User's Manual

SYSTEM DESCRIPTION AND INSTALLATION MANUAL
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CAS/Part No.9005000
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CAS uses data provided by the Flight Management Computer, the
GPS receiver, and other aircraft subsystems to predict a 3-D flight path
based upon a curve-fit extrapolation of the most recent position and
velocity data received. This predicted flight path is then compared with
the internal terrain map of the immediate area and an assessment of the
potential threat of aircraft collision with terrain is computed by the
Collision Prediction and Alerting (CPA) algorithm.
(b) The Premature Descent alert (PDA)
The Premature Decent Alert is based on the forward-looking Collision
Prediction and Alerting (CPA) sensor. If the CPA sensor contacts a
terrain cell plus the Minimum Terrain Clearance Distance (MTCD), then a
comparison is performed. If a 0.25-g level-off maneuver is sufficient to
clear the conflicting terrain, then the PDA alert ”Too Low Terrain” is
annunciated and the Caution Alert Lamp will be activated. Otherwise, the
corresponding CPA alert is annunciated. Therefore, since this alert is
based off of the CPA sensor, any conditions that result in an outage of
the CPA function will also result in the PDA alert being inactive.
(c) Conventional GPWS Modes of Operation
1 Mode 1: Excessive rate of descent with respect to terrain
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CAS meets the requirement for Mode 1 alerts as defined in
RTCA DO-161A. This mode provides not only a reactive short-
term warning as defined in RTCA DO-161A, but also a reactive
medium-term caution when the current flight path is descending
toward the terrain ahead of the aircraft at an excessive rate.
2 Mode 2: Excessive closure rate to terrain
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CAS meets the requirement for Mode 2 alerts as defined in
RTCA DO-161A. This mode provides a reactive short-term
warning as defined in RTCA DO-161A when the current flight
path and the terrain ahead of the aircraft are closing at an
excessive rate. When the CPA function is active, Mode 2 is
conditionally activated when a Terrain Cell in the Terrain
Database is detected to be significantly low. This can be due to a
vertical error, horizontal error or terrain database error. This
detection occurs if the difference between MSL Altitude and
Radio Altitude is appreciably larger than the Terrain Cell Height
in the Terrain DB. This detection is only calculated when Radio
altitude is less than 2500 feet (762 meters).
3 Mode 3: Excessive altitude loss after take-off
T3CAS meets the requirement for Mode 3 alerts as defined in
RTCA DO-161A. This mode provides an alert when there is a
loss of altitude after take-off or during a missed approach. The
mode uses the radar altimeter to determine proximity to the
ground and the barometric altimeter to determine altitude loss. If
an excessive loss of altitude occurs after take-off, a warning is
issued.
Pub. No. 8600200-001, Revision 004
34-45-29
1-33
04 Nov 2014
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