There might be fog or mist present when I depart tomorrow from an uncontrolled aerodrome. The minimum allowable visibility for my departure is half a statute mile, so I need to be able to determine how far I can see along the runway, without a tower, a flight service specialist or an electronic runway visual range (RVR) measurement. I'll have to count runway lights.
I haven't done this in a while--you don't get fog much in the summer--so I had to double check some numbers. I found then in A Quick Reference: Airfield Standards from the US FAA. (Nice little reference. I intend to read it through in its entirety sometime, maybe while waiting for fog). It confirms for me that the lights along the runway edge are spaced 200' apart. There are six thousand feet in a nautical mile, but for some reason ground visibility is measured in statute miles, which contain only 5280 feet apiece. (I had to look that number up, too). That means that I need to be able to see half of that, or 2640' feet along the runway to meet the half-mile minimum visibility. And at this point I realize "well duh: if there is an RVR then a half mile is RVR 2600." So I'm on track. This means that if I pull onto the runway even with one set of runway lights (they are aligned with each other on each edge of the runway) I need to be able to count 2600 divided by 200, or thirteen more pairs of them, stretching away into the foggy gloom, in order to be legal. I'm happy to ignore the extra 40' because RVR values do, and because I'm looking along the hypotenuse of the triangle whose base is on the runway edge, and surely I'll pick up another forty feet there.
Whom am I kidding? This is Cockpit Conversation. We don't make assumptions about trigonometry here, we do trigonometry. The base of the triangle runs from my position at the first runway edge light, to the fourteenth runway light, and is 2600' long. Assume I'm in the middle of a runway, standard width 200', making the height of the triangle 100'. The measurement of the hypotenuse is therefore sqrt((2600 x 2600)+(100 x 100)) = 2602. So no, actually, there is almost no difference between the distance from the first runway light to the fourteenth, and the distance from my eye to the fourteenth light. That's a very skinny triangle. My assumption is wrong. So if I wanted to be a nerd, I could park thirty-eight feet back from the first pair of runway lights. But generally I want to go flying.
In order to save Americans time telling me that the RVR for half a mile is 2400, I'll confirm that in Canada and the other countries I checked researching this post, the RVR for half a mile is 2600. I don't know why Americans use 2400, and neither do the people in this thread on the subject. I especially like the way that the person who initially answers the question there doesn't notice that RVR 2400 for a half mile doesn't add up, until the student points it out. Another thing I expect American commenters to want to tell me today is that "if you have to count runway lights on short final, you should go missed." That's becase their landings are legally restricted by visibility. But Canadian landings are governed only by decision height or MDA. If the runway is visible at DH/MDA, a landing is authorized for us. Our plates have an "advisory visibility" which we can use to calculate whether we expect to be visual at minimums, but its value does not affect our legality to put the airplane on the runway. Once we are past the FAF, the RVR does not restrict us. We do have something called an approach ban which can stop us from legally attempting an approach in terrible visibility, but that's a whole 'nother topic.