With the fog in PER the other night, here are a few questions.
How long into the flight does one decide to divert due to fog?
773 left an hour or so earlier then 481, but had to divert to ADL, where as 481 made it in.
Depends how much fuel you have. In a perfect world, you'd have enough to go all the way to destination, shoot an approach, and then decide at the minima. In practice, Perth is so far from its diversion options, that that becomes unlikely. And, it depends when the fog appeared on the forecasts/actuals. It is quite possible that when flight planning was done, that fog was not on the forecast, in which case there is no need for any alternate to be carried. In that case, the scenario of the earlier departure having to divert, whilst the later one (with, perhaps, a better forecast) carried enough fuel to allow an approach to be made, becomes quite likely.
When I was flying the 767, I recall sometimes carrying the east coast alternates (of course, depending upon where you've come from, you may not be able to carry that much extra fuel). The issue is that it is prone to fog that turns up quite late on the forecasts, the approach aids aren't much chop - Cat II/III is a rarity in Oz, and even where it does exist, has only turned up in the last year or so, it is a long way from anywhere (so you need even more fuel)...
What is the biggest aircraft that can land at KGI? 569 (which was a 763) made it there, but there is roughly 2000 metres, so I would imagine that is not a lot of room?
It's a long time since I've been to Perth, or had occasion to look at the details for Kal, but last time I did look, the max was a 767. Problem too, is that there is only parking space for one, so if two divert there, you end up blocking the taxiway or runway. Your next option after that is Learmonth. As the fog isn't going to clear any time soon, if you do end up there, you're most likely going to be stuck for the night (consider the transport/accommodation implications)...and by the time the fog clears, the crew will be out of hours anyway. It's an alternate of sorts, but one you don't ever want to have to use.
Consider the really long haul operations. There, you're doing the planning based upon a forecast that will have been issued 18 or so hours before your planned arrival. Coming from London to Singapore, the minimum planned fuel on arrival (380) will be about 12 tonnes. That leaves a 5 tonne buffer over the absolute minimum, but it's likely to be eaten into by anything that doesn't go exactly as planned. So, if neither KL or Singapore require an alternate, but being basically untrusting of forecasts, an obvious solution is to carry KL as a alternate. The bare mimimum to do this requires about another 2.5 tonnes at destination, but because that ups the weight for the entire flight, it will cost about 1.5 tonnes, just to carry it. So, you'd need to order another 4 tonnes. Cost of that is about $1600. So, at that point, assuming I don't use any more fuel than planned (i.e. everything goes according to plan), then I will have a bare bones amount of fuel to divert to KL. But, the chances of everything going exactly according to plan are slight, and if I burn even a couple of hundred kilos more that planned, that diversion fuel won't remain sacrosanct, in which case I'll no longer have what was needed to divert....so quite honestly, I may as well not have put any extra on at all. So...now you need some extra to 'protect' that margin. And so it goes. If you divert the 32, it will add about 3 hours to the total journey time, so it is unlikely to make the curfew in Sydney....another complication to consider.