AFDS -OPERATION -LOCALIZER

General
The flight crew uses the localizer switch on the MCP, to select a localizer only approach.
The flight crew uses the localizer only approach when glideslope is not available or uses a different vertical path for descent.
An ILS frequency and course must be set into the FMCF.
The flight crew can use another roll mode to fly the airplane to the localizer. In this example, the flight crew uses the heading select mode.
The LOC mode has these two sub-modes:
*
Localizer arm

*
Localizer active.

Localizer Arm
The LOC switch on the MCP arms only the localizer mode. The PFD shows LOC in white. The LOC switch light comes on.
To remove the arm status, push the LOC switch again before the localizer captures or select another roll mode.
Localizer Active
At LOC capture, the active roll mode changes to LOC. The MCP heading/track window changes to the ILS course.
The localizer capture point is not fixed. The AFDS calculates the localizer capture point as a function of:
*
Ground speed

*
Localizer deviation

*
Intercept angle (must be less than 120 degrees from the selected ILS course).

The AFDS uses the previous data to put the airplane on the localizer beam without large bank angles.
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777 AIRCRAFT MAINTENANCE MANUAL

AFDS -OPERATION -LOCALIZER

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AFDS -OPERATION -APPROACH

General
In approach (APP), the airplane automatically captures and tracks the localizer and glideslope beams. If an autopilot approach continues, the airplane lands automatically.
The flight crew or the FMCF sets the ILS frequency and the ILS course.
The two sub-modes of APP are:
*
LOC and G/S arm

*
LOC and G/S capture.

Autoland Status
If the autopilot is engaged in the approach mode below 1500 feet radio altitude, the autoland status shows on the PFD. The autoland status shows LAND 3, LAND 2, or NO AUTOLAND. The autoland status is the level of redundancy in the autoland system.
LAND 3 shows when there are no failures in any part of the autoland system. LAND 2 shows when any part of the autoland system is one failure away from NO AUTOLAND. NO AUTOLAND shows when the autoland system is not available.
There are more than 30 conditions that can cause a change from LAND 3 to LAND 2 or NO AUTOLAND. Refer to the airplane system schematics for a full list of the conditions that change the autoland status to LAND 2 or NO AUTOLAND.
These are some failures that cause a change from LAND 3 to LAND 2:
*
Single AFDC failure

*
Single backdrive actuator failure

*
Single radio altimeter failure

*
Single multi-mode receiver failure

*
ADIRU status message.

These are some failures that cause a change from LAND 3 to NO AUTOLAND:
*
Two AFDCs fail

*
Approach mode not active at 600 feet

*
Two radio altimeters fail

*
Two multi-mode receivers fail

*
ADIRU failure.

Approach Arm
These occur when the flight crew pushes the approach (APP) switch on the MCP:
*
The approach light on the MCP comes on

*
LOC and G/S show white on the PFD

*
A bus isolation request goes to the electrical power system.

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AFDS -OPERATION -APPROACH

Bus isolation stays active during an autoland until the autopilot disengages or the airplane climbs above 400 feet radio altitude after a go-around starts.
If bus isolation fails, the autoland status changes from LAND 3 to LAND 2.
If you push the APP switch while both LOC and G/S are armed, the approach mode will disarm.
LOC And G/S Capture
The localizer capture point is not fixed. The AFDS calculates the localizer capture point as a function of these data:
*
Ground speed

*
Localizer deviation

*
Intercept angle (must be less than 120 degrees from the selected runway heading).

The glideslope capture point is not fixed. Glideslope capture is a function of these data:
*
The airplane distance from the center of the glideslope beam

*
Glideslope deviation.

BEJ 006, 007
Glide slope capture does not occur until after localizer capture.
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