Rallye EI-BFP wrote:Ryanair flew in grand in the 100m visibility (and did not request CAT2, alas!)
Visibilty is the greater of:
a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background.
b) the greatest distance at which lights in the vicinity of 1000 candles can be seen and indentified against an unlit background.
The range over which the pilot of an aircraft on the centre line on a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.
These are ICAO definitons.
The reported values of these two can be distinctly different. However, TDZ RVR is the measurement used for approach minimums. Usually, but not always, minimums are CAT I...550m, CAT II...300m, CAT IIIA...200m. Company procedures can impose higher, but not lower values. If no RVR is available, met vis values can be used, but have a multiplication factor applied to them.
If an airfield is under LVP, this will be broadcast to aircraft on the ATIS, or informed by ATC. Therefore there is no need, under these circumstances, for an aircraft to explicitaly request a CAT II/III approach.
Aer Arann are brilliant at taking pot shots at cork!
airoshane wrote:The aircraft abuse the RVR rule. The RVR may be 300m and an ATR wants to do a Cat I approach. They usually wait a while and the RVR may jump to 600 m only for a few seconds. ATC announce the new RVR and the aircraft take a "pot shot" at it. The ATC calls out the RVR again when the aircraft are on finals but if it is back below the limit the aircraft still go ahead and try the approach.
I can guarantee it's not a "pot-shot". You may have a good career ahead of you writing for The Sun!!
An aircraft cannot commence an approach, ie leave the IAF unless the current RVR is above the minimum required.
If after leaving the IAF, the reported RVR falls below the minimum required, the approach can be continued as far as the OM/4DME fix/FAF or a point designated as such. On reaching this point, the minimum RVR must be above limits or a missed approach is required.
If all is above limits, but after passing the OM/4DME/FAF point, the RVR falls below minimum, the approach may be continued to DH and a landing made if the required minimum visual reference is attained.
Of course, this is a generic procedure, but most airlines will have SOP`s based around this.
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