Warning Sensor
Caution Sensor
MTCD
Three segments compose the clearance sensors (see Figure 4-8):
1.
A projection of the current flight path (8 seconds for the warning sensor, 20 seconds for the caution sensor)

2.
A vertical maneuver at constant 1.5 G maneuver

3.
A projection of the aircraft climb as per Aircraft Performance

database. In mountainous approach areas, the sensors are linearly reduced to about 30 seconds to avoid undue alerts at low altitude.
If the sensor interferes the MTCD for at least 2 seconds, the T2CAS triggers an aural alert (TERRAIN AHEAD caution or warning). In case of a late reaction from the flight crew or in case of very steep terrain environments, the vertical pull up maneuver may not clear the CFIT risk. In these cases, the T2CAS orders a lateral maneuver (AVOID TERRAIN warning).

On the horizontal plane (see Figure 4-9), the sensor opens up by 1.5° on either side of the track. The aperture of the sensor goes up to 90° during turns. The sensor width starts at 100 m (if GPS error < 100 m) or 200 m (if GPS error > 100m).
To avoid undue CPA alerts when the aircraft is close to the runway, CPA alerts are inhibited in normal take-off conditions or when the aircraft is safely converging towards the runway.
The take-off is considered as normal if:
-
The vertical speed is not negative for more than 2 seconds

-
The distance to the runway threshold is less than 1.9 NM

-Various criteria relative to the variations of MSL altitude, track angle and Radio Altitude are met.

The Runway Convergence protection provides alerts when the aircraft performs a premature descent or flies an unsafe approach flight path. The runway convergence protection is available until the aircraft is within 90 ft above the runway.
The T2CAS considers the runway convergence as safe when:
-Landing gears are down and flaps/slats in landing
configuration
-
The distance to the runway threshold is less than 5 Km (2.7 NM)

-
The aircraft remains in the runway convergence envelope

-
The aircraft track remains in an inhibition range

-If RA is not valid, the vertical speed is in an inhibition range.

If the runway threshold coordinates are not in runway database, the CPA function is deactivated if the aircraft is within 1.9 NM from the airport reference point (horizontal distance) and within 900 ft from the elevation of the airport reference point (vertical distance). The green TERR STBY memo is displayed on ECAM and Mode 2 is permanently activated.
4.1.3.2.2. T2CAS Terrain Hazard Display (THD)
The T2CAS provides only the Standard mode display (compared to EGPWS, see Figure 4-4). In descent, the T2CAS provides an anticipated terrain situational awareness. The Reference Altitude is defined as the aircraft altitude projected 30 seconds along FPA (refer to Figure 4-11).
　
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