etiene

UKtravelbear

etiene

Heathrow Tower

I have written a post in the past about flow control and slot times, but I can't find it right this moment!

The following is a simplified description. Flow control (sometimes referred to by its acronym as ATFM or ATFCM -Air Traffic Flow (Control) Measures) which forms part of the overall Network Management, is an incredibly complex beast.

Firstly, let's talk about 'slot times'. There are two types, one is an airport slot or schedule slot. These are the rights given to an airline to fly from an airport within a certain time window. These are what LHR is short of, and LHR slots are often so valuable they are their own asset class, often changing hands amongst airlines for tens of millions of pounds.

Forget those, we are now going to talk about the ATC slot, or it's official title of Calculated Take Off Time (CTOT). This is a time, with a tolerance of -5/+10 minutes, within which a flight has to take off.

We'll consider a flight from FRA to LHR, all figures quoted hereafter are for illustration purposes only!

In Brussels there's the NMOC, the Network Managment Ops Centre. NMOC has a very large computer that effectively holds all the flight plans for any flight going on in Europe. So NMOC know airport of departure, estimated time of departure, route, speed, airport of arrival.

Also, each airport in Europe will have declared an arrivals capacity to NMOC, in terms of aircraft per hour.

Airspace is divided up into sectors (both vertically and horizontally) by each ATC organisation. Each sector will also have a maximum capacity in terms of aircraft per hour, again, this will have been declared to NMOC.

So, thinking of our flight's route from FRA to LHR, the West departure sector of Frankfurt may declare a capacity of 60, the NW Germany mid-level sector a capacity of 60, NW Maastricht Upper sector; 80, London Clacton sector 45, NE London mid-level 45, Heathrow arrivals 40.

The NMOC computer has all this information. It knows, at any given time, where all the aircraft should be, and it what sectors, and what the capacity of those sectors is. On a benign day, then NMOC might realise that for the hour in which our FRA-LHR flight is esitmating to land at LHR, there are another 45 aircraft estimating arrival. This makes 46, but the capacity is only 40.

NMOC will then start issuing CTOTs to move 6 of those aircraft into the next hour, so delaying the take-offs of the last 6 aircraft in the originial hour by a few minutes each. NMOC will probably also issue CTOTs to most of those in the original hour, but with zero delay, just to ensure they take off and use the slot, and don't depart late causing the same problem in the next hour. Let's imagine our FRA-LHR flight is one of those that is going to depart in the original hour, but there's a flight BRU-LHR which has been given a 10 minute delay on its CTOT to move its arrival to the next hour. If, for some reason (tech, baggage, slow boarding) our FRA flight misses it's slot, then the NMOC computer will start trying to find another canidate to use our place at LHR....BRU is closer to LHR, so it could take our place, so it gets a slot improvement and takes our place in the first hour's arrivals into LHR.

Luckily, the BRU flight was boarded on schedule and the flight/cabin crew and passengers were all ready to go when the improvement came through. In this situation, even they knew they had a delay, as soon as they were ready, the BRU flight crew would have asked ATC to send a 'Ready Message' which tells NMOC that they can take advantage of a slot improvement as in this case.

The above example shows the process with only one (LHR arrivals) constraint on a route. Now imagine the complications that arise when the numbers of flights means that all of the FRA-LHR's flight's sectors are at capacity, and moving a flight's arrival time into LHR back by 10 minutes might then cause another sector on its route to be over capacity, and thus the flight picks up even more delay. Now imagine a flight from Denmark/Norway to France/Spain crossing this route at right angles, or the flights from outside of Europe that have already been airborne for 10 hours, which need to be worked around.

And all this is on a benign day. Imagine the complications when there are thunderstorms around Brighton-Dover, and instead of just the flights from/to central and eastern Europe and London using this route, the flights that would normally go to/from France, Italy, Africa and London have to use the same route, thus doubling the amount of traffic flying through these already 'at capacity' sectors.

And in bad weather, sectors and airports will often reduce their declared capacity. At LHR we might reduce the arrivals capacity to 38, 36 or even 32 per hour. If you imagine lots of the sectors around northern Europe do this, then you can see very quickly hour delays of multiple hours quickly materialise.

CTOT delays, and significant CTOT changes, are common in such scenarios. In ATC we just get a time on each flight's data display in front of the controller. All the calculation is done by the supercomputer in Brussels. We don't just 'decide' who goes first. We have to abide by the CTOT.

