GPS 400 Pilot’s Guide and Reference
190-00140-60 Rev. H
9-7
SECTION 9
TERRAIN
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 (Figure 9-5).
PDA alerting begins when the aircraft is within 15 nm
of the destination airport. PDA alerting ends when the
aircraft is either:
• 0.5 nm from the runway threshold
OR
• at an altitude of 125 feet AGL while within 1.0
nm of the threshold.
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
“Too Low Terrain”“Too Low Terrain”
Runway
Threshold
Figure 9-5 PDA Alerting Threshold
TERRAIN Inhibit
TERRAIN also has an inhibit mode that deactivates the
FLTA/PDA visual alerts. Pilots should use discretion when
inhibiting TERRAIN and always remember to enable the
system when appropriate.
Inhibiting TERRAIN:
1) Select the TERRAIN Page and press the MENU
Key. ‘Inhibit Terrain?’ is selected by default
(Figure 9-6).
Figure 9-6 TERRAIN Page Menu
2) Press the ENT Key. The ‘TER INHB’ annunciation
is displayed in the TERRAIN annunciator field
when TERRAIN is inhibited (Figure 9-7).
Annunciator Field
Figure 9-7 Annunciator Field
Enabling TERRAIN:
1) Select the TERRAIN Page and press the MENU
Key. ‘Enable Terrain?’ is selected by default.
2) Press the ENT Key. The TERRAIN system is
functional again.
