getting to grips with CAT II CAT III operations(8)

on the centreline of the runway can see the runway surface markings or the lights delineating the runway or identifying its centreline (ICAO).
2.4.2 RUNWAY VISUAL RANGE CONCEPT
Categories II and III operations require rapidly updated and reliable reports of the visibility conditions which a pilot may expect to encounter in the touchdown zone and along the runway.
RVR measurements replace the use of Reported Visibility Values (RVV) which isnot appropriate for conditions encountered during the final approach and landing in low visibility, because the visibility observations are often several miles away from the touchdown zone of the runway.
Note: RVR is not the Slant Visual Range (SVR). SVR is the range over which a
pilot of an aircraft in the final stages of approach or landing can see the
markings or the lights as described in RVR definition.
Figure 2.3

2.4.3 RUNWAY VISUAL RANGE MEASUREMENTS
For Category II and Category III operations, the RVR measurements are providedby a system of calibrated transmissometers and account for the effects of ambient background light and the intensity of runway lights (see Chapter 6 for further details).
Transmissometer systems are strategically located to provide RVR measurements associated with three basic portions of a runway:
. the touchdown zone (TDZ),
. the mid-runway portion (MID), and
. the rollout portion or stop end.

For Category II operations the TDZ measurement is required, and for Category III operations the TDZ and MID measurements are mandatory. But for CAT III operations with the lowest weather minima, the three measurements are normally required by FAA.
For CAT III without DH JAR OPS 1 requires only one RVR measuring point on the runway.

2.4.4  ESTABLISHMENT OF RVR MINIMA
In Category II and Category III operations, the minima are expressed in terms of DHand RVR. It is relatively simple to establish the DH. But it is more difficult to establish the RVR to be associated with that DH in order to ensure the required visual reference (three-light segment).
When establishing airfield operating minima, it is recommended to refer toacceptable minima as described in Chapter 3 paragraph 2. The use of those minima has resulted in a high approach success rate.
Theory of RVR minima determination (ECAC document n°17)
The basic principles for the establishment of RVR minima are that the scale of visual reference required by a pilot at and below DH depends on the task that he has to carry out and that the degree to which his vision is obscured depends on the nature of the meteorological phenomena which creates the low visibility conditions.
" ... the task that he has to carry out ... "
Research using flight simulators and flight tests have shown that:
1.  
most pilots require visual contact to be established about three seconds above DH though it has been observed that this reduces to about one second when a fail-operational automatic landing system is being used;
　
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