As expected, the FPL was late filed (less than 3 hours from EOBT). That’s why the initial CTOT was bad.
The slot was due to a flow restriction within LFLL TMA.
I will check what happened on that particular flight. That won’t be before Tuesday. But from the look of the CTOT, I would say that the OP has likely filed the FPL less than 3 hours before EOBT.
Two things I can tell already :
Now regarding ways to improve CTOT, in that case, I would have flown VFR in that airspace (I think the weather was fine yesterday), and you have to go visual at some point to reach LFHN anyway as there is no IAP.
Re-routing further west (ie flying through Clermont-Ferrand airspace) could work too, but you will need to head east at some point to reach LFHN.
Basically flying IFR in the Alps region during « week-end ski » is likely to trigger a CTOT.
They come about if the computer sees too much traffic going through a particular sector at a particular time. Essentially it models all the planned flights in the system and determines where they will be at a particular time. Obv things like strikes reduce capacity and can make it worse but eurocontrol slots are purely algorithmic.
Sure; I said “most”.
Regardless of the actual reason, I think the strategy for the pilot is the same. If this is not correct, would you be able to propose a better one, @Guillaume? Your contributions are always valued.
Most of the CTOTs I have had down there were due to industrial action by ATC. One solution is to file a VFR leg through the affected region; this brings various issues e.g. VRPs, limited altitudes, the need to be VMC prevents effective wx avoidance. A routing around it is another (if you have fuel and time, and you might get a CTOT on that route too because everybody else is doing the same).
The challenge is that you need to see the size of the region. The Eurocontrol website has a map – see below. Doing this job against the clock when trying to get somewhere and you have just a phone is not easy; I recall one instance in 2017 when I ended up losing so much time I had to divert and it cost me hundreds extra and a load of hassle.
But maximising hassle and pain is the exact purpose of industrial action 
As one guy involved in Eurocontrol autorouting said to me: the whole point of a CTOT is to maximise pain. It keeps you grounded while you sit there tearing your hair out about whether you will need to check yourself and passengers into a hotel for the night.
A week ago we were sitting in the ground at Innsbruck with the BA plane delayed by 2hrs due to a faulty gear microswitch. That 2hr delay was “ok” for us because we had a car parked at Gatwick, but for many people on the flight it was a disaster. They missed the last train somewhere, many had to go into hotels, and because the delay was less than x hrs they were not getting the compensation. Imagine doing this with a whole family, with kids…
The general thread on how to beat these industrial action related CTOTs is here.
This is a previous discussion on this topic, and the Eurocontrol map is shown there. It is here and it is on the right (Current Network Situation). It is very small but if you click on Static Map you get a zoomable version
That page also has a validation gateway, under Flight Planning / Free Text Editor etc.
They seem to always be because of Lyon Airspace, anything transitioning their zone has been delayed. When talking to the tower in Calvi, I explained to them I wouldn’t be able to get into LFHN (there is no runway lighting and we’re stuck to the side of a mountain, not a great combination :-))
Calvi Tower called Bastia for me and they negotiated (I assume with Eurocontrol?) They then asked me (after roughly 15 mins) if I was ready for immediate departure, which I was – and gave me the “slot no longer applies” message. When I thanked them, they told me to thank Lyon airspace…
What is the reason for the slots?
Slots like this are very local. I have had fewer CTOTs recently.
Last 5 flights transiting through Lyon Airspace have all had slots. Coming back from Corsica LFKC today I had filed for 1400z. Got a slot for 16.22z which would have gotten me into LFHN past the darkness. Made that point to the tower who got on the phone and got me airborne by 14.00z. Strange world and hoping these slots don’t persist or become the norm.