On a benign day, then there is a very limited opportunity to do a bit of 'CTOT-swapping', but only on very simple situations within one airline. So for example, if the LHR-GLA flight has a 10 min delay, and the LHR-EDI has no delay, and the constraining sector was in the Midlands where both aircraft were going to fly through, and the EDI flight was not going to be ready for its CTOT, but the GLA was ready early, and the swap wasn't going to affect any other flight at all, then it might be done. You can see the number of factors that need to line up there!

Hope that's explained it a bit. Happy to answer any questions. Like I said, that's a very simple version of reality, but hopefully it gets the idea across. Mods, happy for you to separate out if you wish!

KARFA

you were on a non-EU carrier flight taking place outside the EU/EEA so no part of the regulation applies. the prior AMS-DFW flight was covered but as you say there were no issues with it.

corporate-wage-slave

That is a fantastic piece of insight there, Heathrow Tower, thank you very much for putting this here. In discussion with Prospero, we felt it best to preserve it for the FT nation by copying it over also in to the Dashboard Fog thread. It is the sort of thing which explains many details that won't be obvious even to very regular flyers such as myself. Thanks very much for taking the time to do this. I'll also put in a wiki so we can find things more easily.

I think your previous post was this one in the Fog thread:
https://www.flyertalk.com/forum/27632906-post52.html

and you also did this one in the in the Staff thread
https://www.flyertalk.com/forum/24927651-post826.html

(which I found using the search terms "CTOT" and Heathrow Tower)

Thanks again for this.

armouredant

Last nights FAO-LGW landed very late, apparently caused by the aircraft being delayed 1h30m for ZO and ZA (operationally and ATC) at SKG (Thessaloniki) coming to Gatwick, and a further ~1h45m at LGW for ZO and ZY (turnaround). It eventually landed back at LGW 3h15m late, though the delay caused there is reported as ZY.

Fortunately, I was on the Norwegian service on the same route, but was chatting to a few of the pax booked on the BA flight in the lounge. Will they have to fight for compensation, given their flight is reported as being delayed for turnaround only?

etiene

corporate-wage-slave

If FAO-LGW was 3 hours 15 minutes late, BA would need to argue that (say) 20 minutes of it was due to weather/ATC/etc issues on LGW - FAO, and/or FAO-LGW. That seems to me to be a plausible scenario. The skies are absolutely full at the moment, aircraft are being worked for every available minute. The one thing that BA can't use is to blame ATC problems in Greece - which is well advertised at the moment - as a reason for denying compensation to Faro passengers.

Heathrow Tower

You're all very welcome, being here has benefitted me greatly, if sharing a bit of knowledge helps the board then that is little payback as far as I'm concerned.

Pc2k24

Submitted July 28 via online form (during my 5 hour wait at LHR)
Response received Aug 15
Further response received Aug 16 indicating BACS payment, 3-5 days until it will be in my account

The last response was actually quite nice and personal, saying as a valued gold member etc etc and we look forward to welcoming you on your next flight in October to Larnaca

Ive also submitted a further claim on that trip (going to EWR rather than way back)... as BA lost my bag for 48 hours!

Ancient Observer

lhr Tower,
thanks for that. I had read the earlier version too. I'd rather have too many versions rather than none!
Ta
AO

RollAnotherFatOne

Recently had a flight cancelled and then moved onto a flight the following day - downgrade to economy (redemption booking, paid business). short haul in europe, one way.. the original cancellation was caused by weather apparently.
I enquired with BA several weeks ago to complain, and as I did not get a response, I decided to follow up with a compensation claim.
A couple questions. Can I expect more than the difference between avios in economy and business back (4k I think)? and a few pounds in taxes?
How long do I have to give them before I can push this along? maybe a CEDR? it's been over a month now..

corporate-wage-slave

It depends on the route and distance. But the downgrade reimbursement is probably 30% of base fare, see post 5 above. In which case it may be that returning the Avios difference and (e.g.) any APD saved is as good as it gets, but without more information it's difficult to say. If they are doing a full fare recalculation it may well take a few weeks, but you could do you own calculation of what you are owed, and ask them for a CEDR deadlock letter now, however it's unlikely to speed things up at this stage and they may not assist you. After 8 weeks you don't need the deadlock letter, you can go to CEDR at that point.

RollAnotherFatOne

ok thank you - very helpful.