4.1.3.2.3. T2CAS Premature Descent Alert (PDA)
The PDA function provides alerts when the aircraft is descending and the terrain of concern is below instead of ahead of the aircraft. The PDA is an alternative to the CPA TERRAIN AHEAD caution
each time a level off maneuver, rather than a climb one, is sufficient to clear the collision risk. The PDA function triggers an aural alert only (TOO LOW TERRAIN). No indication is displayed on ND.
The T2CAS triggers the Premature Descent Alert when:
-The Caution clearance sensor interferes with the MTCD for more than 2 seconds
-No TERRAIN AHEAD caution had been triggered
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The vertical speed is negative

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The level off altitude is above the MTCD

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The radio altitude is invalid or below a given threshold (1 000 ft in en-route phase, 750 ft in terminal phase, 500 ft in final phase).

4.1.3.3. EGPWS/T2CAS COMPARISON

EGPWS T2CAS 
Mandatory functions  Reactive modes: Imminent Contact Alerting Modes 1 to 5 Mode 2 inhibited when altitude check correct 
Predictive modes: Forward Looking Terrain Alerting Collision Prediction and Alerting (CPA) based on Terrain and Aircraft Performance databases . Terrain Ahead . . Terrain Ahead Pull Up . . Avoid Terrain . 
Premature Descent Alerting . Too Low Terrain . 
Terrain Display Terrain Hazard Display (THD) based on Terrain database 
Voice call outs Managed by FWS (500ft or 400ft call out) 
Other functions  Automatic Deactivation of FMS Source 
FAA audio Caution Terrain instead of Terrain Ahead Terrain Terrain Pull up instead of Terrain Ahead Pull Up 
Bank Angle alerting (A300/310 aircraft only) 
Radio Altitude Blending + GPS altitude activation (Hybrid or Autonomous GPS Architecture) 
GPS Lateral Position (Hybrid or Autonomous GPS Architecture) 
Obstacle database 
Peaks mode 
Compatible with Honeywell Autotilt weather radar  Not compatible with Honeywell Autotilt weather radar 
FLS support (mode 5) 
Runway Awareness and Advisory System (RAAS) 
- 4-14 -

Note: Functions shaded in yellow are basically activated in forwardfit.
4.1.4. INTRODUCTION OF GPS POSITION INTO TAWS ARCHITECTURE

The improvement of TAWS functions relies on the use of the vertical and lateral GPS positions. The use of vertical and lateral GPS positions gets rid of drifts from barometric altitude and FMS position. Such drifts are known to cause spurious alerts (e.g. over-flown aircraft, map shift) and unnecessary go-arounds.
The use of vertical and lateral GPS positions introduces slight modifications in the avionics architecture. According to the aircraft configuration, the new avionics architecture is called:
-Hybrid architecture for aircraft equipped with ADIRU 4MCU able to process GPS position. TAWS uses the pure GPS position from MMR or GPS-SU via ADIRU. It is the most commonly used (A320/A330/A340).
　
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