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Applicable to: ALL
PREDICTIONS FOR ALTERNATES
Predictions for alternates are displayed on the ALTERNATES page.
They are based on:
‐ A default cruise FL equal to 220, if the airway distance is less than 200 nm. Otherwise, it is
FL 310
‐ Simplified wind/temperature models, based on flight crew entries:
• ALT CRZ wind, as entered on the FUEL PRED page
• CRZ temperature interpolated from the temperature model for the primary flight plan.
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT MANAGEMENT
PERFORMANCE - PREDICTIONS
A318/A319/A320/A321 FLEET DSC-22_20-40-30 P 16/24
FCOM 19 AUG 10
‐ Airway distance, or direct distance, as provided by the database (manual entry, if not in the
database)
‐ Cost index = 0 (minimum fuel)
‐ Initial aircraft weight equal to landing weight at primary destination.
Note: 1. No step can be inserted in an alternate flight plan
2. No predictions are displayed for the selected alternate on flight plan pages. However,
the flight crew can read ALTN trip fuel and time on the INIT B page before engine start,
and estimated time and estimated fuel on board at alternate on the FUEL PRED page
after engine start.
RETURN-TO-TRAJECTORY ASSUMPTIONS
Applicable to: ALL
If the aircraft is not on the lateral flight plan, predictions assume an immediate return to the active
lateral leg with a 45 ° convergence angle, or that it will fly directly to the “TO” waypoint, when the
required convergence angle is greater than 45 °.
IDLE FACTOR
Applicable to:
,
,
MSN,,,,
IDLE and PERF factors follow the same principle.
The PERF factor is mainly used for prediction during the cruise phase.
The IDLE factor is dedicated to the FM descent segment.
The aim of the IDLE factor is to adjust FM descent predictions. In particular, the Top of Descent (T/D)
position, with the actual engine idle thrust used during descent.
A positive IDLE factor gives an earlier T/D (shallower path).
A negative IDLE factor delays the T/D (steeper path).
The following graph provides an example (average values) of the IDLE factor's effect on descent
length:
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT MANAGEMENT
PERFORMANCE - PREDICTIONS
A318/A319/A320/A321 FLEET DSC-22_20-40-30 P 17/24
FCOM 19 AUG 10
Example: An IDLE factor of -3 decreases the computed descent length by 5 nm.
IDLE FACTOR AT DELIVERY
The IDLE factor set at delivery is 0 %.
PROCEDURE FOR MODIFYING THE IDLE FACTOR
Refer to DSC-22_20-40-30-B PERF Factor - Procedure to Modify the PERF Factor (On Ground
only).
ENERGY CIRCLE
Applicable to: ALL
The energy circle is a green arc, centered on the aircraft’s position and oriented towards the
current track line. It is displayed on the NDs during descent, when HDG or TRK mode is selected. It
represents the required distance to land from the aircraft’s position down to airport elevation at VAPP
speed, considering all speed constraints on the vertical profile.
PERF FACTOR
Applicable to: ALL
GENERAL
The PERF factor is a positive or negative percentage that is used to correct the predicted fuel
flow, used for fuel prediction computation within the FMGS. It is necessary when the aircraft’s
performance differs from the performance model stored in the FMGS database.
This difference can be due to one or both of the following cases:
1. The FMS contains a performance database, used to compute the predictions and the
performance data. Due to the numerous possible aircraft configurations, the same performance
database is sometimes used for aircraft with slightly different behaviors. In these cases, a PERF
factor is entered to correct the computations performed with a database not exactly tailored for
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT MANAGEMENT
PERFORMANCE - PREDICTIONS
A318/A319/A320/A321 FLEET DSC-22_20-40-30 P 18/24
FCOM 19 AUG 10
the given configuration. As a result, the aircraft or engine type identification on the MCDU’s A/C
STATUS page may not correspond to that of the actual aircraft.
2. Since the actual aircraft drag and engine performance deviate from the nominal model, due to
the aircraft’s age, airline Flight Operations will periodically apply a correction factor to adapt fuel
predictions to actual fuel consumption.
The PERF factor modifies the predicted fuel flow, according to the following formula:
FFpred is the FF used for prediction.
FFmodel is the FF from the aero-engine model.
　
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