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Section 1
Introduction
PDA alerting is provided for both non-published visual approaches and published instrument
approaches to airports. 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.
Limitations
The TAWS function displays altitudes of terrain and obstructions relative to the aircraft’s altitude and
are advisory in nature only. Individual obstructions may be shown if available in the database. Not all
obstructions may be available in the database and data may be inaccurate – do not maneuver or navigate
to avoid obstacles using this information. Terrain information is based on terrain elevation data con-
tained in a database, which may contain inaccuracies. Terrain information should be used as an aid to
visual acquisition – do not use terrain information to navigate or maneuver to avoid terrain.
