Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 91
6.7.3.1.1 Caution Altitude Floor
The caution altitude floor (or terrain floor) is computed as a function of aircraft altitude with respect to nearest runway
altitude and range to the nearest runway threshold position. This parameter represents a distance below the aircraft. The
relationship to the nearest runway threshold location prevents undesired alerts when the aircraft is taking off or landing at an
airport. The system is compatible with terrain clearances allowed for by regulatory approach and departure design criteria.
6.7.3.1.2 Caution Look Ahead Distance
The caution look-ahead distance is computed from aircraft ground speed and turn rate to provide an advanced alert with
adequate time for the crew to react safely. Depending on the situation this distance roughly corresponds to between 40 and 60
seconds of advance alerting.
6.7.3.1.3 Warning Altitude Floor
The warning altitude floor is set to a fraction of the caution altitude floor, as illustrated in the upper part of Figure 6.7-2. The
warning altitude floor is computed as a function of aircraft altitude with respect to nearest runway altitude and range to the
nearest runway threshold position. This parameter represents a distance below the aircraft. The relationship to the nearest
runway threshold location prevents undesired alerts when the aircraft is taking off or landing at an airport.
6.7.3.1.4 Warning Look Ahead Distance
The warning look ahead distance is a fraction of the caution look ahead distance (computed from aircraft ground speed and
turn rate) to provide an advanced warning with adequate time for the crew to react safely.
6.7.4 Terrain/Obstacle Displays and Alerts
The Terrain Awareness alerting and display function maintains a background display of local terrain forward of the aircraft
for optional cockpit display. In the event of terrain or obstacle caution or warning conditions, an aural alert and lamp outputs
are triggered. The background image is then enhanced to highlight terrain or obstacle threats forward of the aircraft.
Obstacle threats forward of the airplane are also enhanced if the adjacent terrain altitude is within a lower terrain layer, or if
the adjacent cells are not illuminated. Obstacle enhancement is only applicable to the 15, 30 and 60 arc second tiers
The background terrain is depicted as variable density dot patterns in green, yellow or red. The density and color are a
function of how close the terrain or obstacle is relative to aircraft altitude. Additionally, the display of terrain based on
absolute terrain elevation is provided if the optional Peaks is enabled. Terrain and obstacle alerts are depicted by painting the
threatening terrain as solid yellow or red.
The terrain display algorithms process the set of digital elevation matrix overlays into a matching set of display matrix
overlays and passed to the radar display output processor. The display matrix overlays hold display attributes rather than
altitude for each matrix element. These attributes are computed for the background and terrain threat areas and kept small
(one byte) to reduce memory requirements and transfer time to the radar display output processor. The aircraft position and
aircraft heading are used at the radar display output processor to extract the radar-like sweeping image ahead of the aircraft
from the display overlays.
Each element of the output display matrix overlays holds a single display attribute byte with fields for the colors, patterns, and
symbols shown below in Table 6.7-1.