Unfortunately there is another side to the flow management restrictions. As well as the CFMU generated slot times which are there for all to see with each delay having a code attributed to define its cause, there are London ACC restrictions in the form of MDI's/ADI's (minimum departure interval or average departure interval. For the sake of this discussion we can treat them as the same thing).
They are generally put on for short term issues such as weather in a sector or perceived bunching of traffic. A restriction will be placed on an airfield and a particular route imposing a minimum departure interval. For instance a CPT departure may normally be a 2 minute IFR route separation but due to weather or a potential over delivery further down the line, the London flow management unit will impose a further restriction. Often this just takes the form of a request not to take the piss but can also be a specified minimum interval such as 1 every 3 minutes. When sectors are really going under with weather I have seen 1 every 10 minutes put on with immediate effect. That can really spoil your day as a departures controller!
These MDI delays are not recorded in the official delay stats like CFMU restriction and are not transparent to the airlines. They are simply applied at the sharp end to the best of our ability.
Don't worry though, technology is here to save us. CDM and the magic TSAT machine will, by a mix of black magic and blind guessing, generate a target start approval time for each aircraft. This is made possible because all operators are so amazing at updating their EOBT to reflect their actual state of readiness.
From this the machine will take account of a myriad of other factors, multiply the result by the square root of Pi and tell us when you should start for an unimpeded taxi to the hold where you will arrive, no doubt fully ready with final figures in hand and be promptly cleared for take off.............
