(a) System inputs The following table lists the inputs required to allow implementation of the TCF function:
 -------------------------------------------------------------------------------| Input | Source | -------------------------------------------------------------------------------| Radio Altitude | External: Radio Altimeter | |----------------------------------|------------------------------------------| | Latitude | External: FMS | |----------------------------------|------------------------------------------| | Longitude | External: FMS | |----------------------------------|------------------------------------------| | Runway Center Latitude | Internal: data base | |----------------------------------|------------------------------------------| | Runway Center Longitude | Internal: data base | |----------------------------------|------------------------------------------| | Navigation mode | FMS | |----------------------------------|------------------------------------------| | Alert Envelope Parameters | Internal: data base | |----------------------------------|------------------------------------------| | 1/2 Runway Length | Internal: data base | |----------------------------------|------------------------------------------| | System Error Factor | Internal: data base | -------------------------------------------------------------------------------
(b)
 Runway Data Base The TCF runway data base consists of data records containing the position of airport runway center points along with 1/2 the runway length. The data base includes all hard surface runways in the world greater than or equal to 3500 ft. in length. The process of generating this data base is certified and includes an end check that validates that the data was not corrupted in the translation process. This data base can be updated without affecting the customer certified system part number. The design of the data base and related software is such that additional runway records can be added in the future without altering the code. The data base provides a means of accessing the runway record of the runway closest to the current aircaft position.

 (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.
R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page A51 1 1 Config-1 Aug 01/05 1 1 1CES 1 Thus the inner and outer radii 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 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. The TCF curve is limited to a minimum value of 245 ft. when it is determined that the aircraft is to the side of the runway.

R  (Ref. Fig. 026) This feature provides improved alerting when landing to the side of the runway.
 (d) Runway Field Clearance Floor (RFCF) The Runway Field Clearance Floor (RFCF) alert envelope is a circular band centered over the selected runway. But unlike the radio altitude based TCF envelope, the RFCF envelope only extends 5 NM past the end of the runway. The bias factor (where the protection starts) is equal to the TCF Bias factor (k) plus an additional offset proportional to the Geometric Altitude Figure Of Merit (FOM).
R  (Ref. Fig. 027) This feature provides improved alerting for cases where the runway is at a high elevation compared to the terrain below the approach path. In these cases the radio altitude may be large enough to inhibit normal TCF operation, but the aircraft could actually be below the runway elevation. Field clearance (height above runway) is determined by subtracting the elevation of the selected destination runway from the current altitude (MSL).
 (e) 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 PULL UP legend of pushbutton switches 4WZ and 5WZ comes on. The legends remain on until the alert envelope is exited.
R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page A52 1 1 Config-1 Aug 01/05 1 1 1CES 1

 Enhances GPWS - TCF Curve Limit R Figure 026
R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page A53 1 1 Config-1 Aug 01/05 1 1 1CES 1

 Enhanced GPWS - Shape of the Alert Envelope R Figure 027
　
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