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message “TOO LOW TERRAIN” will occur. This aural message will occur once
when initial envelope penetration occurs, and one time thereafter for each 20%
degradation in either Altitude (AGL) or Altitude (ASL) depending on which
envelope was violated (TCF or RFCF respectively). EGPWS cockpit alert
annunciations remain illuminated until the alert envelope is exited.
(d) The EGPWS Runway Database consists of data records for all airport runways
offered for the coverage provided by the Terrain Database. For the MK V and MK
VII, all hard surface runways in the world 3500 feet or greater in length are
supported. For the MK VI, all runways 2000 feet or greater in length within the
database region installed are supported. The MK VIII has the ability to select
2000ft or 3500ft runway lengths. The database provides the means of accessing
the records of runways closest to the current aircraft position.
(10) TERRAIN ALERTING AND DISPLAY
(a) The Terrain Alerting and Display (TAD) function monitors aircraft position with
respect to local database-cataloged terrain to provide rapid audio and visual alerts
when a terrain threat is detected. Terrain threats are recognized and annunciated
when terrain violates specific computed envelope boundaries forward of the aircraft
path. The terrain database also includes obstacles (when and where available)
providing similar annunciations when cataloged obstacles violate the same
envelope boundaries.
(b) Terrain Alerting outputs (lights and audio) behaves in the same manner as the
standard GPWS mode alerts. Either caution or warning alerts will initiate a specific
audio alert phrase. The caution aural is “CAUTION TERRAIN” or “CAUTION
OBSTACLE” and the warning aural is “TERRAIN, TERRAIN, PULL UP” or
“OBSTACLE, OBSTACLE, PULL UP” (minor variations exist).
(c) Complementing the terrain threat alerts, the EGPWS also maintains a synthetic
image of local terrain forward of the aircraft for display on Weather Radar
For Training Purpose Only Printed:May 2005
B757-200Aircraft Recur rent Training Manual - 11 -
Indicators. The EGPWS may be configured to automatically de-select the Weather
Display and pop-up a display of the terrain threats when they occur. The logic used
for these configurable controls also provides an external input for predictive
windshear alerts that can override a Terrain Display and revert to the weather
display with the corresponding windshear data.
(d) The EGPWS provides up to two optional external display outputs, each with
independent range-scaling control in the same fashion as weather radar with more
than one indicator. Changes of range scaling to one display do not affect the other
display. Each of these two independent outputs may be used to drive more than
one display.
(e) The EGPWS Terrain Database is the earth’s surface divided into grid sets and
cells referenced to the geographic (latitude/longitude) coordinate system of the
World Geodetic System 1984 (WGS-84). Elements of the grid sets include the
highest terrain altitude (above MSL) in each cells respective area. Grid sets vary in
resolution depending on geographic location. Because the overwhelming majority
of “Controlled Flight Into Terrain (CFIT)” accidents occur near an airport, and the
fact that aircraft operate in closer proximity to terrain near an airport, higher
resolution grids are used around airports. Lower resolution grids are used outside
of airport areas where aircraft altitude enroute makes CFIT accidents unlikely and
for which detailed terrain features are not important to the flight crew. Digital
Elevation Models (DEM’s) are available for most of the airports around the world
today. In cases where data is not currently available, DEM’s are generated from
available topographic maps, sectional charts, and airline approach plates. The
process of acquiring, generating, assembling, and updating the database is
governed by strict configuration controls to insure the highest level of data integrity
for generation of the EGPWS Terrain Database.
The EGPWS Terrain Database is organized in a flexible and expandable manner.
Using digital compression techniques, the complete database is stored in
non-volatile memory within the LRU. Updates and additions are easily
accomplished via a PCMCIA card interface. The Obstacle Database is a separate
file included within the terrain database. Both files are loaded into the EGPWS in
the terrain database PCMCIA card. The obstacle database is accessed by the
EGPWC application software only if obstacle alerting is enabled by installation
configuration. The obstacle data is processed by the EGPWS in the same fashion
　
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