Garmin G1000H Bell 407GX Pilot's Guide - Page 346

HAZARD AVOIDANCE 6.3 HTAWS WARNING: Do not use HTAWS information for primary terrain avoidance. HTAWS is intended only to enhance situational awareness. NOTE: The data contained in the HTAWS databases comes from government agencies. Garmin accurately processes and cross-validates the data but cannot guarantee the accuracy and completeness of the data. NOTE: Terrain data is not displayed when the aircraft is outside of the database coverage region. HTAWS (Helicopter Terrain Awareness and Warning System) is designed to increase situational awareness and aid in reducing controlled flight into terrain (CFIT). HTAWS provides visual and aural annunciations when terrain and obstacles are within the given altitude threshold from the aircraft. The visual and aural alerts and warnings are advisory in nature only. HTAWS satisfies TSO-C194 requirements for certification. HTAWS requires the following to operate properly: • A valid terrain and obstacle database • A valid 3-D GPS position solution HTAWS 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. The data undergoes verification by Garmin to confirm accuracy of the content. However, the displayed information should never be understood as being all-inclusive and data may be inaccurate. HTAWS uses information provided from the GPS receiver to provide a horizontal position and altitude, along with additional altitude input from an optional radar altimeter. GPS altitude is derived from satellite measurements. GPS altitude is then converted to the height above geodetic sea level (GSL), which is the height above mean sea level (MSL) calculated geometrically. GSL altitude is used to determine HTAWS alerts. GSL altitude accuracy is affected by satellite geometry, but is not subject to variations in pressure and temperature that normally affect pressure altitude sensors. GSL altitude does not require local altimeter settings to determine MSL altitude. It is a widely-used MSL altitude source. 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 HTAWS are referenced to MSL. Using the GPS position and GSL altitude, HTAWS 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, HTAWS can provide advanced alerts of predicted dangerous terrain conditions. HTAWS incorporates altitude input from an optional radar altimeter for certain alerts. 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 334 Garmin G1000H™ Pilot's Guide for the Bell 407GX 190-01255-00 Rev. A