Terrain Avoidance and Warning System
Do not use terrain avoidance displays as the sole source of information for maintaining separation from terrain and obstacles. Garmin obtains terrain and obstacle data from third party sources and cannot independently verify the accuracy of the information.
Terrain data is not displayed when the aircraft is outside the installed terrain database coverage area.
Terrain and obstacle alerting is not available north of 89º North latitude and south of 89º South latitude. This is due to limitations present within the Terrain database and the system’s ability to process the data representing the affected areas.
For single pilot aircraft the 'Split COM' function will remove aural alerts from the passenger and co-pilot headsets.
Terrain and obstacle alerting requires Adaptive Terrain or TAWS-B.
The TAWS-B (Terrain Awareness and Warning System - Class B) is a feature that provides increased situational awareness and aids in reducing controlled flight into terrain (CFIT) and obstacles. TAWS-B provides visual annunciations and voice alerts when terrain and obstacles are within the given altitude threshold from the aircraft.
For AXIS™ Flight Display(s) installations that require TAWS-B the configuration will satisfy TSO-C151d Class B requirements for certification. For installations that do not require TAWS-B, Adaptive Terrain will be the standard configuration. The rest of this section will be describing TAWS-B functionality and any mention of pilot configurable options will be in reference to only Adaptive Terrain configuration of the system.
TAWS-B requires information from the following sources:
- A valid terrain and obstacle database.
- A valid 3-D GPS position solution.
TAWS-B uses terrain and obstacle information supplied by government sources. Terrain information is based on terrain elevation information in a database that may contain inaccuracies. Individual obstructions may be shown if available in the database. Garmin verifies the data, per TSO-C151d. However, the displayed information should never be understood as being all-inclusive and data may still contain inaccuracies.
TAWS-B uses information from the GPS receiver to provide a horizontal position and altitude. GPS altitude is derived from satellite measurements. GPS altitude is then converted to a height above geodetic sea level (GSL), which is the height above mean seal level (MSL) calculated geometrically. GSL altitude accuracy is 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 find the MSL altitude. Therefore, GSL altitude provides a highly accurate and reliable altitude to calculate terrain and obstacle alerts.
The terrain and obstacle databases are referenced to mean sea level (MSL). Using the GPS position and GSL altitude, the system displays a 2-D picture of the surrounding terrain and obstacles relative to the position and altitude of the aircraft. Furthermore, the system uses GPS position and GSL altitude to calculate and project the aircraft’s flight path in relation to the surrounding terrain and obstacles. In this manner, TAWS-B can provide advanced alerts of predicted dangerous terrain and obstacle conditions.
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 displayed GPS-GSL altitude. This variation results in the aircraft’s true altitude differing from the baro-corrected altitude.
- Forward Looking Terrain Avoidance (FLTA) Alerting, which consists of:
- Required Terrain Clearance (RTC) / Required Obstacle Clearance (ROC) Alerting.
- Imminent Terrain Impact (ITI) / Imminent Obstacle Impact (IOI) Alerting.
- Premature Descent Alerting (PDA)
- Excessive Descent Rate (EDR) Alerting.
- Negative Climb Rate (NCR) after takeoff Alerting.
- Altitude Voice Call Out (VCO) Alerting.