Garmin GNS 430 Pilot's Manual And Reference page 198

SECTION 11
TERRAIN
PDA Alert
• CAUTION - The annunciation and pop-up terrain
alert shown in Figure 11-10 are displayed during
a PDA alert.
Premature Descent Alerting (PDA) using GPS-
MSL Altitude
The Premature Descent Alert (PDA) is annunciated
when the aircraft is below the recommended minimum
altitude for an approach (Section 11.3, Premature Descent
Alerting (PDA) for details). The aircraft' s GPS-MSL altitude
is compared to a calculated Above-Ground-Level (AGL)
altitude along the estimated path to the airport. When the
aircraft descends below the calculated minimum altitude
for the estimated path, the PDA alert is annunciated.
PDA alerting is provided for both non-published visual
approaches and published instrument approaches to
airports.
11-10
Figure 11-10 PDA Alert
GNS 430 Pilot's Guide and Reference
When an instrument approach is loaded in the active
flight plan, the PDA function can provide a greater margin
of safety by alerting for altitude deviations below minimum
approach altitudes. Minimum altitudes are typically
known for the segment prior to the final approach fix and
for step-down fixes prior to the missed approach point.
Minimum descent altitudes, decision altitudes, and decision
heights are not known for instrument approaches. These
minimum altitudes, as shown on instrument approach
procedures, are published as MSL altitudes and flown by
reference to barometric altitude provided on the altimeter.
Non-standard atmospheric conditions, as described in
the previous section, may result in differences between
GPS-MSL altitude and Baro-Corrected altitude. These
differences may reach a level that can trigger a PDA when
flying the published minimum altitude as indicated on
the altimeter. Operations in below-standard-temperature
environments may require the pilot to compensate for the
non-standard temperature by increasing the minimum
altitude that is defined in the approach procedure for
standard atmospheric conditions.
Differences between GPS-MSL altitude and Baro-
Corrected altitude typically increase as height above
ground level increases due to non-standard atmospheric
effects. In response to PDAs, the pilot should always
ascertain the appropriate altitude is being flown in
consideration of atmospheric conditions.