getting to grips with surveillance(67)

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The EGPWS crosschecks the FMS position with navaids data, altimeter and heading data, and stored terrain data to guard against navigation errors. When the FMS position crosscheck is positive, the Envelope Modulation function uses the aircraft lateral position and the geometric altitude (refer to 4.1.4.1 – EGPWS Geometric Altitude) to reduce the caution and warning envelopes during the
approach on specific areas.
4.1.3. PREDICTIVE TAWS FUNCTIONS
EGPWS or T2CAS only (not GPWS) includes predictive TAWS functions (also known as the enhanced functions for EGPWS). EGPWS and T2CAS provide different methods for the prediction of collision. The following paragraphs describe their respective predictive functions. A synthetic table provides at the end of this section the equivalence between these different predictive functions.

4.1.3.1. ENHANCED GPWS FUNCTIONS
4.1.3.1.1. EGPWS Terrain Awareness and Display (TAD)
The TAD function analyses the terrain in caution and warning envelopes (see Figure 4-3) ahead and below the aircraft. When a terrain penetrates one of these envelopes, the TAD function triggers visual and aural alerts.
The envelopes are defined by:
-
A centerline that lines up with the ground track. A lead angle is added during turns.

-
A width that starts at . NM (460 m) and gets wider ahead of the aircraft with an aperture of 3 degrees on either side of the centerline.

-An altitude floor that is computed according to the aircraft altitude, the nearest runway altitude, and the range to the nearest runway threshold. It prevents irrelevant alerts at take-off and landing.
-
A slope that varies with the aircraft Flight Path Angle (FPA).

-
A look-ahead distance that is computed from the aircraft ground speed and turn rate. It provides an advance alert with adequate time for the flight crew to safely react. The caution look-ahead distance provides 40 to 60 seconds of advance alerting. The warning look-ahead distance is a fraction of the caution look-ahead distance.

 

The  TAWS  displays  the 
surrounding terrain  on ND 
according  to  the  aircraft 
altitude.  A  color-coding  is 
applied as in Figure 4-4. 
The Reference Altitude is 
projected  down  along  the 
flight path  from the  actual 
aircraft altitude to provide a 
30 second  advance  display 
when  the  aircraft  is 
descending more than 1 000 
ft/min. 
The  EGPWS  provides  two 
different  modes  of  terrain 
display on ND:  Figure 4-4 : EGPWS co lor coding  with  peaks option 
- Standard  mode:  the  terrain  is  displayed  according  to  the  vertical 
　
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