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TCF/TAD
INOP and
INHIBIT
Geometric
Altitude
38
The EGPWS TCF and TAD functions are available when all
required data is present and acceptable. Aircraft position and
numerous other parameters are monitored and verified for
adequacy in order to perform these functions. If determined
invalid or unavailable, the system will display Terrain
inoperative or unavailable annunciations and discontinue the
terrain display if active.
TAD/TCF functions may be inhibited by manual selection of a
cockpit Terrain Inhibit switch. Neither loss nor inhibiting TAD/
TCF effects the basic GPWS functions (modes 1-7).
If Peaks affects is not active and TAD becomes unavailable
due to position error, terrain inoperative or unavailable is not
indicated if the aircraft is greater than 8000 feet above the
highest terrain or obstacle within a 320nm radius. If
indicated below the 8000 foot threshold, it is extinguished
when the aircraft climbs above, and is again displayed once
the aircraft descends below the 8000 foot threshold. This
eliminates potentially longterm illumination of the condition
during the high enroute phase of flight.
Based on GPS altitude, geometric altitude is a computed
pseudo-barometric altitude (Above Sea Level - ASL) designed
to reduce or eliminate errors potentially induced in Corrected
Barometric Altitude by temperature extremes, non-standard
pressure altitude conditions, and altimeter miss-sets. This
ensures an optimal EGPWS Terrain Alerting and Display
capability. Geometric Altitude also allows EGPWS operations in
QFE environments without custom inputs or special
operational proceedures.
Geometric Altitude requires GPS Altitude input with its associ-
ated Vertical Figure Of Merit (VFOM) and
Receiver Autonomous Integrity Monitoring (RAIM) failure
indication, standard (uncorrected) altitude, Radio Altitude,
Ground Speed, Roll Angle, and aircraft position (Latitude and
Longitude). Additionally, corrected Barometric Altitude, Static
Air Temperature (SAT), GPS mode, and the number of
satellites tracked are used if available.
The Geometric Altitude is computed by blending a calculated
Non-Standard Altitude, Runway Calibrated Altitude
(determined during takeoff), GPS Calibrated Altitude, Radio
060-4241-000 • Rev. D - March 2000
MK V & MK VII EGPWS Pilot Guide
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