Tuesday, September 22, 2015

I See Fourteen Lights

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.


Sarah said...

I see four lights!

But no, really? I am out of somewhat out of practice with US IFR rules, but I believe the visibility rules are minimums for beginning an approach. Part 91 pilots can finish the approach if key elements are visible - the runway threshold, approach lights, so on. ( 91.175 ).
Fancy commercial 121 (scheduled) or 135 (chartered) operators do need the visibility minima to start the approach.

And part 91 pilots can take off with 0-0 visibility, but that would be crazy.

Unknown said...

IFR flying along with its permutations and combinations of rules and situations makes my head hurt. I seriously want a 1-800-CALL-MY-LAWYER number sometimes.

Case in point: KEWR weather is low enough to require CATII Rwy 04R landings. But the rules say that the centerline lights (among other things) must be functioning. The last (far end) 2,000 feet of CLL are NOTAMed U/S. Can I do the approach?

I decided yes because the CLL were functioning on the portion of the runway that I would use. Other flights holding must have liked my decision because they started asking to leave the hold and follow me on the approach. No one violated me as far as I know. Later in the day on a return flight I noticed that the NOTAM had been changed to read: RWY 04R last 2,000 feet closed.

I can only imagine how many discussions in how many flight decks and control towers this situation caused... Could have all been settled with one call to 1-800-CALL-MY-LAWYER! ;-)

Anonymous said...

There are six thousand feet in a nautical mile? Yes. But if you take six thousand feet out of a nautical mile, you still have 76 feet left.
A nautical mile is 1,852 m, which is 6,076 ft in 'Imperial units'.
1,852 m is the 'international nautical mile'. Of course the U.S. and Britain (does that include Canada?) don't use the unit that the rest of the world uses. The U.S. nautical mile is 1,853.248 m or 6,080.20 U.S. feet. The British 'Admiralty mile' is 1,853.184 m or 6,080 'international feet'. But in 1954 th U.S. actually did start using the 'international nautical mile'. The Brits held out till 1970.
How do I know all this? The 1,852 m I just know, like I know that a U.S. mile is 1,609 m. Comes in handy if you have an interest in aviation and live in Germany. But to make sure I looked it all up on Wikipedia: Nautical Mile
Why don't the regs specify the RVR in runway lights? In Italy along the autostrada they have big signs telling motorists how fast they are allowed to drive in fog, depending on how many side-marker posts they can see.
Love your blog, because you really do the trigonometry (and write well).