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In US AIP differences from ICAO:

12.3.1.7 d/e/f
U.S. controllers do not give wind speed, visibility, or RVR values in metric terms. RVR values are given in 100- or 200-foot increments while RW values are given in Venule increments. [emphasis mine]

Same AIP's abbreviation list does not mention RW. Google wasn't helpful.

Here 12.3.1.7 d/e/f references chapter 12 of Doc 4444 15th edition (not the latest). The same section in the latest 16th edition is irrelevant (phraseologies regarding traffic information). If I find the older 15th edition I'll update.

The same entry is present in the Nov 2016 AIP edition, so it's neither new, nor a likely typo (PDF; 20 MB).

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The best way to get a definitive answer is likely to email the FAA (there's a link at the bottom of the AIP page) but my strong suspicion is that both RW and Venule are errors caused by scanning in an old document, or maybe manually entering it.

First, if you go back to a 2002 version of the FAA's ATC orders 7110.65, there are references to "RVR/RVV" values. I suspect that "RVV" was scanned or manually entered at some point as "RW" and no ever noticed. In fact, if you search the FAA site for "RVR/RW", the only hits appear to be old, scanned copies of paper documents.

I also found an explanation that shows that RVR and RVV were often used together, which to me supports the idea that "RVV" is the correct abbreviation:

g. RVR/RVV.

  1. Provide RVR/RVV information by stating the runway, “visual range” or “visibility value,” as appropriate, and the indicated value. The abbreviations “R-V-R” or “R-V-V” may be spoken in lieu of “visual range” or “visibility value.”

The latest 7110.65 doesn't refer to RVV any more, by the way:

This changes [sic] removes all references to Runway Visual Value (RVV) from this order.

Second, "Venule". Based on the dictionary definition and Googling, it's clearly a medical term, not an aviation or meteorological one. So what might it have been? The PC/G definition from the old 7110.65 linked above says:

Runway Visibility Value (RVV)- The visibility determined for a particular runway by a transmissometer. A meter provides a continuous indication of the visibility (reported in miles or fractions of miles) for the runway. RVV is used in lieu of prevailing visibility in determining minimums for a particular runway.

I can't think of a word close to "Venule" that means "in miles or fractions of miles" but possibly the original text was "1/4 mile increments" or "¼ mile increments" (or another fraction). Depending on the original text size, font etc. it's possible that "1/4 mile" become "Venule".

Anyway, this is all speculative even though I think it's very plausible. I'd definitely email them if you want an authoritative answer.

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  • $\begingroup$ You gave me an idea. I found a copy in the National Technical Reports Library. Your answer is spot on. Thanks again! $\endgroup$ – ymb1 May 1 at 22:25
  • $\begingroup$ I also suggest to email the FAA $\endgroup$ – Raffles May 1 at 22:25
  • $\begingroup$ @ymb1 You're welcome! And nice find on the other copy that confirms my guess! :-) $\endgroup$ – Pondlife May 1 at 22:27
  • $\begingroup$ @ymb1 amusingly that copy seems to have a typographical error too in the next row: "wind is 1 kt but 6 kt" presumably should have been "wind is >1 kt but <6 kt". $\endgroup$ – hobbs May 2 at 1:53

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