2.2.9 TERRAIN CLEARANCE FLOOR AND RUNWAY FIELD CLEARANCE FLOOR
The Terrain Clearance Floor (TCF) alerting function adds an additional element of protection to the standard Ground Proximity Warning System. It creates an increasing terrain clearance envelope around the airport runway to provide CFIT protection against situations where Mode 4 provides limited or no protection. TCF alerts are based on current aircraft location, destination runway center point position, and Radio Altitude (altitude AGL). TCF is active during takeoff, cruise, and final approach. TCF complements the existing Mode 4 protection by providing an alert based on insufficient terrain clearance even when in landing configuration.
The TCF function is enhanced in all models (beginning with release –210-210 for the MK V/VII) with the addition of a Runway Field Clearance Floor (RFCF) alerting function. RFCF is based on current aircraft location, destination runway center point position, and Geometric Altitude or altitude Above Sea Level (ASL) relative to the destination runway. RFCF provides short landing protection for runways that are significantly higher than the surrounding terrain.
When an aircraft penetrates either the TCF or the RFCF alert envelope, the aural message “TOO LOW TERRAIN” will occur. This aural message will occur once when initial envelope penetration occurs, and one time thereafter for each 20% degradation in either Altitude (AGL) or Altitude (ASL) depending on which envelope was violated (TCF or RFCF respectively). EGPWS cockpit alert annunciations remain illuminated until the alert envelope is exited.
2.2.9.1 RUNWAY DATABASE
The EGPWS Runway Database consists of data records for all airport runways offered for the coverage provided by the Terrain Database. For the MK V and MK VII, all hard surface runways in the world 3500 feet or greater in length are supported. For the MK VI, all runways 2000 feet or greater in length within the database region installed are supported. The MK VIII has the ability to select 2000ft or 3500ft runway lengths. The database provides the means of accessing the records of runways closest to the current aircraft position.
2.2.10 TERRAIN ALERTING AND DISPLAY (OPTIONAL)
The Terrain Alerting and Display (TAD) function monitors aircraft position with respect to local database-cataloged terrain to provide rapid audio and visual alerts when a terrain threat is detected. Terrain threats are recognized and annunciated when terrain violates specific computed envelope boundaries forward of the aircraft path. The terrain database also includes obstacles (when and where available) providing similar annunciations when cataloged obstacles violate the same envelope boundaries.
Terrain Alerting outputs (lights and audio) behaves in the same manner as the standard GPWS mode alerts. Either caution or warning alerts will initiate a specific audio alert phrase. The caution aural is “CAUTION TERRAIN” or “CAUTION OBSTACLE” and the warning aural is “TERRAIN, TERRAIN, PULL UP” or “OBSTACLE, OBSTACLE, PULL UP” (minor variations exist).
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