USING THE MONITORED FUEL FACTOR
The predicted fuel flow is modified according to the following formula:
. PERF FACTOR(%) .
FF = FF × 1+
PRED MODEL ..
. 100 .
where . FFPRED is the fuel flow used for prediction
.
 FFMODEL is the fuel flow out of the FMS aircraft performance database

.
 PERF FACTOR(%) is the performance factor entered in the MCDU in percent

This correction is applied throughout the flight.
The PERF FACTOR can only be entered/modified on ground. It is entered in the AIRCRAFT STATUS page, like any other data. Read paragraph 0-2.6. Procedure to change the PERF FACTOR.
The PERF FACTOR is the sum of two different factors: -The basic FMS PERF FACTOR -The monitored fuel factor, using an aircraft performance monitoring method
(read chapter B-3 The cruise performance analysis methods)
In the following, the FMS performance factor will be referred to as FMS PERF FACTOR or PERF FACTOR.

2.4.2. Basic FMS PERF FACTOR
As a reminder, the nominal performance level of the aircraft is what we call the IFP level or book level (read paragraph D-1.The book level for more details). There is one IFP level per aircraft model. Several aircraft models may have the same IFP level if they strictly have the same in-flight performance.
On the other hand, the size of the FMS performance database is not sufficient to contain all the different aircraft performance models. Depending on the aircraft/engine combination, the FMS performance model may not be exactly the one of the aircraft on which it is installed. As a consequence, the engine type that is displayed on A/C STATUS page may not correspond to the installed one.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring

USING THE MONITORED FUEL FACTOR

Therefore, a correction should be applied when the aircraft FMS database does not exactly fit to the aircraft model. It results in FMS predictions consistent with the aircraft book level.

2.4.3. Monitored fuel factor
On the other hand, the actual aircraft drag and engine performance deviate from the nominal model due to the aircraft's aging process. Applying a correction will shift the performance level from the book level to the actual performance level enabling better fuel predictions. As a reminder, the monitored fuel factor can be obtained from one of the aircraft performance monitoring methods.
In the absence of measurements, the monitored fuel factor makes a default assumption in terms of fuel quality. Basically, FMS predictions are performed with a basic FLHV equal to 18400 BTU/LB (which is very conservative). As of a consequence, and in order not to over-penalize the FMS prediction, the monitored fuel factor should be corrected for the FLHV effect.

Book level (18590 BTU/LB)
　
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