Can TAF's visibility replace RVR?
 

Regarding TAF visibility and Appraoch WX minimum RVR.

When dispatch long distance flight (more than 2hour / In DISP PHASE)

- TAF Forecast visibility : 600m
- AIRPORT appraoch minimum : CAT 1 / VIS 800m (or RVR 550m)

Is it possible to dispatch in this case? (with one alternate airport)

TAF is below AIRPORT VIS minimum but above RVR

if you can help me with this, I’ll really appreciate it.
Best Regards!

Fake TAF, real one would have RVR predictions for VIS below 2000. Or something like that.

Also, the question is impossible to answer until you state which regulatory framework you operate under.

Regards.

never saw RVR in TAF

RVR is always an observation, never a forecast. Therefore, it wouldn't be in a TAF.

Under EASA when vis drops below 800m, RVR is required. Since you CANNOT have the RVR, because it's not a forecast, but an observation, I would consider the wx below minima. It is permissible to dispatch in such a case, provided two alternates have been selected and fuel for the furthest is carried. Again, this is under EASA, other jurisdictions may have different rules.

Thank you for your reply.
but It is not Fake TAF.
actually the last year, RKSI/ICN TAF had been forecast VIS 600m.
At that time, our company discussed this matter for dispatch aircraft (CAPT qualification CAT1)
and I'm just curious about the others opinions
many thank you

Thanks for correcting.

Hi,

the forecasted visibility is above the RVR required, so you can legally dispatch with 1 alternate.

Thank you for your reply.
I think so too
RVR is just observation value.
So if I want to dispatch this VIS 600m, I have to provide two alternate. (except short flight)

and If possible, can you tell me where I can see EASA regulation regarding this matter?

There are two opinions. (both my company colleagues and PPRuNe forums)

1. RVR is observation, not forecast.
so I have to compare TAF visibility 600m and appraoch minima's visibility 800m (not RVR 550m)
and It should be considered WX below minima

2. TAF visibility 600m is above appraoch minima's RVR 550m
so there is no problem, dispatch with 1 alternate

I'm not sure which is right.
It is not clear in my company regulation.
and I just wonder what ICAO/FAA/EASA ruels and others aviation workers opinion.

Thank you for many reply.
and please give us more other thinks.
many thks.

The answer to this question will depend on national regulations and/or company regulations.

I can’t add any more to the regulatory side of things, but is it not the case that if we assume a uniform distribution of cloud/mist/fog giving a met vis of 600m, we could reasonably expect an RVR measurement to give more than than 600m?

Of course it doesn’t always work like that in the real world - especially in the ranges of visibility we’re talking about here. It’s not unforeseeable that a rogue fog bank could envelope the TDZ RVR on such a day. But the chances of that happening on a day when one didn’t have the ability to carry extra fuel for another div and or holding without - say - offloading freight are pretty low, and one would still have their original alternate which must have met the more stringent requirements - from an EASA OPS pint of view. In that case - if you gotta go (to the alternate) then you gotta go...

SL, the assumption that fog has a uniform distribution would be ill-founded.

Fog has many different structures, particularly variations between forming and dispersing.

Stratiform cloud-like 'onion layers' in compression (forming) with significant vertical variation in visibility in each layer. Alternatively 'pork-pie', mini cumulus with Horizontal variation in visibility and ambient light levels during dispersal.

A critical time for variable fog structure in wet fog is around Cat 2 RVRs, where a 'land' decision above DH can turnout to be poorly judged. Whereas Cat 3 RVRs are more likely to be a stable fog with little variability.

Then there is a range of visibility distributions associated with different particles; wet / dry cloud, snow - type or rate of snowfall, dust, smoke, etc.

RVR originated from extensive research into these areas and is a compromise between what can be measured accurately, and the pilots need for slant visual range - what might be seen at different heights.

I agree, in practical terms, low viz is a highly unpredictable situation, even in the short term. Often even a METAR often goes out of date as soon as it’s published, let alone a forecast.

There’s often been times I’ve thought I’m glad not to have been a Met forecaster.

Fake TAF or not, the same concept would apply if the destination had a non precision approach or similar.

