Firstly I’m not going to second guess what any of my colleagues have done, just add some information/context to hopefully answer some of the questions because it is reasonable to ask why one and not other flights were affected to such an extend.

The reports I linked earlier come from the aeronautical meteorologist at BLR and are official reports, the forecast has changed whilst the aircraft was Enroute and the actual weather reports were worse than forecast.

More fuel was loaded with weather at destination being the reason, and given my experience with Indian stations I’d have done the exact same.

I’ve spend a bit of time looking at FR data and there are good gaps between the arrivals surrounding the approach made by BA119. They are all using the northern of the two runways and then a subsequent arrival uses the southern runway and makes a successful approach. The runways in use are determined by ATC and based on things such as fire cover available, and local restrictions, Equiptment for the runways etc etc etc. the southerly runway has a lower (more chance of getting in) minima than the northern, however that southern runway also has a history of approach failure due to losing the ILS signal, which is an instant go around in low visibility.

When it comes to fog, there is no such thing as homogenous or uniform fog. It comes and goes in patches with varying visibility, all within a relatively short radius or distance. One minute you might have the required visual references to continue if low down, or the visibility required to legally continue below 1000feet (due to the approach ban rule), the next minute you won’t.

So with an approach on the northern runway unsuccessful, probably a ‘pea souper’ at the threshold at that point in time, a hold whilst ATC get the southern runway up and running and also co-ordinating a diversion, burning fuel during the wait and knowing there is no guarantee of success due to a history of ILS signal problems on 09R, but a good bolt hold just down the road (relatively speaking) and legal limits to comply with, I’m not surprised with the decision made to divert. MAA was also not the filed alternate, but was the most convenient option that complied with A, B and C below.

As others have correctly said it’s all down to A - safety and B - the law and C - minimising the buggeration factor.

So in short with visibility that low and changing from one minute to the next, I’m not surprised some got in and one did not. Others were delayed in arrival too. For departures, those flight arriving outside of the worse period of fog wouldn’t have been affected, and aircraft having night stopped at BLR or being based there, just starting their day of flying would equally not be affected.

Hope that answers some of the questions.