LOC Mode
Figure 010
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1 C. Longitudinal Modes
(1)
General (Ref. Fig. 011) To ensure aircraft guidance on the longitudinal axis in AP/FD, the pilot can use:
-
the modes which ensure capture and holding of a level,
-
the modes which ensure control of level changes.
The level change control is "selected" when the pilot imposes the
guidance parameters via the FCU.
The level change control is "managed" when the guidance parameters
are determined by the FM part.
The list of longitudinal modes and sub-modes active in cruise and the
control laws and associated references are given in the figure.
(2)
Altitude Acquisition and Hold Modes
(a) Altitude acquisition mode (ALT ACQ)
1 Characteristics
_ This mode permits to acquire the altitude selected in the ALT counter of the FCU or the altitude provided by the FM part when the passage of an altitude constraint in the longitudinal flight plan requires levelling of the aircraft. The ALT ACQ mode includes an arming phase in which a support mode for level change control ensures convergence toward the desired level. The ALT ACQ mode becomes active when the capture condition is satisfied i.e. when the aircraft altitude deviation with respect to the target level is lower than a value dependent on the vertical speed. When the target level is actually reached, the altitude hold mode is automatically engaged and replaces the ALT ACQ mode.
2 Operational use
_ The ALT ACQ mode is always armed except in the cases below:
-in altitude hold or ALT ACQ mode
-after glide capture
-in V/S-FPA mode when the aircraft deviates from the selected altitude.
a FCU level acquisition
_ With the ALT ACQ mode armed, the operating sequence is as follows:
-selection of a flight level on the FCU (during selection, the ALT ACQ mode is inhibited)
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-engagement of a support mode which permits to reach the selected level (ALT in cyan on the FMA)
-capture of the selected level (ALT * in green on the FMA)
-switching to the altitude hold mode when the altitude
deviation becomes lower than 20 ft. (Ref. Fig. 012) This operating sequence can be modified if the pilot changes the reference altitude:
-
During the arming phase: If, when the pilot stops his selection the capture condition is met, the ALT ACQ mode is immediately engaged. (Ref. Fig. 013)
-
During the capture phase If, in spite of the reference change the capture condition is still met, the ALT ACQ mode remains active but a performance degradation may occur. If the capture condition is no longer met, the V/S or FPA mode is engaged with the current aircraft Vz or FPA taken as a target at mode engagement. (Ref. Fig. 014)
b Acquisition of an altitude constraint
_
With the ALT ACQ mode armed, the altitude constraint
delivered by the FM part will be captured:
-if it is located between the level selected on the FCU and the aircraft level
-and if the support mode is a mode for automatic control
of level changes. During the arming phase, the ALT message is displayed in magenta on the second line of the FMA. The condition for activation of the ALT ACQ mode is the same as for the FCU level capture. The FMA messages associated to the modes of capture and holding of an altitude constraint are displayed in magenta. If the conditions required to follow the flight plan disappear (loss of NAV mode or loss of longitudinal flight plan) during the capture, the AP/FD switches to the V/S FPA mode and holds the A/C vertical speed. The FCU level capture is armed again.
3 Barometric correction
_ (Ref. Fig. 015) The captured altitude is dependent on the barometric correction selected on the FCU. Depending on the standard or BARO selection made on the associated side of the FCU:
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AP/FD Longitudinal Modes Active
Figure 011
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1
ALT ACQ Mode
Figure 012
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