many times on the net I have read that cat-1 ils can serve till the runway visbility remains atleast 550m. at my local airport, operators close the cat-1 runway for landing when visibility drops below 1600m, saying that they wont use it before RVR instrument is installed for the runway. the adjacent cat-111b runway has RVR instrument installed on it. should not the visibility info from it be sufficient to continue operations on the cat-1 runway?is an RVR machine exclusively required to operate in low visibility for a particular runway?

Visibility and RVR (runway visual range) are two different things.If the RVR is 550m the viz might be as low as 100-200m.For a precise RVR measurement you need three measuring points very close to the runway.Touchdown zone, mid point and stop end.(three for each runway)So you can't use the RVR of a different runway as this would not give you a precise enough value. RegardsBernt Capt 767

>Visibility and RVR (runway visual range) are two different>things.>If the RVR is 550m the viz might be as low as 100-200m.can you please point out the difference b/w the two terms>For a precise RVR measurement you need three measuring points>very close to the runway.>Touchdown zone, mid point and stop end.(three for each>runway)>So you can't use the RVR of a different runway as this would>not give you a precise enough value. >>Regards>>Bernt >>Capt 767how precise the value need be?I guess visibility shouldnt vary much for two runways at the same airfield

>>Visibility and RVR (runway visual range) are two different>>things.>>If the RVR is 550m the viz might be as low as 100-200m.>>can you please point out the difference b/w the two terms>"Visibility" is a general observation by the people in the control tower of what they can see. RVR is what the transmissometers at the specified locations along the runway is reporting. These are different reports from very different locations. Often, the people in the tower may be reporting 0 visibility while the RVRs are reporting adequate values for operations. RVR is always a better and more accurate value than visibility. If both visibility and RVR reports are available, the RVR will supercede for that runway.>>For a precise RVR measurement you need three measuring>points>>very close to the runway.>>Touchdown zone, mid point and stop end.(three for each>>runway)>>So you can't use the RVR of a different runway as this would>>not give you a precise enough value. >>>>Regards>>>>Bernt >>>>Capt 767>>how precise the value need be?>I guess visibility shouldnt vary much for two runways at the>same airfieldRVR for a runway can vary greatly between one end to the other, let alone different runways on different sides of the field. One runway might be socked in by a fog bank and be in CAT III conditions while another one may be good enough for visual approaches on the other side of the field. Since the visibility minimums specified in ILS approaches are measured in 100's of feet (or 10's of meters), the RVRs need to be that precise.

>"Visibility" is a general observation by the people in the>control tower of what they can see. Kevin,Do they only use their experience or do they have some objective tools to assess what this visibility is?Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

Known objects and landmarks visible from the tower indicate the visibility extent during the observation.

Short answer; Visibility.....no lighted objects.RVR.....lighted objects. From the AMS Glossary;"runway visual range

>RVR for a runway can vary greatly between one end to the>other, let alone different runways on different sides of the>field. One runway might be socked in by a fog bank and be in>CAT III conditions while another one may be good enough for>visual approaches on the other side of the field. Since the>visibility minimums specified in ILS approaches are measured>in 100's of feet (or 10's of meters), the RVRs need to be that>precise.>but I still dont understand as to why on unavailability of RVR instrument, the cat-1 rwy is shut for landings when visibility is less than 1600m, almost three times what cat-1 can handle

>but I still dont understand as to why on unavailability of RVR instrument, the cat-1 rwy is shut for landings when visibility is less than 1600m, almost three times what cat-1 can handleDepends on e.g. (but not only) the runway lighting facilities as well.550m is the absolute minimum.E.g. a note on our charts says: The whole RWY lighting may be failed for CAT I operations if the RVR is greater than 1000m.RegardsBernt Capt 767

>Depends on e.g. (but not only) the runway lighting facilities>as well.>550m is the absolute minimum.>E.g. a note on our charts says: The whole RWY lighting may be>failed for CAT I operations if the RVR is greater than 1000m.failing for visibility greater than 1000m? - sounds wierd.talking about the cat-1 rwy, it doesnt have centerline lights, but I was talking in reference to the RVR instrument.the people at the airfield are currently setting up the RVR instrument and say that it will make possible the rwy to be used in visibility lower than 1600m.

>failing for visibility greater than 1000m? - sounds wierd.means that if no lighting is available, the viz needs to be at least 1000m.>talking about the cat-1 rwy, it doesnt have centerline lightsCL is only needed for CATII and CATIII ops. RegardsBernt Capt 767

>>but I still dont understand as to why on unavailability of RVR>instrument, the cat-1 rwy is shut for landings when visibility>is less than 1600m, almost three times what cat-1 can handleThat does sound unusual. You'll have to provide more information, such as what airport and runway you are asking about.

"The whole RWY lighting may be failed for CAT I operations if the RVR is greater than 1000m."What that means is that CAT I operations are permitted when the runway lighting is failed (ie out of commission) if the visibility is greater than 1000m.