As I understand the system (and I'm certainly no expert),
The Appoved Departure Time, is as it says, the time your departure is approved for i.e the time filed in your plan.
The Calculated Take Off Time is basically a departure slot allocated by flow control, the system works on an estimated off blocks time and a nominal taxy time for the aerodrome (EGPF 10mins) and the delay is determined from that.
The CTOT has some tolerance at some units, usually -5mins to +10mins around the CTOT.#
Flow are a lot stricter now and no leeway outside these times is permitted except under extreme circumstances which are listed in the unit MATS part2.
Hope this helps, no doubt somebody from flow will correct something I've said
As an aside, any of you fly guys out there, when you have a slot, if its busy, call ground a little bit earlier for push/start that way we won't have to refuse/send you back to stand because you've got stuck in a queue, we may tell you to wait, but a least we'll get you going in time for the slot
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Turn left heading 230, close from the left, report established
[This message has been edited by form49 (edited 16 November 2000).]