Originally Posted by
CPH-Flyer
A question on delays. If you can talk about that part of it, how does it get decided who is on time, who gets delayed, and who cancels/diverts?
Reason for asking, I came in JL Saturday with one of the shortest holds of the 9 JL arrivals in LHR I have had this year. Despite the fog. Does JL with only one flight a day get a certain priority to the available landing slots? Or is it more random than that?
Another good question!
There are three mechanisms at LHR.
If there is bad weather forecast, ATC will provide Heathrow Airport with a landing rate prediction for the following day broken down by hour. If that prediction falls below the demand of the schedule, something needs to be done.
There is a process called 'HADACAB' which the airlines have signed up to where Heathrow Airport can force an x% reduction of the schedule across the board. Which flights get cancelled are always down to each airline.
There is another, non-mandatory process we call Demand v Capacity where Heathrow Airport and ATC will inform all the airlines that if nothing is cancelled there will be x delays, we recommmend reducing the schedule by x%. Again, it is up to each airline to decide whther to cancel, and which flights to cancel.
Always working in the background is Air Traffic Flow Management, which results in the ATC slot time that you may hear pilots talk about on the PA. The following is a very simplified explanation!
Within Europe, all flight plans are fed into a big computer is Brussels, at the Network Manager Operations Centre (NMOC). ATC organisations also feed into this computer the capacity, in terms of aircraft per hour, each little parcel (or sector) or airspace has - including the landing capacity of each airport.
Let's say that LHR has an arrival capacity of 42 per hour. NMOC can see that for the hour of 0700-0800, taking into account flight plan departure times, speeds, routes etc, that 44 aircraft are actually forecast to turn up in the 0700-0800 hour.
NMOC knows that they have to delay 2 aircraft into the next hour, so they issue those slot times (called Calculated Take Off Times (CTOTs)) with delays to ensure they don't arrive before 0800. It might also issue CTOTs to other flight, but they correspond with the planned take off time anyway (ie. the slot time does not actually 'delay' the flight) to ensure that aircraft gets airborne when NMOC is expecting it to. No point delaying some aircraft when the aircraft you let through with no delay turn up late! Everybody loses.
If an aircraft with a Heathrow Arrival CTOT misses its slot, that info will get fed back to NMOC, and it may re-arrange things. For example, if the missed slot is an aircraft in Zurich, then an aircraft being delayed on the ground at Brussels or Paris might get an improvement on their slot, so as to take the Zurich's flight position in the planned sequence. This potential for improvement is why crews like to get eveyone on board ready to go even with a delay.
A proportion of flights will be what are called 'out of area' arrivals, those flights that take off from airports outside of Europe, where there is, at the moment, no mechanism to delay them at their departure airport, so it's only European departures to LHR that will potentially get slots.
I think that gives an overview.