Either way, in my region you could dispatch by factoring the visibility. If, for example, the destination had HIALS and it was night, the factor would be 2. So 600m becomes 1200m, thus above the minimum of 800m so you can launch.


edited to add that the 1200m is now what we call "equivalent RVR"

As 'safetypee' and 'ShyTorque' said,
Low VIS is a highly unpredictable situation.

In this case,
We have some regulation for Weather forecast destination airport

* When Below the weather minimum or weather information is not available.
1. More than 2 hours of flight time : NO DISPATCH
2. When flight time is less than 2 hours, If Current weather at the destination airport is above weather minimum : DISPATCH

So If METAR or AMOS's RVR is above WX minimum, We can dispatch less than 2 hours flight regardless of TAF

But More than 2 hours flight, I don't know which one to apply VIS 800m or RVR 550m.
Because No RVR is forecast for TAF.

Thank you for your opinion.

We also have "equivalent RVR" table.

But There are special conditions for applying this regulation in our company.

* RVR Conversion Table Note
1. This table may not be used for takeoff minima determinations, CAT 2 or CAT 3, or if RVR is available and reported
2. This table may not be used for landing minima determinations on Flight Planning Stage.
3. Equivalent RVR = (Reported Prevailing Visibility) X Factor

anyway many thank you for your reply.

Been back "home" (Oz) for some time now, but if my memory serves me right when in the M.E. we had a correction factor to go from Vis to RVR if only one was stated and you needed the other, factor was 1.5. When I was in HK I'm pretty sure that the company had a nominated destination VIS requirement, not given as an RVR.

Also, are not Vis and RVR different?

Edit: can see some have covered this.

https://www.universalweather.com/blo...ps-visibility/

3. Calculation of visibility

Visibility values are determined by both human observers and the Automated Surface Observing Systems (ASOS). The human observer determines visibility by identifying objects and landmarks at known distances throughout a 360 degree circle around the observation point. The greatest visibility observed over 50% or more of the 360 degree area is the prevailing visibility. If, however, there’s a sector of the 360 degree area that significantly differs from prevailing visibility, the observer may add a remark. ASOS measures and converts sensor-driven values to visibility values corresponding to what the human eye can see. Pilots must be mindful that nearly half the area around an airport may have lower conditions than the reported prevailing visibility.

5. RVR considerations

RVR is distance over which a pilot of an aircraft, on the centerline of a runway, can see delineated runway surface markings and centerline. RVR values are normally determined by the human eye or with an Instrumented Runway Visual range (IRVR) trasmissometer. RVR is important as it provides the main criteria used to determine category of visual aids operational at an airport as well as criteria/minima for instrument approaches.

You may be considering when you have a legal TAF but a METAR below minimums as you are very unlikely to get RVR in a TAF. For planning purposes, you would use the TAF to determine dispatch requirements.

For EASA land CAT operations, the following would be applicable:

The table in GM1 CAT.OP.MPA.185 has useful guidance for applying TAF criteria to determine suitability of weather from a TAF. Once you have determined that the forecast is suitable (or not) then CAT.OP.MPA.185 would also require alternate weather of better than NPA RVR/VIS with ceiling at or above MDH for the most applicable runway as only CAT I is available at destination. Provided those conditions are met then you could dispatch with one alternate.

If the destination forecast is below limits, as per yunje’s example of RKSI/ICN TAF of 600m visibility (800m visibility required), then you would require two alternates with suitable weather forecasts as per CAT.OP.MPA.185.

Once you arrive at the destination then CAT.OP.MPA.305 becomes applicable. This requires that you cannot proceed below 1000 ft above the aerodrome if the reported RVR/VIS is less than the applicable minimum.

The conversion of reported met vis to RVR, detailed in AMC10 CAT.OP.MPA.110, would not be applicable at the planning stage.

It’s not unlikely, it’s impossible to have RVR in a TAF as it is an actual measurement.
AMC10 CAT.OP.MPA.110 does not state that the CMV is not applicable at planning stage.

True, but given that it is reported met visibility it is unlikely to be of value for when you arrive two hours later in the example given by the OP. The only time it would possibly be applicable for planning minima, IAW CAT.OP.MPA.185, is when the reported met visibility is given to you before you depart and you will arrive in one hour or less.