TwistedFirefighter, your e-mail is "blocked by the administrators", so here's your reply.
Okay, at 1340 Geneva alerted us to an impending overload in their sector between
1500 and 1600 UTC.They say it was due to aircraft climbing above their FPL level
(they would not previously have been counted) and requested that we reduce the traffic
to the correct number. This was done at 1344, the usual 30 minutes parameter was
overridden otherwise there would have been an insufficient reduction in traffic.
This is when you received your first slot revision.
At 1346 EGBB TWR sent an REA message telling us that you could be airborne within
10 minutes. This prompted the two subsequent revisions as the machine reallocated
any vacant slots. Why there were vacant slots in such a situation is not clear, I
would have to look at the messaging for the other affected flights (about 50 of them),
but a Zurich sector was modified at about the same time- that could have moved some
of the other traffic.
Just to explain the first bit, we have a big problem with aircraft being filed below
regulations and then climbing into them. Some do this deliberately, queue jumping
if you like. Others use it as a legitimate way of avoiding delay, unfortunately if the
crews are not briefed accordingly they will accept climb offered by other ATC units (who
are normally unaware of problems which may be several sectors down the line).
Most of the overloads we investigate are due to either this problem or aircraft
departing outside slots. Don't forget that even the existing slot window of -5 +10
in a sector with a capacity of 60 per hour can deliver 75 flights (10 from the
previous hour, 5 from the next hour).
So the closer the system gets to its limits the more rigid it becomes. We either have
to accept less flexibility or higher delays overall.
Flowman.