AH is only linked to the probability of failure(s) of the automatic landing system. Operators are free to select an AH, lower than AH indicated in AFM, but not a higher value, AH is evaluated during Aircraft Evaluation. For A319/A320/A321/A330aircraft AH it is specified as 100 ft. in the FCOM.

3.9. Runway Visual Range (RVR).
RVR is the range over which the pilot of an aircraft on the Centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its Centre line. The height of the pilot’s eye is considered to be 15 ft. above the runway. For CAT II/III A/B ops, generally three RVR systems (Transmissometers) are installed along the Rwy, termed as: i) Touchdown RVR: installed in the touchdown area 1000ft from the threshold in down wind direction and within 400ft (120M) from centerline of the runway ii) Mid RVR: installed in the Middle side of the runway in the line of touchdown RVR

iii) Rollout RVR (End RVR): installed in the rollout area RVR may be more or less than the reported visibility. The difference depends on the existence of weather phenomenon and the location of the RVR system.
Touchdown RVR and mid RVR are mandatory for CAT-II and CAT-IIIA Rollout RVR is recommendatory for CAT-IIIA

3.10. Transmissometers: An instrument that takes a direct measurement of the transmittance between two points in space,
i.e. over a specified path length or base line.
3.11. Slant Visual Range (SVR): SVR is the range over which the pilot of an aircraft on final approach can see the visual aids specified in RVR definition. In shallow fog or when fog is confined to ground layers, SVR is more than RVR. In matured fog the density of fog increases with height and so the SVR is less than RVR.

3.12. Successful Approach:
A successful approach (vide DGCA Operations Circular No.2/99) is one, which at 100 feet point:

i) The airplane is in trim so as allow for continuation of normal approach and landing. ii) The Indicated Air Speed and Heading are satisfactory for a normal flare and landing. With auto throttle control system, the speed must be ± 5 kts of programmed airspeed but may not be less than computed threshold speed. iii) No unusual roughness or excessive attitude changes occur after leaving middle marker. iv) No unusual deviation in the touchdown and roll out performance.
Note: Every operator shall establish and maintain a suitable system for recording success and failure of approach and automatic landing to monitor the overall safety of the operation in accordance with the DGCA Ops. CircularNo.2 of 1999.
3.13. ILS critical area: An area of defined dimensions about the localizer and glide path antennas where vehicles, including aircraft, are excluded during all ILS operations.  The critical area is protected because the presence of vehicles and/or aircraft inside its boundaries will cause unacceptable disturbance to the ILS signal-in-space.
　
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