-
A/C V/S is negative
-CLR altitude is below crossover altitude, the system computes and displays the corresponding CAS on the MCDU.
b A/C in DESCENT phase if:
_ -a CAS is selected on the FCU (and no Mach is defined)
-
A/C ALT is at or below the crossover altitude the corresponding to the CAS value
-
A/C V/S is positive
-CLR altitude is above crossover altitude, the system computes and displays the corresponding Mach on the MCDU.
(d) Speed envelope (used for predictions) Selected speeds are limited in the computation of predictions by the speed envelope of selected mode.
C. Vertical F-PLN Management
(1) Overview The vertical F-PLN management provided by the FM mainly consists in two tasks which are computations of predictions and vertical guidance.
(a) Overview of predictions The FM is in charge of predicting aircraft performance along the active flight plan. From the current state of the aircraft and of its environment, these predictions are computed versus defined lateral path, predicted atmospheric conditions and flight strategy defined for each flight phase. The current state of the aircraft is defined by: GW, CG, position, altitude, speed, active control modes (managed/selected in lateral, vertical (Ref. NOTE) and speed). Its environment includes time, wind and temperature. The lateral path considered is composed of all lateral legs except HM leg which is initially supposed not to be flown. But if the holding is still inserted while the aircraft approaches the entry fix, a complete holding pattern is supposed to be flown; once the aircraft is in the HM leg, predictions assume the aircraft will exit the holding when overflying the fix again. If there is a lateral discontinuity between two points in the flight plan, predictions assume the aircraft will fly directly between the two points (direct distance is used). The flight strategy is defined phase by phase in para. (2) assuming managed (lateral, vertical and speed) control active. When lateral path, vertical path or speed is in selected control, specific assumptions are used which are described in para. (3),
(4) and (5).
These computations involve the use of a performance data base
which contains aero/engine models of the aircraft and atmospheric
models.
NOTE : Managed vertical control = LVL CHANGE managed control or
____
ALT ACQ or ALT mode.
1 Display of predictions
_ Some data resulting from the computation of predictions are displayed on the MCDU. The flight plan pages display predicted time, speed, altitude, wind, and remaining fuel at each waypoint and pseudo-waypoint of the flight plan. Extra and remaining fuel at each fixed waypoint are displayed on the VERT REVISE page. Time and remaining fuel at destination are displayed on the FUEL PRED, PERF CRZ and DES pages. From takeoff transition, predictions of time are in Universal Coordinated Time (UTC) (if the A/C clock is valid). Predictions of speed are in CAS or MACH according to the target used.
Predictions of altitudes are in feet below transition altitude
and in flight levels above.
(Ref. Fig. 008)
2 Update of predictions
_ Predictions are regularly updated according to current parameters. Moreover, they are recomputed in case of modification of:
-lateral flight plan (new waypoint, new dest, etc.)
-vertical flight plan (cruise altitude, constraints, etc.)
-forecast atmospheric conditions entered by the crew (wind, temperature)
-cost index
-speed control (selected/managed).
In that case, during recomputation time, predictions are
dashed on the MCDU pages.
(b) Overview of vertical guidance The task of the FMS concerning the vertical guidance is totally dependent on the selection of managed or selected controls (lateral, vertical and speed). It may be to provide the targets sent to autothrust and autopilot or limited to the display of messages or advisory data.
(2) Managed lateral, vertical and speed control Prior to a phase-by-phase presentation of predictions and guidance in managed control, it is necessary to give a general view on guidance in managed control.
(a) Overview on vertical guidance in managed control Managed vertical control consists in LVL CHANGE (i.e. CLIMB or DESCENT mode), ALT ACQ or ALT mode. Three levels are considered: mode, submode and targets.
1 Mode
_ The FM part of the FMGC makes a request to the FG part for the selection of the active mode, CLIMB or DESCENT or for the capture of an altitude due to a constraint. But the final choice of the active mode among ALT ACQ, ALT, CLIMB or DESCENT is made by the FG part versus FM requests and pilot orders via the FCU (that is why in the description phase-by-phase given below, the mode requested by the FM part is differentiated from the active mode). The rules for mode engagement are described in 22-12-00.
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