If the aircraft is in cruise within 180 NM of the destination and destination data (QNH, WIND, TEMP) have not yet been entered, the ENTER DEST DATA message is displayed.
 2 Theoretical descent path
_ a General
_ Descent predictions and guidance are based on a theoretical descent path, which is an altitude, speed and thrust profile, function of distance to destination. It is constructed according to the entered atmospheric data, vertical and lateral F-PLN data: all descent constraints are taken into account, and flight plan leg transitions are estimated. However holding patterns (HM legs) are not taken into consideration. In case of conflict between altitude constraints, priority is given to the highest one. The descent path is computed backwards from the DECEL point (Ref. APPROACH phase below) up to the CRUISE FLIGHT LEVEL, or aircraft lateral position (or HM exit fix it flying an HM) if it is being recomputed during the descent. It can consist of three parts: (Ref. Fig. 017)
 -a geometric path, if one is required to observe some altitude (or flight path angle) constraints
 -an idle path, if possible above the highest restrictive altitude constraint
 -a repressurization segment, if necessary to meet the specified cabin rate.
 b Geometric path
_ If necessary, a geometric path is built from the DECEL point up to the highest altitude constraint to which a vertical path based on the holding of IDLE thrust and descent speed (ECON or AUTO, with regards to speed constraints and limits) cannot be constructed. The geometric path altitude profile is composed of successive constant slope segments defined to meet restrictive altitude and flight path angle constraints. The geometric path speed profile is integrated backwards from the DECEL point assuming altitude profile elevator control and thrust speed control in order to meet speed limits and constraints.

 Theoretical Descent Profile Construction
 Figure 017

 Speedbrakes are assumed to be extended where necessary. However some of the segments may be too steeps to be flown even with speedbrakes extended to mid travel, then a TOO STEEP PATH marker is displayed on the F-PLN page to highlight the resulting profile discontinuity, and the TOO STEEP PATH AHEAD message is displayed before the descent.
 c Idle path
_ An idle path is built above the highest restrictive altitude constraint. It is based on the holding of descent speed (ECON or AUTO, with regards to speed constraints and limits) and theoretical thrust (IDLE + DELTA).
 d Repressurization segment
_ If it is necessary to meet the specified cabin rate, a repressurization segment is inserted at the upper end of the idle path, based on the holding of descent speed and a vertical speed derived from the specified cabin rate value. If the specified cabin rate cannot be met, the computed value is displayed instead and the message CHECK CABIN RATE is displayed before the descent. (Ref. Fig. 018)
 e Recomputation of the theoretical descent path
_ The entire descent path must be recomputed to account for changes in the F-PLN or atmospheric forecast data, prior to DESCENT phase if any of the following are modified:
 -lateral F-PLN
 -vertical F-PLN
 -cost index
 -descent forecast winds and cabin rate
 -destination temperature, wind and preselected QNH
 -descent optimum MACH or CAS
 during DESCENT phase whenever:

 -
the lateral F-PLN is modified

 -
the vertical F-PLN is modified

 -
the aircraft enters an HM leg (holding pattern)

 -
the aircraft reaches a TOO STEEP path. It is to be noted that unlike predictions, the path is not recomputed when SPEED CONTROL mode is switched from AUTO to MANUAL (or vice versa). The path is computed assuming SPEED AUTO CONTROL.

 3 Predictions
_ Predictions assume the aircraft will fly the theoretical profile.

 Repressurization Segment
 Figure 018

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 If the A/C is off that profile in altitude and/or speed,
 predictions suppose an immediate return to the theoretical
 profile from A/C present position to interception of the
 profile in altitude and speed.
 The way to return to the profile depends on the current
 position of the A/C versus alt profile as described below.

 a_ A/C above altitude profile (A/C beyond T/D).
 Return to the profile is assumed to be performed with: idle
 thrust, speedbrakes extended to mid travel, hold of the
 ECON descent speed plus a margin, until the interception of
 the altitude profile.
 The speed margin is equal to 20 kts above an optimum speed
　
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