Premature Descent Alerts (PDA)
TAWS generates alerts when the aircraft violates the Minimum Ground Clearance Boundary
(MGCB) protection floor around an airport. The MGCB is generated from the runway end or
airport reference point.
The system generates a “
TOO LOW TERRAIN
” alert when the aircraft is hazardously below
the normal approach path angle of three degrees. The chart below describes this alert
region.
TAWS utilize either a valid radio altitude or a height above the terrain value obtained by
comparing the barometric altitude to the terrain elevation from the TAWS system. This
permits the Premature Descent Alerts to function and permits the FLTA alerts to be available.
Premature Descent Alerts are available when the aircraft is below the runway elevation. This
occurs when the airport is on a mesa or hilltop and the minimum radio altitude for the alert is
not obtained due to the runway location.
Fokker 50 - Flight Navigation Instruments
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