NAV Engagement
Figure 034
NAV Engagement Display
Figure 035
(b) NAV control law parameters When NAV is engaged, the determination of the control law to be used depends on the type of leg and transition.
-
If the active leg is a course leg (CX except CF leg), the TRACK control law is used to capture and hold the track target.
-
If the active leg is a heading leg (VX legs), the HEADING control law is used to capture and hold the heading target.
-else, the horizontal path control law is used to guide the A/C on the path. The nominal bank angle computed using the leg geometry is corrected by XTK and TKE values.
For transitions, the lateral control law also depends on the type of transition:
-
Type I : The nominal bank angle is adjusted by XTK, TKE values to follow the defined arc.
-
Type II : Crosstrack error (XTK) and track angle error (TKE) are used to capture the desired track and then the fixed path control law is used (for holding pattern entries, Ref. Para. E.(1)).
-
Type III : The heading control law is used to capture and hold the heading target.
-
Type IV : The horizontal path control law is used for both the circular path and the straight path leg.
-
Type V : The track control law is used to capture and hold the
track target. The lateral control law used to capture a leg, when NAV is engaged, depends on the leg to be captured.
-
For heading legs, the heading control law is used to capture the desired heading.
-
For course legs (except CF leg), the track control law is used to capture the desired track.
-
For a DF leg, the horizontal path control law is used.
-
For other legs the crosstrack error is compared to the crosstrack limit. If crosstrack error is greater than crosstrack limit, then the heading control law is used to hold the current heading until the crosstrack error equals the crosstrack limit when the horizontal path control law is used to capture the leg. In this case, Nav only engages at that time. Else the horizontal path control law is immediately used to capture the leg.
If the flight plan is changed while in a capture procedure, the capture rules are superseded by the control law that would normally apply for the new active leg.
(3) Approach area guidance submodes In approach area, there are two types of guidances :
-RNAV regrouping lateral path, heading, track submodes
-ILS (LOC)
(a)
RNAV guidance This mode may be used on any leg of a VOR, RNAV or NDB approach. The submodes used for the guidance in approach are the same as for the enroute case : lateral path, heading, track, and are as defined in para. G.(2)(a).
(b)
ILS (LOC) mode ILS guidance may be used only on final LOC segments. Before this point, RNAV guidance may be used. This mode computed in FG part, is selected through FM part if one of the following conditions occurs :
-an ILS or LOC only approach is selected in the active flight plan at destination
-any RMP is set to NAV mode
-
no destination, or no approach, or a runway only approach exists in the active flight plan at the destination
-
no flight plan is active
When RNAV guidance is selected for approach and an ILS/LOC is
tuned on the MCDU RADNAV page, the message 'CHECK APPR GUIDANCE'
is displayed once on the MCDU and the EIS at the moment when the
A/C is at 100 NM of the top of descent or DES phase is active. It
is displayed when the APPR phase is active.
The message is cleared when conditions for display are no longer
true or if a pilot clears it.
The PERF APPR page displays the DH field when ILS guidance has
been selected.
The MDA/MDH field is always displayed on the PERF APPR page, and
entry always possible.
(Ref. Fig. 036)
The following logic is used to ensure the capture of the LOC axis
in case of deficient navigation computations:
Condition A:
-LOC mode is armed for 3 s at least
and
-ILS approach is selected in the flight plan
and
-NAV is engaged
and
-ILS actually selected frequency is equal to the F-PLN ILS frequency and
-the aircraft is at less than 20 NM from the LOC station.
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