Garmin GPS Receiver 400/500 User Manual download pdf (Page 48)

Section 3

TERRAIN Alerts

Premature Descent Alerting (PDA)

The TERRAIN system 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 thresh-

old for alerting based upon various parameters (Figure 3-3). 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 3-4). PDA alerting ends 0.5 nm from the runway threshold OR at an altitude of 125’

AGL while within 1 nm of the threshold.

5 n

Runway Threshold

Final Approach Fix

200' below FAF Altitude

15 nm

CAUTION

Figure 3-3: Example of Non-Precision Approach PDA alert

threshold. The threshold for the descent is computed by algo-

rithms based on varying ﬂight conditions.

Runway Threshold

Final Approach Fix

200' below FAF Altitude

15 nm

CAUTION

PDA Alert is

0.7 Below Glideslope

Glideslope Intercept

CAUTION — The above annunciation and pop-up ter-

rain alert are displayed during a PDA alert.

PDA Severity Levels

Figure 3-4: An example of a PDA alert threshold for an ILS

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Garmin GPS Receiver 400/500 User Manual Page 48