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 advisories generated by GPWS are the aircraft's altitude above the ground as calculated by the radio altimeter.

1.18.2.2 Enhanced Ground Proximity Warning System Enhanced GPWS has the ability to "look ahead" of the aircraft to determine the terrain elevation along the flightpath. Enhanced GPWS can therefore provide pilots with visual and aural alerts in advance of an impending impact with terrain, thus allowing pilots more time than with traditional GPWS to determine the necessary corrective actions to be taken.

Enhanced GPWS compares the aircraft's position, as determined by its on-board navigational systems (that is, the flight management system [FMS], inertial reference system, or GPS), with a stored terrain database. Terrain and ground obstructions that may pose a collision threat along the flightpath of the aircraft result in aural and visual warnings. The visual warning information is provided to the pilot using the color graphics capabilities of a dedicated display screen, the color weather radar, or an Electronic Flight Instrument System map display (depending on the particular installation).

Further, unlike traditional GPWS, enhanced GPWS utilizes an airport position database to establish a "terrain clearance floor" around all airports. This feature ensures sufficient terrain clearance regardless of the airplane's landing gear and flap configuration.

1.18.2.2.1 Notice of Proposed Rulemaking for Enhanced Ground Proximity Warning Systems

On August 26, 1998, the FAA issued a notice of proposed rulemaking (NPRM) addressing the development and installation of a TAWS (Docket No. 29312, Notice No. 98-11). The NPRM stated that Technical Standard Order (TSO) C151, titled "Terrain Awareness and Warning System," was being developed through the FAA's TSO process and that, once the TSO has been completed, the FAA would issue an advisory circular (AC) addressing an acceptable means of obtaining installation approval.

The FAA believed that the installation of a TAWS would ensure that all applicable airplanes operated under Parts 91, 121, and 135 would have state-of-the-art equipment to aid in the prevention of CFIT accidents. The FAA's proposal also applies to operators conducting flights under Part 125 and operators of U.S.-registered airplanes under Part 129.

The FAA proposed that, for operations conducted under Part 121, the rule would apply to all turbine-powered airplanes and that, for operations under Parts 91, 125, 129,