Why is this wrong?

[chuychanga]

I have a feeling that I'm misinterpreting something here, but it makes sense to me. Is this wrong? If so, why?

FAR 1.1 defines Flight Visibility as the average forward horizontal distance FROM THE COCKPIT OF AN AIRCRAFT IN FLIGHT....

RVR is the the maximum distance in the direction of takeoff or landing at which the runway, or specified lights or markers delineating it, can be seen from a position corresponding to the average eye level of pilots AT TOUCH DOWN.

This implies that RVR and Flight Visibility are two different things measured from two different places.

So if Flight Visibility is measured from the cockpit, can we assume that only the pilots can determine what the Flight Visibility is, while the RVR reports the Ground Visibility?

FAR 91.175 (C)(2) says you must have the required FLIGHT VISIBILITY to go below DH...

FAR 91.175 (D) says you can't land unless you have the prescribed FLIGHT VISIBILITY...

Here's what I'm getting at:
Let's say the approach minimums are 1800 RVR. I have the minimums prior to starting the approach and then inside the marker it goes down to 1000 RVR. I continue down and then at DH I see the runway threshold. A 3 degree glideslope reaches 200 vertical feet about 3500 feet from the threshold (depending on the length of the runway). 3500 feet is well over 1/2 mile. So if I see the runway threshold from there, can I assume that I have the required Flight Visibility for the approach even though the controller is calling out a lower RVR? Can I land?

I know this is semantics and I'm not the type of person to try to find loopholes in rules that were put there for my own safety. I've never gone below DH without the proper RVR. I'd always assumed that you could not land in that situation, not because of the danger of missing the runway, but because of the danger of a runway incursion with someone in the fog after you land.

But the fact that 91.175 states Flight Visibility instead of Ground Visibility leads me to believe that you could land.

I know that RVR is controlling over prevailing visibility, but is it controlling over flight visibility as well?

 

[skeezer]

If you have the required FLIGHT visibility, and the required visual references you can land.

Some air carriers (mine included), do not allow you to land if the reported visibility is less than mins, even if you have the required flight visibility.

Peace

Skeezer

 

[avbug]

Flight visibity is used to determine visibility for landing. Under part s 121 or 135, you may not begin the approach unless you have the requisite approch minimums. Opertions Specifications call for touchdown RVR as controlling. If RVR is not reported, then the reported visibility controls weather or not you can fly the approach.

Ground visibility is that reported by a ground observer. RVR is eletronically determined using a transmissiometer. We have all seen cases in which a little frost on the trasnmisiometer has lead to it reporting low visibility, when in fact we can see that the visibility is fine. In such cases, where it's reported, it's still controlling.

Under Part 91, you may fly the approach regardless of the reported weather minimums. If you have adequate flight visibility to land, or continue the approch beyond minimums, then you may do so at your discretion. Be aware, however, that if official weather reports are showing the weather below minimum and you do continue, you may be required to defend your actions in the event of a mishap or an investigation.

You may not do anything wrong, but it may be the ground incursion from the guy you don't know about taxiing downfield, that draws the attention that leads to an investigation. Anyting could, and in that case you may likely be called upon to defend your actions and show that you had the adequate flight visibility. You can't do this; it's only your word...and as you know, under the FAA's administrative law, you are guilty until proven innocent.

 

[bigsky]

I have landed- with very good flight visibility when the airport is calling it below minimums-- I think that is exactly why it is written that way. At big airports you can have varying conditions at different parts of the airport. I also have had to go missed approach- even though rvr was more than adequate but flight vis was not!
Once landing at JFK on 13L it was well below mins however the fog bankdidnt start until 2000 feet down the ry- we were able to land in the clear and the trouble didnt begin until on the rollout we couldnt see anything.