10-18
GTN 625/635/650 Pilot’s Guide
190-01004-03 Rev. P
Terrain
10.5.6.3 Premature Descent Alerting
A Premature Descent Alert (PDA) is issued when the system detects that the
aircraft is significantly below the normal approach path to a runway.
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 feet AGL while within 1.0 NM of the threshold. During
the final descent, algorithms set a threshold for alerting based on speed, distance,
and other parameters.
700
600
500
400
300
200
100
Height Above Terrain (Feet)
Distance From Destination Airport (nm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PDA Alerting Area
PDA Alerting Area
Runway
Threshold
Figure 10-10 PDA Alerting Threshold
PDA and FLTA aural and visual alerts can be manually inhibited. Discretion
should be used when inhibiting terrain alerts and the system should be enabled
when appropriate. When terrain alerting is inhibited, the alert annunciation
“TER INHB” is shown.
10.5.6.4 Inhibiting/Enabling PDA/FLTA Alerting
Inhibit mode deactivates the PDA/FLTA aural and visual alerts. Pilots should
use discretion when inhibiting terrain alerts and always remember to enable
the system when appropriate. Only the PDA and FLTA alerts are disabled in the
inhibit mode. After cycling power, the terrain alerting function will no longer
be inhibited.
Summary of Contents for GTN 625
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