The weather at the airport is reported below minima — is it legal to try the approach anyway, just to take a look, before heading to your alternate? This is one area where the rules in Canada and the U.S. seem to differ quite a bit.
In the U.S., aircraft operating under FAR Part 91 (private and some commercial operations) have no restriction at all: they are free to try an approach even if the weather is reported as 0 ceiling and visibility. U.S. aircraft operating under FAR Part 135 (charter and commuter, sort-of) are not allowed even to attempt an approach unless the weather is at or above minima. This is probably a gross oversimplification, but it gets the rough idea across.
Canada falls somewhere in between these two extremes. We have something called the Approach Ban, laying in out gory detail when the pilot of any flight (private or commercial) is allow to attempt an approach based on Runway Visual Range (RVR) reports from transmissometers located at the runway threshold (RVR “A”) and mid-runway (RVR “B”):
- When only RVR “A” or RVR “B” is available for the approach runway, it must be reporting at least 1,200 ft.
- When RVR “A” and RVR “B” are available for the approach runway, RVR “A” must be reporting at least 1,200 ft, and RVR “B” must be reporting at least 600 feet for fix-wing aircraft (any value is OK for rotorcraft).
If these conditions are not met, then an aircraft is not allowed to complete an approach past the outer marker or final approach fix. There are a whole bunch of exceptions, though:
- if the aircraft is already inside the OM or FAF inbound when it receives the report, it may continue;
- if the aircraft has informed ATC that it is on a training flight and plans to go missed, it may continue;
- if the RVR is fluctuating above and below, and the reported airport visibility is at least 1/4 SM, the aircraft may continue; and
- if the aircraft is conducting a CAT III approach, it may continue.
Even more importantly, the ban applies only to airports operating transmissometers on the approach runway — if you’re flying into a medium or small airport, or even approaching a smaller runway at a big airport, there are usually no transmissometers, and therefore, no approach ban.
That said, as far as I understand (never having sat through commercial ground school), commercial operations all have Transport-Canada-approved operating manuals that can place additional restrictions on what pilots flying for the operator are allowed to do, and these may go far beyond the approach ban (especially since they’re usually ammended after every an accident or incident).
Corrections and elaborations are welcome in the comments, especially from pilots who have sat through commercial groundschool on either side of the border.