Premature Descent Alerting (Pda) Using Gps-Msl Altitude; Limitations - Garmin GNC 420W Pilot's Manual Addendum

Part Three: Section 1
TAWS Introduction
atmosphere conditions seldom match the standard
conditions defined by the International Standard
Atmosphere model (where pressure, temperature, and
lapse rates have fixed values), it is common for the
Baro-Corrected Altitude (as read from the altimeter) to
differ from the GPS-MSL Altitude.
Premature Descent Alerting (PDA) using GPS-MSL
Altitude
The Premature Descent Alert (PDA) is annunciated
when the aircraft is significantly below the
recommended minimum altitude for an approach.
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.
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Limitations

NOTE: The data contained in the TAWS databases
comes from government agencies. Garmin accurately
processes and cross-validates the data, but cannot
guarantee the accuracy and completeness of the data.
TAWS displays terrain and obstructions relative
to the altitude of the aircraft. The displayed alerts
and warnings are advisory in nature only. Individual
obstructions may be shown if available in the database.
However, all obstructions may not be available in the
database and data may be inaccurate. Never use this
information for navigation or to maneuver to avoid
obstacles.
Terrain information is based on terrain elevation
data contained in a database that may contain
inaccuracies. Terrain information should be used
as an aid to situational awareness. Never use it for
navigation or to maneuver to avoid terrain.
TAWS uses terrain and obstacle information sup-
plied by government sources. The data undergoes veri-
fication by Garmin to confirm accuracy of the content,
per TSO-C151b. However, the displayed information
should never be understood as being all-inclusive.