User's Manual

11-41190-01007-03 Rev. J
Garmin GTN 725/750 Pilot’s Guide
Foreword
Getting
Started
Audio &
Xpdr Ctrl
Com/Nav
FPL
Direct-To
Proc
Charts
Wpt Info
Map
Traffic
Terrain
Weather
Nearest
Services/
Music
Utilities
System
Messages
Symbols
Appendix
Index
affected by factors such as satellite geometry, but it is not subject to variations
in pressure and temperature that normally affect pressure altitude devices. GSL
altitude does not require local altimeter settings to determine MSL altitude.
Therefore, GSL altitude provides a highly accurate and reliable MSL altitude
source to calculate terrain and obstacle alerts.
The terrain and obstacle databases used by TAWS-A are referenced to mean
sea level (MSL). Using the GPS position and GSL altitude, TAWS-A displays a
2-D picture of the surrounding terrain and obstacles relative to the position and
altitude of the aircraft. Furthermore, the GPS position and GSL altitude are used
to calculate and “predict” the aircraft’s flight path in relation to the surrounding
terrain and obstacles. In this manner, TAWS-A can provide advanced alerts of
predicted dangerous terrain conditions.
Class A TAWS incorporates radar altimeter input with the GSL altitude to
provide a more accurate position reference when at lower altitudes for certain
alert types, and to retain a level of ground proximity warning capability in the
unlikely event of an airport, terrain or obstacle database failure.
Baro-corrected altitude (or indicated altitude) is derived by adjusting the
altimeter setting for local atmospheric conditions. The most accurate baro-
corrected altitude can be achieved by frequently updating the altimeter setting
to the nearest reporting station along the flight path. However, because actual
atmospheric conditions seldom match the standard conditions defined by the
International Standard Atmosphere (ISA) 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. This variation
results in the aircraft’s true altitude differing from the baro-corrected altitude.
TAWS-A provides the following alert types:
• Forward Looking Terrain Avoidance (FLTA) Alerting, which consists of:
- Required Terrain Clearance (RTC) / Required Line Clearance (RLC) /
Required Obstacle Clearance (ROC) Alerting
- Imminent Terrain Impact (ITI) / Imminent Line Impact (ILI) /
Imminent Obstacle Impact (IOI) Alerting
• Premature Descent Alerting (PDA)
• Ground Proximity Warning System (GPWS) Alerting, which consists of:
- Excessive Descent Rate (EDR) Alerting
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