400/500 Series Garmin Optional Displays

Premature Descent Alerting (PDA)
Garmin TAWS issues a Premature Descent alert when the system detects that the aircraft is significantly
below the normal approach path to a runway. There are three different scenarios to consider with PDA:
• No Approach Loaded—PDA alerting begins when the aircraft is within 15 nm of the destination
airport and ends when the aircraft is either 0.5 nm from the runway threshold OR is at an altitude of
125’ AGL while within 1 nm of the threshold. During the final descent, algorithms will set a threshold for
alerting based on altitude, distance, and other parameters.
• Non-Precision Approach Loaded—PDA alerting begins when the aircraft is within 15 nm of the
destination airport and the aircraft is on the flight plan path. Again, algorithms are used to set a threshold
for alerting based upon various parameters (Figure 3-2). PDA alerting ends at 0.5 nm from the runway
threshold OR at an altitude of 125’ AGL while within 1 nm of the threshold.
• ILS Approach Loaded—PDA alerting begins when the aircraft is within 15 nm of the destination airport
and the aircraft is on the flight plan path. Prior to crossing the FAF, the pilot will receive a PDA alert
if the aircraft descends more than 200’ below the FAF crossing altitude. Once the aircraft crosses the FAF
and is inbound to the runway, PDA will alert the pilot if the aircraft descends 0.7° below the ILS glideslope
(Figure 3-3). PDA alerting ends 0.5 nm from the runway threshold OR at an altitude of 125’ AGL while
within 1 nm of the threshold.
Runway Threshold
0.5 nm
PDA Alert is
0.7 Below Glideslope
15 nm
Final Approach Fix
CAUTION
Figure 3-3: An example of a PDA alert threshold for an ILS approach
Glideslope Intercept
200' below FAF Altitude
Runway Threshold
0.5 nm
Section 3
TAWS Alerts
PDA Severity Levels
CAUTION — The above annunciation and pop-up terrain
alert are displayed during a PDA alert. For TAWS,
the PDA alert is acompanied by the aural message “Too
Low, Terrain”.
Final Approach Fix
15 nm
200' below FAF Altitude
CAUTION
Figure 3-2: Example of Non-Precision Approach PDA alert
threshold. The threshold for the descent is computed by algorithms
based on varying flight conditions.
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