Tower Visibility vs AWOS Visibility

[flydude]

Recently we came accross a Metar that was reporting 1/4 mile visibility with a "Tower visibility" of 1/2 mile. The question was raised as to which visibility is controlling. Is it the visibility reported by the AWOS or ASOS, or is it the visibility reported by the tower?

In other words could you begin the approach in this situation?

We tried to research tower visibility and could not find any info.

 

[Vector4fun]

Flydude,

Normally, the lower of the two values is considered the "Official" visibility. However, RVV or RVR is most often the final word where such equipment exists.

From FAA 7900.5B, Surface Weather Observing

9-15. CONTROL TOWER OBSERVATIONS AND ACTIONS AT CO-LOCATED SITES

Control tower personnel certified to take visibility observations shall:

a. Notify the weather station or contract observer when they observe tower prevailing visibility to decrease to less than, or increase to equal or exceed, 4 miles.

b. When the prevailing visibility at the tower or the surface is less than 4 miles, report all changes of one or more reportable values to the weather station or contract observer.

c. As required by FAA directives, use the lower of either the tower or the weather station/contract observer visibility for aircraft operations.

As you might guess, different WX phenomenon cause the vis to differ at runway level vs. the Tower cab level. My cab sits at around 100', and there are sometimes days when we cannot even see the ground except by going out on the catwalk and looking straight down. However, the vis at the surface might be a half mile or more. Doesn't matter much to GA because the vis at 200' (ILS DH) is probably gonna be worse than ours....

 

[bigsky]

was the tower visibility in the remarks or the main body of the report?

 

[flydude]

Bigsky, sorry it took so long to reply, Yes it was in the remarks, and I do agree that the 1/4 mile is probably controlling, I just can't find anything in writing that explains this.

 

[bigsky]

I know somewhere- whether it be a far or something in op specs- but says that what is in remarks is info only and the main body is controlling.
If I ever get home again Ill try to find reference

 

[Dave Benjamin]

Main body will be controlling in this instance.
RVR is runway specific so if you had an RVR value that met the approach mins you'd be good to go for an approach to the runway with that RVR value even with lower prevailing visibility.

 

[A Squared]

Progress?
"I'm sorry Dave, I can't let you do that" - from Orwell's epic 1984

"20, 10, RETARD, RETARD" - what you hear in the flare on Airbus 320

Actually, that was from "2001: A Space Odyssey" and the real quote is:

"I'm sorry, Dave, I'm afraid I can't do that."


Sound file of HAL

Little known facts:

Sherlock Holmes never said "Elememtary, My dear Watson" in any of the Aurthur Connan Doyle books

Rick (Humphery Bogart) did not say "play it again Sam" in Casablanca

 

[slider]

FAR's

flydude...
I've read some of the replys to your inquiry, dont listen to any of them they'll get you in trouble! In dispatch, this scenario is encountered quite often. To answer your question, the ONLY flight that would be legal to begin an approach to that airport under those conditions would be one operated under part 91 (ref, 91.175). Comments and observations found in the remark section of a METAR are never controlling (ref, 121.655). You should never see the term "Tower Visibility" used in the main body of the METAR, conversely, an RVR report should never be in the remark section of the METAR. If you ever run across this, I hope you would question it and bring it to the attention of a FSS. Remember NO ONE, not the NWS, the FAA, not even the pencil necked lawyer(s) who wrote the regulation are perfect and beyond questioning! In addition, a part 121 air carrier op spec's (scheduled or supplemental) have absolutely nothing to do with which value is controlling.