VASI Distance - SM or NM

[80/20]

As avbug points out there could be two questions:
(1) Distance for obstacle protection - described in TERPS
(2) Distance that the VASI can be seen (FlyLawyer's question). AIM give a general rule of thumb good enough for most of us. Detail seekers can find more information in various FAA Orders and even more fine points from vendors that sell VASI light systems. No matter how deep you dig there will always be a bit more to learn and someone who knows more details.

I like Avbugs practical and obvious point: "Distance that the VASI can be seen is entirely dependent on the prevailing visibility on any given day."

 

[The FNG]

Prevailing visibility is irrelevant. It's the flight visibility that matters. You see it when you see it.

This isn't a tech question either. It's an HR question to see who trips over the mouse turds.

 

[avbug]

I have no idea what that comment means, but prevailing visibility IS flight visibility...the visibility prevailing at any given time. If very low visibility prevails then you can only see the VASI/PLASI/whatever at a very short distance...this isn't rocket science.

The distance is entirely dependent on the prevailing visibility. Prevailing visibility is flight visibility. It's also ground visibility. In practical terms, it's how far you can see, whoever you are, wherever you are.

The ONLY issue with the VASI is obstacle protection, and this varies with each installation, and can only generally be set. As already given, repeatedly, the obstacle protection distances are expressed in nautical miles.

 

[Snoopy58]

As I had posted before the boards went down...

The comment that...

Prevailing visibility is flight visibility. It's also ground visibility. In practical terms, it's how far you can see, whoever you are, wherever you are.

... isn't really accurate. Prevailing Vis and Flight Vis are two different concepts, used for different purposes, and can be different values at the same place at the same time.

Prevailing vis is what you hear reported on ATIS & in the METARS generally (although other things like RVR sometimes come up as well), and is defined by an observer looking around the horizon for a set of defined landmarks. If he can see, for instance, all his "2 mile" landmarks, most of his "3 mile" landmarks, but only one of his "4 mile" landmarks, then he would report the PV as 3 miles. The actual vis may be greater or less in some directions than in others, but the "median" vis is what he reports. Sometimes you'll see runway-specific RVR's reported as well.

Flight vis is how far you can see in flight.

How can they be different? Picture a wall of sea-fog approaching an airport close to the ocean... in some directions (as the observer looks toward the water), the vis won't be very good. In the opposite direction, the vis might be excellent. If you're shooting the approach from the "good" vis side, you may see the threashold plainly from miles away, but not see the far end of the runway even at touchdown. Conversely, if you're approaching from the fogbank side... you DID look at the Missed Approach, didn't you?

Another example: I'm flying at 5500' MSL above KABC airport. My flight visibility is 10+ miles, so I'm legal to fly VFR since your "1/2/3/5" visibility is FLIGHT vis. The airport below me may be reporting 1/2 mile in light rain, prevailing vis, but I'm legal to do what I'm doing.

From http://www1.faa.gov/atpubs/PCG/v.htm

VISIBILITY- The ability, as determined by atmospheric conditions and expressed in units of distance, to see and identify prominent unlighted objects by day and prominent lighted objects by night. Visibility is reported as statute miles, hundreds of feet or meters.

(Refer to 14 CFR Part 91.)

(Refer to AIM.)

a. Flight Visibility- The average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night.

b. Ground Visibility- Prevailing horizontal visibility near the earth's surface as reported by the United States National Weather Service or an accredited observer.

c. Prevailing Visibility- The greatest horizontal visibility equaled or exceeded throughout at least half the horizon circle which need not necessarily be continuous.

 

[avbug]

I read your post, and without playing semantics here...I believe it's very obvious from what I wrote before exactly what I meant.

The value of the VASI depends on the visibility as it happens to exist at any moment in time. Not a tinkers **CENSORED****CENSORED****CENSORED****CENSORED** if it's ground, flight, or any other visibility. It's how far you can see. Period.

That being the case, there is NO value at all to making any conjecture about how far the VASI is good for, with respect to begin seen.

The ONLY issue is how far the vasi, plasi, whatever, is good for in terms of obstacle protection, and this has been beaten to death. Where given, this distance is in nautical miles.

The distance at which the VASI may be seen is entirely dependent on the existing, prevailing, ambient, as-you-see-it, good-lord-it's-as-thick-as-pea-soup-done-wrung-through-a-pig's-bladder-out-here visibility. Call it whatever you want. It's how far you can see. Clear enough??

 

[The FNG]

The point is, prevailing visibility IS NOT the same as flight visibility. The statement that "Prevailing visibility is flight visibility" is completely WRONG.

BFD you say? BFD indeed. Example:

To continue past the FAF prevailing (reported) visibility is controlling, once inside the FAF flight visibility is controlling. The terms have distinct and separate meanings and are not interchangable

Pilots have been violated for continuing past the FAF with prevailing vis below mins and flight visibility above mins mistakenly believing, as apparently others do as well, that "Prevailing visibility is flight visibility". Well, that's as wrong as two boys F***ING. (As those crews later found out)

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121.651

(b) Except as provided in paragraph (d) of this section, no pilot may continue an approach past the final approach fix, or where a final approach fix is not used, begin the final approach segment of an instrument approach procedure--
(1) At any airport, unless the U.S. National Weather Service, a source approved by that Service, or a source approved by the Administrator, issues a weather report for that airport; and
(2) At airports within the United States and its territories or at U.S. military airports, unless the latest weather report for that airport issued by the U.S. National Weather Service, a source approved by that Service, or a source approved by the Administrator, reports the visibility to be equal to or more than the visibility minimums prescribed for that procedure. For the purpose of this section, the term "U.S. military airports" means airports in foreign countries where flight operations are under the control of U.S. military authority.
(c) If a pilot has begun the final approach segment of an instrument approach procedure in accordance with paragraph (b) of this section and after that receives a later weather report indicating below-minimum conditions, the pilot may continue the approach to DH or MDA. Upon reaching DH or at MDA, and at any time before the missed approach point, the pilot may continue the approach below DH or MDA and touch down if--
(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and where that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;
(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach procedure being used
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[ksu_aviator]

Funny

Any body else think its funny that in this post one simple question was asked and yet of the 10 or so people that responded to the question only two people directly answered the question?

 

[Snoopy58]

The question itself is an utterly pinhead question... anyone who would ask it in an interview is either an utter idiot, or asking it to see how the applicant deals with nonsensical questions (i.e. are you condescending when a non-pilot asks basic or even stupid questions -- a fair thing for an HR type to be interested in). Or to see who has "insight" (i.e. gouge) into the questions being asked.

But I'd lose all respect for any pilot who actually thought I ought to know the answer to such a question in an interview.