Not sure why I'm bothering but here goes.
Using one of TACs examples, I'm in the stack at FL200 because airfield requires alternate due fog.
If they ask me for latest divert time, I can have two possible numbers. One is the latest time I can shoot an approach, miss out and get to my alternate. This is the figure you keep using TAC. The second time will be slightly later one whereby I can divert from my hold to a suitable alternate airfield.
Obviously then, I have two options up my sleeve. If ATC can clear me for an approach before the first time then yahoo- all is well and good. If they can't do that then I can STILL CONTINUE TO HOLD on the chance the weather clears ABOVE THE ALTERNATE CRITERIA. Given the way the fog works in Sydney, there are LOTS of times when this second occasion happens. Further, I know of lots of flights that have employed this second technique when late in the sequence but with the weather improving. Using rough numbers and fuel burns for the 767 TAC, this second figure could well be about 20-30 minutes after the original number.
So, the time you give ATC would mean that you'd be sometimes diverting when you didn't have to? That IS what you're saying. The airport may be wide open now but because you HAD to shoot your approach 20-30 minutes earlier (when it was below the alternate criteria), you are now on your way to a different aerodrome- even though every man and his dog is now making an approach to an aerodrome that is above the SLAM.
Thankfully I'm not a 'crap magnet' like some of the other QF guys but I've seen the above occur a bunch of times at SYD, MEL and BNE. Arrive at holding with airfield requirint alternate and 20 minutes later there are no requirements at all. Ten minutes after that and we're CAVOK! Still, you'd be enjoying your diversion on min gas right about then!
