Europe’s answer to a US-style ground delay program is CASA — the Computer Assisted Slot Allocation algorithm that has assigned Calculated Take-Off Times since 1995. When several active regulations hit the same flight at once, the documented rule is blunt: the most penalising regulation’s delay is forced into every other regulation that flight crosses.
A filed flight plan doubles as the slot request. A Slot Allocation Message arrives at least two hours before departure, and the standard takeoff window runs CTOT minus five to plus ten minutes.
Ask a dispatcher what CASA does and you’ll usually get the fairness story: first planned, first served. Official materials lean into that version too. An academic paper will call CASA a baseline heuristic.
None of them explains what happens when one flight gets caught by several capped sectors at once — the exact situation peak summer days in Europe produce routinely.
The confirmed mechanics are clear. A regulation caps the entry rate, the slot list matches declared capacity, and a filed flight plan is the slot request.
But once several regulations land on a single flight, the public record narrows to a single sentence buried in the EUROCONTROL Users Manual. How the system actually balances fairness inside that sentence is the part nobody documents.
How CASA builds a slot list
European airspace is some of the most congested in the world, and demand routinely overshoots sector capacity. When that happens, the EUROCONTROL Network Manager activates a regulation that caps the entry rate, defines the affected traffic flows, fixes a time window and names a reference location.
Once the regulation is live, CASA builds the slot list automatically — no manual calls between dispatchers and flow managers. The list is built to match the declared capacity rate, not a rough estimate. Positions are then assigned on the First-Planned-First-Served principle: the earlier a flight plans to reach the regulated point, the earlier its takeoff time.
One quirk saves operators a step. Because a filed flight plan counts as a slot request in the system’s eyes, there is no separate manual request to miss. Medical emergencies and specific airborne traffic get forced positions based on estimated arrival times, exempted from that standard sequence.
The CTOT compliance window
At least two hours before departure, the system sends a Slot Allocation Message — the SAM — to the departure aerodrome and the operator for any regulated flight. If a new regulation is applied after that two-hour boundary, the slot is issued immediately rather than held.
The takeoff window itself is narrow. Standard practice across the Romanian and Czech AIP texts is CTOT minus 5 to plus 10 minutes. The Czech publication extends that to minus 5 to plus 20 minutes during alert and activation phases — an exceptional contingency allowance, not the routine tolerance.
There is no haggling at the gate. If a flight does not adhere to its slot, ATS units are required to withhold start-up clearance. The same national authority that publishes the tolerance enforces it.
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What happens with multiple regulations
Here is where CASA stops being transparent. The documented rule for a flight caught by several regulations is one sentence from EUROCONTROL: it takes the most penalising regulation and applies that delay to every other regulation the flight crosses. In the slot messages themselves, the most penalising regulation is listed first in the REGUL fields — a formatting detail that doubles as a diagnostic.
When a regulation is modified, proportional delay updates apply across every regulation the flight crosses, not just the changed one. And a flight-plan update inside the two-hour window that changes the profile calculation can mark the flight a EUROCONTROL-defined “late updater” — adding delay that again propagates through each regulation it touches.
Then the documentation goes quiet. What it does not explain is how CASA balances equity when several regulations compete for one flight’s sequence, how it orders those most-penalising rules internally, or how it rebalances when a regulation shifts mid-sequence. That gap is precisely where CTOT behavior becomes least predictable.
One week in early September 2025 puts the scale in human terms. 53,873 flights ran ATFM delays — roughly 7,696 departures a day, averaging 3.2 minutes each — all tethered to CASA-generated CTOTs and a takeoff window measured in minutes.
The legal skeleton underneath the algorithm
CASA’s sequencing rule is not just an internal convention; it sits on EU law. Commission Regulation (EU) No 255/2010, issued on 25 March 2010, is the operative measure for air traffic flow management. Article 2(7) defines an ATFM departure slot as a calculated take-off time attributed by the central unit, with the local ATS unit managing the tolerance.
Article 3 then encodes the priority rule in legal form: flights are ordered according to their planned entry into the location where the measure applies, unless a formally agreed alternative benefits the European network. That is FPFS with a regulatory spine, and it is why a national ATS unit that denies start-up clearance is applying the same sequence CASA used to build the list.
When the consolidated text was re-checked in mid-2025, this regulation was still the operative EU measure. Through the 2024–2026 window, EUROCONTROL publications continued to describe CASA as the working slot allocation procedure inside ETFMS — with no named successor. The law and the live algorithm still line up.
What this means for you
For a traveler watching a departure time slip, a CTOT reads as arbitrary. It isn’t. It is the output of one algorithm responding to one capacity cap on one sector, applied as a takeoff window measured in minutes.
Dispatchers can treat the two-hour boundary as a planning fence. Later profile edits carry a documented risk — the earlier an update goes in, the lower the downside.
Then there is the known unknown. Multi-regulation handling reduces to a single sentence in the public record, and that is exactly the scenario where CTOTs become least predictable. Until EUROCONTROL publishes a real version history, assume the documented one-sentence rule is the whole public story — and build schedule risk around it when several regulations stack.
Questions? Answers.
What happens if an operator updates the planned departure time within two hours of the last filed EOBT?
If the update changes the profile calculation, the flight is treated as a late updater, which can add delay across every regulation it crosses. Slots are normally allocated at SIT1 — two hours before EOBT — and will not deteriorate after that moment unless one of the crossed regulations is modified.
Does EU law tell CASA how to prioritise flights for departure slots?
Yes. Commission Regulation (EU) No 255/2010 defines an ATFM departure slot and requires priority for flights according to their planned entry into the location where the measure applies, unless a formally agreed alternative benefits the European network.
Is there a publicly documented version history or replacement timeline for CASA?
No. As of mid-2025, EUROCONTROL has not published a named CASA version history, release notes, or an announced production replacement within ETFMS over the last decade.
