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represents terrain and / or obstacles above, level with or below the
airplane’s altitude based upon airplane position relative to the geographic
database. If the airplane is in an area not covered by the database
(typically near the poles) the display is low density magenta. Terrain that is
more than two thousand feet (2000’) below the airplane is not displayed,
nor is terrain within four hundred vertical feet (400’) of the elevation of the
nearest airport runway. See the table at the end of this topic for a full
description of the significance of the colors and densities of the TAD
function.
With the incorporation of the Peaks function, the TAD presents a digital
readout of the elevations of the highest and lowest terrain and / or obstacle
currently displayed. The numerical values of the readout are in hundreds of
feet above mean sea level (MSL), thus a display of 125 equals (=) 12,500
feet MSL. The elevation values are displayed in the same color as the
terrain of that elevation. The elevation of the highest terrain indicated in
red, the lowest in green. If there is no significant variation in terrain
elevation, for instance over flat terrain, only the highest elevation is
indicated numerically. The numerical elevation values are no longer
displayed when the airplane is five hundred feet (500’) or less above the
terrain (250’ if the landing gear is extended).
When the image is initially presented, the ten (10) mile range is the default
value - other range values must be manually selected. When potential
terrain conflicts prompt caution or warning alerts, the terrain and / or
obstacle is depicted in solid yellow for cautions and solid red for warnings.
Additionally, during alerts the image scale of the area immediately
surrounding the hazard is enlarged to better identify a small obstacle or
terrain feature.
OPERATING MANUAL
2A-34-00 PRODUCTION AIRCRAFT SYSTEMS
Page 32
October 11/01
Revision 5
Color Indication
Solid Red Terrain / Obstacle Threat Area - Warning
Solid Yellow Terrain / Obstacle Threat Area - Caution
50 % Red Fill Terrain / Obstacle that is more than 2000 feet
above airplane altitude
50 % Yellow Fill Terrain / Obstacle that is between 1000 feet
and 2000 feet above airplane altitude
25 % Yellow Fill Terrain / Obstacle that is 500 feet (250 feet
with landing gear extended) below to 1000 feet
above airplane altitude
Solid Green (Peaks display) Shown only when no Red or Yellow Terrain /
Obstacles are within range of the display. The
highest Terrain / Obstacle is not within 500 feet
(250 feet with landing gear extended) of
airplane altitude
50 % Green Fill Terrain / Obstacle that is between 500 feet
(250 feet with landing gear extended) and
1000 feet below airplane altitude
50 % Green Fill (Peaks display) Terrain / Obstacle that is in the middle
elevation band when there is no Red or Yellow
Terrain / Obstacle within range on the display
16 % Green Fill Terrain / Obstacle that is between 1000 feet an
2000 feet below airplane altitude
16 % Green Fill (Peaks display) Terrain / Obstacle that is in the lower elevation
band when there is no Red or Yellow Terrain /
Obstacle within range on the display
Black No significant Terrain / Obstacle
16 % Cyan Fill (Peaks display) Water at mean sea level elevation (0 feet
MSL)
Magenta Fill Unknown terrain. No terrain data in the
database for the magenta area shown
F. Geographic Altitude:
The alerts and displays generated by the EGPWS are most accurate when
airplane altitude can be determined with a high degree of certainty. To
obtain the highest accuracy in measuring airplane altitude, the EGPWS
computes a Geographic Altitude (GA). GA is a blended altitude derived
from all altimeter data sources available, and includes:
• Non-corrected standard altitude
• Runway calibrated altitude computed during takeoff
• GPS calibrated altitude
• Radio altitude calibrated during approach
• Barometric altitude, corrected for local conditions if available
For each of these readings, a Vertical Figure of Merit (VFOM) is
determined in order to calculate the importance of the individual reading in
blending the final GA computation. The final computed GA value is more
accurate than the value of individual sensor readings and allows a more
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-34-00
Page 33
October 11/01
Revision 5
precise determination of separation between the airplane and the terrain in
the EGPWS database.
NOTE:
A terrain awareness display with degraded accuracy
　
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