System alerts
Forward looking terrain alerts
Flight phases of En-route, Terminal, Approach and Departure are determined by the aircraft’s
position and inputs from the FMS, landing gear position, flap position and glide slope. TAWS
utilizes flight phase to generate values for RTC alerting.
Along track and along arc lateral/vertical envelopes are generated based on the aircraft’s
current position, track, turn rate and ground speed. Vertical envelopes are generated in
climb, level and descent regions.
Required terrain clearance alerts
Required Terrain Clearance alerts are generated when the aircraft is above terrain ahead on
the flight path, but the projected clearance is considered unsafe for the phase of flight.
Terrain Caution is generated approximately 60 seconds from the point over the ground
where the required terrain clearance is less than the vertical clearance required for the flight
phase. At approximately 30 seconds a Terrain Warning will occur.
Since RTC is reduced in terminal, approach and departure, the system favors en-route
whenever the information available to the TAWS computer is ambiguous.
Imminent terrain impact alerts
Imminent Terrain Impact Alerts are generated when the aircraft is currently below the
elevation of terrain ahead at or above the projected vertical flight path. Terrain Caution is
generated approximately 60 seconds prior to predicted terrain conflict. At approximately 30
seconds prior, a Terrain Warning will occur.
High terrain impact alerts
High Terrain Impact Alerts are generated when the terrain ahead along the flight path is
higher than 1500 feet above the projected vertical path. The high terrain warning will occur
earlier based on the configured climb rate for the aircraft. This adds additional response time
for terrain significantly higher than the aircraft.
Flight plan look ahead alerts
TAWS generates static and dynamic terrain envelopes along and down the flight path. Static
lateral and vertical envelopes are used for the terrain on the flight plan advisory feature. The
terrain on the flight plan advisory feature uses the vertical and horizontal flight plan
information to look down the flight plan for terrain threats. For dynamic terrain caution and
warning alerts, TAWS uses estimated position error, and current ground speed.
Circle to land and VFR approach operations
When TAWS is operated in the visual portion of circle to land approaches at airports where
sharp terrain contours exist in the circling environment, it may be necessary to activate the
Terrain Inhibit function to eliminate nuisance alerts from TAWS. This is not a fault with
TAWS, but a result of violating the Required Terrain Clearance.
Fokker 50 - Flight Navigation Instruments
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