(c)
 Alert Envelope The TCF alert envelope is a circular band centered over the nearest runway.

R  (Ref. Fig. 024) The distance from the runway center to the inner envelope edge is equal to 1/2 the runway length plus the envelope bias factor. Thus the inner and outer radius of the envelope are modulated based on the runway length and envelope bias factor. Runway length varies from one runway to the next, and the envelope bias
R 1EFF : 001-049, 051-099, 101-105, 151-199, 1 34-48-00Page A45 1201-214, 1Config-1 Aug 01/05 1 1 1CES 1 factor is typically 1/2 NM to 2 NM and varies with position accuracy. The outer alert envelope boundary extends to infinity, or until it meets the outer alert envelope boundary of another runway. The alert is inhibited below radio altitude of 30 ft. This cutoff value was chosen to match the current Mode 4 lower cutoff value.

 (d) System Outputs When an aircraft penetrates the TCF alert envelope the following aural message occurs: TOO LOW TERRAIN. This aural message occurs once when initial envelope penetration occurs, and one time thereafter for each 20 per cent degradation in radio altitude. At the same time the GPWS legend of pushbutton switches 4WZ and 5WZ comes on. The legends remain on until the alert envelope is exited.
 (3)
 Not Applicable

 (4)
 Not Applicable

R **ON A/C 106-149, 220-299, 301-399, 401-499,
 Post SB 34-1287 For A/C 220-228,301-302,
 (2)
 Airbus Corrected Barometric Altitude Airbus Corrected Barometric Altitude is available on A318/A319/A320/A321 series aircraft. It provides accurate corrected barometric altitude data regardless of the altimeter reference setting (QNH or QFE). Airbus corrected baro altitude allows the Terrain Awareness and Display function to remain enabled during QFE approaches conducted by aircraft not equipped with GPS, and provides a more accurate corrected baro altitude input to the Geometric Altitude algorithm for GPS-equipped aircraft. The Enhanced GPWS examines the selected baro reference (QNH, STD or QFE) from the Flight Control Unit (FCU). If the selected baro reference is QFE, then the Enhanced GPWS computes corrected barometric altitude using destination QNH data from the Flight Management System (FMS) and standard (uncorrected) altitude. For aircraft not equipped with GPS, Airbus corrected barometric altitude is used as the altitude source for the Terrain Awareness and Display functions and the Envelope Modulation function.

 (3)
 Geometric Altitude Geometric Altitude is a computed aircraft altitude designed to help to ensure optimal operation of the Enhanced GPWS Terrain Awareness and Display functions through all phases of flight and atmospheric conditions. Geometric Altitude uses an improved pressure altitude

R 1EFF : 001-049, 051-099, 101-149, 151-199, 1 34-48-00Page A46 1201-214, 220-299, 301-399, 401-499, 1Config-1 Aug 01/05 1 1 1CES 1 calculation, GPS altitude, radio altitude, and terrain and runway elevation data to reduce or eliminate errors potentially induced in corrected barometric altitude by temperature extremes, non-standard altitude conditions, and altimeter miss-sets. Geometric Altitude also allows continuous Enhanced GPWS operations in QFE environments without custom inputs or special operational procedures. With the Geometric Altitude function, Enhanced GPWS can operate reliably throughout extreme local pressure or temperature variations from standard, is not susceptible to altimeter miss-sets by the flight crew, and will not require any custom inputs or special procedures by the flight crew when operating in a QFE environment.

 (a)
 Required inputs. The Geometric Altitude algorithm requires GPS altitude with Vertical Figure of Merit (VFOM) and RAIM failure indication along with standard (uncorrected) altitude and radio altitude. Ground speed, roll angle, and position (latitude and longitude) are used indirectly and are also required. Additionally, corrected barometric altitude, static air temperature (SAT), GPS operational mode and the number of satellites tracked are used if available. The required GPS signals are provided via ADIRU on the GPIRS bus or directly from the GPSSU or MMR. Standard altitude, corrected barometric altitude, and SAT are provided directly from the ADR. If SAT is not available, geometric altitude is computed using standard altitude with a corresponding reduction in accuracy.

 (b)
 Altitude calculation The Geometric Altitude consists of three main functions:

 -
computation of non-standard altitude

 -
computation of the component altitudes and VFOMs

 -final altitude-signal blending. Additional logic exists to handle reversionary modes and signal reasonable checking for each component altitude.
R (Ref. Fig. 025)
R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page A47 1 1 Config-1 Aug 01/05 1 1 1CES 1
　
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