight hours. The maintenance done on the aircraft may also result in different consequences for cruise performance analysis (engine change on a specific aircraft will definitively change the monitored fuel factor for the concerned aircraft). To sum up, each individual aircraft has its own history.
Airlines usually tailor the performance factor to each individual aircraft. Refining the cruise performance analysis at the tail number level allows to adjust the book level to the actual aircraft performance of each tail number. Thus, for a given tail number, the computerized flight planning, the FMS predictions and any route study will be customized to each individual aircraft.
It is worth mentioning that the other advantage of routine performance monitoring is that analysis result may evidence unusual conditions by comparing each tail number to the rest of the fleet. Thus, this procedure may also contain trigger conditions for warning the airline maintenance department, in order to keep the aircraft as good as possible.
4. CHANGING THE FUEL FACTOR
4.1. Introduction
Changing the fuel factors is defined in each airline fuel policy. It may vary a lot depending on the airline structure and means available for flight planning and flight operations. The following will show some examples, which cannot be put into place "as is" but should anyway be adapted to each individual airline’s needs.
Basically, the fuel factor(s) has(ve) to be updated following noticeable modification of the fuel consumption. Specific attention is required after major maintenance actions (engine change for instance). Such a modification is of course determined based on the aircraft performance history. The point is to identify what lies behind "noticeable modification". This definition is the airline’s responsibility.
Indeed, some airlines change the fuel factor as soon as an evolution is detected/monitored, while some others use various smoothing techniques. The difference between the two is of course the margin for conservatism.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring

POLICY FOR UPDATING THE FUEL FACTOR
Whatever the airline policy, some techniques are usually used to monitor the trend of the fuel factor evolution versus time.
　
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