Originally Posted by
flying_pig
Thank you VERY much Heathrow Tower for this interesting explanation.
Out of curiosity, during a normal day at LHR, what % of flights takes off using the actual slot that was assigned when logging the flight plan? (ie: how many flights have to request a new slot?)
Also... does it ever happen that a flight misses the slot time while being in a queue for take-off, taxiing to get to the runway? I wonder what would happen in such circumstance: flight X is trying to get to the runway, there are 10 more planes ahead and flight X physically can't get to the runway before such planes take off, flight X misses the slot and Eurocontrol assigns a new one 1 hour later... Would flight X have to return to its gate? In my experience this never happened; I am wondering what are the processes in place to avoid such occurrence.
I'd say around 70-80% get airborne on their original slot time, perhaps more.
Sometimes a flight without a slot time is given one while it is taxying out (or a flight already with a slot is given a new one) which brings significant delay. In that case one of our assistants would phone the UK Flow Unit at Swanwick, to discuss.....usually we explain that it was already taxying out (or was going to make its original slot time with no problem). Unless the reason for the slot time is serious (airfield closed/snow/thunderstorms etc), we manage to allow the aircraft to depart under the original timescale.
This also occurs if a flight begins the push-back in time to make the slot time, but is held up on the taxiway somewhere due to other traffic, and we don't think it will manage to get airborne in the slot window, then we'll call UK Flow and can usually get a 5 minute extension on the slot time.