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Met Vis vs RVR

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Met Vis vs RVR

Old 22nd Feb 2003, 09:57
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Met Vis vs RVR

Can I draw on your collective experience to seek an answer to a question that is bothering me. I operate under JAR Ops rules where it is not permissible to proceed beyond the outer marker if the visibility/RVR is below the appropriate minima.

If Met vis is reported as good (relatively) but the RVRs are below Cat 1 minima (I guess this is caused by a layer of very shallow fog) would it be permissible to ignore the RVRs and continue a Cat 1 approach? [e.g. Met vis 1400m, RVRS 250m/250m/300m, no Cat 2/3 available].

What if the runway environment could be seen from the outer marker?

I suspect there will be many answers, but if anybody can quote a reference from JAROps (or similar) I would be most grateful.
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Old 22nd Feb 2003, 10:37
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Factored met vis is only to be used when there is no RVR available. If RVR is available it cannot be ignored or bypassed.

Low RVR but lights etc visible from the OM is the classic indicator of shallow fog which gives a potentially lethal illusory effect.
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Old 22nd Feb 2003, 21:15
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Sorry, don't have chapter and verse but, bearing in mind the warning above of illusory effects, what you see out of the window is what you see out of the window.

To recap, RVR's over-rule Metviz, and to continue an approach beyond FAF, or OM or 1000' above threshold if FAF is not specified, then you must have an RVR or failing this a corrected metviz(table 5) equal to or above that required for the approach. For non-precision approaches the minimum cloud base (bkn or ovc) must be equal to or greater than the MDA for the approach.

If you have the runway environment in sight then you can continue because the conditions for continuing below the decision altitude are already fulfilled.

Its quite an unusual condition, especially when the approach is over water.
I remember the first time, seeing the airport from 20-30 miles out one night and RVR.s varying between 600-700m.

I was considering a visual approach but decided that, just as discretion is the better part of valour so is cowardice the better part of discretion, and made an ILS. The fog was only 20'-50' thick and a visual would have worked!

It is not unknown for the RVR sensors to get contaminated, svere frost for example.
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Old 22nd Feb 2003, 22:59
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JAR–OPS 1.405 Commencement and
continuation of approach
(a) The commander or the pilot to whom
conduct of the flight has been delegated may
commence an instrument approach regardless of the
reported RVR/Visibility but the approach shall not
be continued beyond the outer marker, or equivalent
position, if the reported RVR/visibility is less than
the applicable minima. (See IEM OPS 1.405(a).)
(b) Where RVR is not available, RVR values
may be derived by converting the reported visibility
in accordance with Appendix 1 to JAR-OPS 1.430,
sub-paragraph (h).

The above is the JAR chapter and verse.
If you have the runway environment in sight then you can continue because the conditions for continuing below the decision altitude are already fulfilled.
I don't think so. Even if you can see the runway from 100 NM away, the test is still applied at the OM. It's no good declaring a visual approach either, because there is an 800m RVR AOM for that too.
It is not unknown for the RVR sensors to get contaminated, svere frost for example.
And how will a pilot on approach know this, even if correct (it's unknown in my experience)? Regardless, in the UK the RVR is legally whatever the aerodrome says it is.

Last edited by Hew Jampton; 22nd Feb 2003 at 23:18.
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Old 22nd Feb 2003, 23:41
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So how do you explain the discrepency when you taxi out for a low viz take-off?

If you require 90m you go out and count the lights even though the RVR says 125.

You are clearly over-ruling the RVR with your actual observation.
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Old 23rd Feb 2003, 07:18
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Miser - my understanding of this is that you are CONFIRMING that the RVR is AT LEAST as reported - ie that the RVR is not OVER-READING for what you actually have in the take-off path? If you see LESS than your specified minimum (due to fog banks etc - remember the transmissometers are not ON the runway) you do not go, non?

Regarding 'ignoring' RVRs, this happened at Aberdeen last month when the heavy snow had fallen, and there was a degredation of RVR readings due to blowing snow. Vis was a genuine 40k+ and I guess you need ATC to confirm a 'fault' with the RVR machine and give a 'human obs'? I also remember a problem at Edinburgh ?last year? when a fine drizzle brought the 'RVR' down to 120m when the actual vis was 10k+. Again, I think ATC 'killed' the RVR system.
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Old 23rd Feb 2003, 11:52
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Thanks Guys,

this is great feedback.

The quote from JAR is almost exactly what our Ops Manual says. The confusion (in my mind) is the term vis/RVR. As you point out, had the RVR not been available, we could have continued the approach.

However, it is possible that we might have lost the required visual references very late in the approach, causing a late GA.

Thanks for the info, I am away for 3 days but do keep the feedback coming.
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Old 23rd Feb 2003, 12:38
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If RVR is U/S you can continue. What did we do before RVR technology was available? Surely attempting the approach and doing the published missed if not visual is still safe. Or is it??

Would be interesting to hear the ops view in this, as it seems this rule would, make alot more aircraft divert before even having a go atleast, Gets expensive diverting. I can understand shooting an approach and diverting, atleast you have tried. But not to try....Of course this depends if previous aircraft were getting in or you are the first cab off the rank.

If RVR is too low for your category, brings to mind the question when do they close the Airport? Do they take into consideration RVR readings .If not they should, if your JARs have it in Balck and White for the Pilot.

But as the previous poster has stated, they dont, they just make the equipement U/S. So if the controllers take the readings with a grain of salt ,maybe the JAA should.

Maybe they should review this ruling , to give the Captain the option. It seems to be too constrictive to me.


Regards
Sheep

PS. Hey I havent flown in snow , so maybe I m missing the point here.

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Old 23rd Feb 2003, 18:38
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A consideration is the increased risk if a missed approach is required.

The level of risk during an instrument approach is based on probabilities for different stages of the approach. The overall risk is predicated on the completion of the approach ie a landing.

A higher risk occurs during the missed approach. The level of probability used to determine a particular missed approach involves a higher risk than concluding the procedure with a landing.
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Old 23rd Feb 2003, 19:31
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Before interferometers came in RVR was assessed by humans, and this is still a valid way of measurement. A pilot sitting on the button waiting for takeoff has the accurate and overriding measurement of RVR and should act according to his reading.

The factoring system has caveats. It must not be used if an RVR is available from ATC. It must not be used for Cat ll or Cat lll ops, and not for takeoff, for the reason given above.

It is not funny to arrive in the flare on a visual approach and find you can't see anything outside. Stick to the rules, even if you can see the runway from way out it is the last few feet that count.

Dick W
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Old 23rd Feb 2003, 21:16
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You're right there, Dick.
But it applies at any time, if you lose the required visual reference (or screw up the flare for some other reason) you make a missed approach. In this situation, though you are better prepared for it.

A point to bear in mind is the actual conditions which are prevailing.
So on checking in with ATC you say you've got their ATIS and they tell you what the RVR is right now.
Then they ask you what your minimum is and clear you for the approach.
You tell them "550" for example, and the next you hear from them is, "RVR 550, 600, 550. Call beacon inbound."

When you call you've passed the OM and are legal to continue to minimum anyway.
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