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[I do not believe this question duplicates a previous question, which involves different conditions and circumstances]

I'm interested in the airline's response to a query of mine about why a flight was cancelled due to weather when destination airport had visibility of at least 12 SM. I asked them and the airline wrote back, attaching relevant METAR and TAF reports.

The destination airport was Victoria International (CYYJ) which is not category III (maybe cat II?). All METAR reports indicated 12SM visibility with the lowest ceiling FEW170. The airline acknowledges that the actual conditions were clear. The flight was scheduled to depart 0716 ZULU, 33 min. duration, 2 alternates with CAT III available at 16 min (Vancouver, CYVR) and 33 min (Seattle, KSEA) flying time.

The aircraft was a Dash 8. A competing airline flew the same route with a CRJ 15 minutes before the scheduled departure without incident. This was the only cancellation at YYJ that evening (there were perhaps a dozen arrivals, mostly Dash 8s or Q400s but there was an A320 and at least one 737).

First question

The airline claims a TEMPO condition of 1/4SM made it illegal to dispatch. But the TAF at 335Z said TEMPO 1 1/2SM BR SCT002, not 1/4SM. The bases prediction was P6SM SCT250 but there was also a PROB40 for 1/4SM and fog (FG).

Is a PROB40 for fog and 1/4 SM visibility enough to cancel? Is the airline required to cancel at that point? The 335Z TAF also had RMK next forecast by 0600Z, 1 hour before departure.

Second question

The new TAF issued at 538Z did not have TEMPO but had PROB30 (I can't find PROB30 in my TAF/METAR definition list, as PROB40 is supposed to be 30-50%, right?) for 1/4SM and fog (FG) again with what appears to be a main prediction of P6SM FEW150 SCT250. Was this enough to reasonably cancel? Was the airline legally required to cancel on this basis?

Update: a poster asked me if I could post the full set of TAF and METAR reports, as received from the airline. Here they are (assuming no typos on my part because I received these hard-copy not electronic):

Scheduled flight departure from SEA Zulu 7:16 Flight duration: 33 min. Arrival YYJ

METARS for YYJ:

0700Z 27004KT 12SM FEW170 BKN250 01/01 A3043
0600Z 27004KT 12SM SCT250 M01/M01 A3041
0500z 27004KT 15SM FEW250 00/M00 A3041

TAFs for YYJ:

CYYJ 1005338Z 1006/1106 VR03KT P6SM FEW150 SCT250
PROB30 1006/1016 1/4SDM FG VV002 FM101800 29008KT P6SM FEW004 SCT100
FM102000 35005KT P6SM SKC RMK NXT FCST BY 101200Z

CYYJ 100335Z 1003/1024 VRB03KT P6SM SCT250
TEMPO 1003/10-18 1 1/2SM BR SCT002
PROB40 1005/1016 1/4 SM FG VV002 FM101800 29008KT P6SM
FEW004 RMK NXT FCST BY 100600Z

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    $\begingroup$ Welcome to Aviation SE. A little bit of formatting never hurt anyone though. $\endgroup$
    – Stelios Adamantidis
    Commented Feb 16, 2018 at 16:22
  • $\begingroup$ For operations under FAR Part 121 in the U.S. "The FAA Office of Chief Counsel has consistently interpreted these regulations to mean that the worst weather condition in any of the reports or forecasts used to control a flight movement is the controlling factor." (see FAA FSIMS 3-2046). So, in the U.S. if the PROB40 was 1/4SM and if there was not an Instrument approach the crew could fly to 1/4SM mins, then dispatch would not be allowed. Also, for YYJ I don't see a Cat 2 or 3 procedure. Lastly, other carriers may have approval for lower mins. I don't know if the rules are the same in Canada. $\endgroup$
    – user22445
    Commented Feb 16, 2018 at 17:30
  • $\begingroup$ You haven't told us the departure airport or the airline so we can't say what regulations applied to the flight, and individual airlines have their own operating procedures too. It's possible that non-weather reasons also applied here (see the answer that @fooot linked to) so I don't know if you'll get a clear answer on this. You might consider asking instead what the applicable regulations are in the country that interests you (USA? Canada?), remembering that an airline's OpSpecs might override them. $\endgroup$
    – Pondlife
    Commented Feb 16, 2018 at 17:32
  • $\begingroup$ Flight was Alaska subsidiary Horizon, origin airport was SEA. Thanks to 757toga for the info (if I've got it right, PROB40 is ruling and nothing can override that (clear actual condition reports, etc.) though other carriers may have approval for lower mins (hence the "no problem" status of an at-the-same-time Delta flight). Horizon seems to have operated under US rules: "EWINS forecast was not considered as an option. EWINS is authorized in Canada (DSM 5.13) and this may have allowed our flight to operate" (this was the VP Flight Ops quoting the answer he got from the Chief Dispatcher. $\endgroup$
    – Douglas Baer
    Commented Feb 16, 2018 at 19:25
  • $\begingroup$ @Douglas Baer, you are correct (in my opinion). PROB40 (or lowest noted in body or remarks of TAF) is ruling as per FAA interpretation noted above. Approved EWINS could prevail (for TAF) but unless the METAR is changing and indicates that the TAF would also change if it were being reissued right away, a correctly utilized EWINS program would likely not be a benefit. Not all carriers have EWINS approval. $\endgroup$
    – user22445
    Commented Feb 16, 2018 at 23:58

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