b A/C below altitude profile (A/C in DES phase)
_ If A/C is above the highest restrictive altitude constraint of the descent, hold of V/S = -1000 ft/mn and ECON descent speed until either the altitude of the constraint or interception of the altitude profile. If A/C is below the highest restrictive altitude constraint:
-When there is an AT or AT OR ABOVE alt constraint which applies at predicted A/C position, level flight is flown with ECON descent speed until it is left behind.
-Otherwise hold of ECON descent speed and of a constant flight path angle equal to half the angle described by the theoretical descent path at the A/C altitude.
This applies up to the predicted interception of the
altitude profile.
(Ref. Fig. 019)
c Consideration of HM leg
_ (Ref. Fig. 020) Before entering the HM leg, the holding is supposed to be flown. When the aircraft is in the holding, predictions assume that the leg is flown at holding speed with a vertical speed equal to - 1000 ft/mn until reaching a restrictive altitude constraint, the FCU altitude or the exit fix. If FCU or constraint altitude is reached first, the rest of the pattern is assumed to be flown level at that altitude. If the predicted exit fix altitude is not on the theoretical descent profile, return to the path is made according to the rules defined in previous paragraphs (A/C below profile; A/C above profile).
Return to Path Predictions
Figure 019
Consideration of HM Leg in Predictions
Figure 020
R 1EFF : 001-049, 051-099, 101-105, 151-199, 1 22-73-00Page 45/46 1201-212, 1Config-1 May 01/05 1 1 1CES d Recomputation of predictions
_ Predictions are continually refreshed. They are recomputed whenever the theoretical path is rebuilt, when SPEED CONTROL mode is changed, and at each HM fix overfly.
4 Guidance
_
a Path reference
_ (Ref. Fig. 021) Beyond the top of descent and during the descent, the vertical FLIGHT PATH ALTITUDE corresponding to the computed altitude profile at the computed aircraft position is displayed on the PFD altitude scale as a small magenta circle.
b DES mode
_ It can be armed or engaged in LATERAL AUTO CONTROL, above FCU selected altitude, only if a vertical path is defined or during a limited time while the path is being computed. If the 20-second allowance is exceeded, a reversion to V/S or FPA mode occurs. When a level off is required for an altitude constraint, ALT ACQ mode is engaged automatically, and DES mode is armed for automatic re-engagement when the constraint is passed. Predictions are normally used to determine whether a level off is necessary. However while predictions are being computed, the system will target altitude constraints within a limited altitude range. The AIRBRAKES message can be displayed on the PFD if the aircraft is too high and is going to miss an altitude constraint, according to predictions. In DES mode, guidance depends on computed aircraft position relative to the vertical flight path and whether aircraft speed is correct. Elevators are used to control the aircraft to the path. Thrust can be applied to prevent underspeeding or to maintain speed. In case of overspeeding, the aircraft peels away above path. Idle thrust and speed elevator control is used when the aircraft is above path to dive back down. If the aircraft is below path, vertical speed or flight path angle elevator control is used to converge towards the path, and speed is held with thrust. For an HM leg, V/S = -1000 ft/mn is targeted once holding pattern speed is reached and held with thrust. In SPEED AUTO CONTROL, to minimize throttle control levers movement, aircraft speed is allowed to vary within a range which is displayed on the PFD, except when decelerating or
controling to an HM speed. The underspeed allowance is 20 Kts. The overspeed allowance is 20 Kts unless a speed constraint or limit applies, in which case it is only 5 Kts. The speed range is limited from max (VMAN/VLS) to min (VMAX/VMO-3/MMO-0.006). On the idle path, engines are allowed to remain at IDLE or IDLE+DELTA thrust within the speed range. Return to path from above uses the maximum overspeed allowance. In SPEED MANUAL CONTROL, there are no margins, FCU selected speed is maintained. Selected submode can be:
--------------------------------------------------------------------------| | above path |on idle path | on path or | below | below | | DES MODE | or |(SPEED AUTO |underspeeding|geometric| path or | | |overspeeding|CONTROL only)| on idle path| path | in an HM| --------------------------------------------------------------------------|Elevator | SPEED | VPATH | VPATH | FPA | V/S | |target | | | | | | |-----------|------------|-------------|-------------|---------|---------| |Thrust | THRUST | THRUST | SPEED | SPEED | SPEED | |target | | | | | | --------------------------------------------------------------------------
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