For takeoff phase, the predicted profile starts with a first segment at V2 + 10 kts and a Max takeoff thrust until the thrust reduction altitude is reached. This is followed by a second segment at V2 + 10 kts and Max climb thrust up to the acceleration altitude. These predictions (time, distance, fuel) are based on precomputed allowances. Predictions in climb start with an acceleration segment from V2 + 10 kts until the selected climb speed or the lower speed between :
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the optimum,
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the climb speed limit,
-any climb speed constraint is reached. The climb speed profile satisfies all the speed constraints and limits. The altitude profile, based on this speed profile and max climb thrust is limited by any AT or BELOW altitude constraint (if any). The speed profile is not modified to satisfy high altitude constraint. The constraints in climb are outlined by a star "*" on the corresponding field in the F-PLN page. The star is normally magenta but can be amber if the system predicts that the constraint will be missed. The end of the climb phase is indicated by the pseudo waypoint T/C on the F-PLN page. For cruise, predictions compute speed, time and fuel assuming altitude hold at the cruise flight level. A step climb or descent may exist. The speed profile in optimum mode (ECON) varies as the weight decreases during the flight. The T/D pseudo waypoint which terminates the cruise phase is computed by the descent profile calculation. From takeoff to cruise phase, the predictions and the profile are recomputed periodically or following modification of any flight plan elements or speed or weight. In order to get predictions for the descent phase, a descent path is computed backwards from the first approach point (DECEL) to the cruise (or step) flight level. This path is divided into 3 parts. The bottom part is a geometric path defined by straight segments in space between the various altitude constraints so that a minimum of pitch maneuver is required. Above the highest restrictive descent altitude constraint, the system computes an idle path based on holding of the descent speed and idle thrust + delta. This segment applies up to the cruise flight level where the T/D pseudo waypoint is defined. However, if a specified cabin rate must be reached, a repressurization segment is inserted at the top of the idle path, and a new T/D is defined. It is based on hold of the descent speed and a vertical speed computed from the specified cabin rate value.
The predictions compute the flight plan parameters (time/fuel) based on this path. The total descent time is also computed and from the cruise flight level and the airport elevation, the descent cabin rate is computed and displayed on the MCDU if it exceeds the specified cabin rate. The descent path and the predictions are recomputed after each flight plan modification or speed selection change or any destination parameter modification, when the aircraft is not in descent. If the aircraft is in descent phase, the descent path is also recomputed when the aircraft reaches a holding pattern or a too steep path. Note that an HM holding pattern is never considered as part of the descent path. When the aircraft is offpath, the system computes predictions with specific "return to path" assumptions (hold vertical speed if below the path, idle thrust and half speedbrakes extended if above the path). If the "return to path " from above does not intercept the path before the next altitude constraint, then a message requesting the extension of speedbrakes is displayed on the PFD. It is also displayed if a deceleration is required while above path. The approach phase starts at the DECEL point where the aircraft should start decelerating to reach VAPP at 1000 ft. AGL. The approach profile is then also defined backwards according to approach procedure data. Idle thrust is assumed until reaching the descent speed where the DECEL pseudo waypoint is defined. The various flap/slat configurations are assumed to be extended at the maneuvering speed. If an altitude constraint exists, an idle descent at constant speed is defined just after the constraint. If a speed constraint exists, a speed hold segment is defined after this constraint until the next point where a deceleration is required to reach the final conditions. Once the approach profile is determined, predictions on the approach are computed and shown on the MCDU. There are no predictions for go around.
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