Altitude/Visibility determination

[ElNull]

A simple question that may look dumb to some of you, but how do you determine the slant visual range? lets say during low visibility and after landing you want to report visibility. (is that even the correct definition of the slant visual range)?

[BOAC]

Probably an over-simplification, but all pilots really need to know is the HEIGHT at which you saw the required visual reference (ie where YOU made the 'decision') - which is naturally 'slant range' and nothing else - so that DAs and DH's can be assessed.

[ElNull]

How would you measure the SVR at a specific height? Is it the same distance in feet as your height? so lets say a visiual reference is made at 1000 ft AGL, does it mean the SVR/oblique visibility equals approx 300m?

[BOAC]

NO! I cannot see your 'qualifications' here in your profile to answer you (what are they?), but do you understand Glideslope angles and the like?

IE What is the 'slant' range (feet/metres/bananas as you wish) from 1000' on a 3 degree slope?

[ElNull]

I do understand Glideslope angles and the like. How would you measure visibility at a specific height during approach? does it require a glidepath angle at all? I need an answer... thanks!

[BOAC]

What is the 'slant' range (feet/metres/bananas as you wish) from 1000' on a 3 degree slope? - any idea?

[safetypee]

Although there is a geometrical relationship between slant visual range and RVR (or met vis), SVR is specific to the met conditions at that time; these vary considerably due to the nature of the fog structure (thickness/density), cloud type, snow, rain, etc. Thus SVR is not used for reporting – for practical purposes in normal operation it cannot be determined.

The first fig shows that SVR is independent of GS, depending on altitude, ‘fog thickness’, and RVR / met vis.
The second fig indicates the range of variation of SVR against RVR for extremes of ‘fog thickness’.

http://i59.tinypic.com/28roa5v.jpg

http://i58.tinypic.com/2ni3alg.jpg

[BOAC]

Hey - let's repeat Q1? Altitude/Visibility determination
A simple question that may look dumb to some of you, but how do you determine the slant visual range? lets say during low visibility and after landing you want to report visibility. (is that even the correct definition of the slant visual range)?

Forget RVR?

[MarkerInbound]

Somewhere out of the fog (in my brain) something's coming, ah, yes, I can see it now -

A(squared)+B(squared)=C(squared)

As if anyone cared.

[BOAC]

Yup - two to go!

[Tu.114]

It is basic geometry, as has been mentioned.

But what is the operational use in knowing this figure or spending time to calculate it? Will knowing this number make any difference to You on the approach, seeing that You will get a RVR/visibility and a ceiling or VV provided? Maybe I have missed something until now, but I never felt the need for such a number.

If You want to determine the visibility on the ground on the runway, why not count the runway centerline lights? The number seen times 15 will be the vis. in meters; if other units are preferred, just apply a factor.

[safetypee]

As above, there is little operational practicability in SVR, and even if calculated by crew observation there is no guarantee that this relates to a ‘true’ value of ‘visibility’.
Of greater benefit is a report first contact height during the approach and relating that to DH, but even so such information should be used with caution. Fog ‘thickness’ can vary with altitude as well as 'type', and thus what one crew saw another may not, or even what is seen at DH can be lost at lower altitude.

When on the ground, an estimate of RVR can be made against the number runway lights seen vs their spacing, but whatever is assessed should not be passed on as RVR as this has a specific meaning. There may be alleviations for this when lined up for take-off.

[BOAC]

Refer post #2?

OP has disappeared, by the way, whatever his/her involvement.

[ElNull]

Thanks for your answer safetypee and Tu.114. I Knew how to determine RVR on the runway, however I got asked by the tower once (after landing) about the visibility during approach. My answer was the altitude at which visual reference was made. Thought there should be a simple calculation that determine visibility in relation to altitude.

[sodapop]

My rule of thumb for the altitude at which you should expect to see the threshold is RVR/3 + 50'....i.e. 600 meters vis means you should see the rwy at 250' (obviously we are talking about fog with NSC).

By this reasoning, to calculate the vis from an altitude at which you first see the rwy, simply subtract 50' and multiply by 3. I.e. see rwy at 250' = 200 x 3 = 600m vis.

Very TLAR but found it's usually close!

[safetypee]

sodapop, I like rules of thumb, but don’t rely on them. Always be prepared to be surprised.
See http://www.pprune.org/questions/531730-cat-i-approach-question.html#post8263003 which shows that the height at which contact with the threshold might be made varies considerably with fog type (and perhaps with seat height).

[ElNull]

Thanks soda... do you know a reference for that? Also isn't that supposed to be a height (not altitude)?

[sodapop]

Yes, I should have written height instead of altitude but I presume everyone understood that I wasn't referencing an approach to La Paz or Denver or Munich.

(PS: you used the term altitude in your thread title)

Reference??? Just the boys talking :mad: over beers in the pub. But maybe, Al Parma mentions it in his "Rules of Thumb" booklet? It's all 60to1 stuff. My navs in the AF used to harp on this stuff whilst doing airborne radar approaches into :mad: airports in the ex-CCCP on nuclear treaty verification flights where there were no published min vis and we had to pick up a Russian nav in Tempelhof to be our radio transmitter. Great fun, especially the arrival lunches with beverage choices of vodka, vodka or vodka. But I digress...

SafetyP, "don't rely on rules of thumb"... No kidding. That's why we have decision heights for Cat 1, 2, and 3A/B. Further, yes, seat height and eye sight and fog type (density) all contribute to everyone's individual point of rwy contact, hence the use of the term "rule of thumb". If it was exact, scientists would be writing about and choosing the apes to do it.

[ElNull]

Appreciate your reply, thanks again!

[sodapop]

Cheers mate!

When operating in low vis, always remember the immortal words of Sam Snead who supposedly (hopefully) said,

"If you're not thinking about :mad:, you're not concentrating"

and then remember to remember, "cleared the approach also means cleared the missed approach" and apply accordingly.