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Interesting comment about your charter and the pax being late etc etc affecting slots.
I can remember a few years ago that same problem arose in Europe with charters being late and Eurocontrol simply told the operators that a slot is just that - it is up to the charter operators to educate their pax and impress on them the need to arrive at the time they want the a/c. You only need to catch a few big fish and everyone learns. By impressing on your charterers the need to be on time it is going to make it a lot easier for you.
I remember the classic case of a SA businessman a few years ago who arrived late at Le Bourget to go to Luton and when the crew tried to get a new slot the earliest they could get was 6 hours down the line. Well said businessman threw his toys and demanded to go and they just ignored him. But now he has learnt his lesson and when he flies in Europe he is always on time.
So the slots system is a 2 way thing - you be on time and it works and ATC handles you, but you need to ensure your pax understand that. And once they learn how it works they will always be on time.
Oh and if you get a pm from Springbok 702 about questions be careful what u say - it is Radio702 in disguise - you have been warned!