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during approach, using standard altitude (label 203) from ADIRU 1 and QNH baro correction entered by the crew via the MCDU. On the MCDU, a prompt comes into view adjacent to the line key related to QNH information for input when < 180 NM from destination. (Ref. Fig. 017) Other aircraft inputs include aircraft heading (from the IR), roll attitude (from IR) and flight path angle (Gamma, derived by the Enhanced GPWC). Aircraft altitude for the Terrain Awareness functions is also computed from pressure altitude and SAT received from the air data portion of the ADIRU, altitude from the Global Positioning System (if available), and height above ground provided by the Radio Altimeter (RA). When GPS parameters are available via a specific Pin-Program, and from GPIRS, the Enhanced GPWS computes a Geometric Altitude.
Control Inputs Installations provide discrete terrain display select pushbutton switches (TERR ON ND) in the cockpit for each display. These are momentary-action pushbutton switches that are processed by the Enhanced GPWS input processing and signal selection block as input to display control logic. Additionally, for aircraft with predictive windshear radar capability (if WR/PWS installed), a windshear override boolean is computed using alert level bits from label 077 on WXR hazard bus for the display control logic. (Ref. Fig. 004A) This boolean allows the Enhanced GPWC to manage priority between Enhanced GPWC alerts and PWS alerts. The installation
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requires input of cockpit-selected range scales provided by the FCU for each ND.
(b) Local Terrain Processing The local terrain processing block extracts and formats local topographic data and terrain features from the related data bases creating a set of digital elevation matrix overlays for use by the terrain threat detection and display processing functions. Additionally, data for the nearest runway are also extracted for use by the terrain threat detection and display processing functions. Processing for each topographic data base and the runway data base are described in the following sub-sections.
1_ Terrain Surface Data
Local terrain processing of topographic surface data updates a
set of digital elevation matrix overlays that are positioned
with respect to aircraft position. Each matrix element
contains the highest terrain altitude with respect to mean sea
level in that element area. Elements where terrain data are
not available are marked invalid. These unknown data are
displayed in magenta low density on ND whatever the altitude.
2_ Obstacle Data
In addition to terrain surface data, the terrain database
contains obstacle data. The obstacle data are displayed as
terrain data (same coloring scheme) and cause visual
indications of warning and caution alerts as terrain. The
current obstacle database is obtained from N.O.A.A. (National
Oceanic and Atmospheric Administration). It includes obtacles
in the United States and part of Canada, Mexico, the Bahamas
and Europe. Obstacle alerting and display is activated via a
specific programming pin.
3_ Nearest Runway Data
Data for the nearest runway are extracted and processed for
use by the terrain threat detection and display processing
functions. Data are extracted from the same airport data base
used by the Terrain Clearance Floor functions (Ref. para.
D.(2)). This data base contains data on all hard-surface
runways 3500 ft. or more in length with published coordinates.
The contents of the data base are processed by the local
terrain processing into nearest runway center position,
nearest runway threshold position, and nearest runway altitude
for use by the Enhanced GPWS. These data are updated when the
terrain threat detection and display processing functions are
performed.
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(c) Terrain threat detection The terrain threat detection and display processing block performs the threat analysis on the terrain data within computed caution and warning envelope boundaries below and forward of the aircraft path. Results of these threat assessments are combined with background terrain data and data for the nearest runway and formatted into a terrain display image which can be displayed on Navigation Display in place of the weather image. In the event of terrain caution or warning conditions, a specific audio alert is triggered and the terrain display image is enhanced to highlight each of the types of terrain threats.
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