VMC erroneous



  • Earlier today I saw this METAR with the blue tag MVFR:

    EBCI 051150Z 26010KT 240V320 9999 FEW015TCU SCT025 09/05 Q1012 TEMPO 4000 SHRA BKN012CB

    There are two problems with that:

    1. MVFR: I suppose it refers to "Marginal VFR", but this is specific to FAA which has no authority in EU.
    2. The ceiling is lower than VMC (SERA 5005b), so VFR is not possible. You cannot take off nor land, you cannot fly low enough to stay clear from clouds.

    The VMC are defined as follows:
    a) SERA.5001

    • alt >= 10000 ft AMSL (or FL100):
      • vis >= 8 km, distance from clouds >= 1500 m hor and 1000 ft ver
    • 3000 ft <= alt < 10000 ft (or height >= 1000 ft AGL):
      • vis >= 5 km, distance from clouds >= 1500 m hor and 1000 ft ver
    • alt < 3000 ft and height < 1000 ft:
      • controlled airspace (A-E): vis >= 5 km, distance from clouds >= 1500 m hor and 1000 ft ver
      • uncontrolled airspace (F,G): vis >= 5 km, clear of clouds and surface in sight

    b) SERA.5005b VFR Flights at Aerodromes
    Except when a special VFR clearance is obtained from an ATC unit, VFR flights shall not take off or land at an aerodrome within a CTR, or enter the ATZ or aerodrome traffic circuit when the reported meteorological conditions at that aerodrome are below the following minima:

    • the ceiling is less than 1500 ft; or
    • the ground visibility is less than 5 km.

    c) SERA.5010 Special VFR
    Within CTR subject to ATC clearance

    • day only
    • by the pilot:
      • clear of clouds and surface in sight
      • vis >= 1500 m (helicopters: 800 m)
      • speed <= 140 KIAS
    • by the ATC:
      • vis >= 1500 m (helicopters: 800 m)
      • ceiling >= 600 ft

    I hope this clarifies and will help give more correct information in the future, I'm aware this is still WIP :)


  • administrators

    Yep I know. We will fix that



  • @redglyph
    you say: "b) ...... when the reported meteorological conditions..."

    In the above mentioned METAR the REPORTED ceiling is not less than 1500ft, as the clouds are FEW015TCU SCT025. Also the REPORTED visibility is 10+ km.
    The term "TEMPO 4000 SHRA BKN012CB" is not an observation but a supplemental information (2-hour forecast trend).



  • Also, I want to mention that we must not confuse ceiling with significant cloud height (SCH).

    CEILING is the lowest height with broken (BKN) or overcast (OVC) reported by METAR.

    SIGNIFICANT CLOUD HEIGHT is the same as ceiling in most of European countries.
    Only in the United Kingdom, Belgium, France and the Netherlands the criterion is 3 octas or more, so SCH takes into account even scattered (SCT) clouds (3~4/8).

    SCH is used in COLOUR STATE of an airfield.
    https://en.wikipedia.org/wiki/Colour_state



  • @meteo-gr Thanks for the information!

    I'll have to check the status of a TEMPO in the METAR (it was not in the TAF) and whether it qualifies as criterion in VMC. If that's not the case, the "MVFR" tag had no place either, it should simply have been shown as VFR-authorized. But in practice, I called the ATC and those were the observed conditions, even the TEMPO showed them as temporary conditions, and VFR flight was definitely not allowed.

    I've never heard of the SCT being used as ceiling in those countries, do you have a pointer in an AIP or other document? That would be most helpful! I saw your link to Wikipedia (which I usually don't trust) but there is no valid source for this information there.



  • @meteo-gr
    (1) I've checked the TEMPO issue. This is from the UK ATPL course, which is based on EASA:

    "A TREND forecast is valid for 2 hours afer the time of the observation of the METAR, and constitutes the final section of the METAR. The change in weather conditions indicated by the code, TREND, can be further qualifed by the codes, BECMG, meaning becoming, or TEMPO meaning temporarily."

    It further clarifies, "BECMG indicates that the change in the present weather will be long-lasting. TEMPO, on the other hand, means that the change is temporary, and that the different conditions will prevail for periods of less than one hour, only, and no more than half the time period, in aggregate."

    That seems to confirm the reported conditions were indeed BKN012. What you mentioned was a TREND section (which is interesting, I've never seen one).

    (2) Regarding what is considered as ceiling. The latest SERA.5005b still specifies that VFR flights within a CTR are, among other conditions, only cleared for a ceiling being at least 1500 ft high. This is valid for UK - see ORS4 No.1195, Belgium - see EB-ENR 1.2-1, and France for instance. So we have to look for BKN or OVC heights, not SCT.

    Interestingly, VFR at night in the UK would be authorized for a minimum cloud ceiling at 1500 ft AMSL. I'll assume it's a typo on the CAA website (http://caa.co.uk/Commercial-Industry/Airspace/Rules-of-the-air/SERA-Implementation/).

    The cloud base, defined for instance in Belgium as at least 3/8, is used along with visibility to define a COMOPSAIR weather colour state, which "is a guide for pilots and controllers as to the existing weather conditions. ATC will not refuse an approach clearance for the sole reason that the weather report indicates conditions below the published procedure minima." (AIP Be-Lux, AD 1.1-2, 15 Sep 2016).

    Moreover, this is used in the military aviation only. So I'm not sure how this is relevant to the original post above?

    I'm pretty sure that's also the case for the Wikipedia link you mentioned, whose source are not available anymore but for which one of the main links is labelled "Department of the Air Force. 2010-12-24."







  • @meteo-gr

    Thanks for the link, this document, although very old and not precise, confirms what I said about TEMPO. It's not pg 16, which only suggests TEMPO is the same as in TAF, but pg 21 which says it represents temporary conditions, as said before.

    The other document seems to be the same, only more recent, I suppose that's what you meant to post in the first place. But the content does not change regarding this point.

    So that confirms those were the actual observed meteorological conditions, I'm glad we could sort that out :)

    PS: Unfortunately I cannot post an URL since I only have the printed version of it, but the excerpt I gave above with the precise meaning of TEMPO in a METAR is coming from the Oxford ATPL course, 2014. Chapter 25, pg 477 if that helps.



  • @ivo said in VMC erroneous:

    Yep I know. We will fix that

    Another weird one today: "110620Z 20009KT 6000 -RA SCT015 BKN030 06/05 Q1013 NOSIG" as "MVFR"

    Except for the time of day, it's simple VFR since we have 6 km vis, 3000 ft ceiling.

    It's limit night VFR ;-) 31 minutes before sunrise. Actually I'm curious how you calculate that for sunset? It's obviously not done with the usual 30' approximation, is that the 6° centre-of-the-Sun rule?

    Sunrise: 7:51, sunset: 17:03
    Dusk (end of VFR day): 17:39


Log in to reply
 

Windyty, S.E. - all rights reserved. Powered by excellent NodeBB
NodeBB & contributors, OSM & contributors, HERE maps

Looks like your connection to Windy Community was lost, please wait while we try to reconnect.