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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.
0.
5n
m
RunwayThreshold
FinalApproach Fix
200'belowFAFAltitude
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.
RunwayThreshold
FinalApproach Fix
200'belowFAFAltitude
15 nm
CAUTION
PDAAlert is
0.7 Below Glideslope
GlideslopeIntercept
0
.
5
n
m
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
