.  
Reduces the dependence between the navigation (i.e. FMS) and the surveillance (i.e. TAWS).

Within the former TAWS architecture (refer to Figure 4-13), the position source is:
1.
The FM position first, then 

2.
IR data if the FM position is unavailable.

Within the hybrid (refer to Figure 4-14) or autonomous architecture (refer to Figure 4-15), the selection of the position source applies the following sequence (by order of priority):
1.  
GPS position, then

2.  
IR data (from ADIRU 1 on A320/A330/A340 aircraft or IRU 1 for A300/A310 aircraft) combined with the last valid GPS data (GPIRS), then

3.  
FM position (from FMGEC 1 for A320/A330/A340 aircraft or FMS 1 for A300/A310 aircraft)

When all these sources are not valid or not accurate enough:
-
If the automatic deactivation of predictive TAWS functions has been selected (pin-programming), TAWS automatically deactivates predictive functions (basic TAWS functions remain active).

-
If the automatic deactivation of predictive TAWS functions has not been selected, the flight crew must manually switch predictive functions to OFF (TERR to OFF, refer to 4.1.6 – TAWS Controls).

4.1.5. TAWS INDICATIONS
This section briefly describes indications and controls. For more details, please refer to your FCOM.
4.1.5.1. TAWS BASIC MODE INDICATIONS
Please refer to 4.1.2 for the aural alerts provided in TAWS basic modes.
For EGPWS or T2CAS:
When the TAWS triggers a caution, the When the TAWS triggers a warning, the GPWS light comes on. PULL UP light comes on.

4.1.5.2. TAWS PREDICTIVE FUNCTIONS
The TAWS display on ND is available in ROSE and ARC modes.
TERRAIN AHEAD Caution

TERRAIN AHEAD1
-EGPWS: Every 7 seconds until conditions disappear.
-T2CAS: When the caution sensor penetrates the MTCD for at least 2 seconds. Every 5 seconds until conditions disappear.
TERRAIN AHEAD Warning

TERRAIN AHEAD – PULL UP2
-EGPWS: Repeated until condition disappears.
-T2CAS: When the warning sensor penetrates the MTCD for at least 2 seconds. Repeated until conditions disappear.

1 The European EASA regulations require the wording TERRAIN AHEAD. The FAA regulations require
CAUTION TERRAIN.
2 The European EASA regulations require the wording TERRAIN AHEAD – PULL UP. The FAA
regulations require TERRAIN TERRAIN – PULL UP.

TOO LOW TERRAIN

-EGPWS TCF: When the aircraft penetrates the TCF envelope and each time the AGL altitude deteriorates by 20%.
-T2CAS PDA: Refer to 4.1.3.2.3 – T2CAS Premature Descent Alert
　
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