Chapter-4  DECISION HEIGHT AND VISUAL REFERENCES  Page-26 

Cockpit Cut-off Angle   

Height of pilots eye  Distance obscured by aircraft nose  SVR required for Cat-II/IIIA 
169 ft. (RA 150 ft.) 119 ft. (RA 100 ft.) 69 ft. (RA  50 ft.)  180 m. 127 m. 76 m.  300 m. 247 m. 136 m. 

(The SVR given is based on required visual reference of 120m. for CAT-II Ops and 60 m for CAT-IIIA Ops.)

4.9 Acquiring Visual Reference.
The visual segment and the visual cues at various heights during approach would depend on not only RVR but on following factors also:

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-Glide path angle
-Cockpit cut-off angle
 -Approach configuration
 -Pitch Attitude

The RVR measurement is based on the following factors:
 -the atmospheric transparency.
-the background luminance.
-the intensity of runway lights.

However the pilot’s range of vision is also affected due to factors such as
-Transparency of the windscreen
-Water due to rain etc. on the windscreen
 -Cockpit lighting
-Prior exposure to other illumination

Research using flight simulators and flight tests has shown that most pilots require the following: -
i) Visual contact to be established about 3 seconds above DH.
This reduces to about one second when a fail operational automatic landing system is used.
ii) To establish lateral position and cross-track velocity
to be able to see not less than a 3-light segment of the centerline of approach lights or runway center line lights or barrettes of the touchdown lighting. iii) To maintain wings level:  to be able to see an approach lighting cross bar or the threshold or a barrette of the  Touchdown zone lighting.
iv) To make adjustment to the flight path in the vertical plane using purely visual cues, to be able to see a point

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which has a negligible apparent movement relative to the aircraft.
For acquisition of Visual Reference the following informations can be very useful during final approach:
i) Seat position should be properly adjusted. A too low seat adjustment may greatly reduce the visual segment. A319/ A320/ A321/ A330 aircraft are equipped with an eye position indicator. The optimum eye position is obtained when the pilot sees the red indicator ball covering the white ball.
ii) Use of landing lights and strobe lights in low visibility condition can be detrimental to acquisition of visual reference. Reflected light from water droplets or snow may actually reduce visibility. Landing lights and strobe lights would therefore not be used in Cat-II/IIIA/IIIB conditions. 
iii) The portion of approach lights and runway lights constituting the visual reference and their pattern must be understood and anticipated as the aircraft approaches to DH.
　
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