The FMGS (Flight Management and Guidance System) contains a database that aids in predicting aircraft performance and calculating necessary data. However, variations in aircraft configurations can lead to discrepancies between the database information and the aircraft’s actual performance. To address this, the FMGS applies corrections to its computations.
Over time, factors such as aircraft drag and engine performance may deviate from the database’s baseline figures. This necessitates periodic adjustments to ensure accurate performance predictions.
PERF Factor:
The PERF factor is used by the FMGS to adjust the predicted fuel consumption. For example, if the PERF factor is set to +1.8, it indicates that the aircraft’s fuel consumption is 1.8% higher than the baseline performance model. Typically, older aircraft require more significant corrections.
IDLE Factor:
The IDLE factor is used to fine-tune the aircraft’s vertical profile during descent, specifically from the Top of Descent (TOD) to the first altitude constraint. It modifies the idle thrust by adding a delta thrust, allowing the aircraft to maintain the computed vertical profile despite unexpected wind conditions.
1.A positive IDLE factor results in a less steep descent profile, causing the descent to begin earlier.
2.A negative IDLE factor makes the descent profile steeper, leading to a later start of the descent.
These adjustments ensure the aircraft follows an optimal descent path.
