CTOT, slots and A-CDM, explained: CTOT, TOBT, TSAT and what even experienced captains get wrong

By Arnthor Birgisson, business aviation captain and founder of Galley. Facts checked on 17 October 2026 against the official texts listed under Sources.

This is a guide, not an operating procedure. Your operator’s procedures, the AIP and ATC’s instructions come first.

The slot message arrives when the passengers are already at the FBO, asking when we leave. Ops are on the phone, the handler wants a time for the catering, and somebody asks whether we can “use the ten minutes”. That is the moment when most of us, me included, mix up the abbreviations, the windows before and after the slot, and the taxi time. We are human, and under pressure the similar-sounding times blur.

A slot is teamwork: the crew, ops, the handler, the tower and the Network Manager each hold a piece of it. This guide puts each term and each number next to the text that defines it, so that everyone in that chain can work from the same words: EUROCONTROL’s ATFCM and IFPS Users Manuals, its A-CDM manual and specification, the EU regulation on flow management and the States’ AIPs.

1. The CTOT is a take-off time

When a flow regulation affects your flight, the Network Manager (NM) gives it a slot. You don’t ask for one: the ATFCM Users Manual says “the act of filing a flight plan effectively constitutes a request for a slot” (§8.1.1). The slot itself is defined in §8.2.3.1:

“A slot is issued as a Calculated Take-Off Time (CTOT). The CTOT is defined as a time at which the aircraft shall take-off.”

EU law says the same. Regulation (EU) No 255/2010, Article 2(7), defines an ATFM departure slot as “a calculated take-off time attributed by the central unit for ATFM with a time tolerance managed by the local ATS unit”.

The CTOT is not an off-blocks time and not a start-up time. The manual puts the planning on the operator: “AOs need to plan the departure of a flight so that the aircraft will be ready for start-up in sufficient time to comply with a CTOT taking into account the taxi time” (§8.2.1). Operator and ATC “are jointly responsible for achieving CTOT compliance at departure aerodromes” (§8.2), the CTOT is part of your ATC clearance (§8.2.2(b); Regulation 255/2010, Article 6(6)(a)), and receiving it is the operator’s job: “The AO is responsible for making arrangements to ensure receipt of any CTOT issued by the NM” (§11.4).

The first CTOT comes in a Slot Allocation Message (SAM), sent to the operator and the departure tower “at the earliest 2 hours before the last received EOBT”, or straight away when a regulation starts later (§8.2.3.1).

2. The −5/+10 window belongs to ATC

Most of us know the slot window: 5 minutes before the CTOT to 10 minutes after it. It is easy to forget whose window it is. Appendix F of the ATFCM Users Manual:

“The slot tolerance (-5’ to +10’) is available only to ATC and only to organise the departure sequence. If there is no departure sequence, the CTOT shall be strictly adhered to.”

The UK and Spanish AIPs say it in ENR 1.9 in the same words: “A slot window is available to ATC to optimise the departure sequence. This is not for use by AOs who should plan an EOBT consistent with the CTOT.” Estonia’s AIP says it most plainly: “A tolerance of - 5' to + 10' of the CTOT is defined as the period during which the FLT shall take off.” Outside the window you don’t go: the tolerance is one “for which the aircraft must not depart outside”, and ATC “may deny start up clearance to a flight unable to meet its CTOT” until a revised CTOT is issued (§8.2.2(e), (f)).

So plan to be airborne at the CTOT itself. If you plan for CTOT +10, a short wait behind another aircraft at the holding point puts you outside the window. Some airports ask for more: Heathrow’s AD 2.20 says “Any aircraft with a CTOT should plan Reduced Engine Taxi to be ready for departure at CTOT - 5 minutes”.

NM can widen the window for a whole airport for a while (de-icing, for example), but the ATFCM Operations Manual rules that out for one late flight: “The procedure shall not be used in case of an individual flight that cannot make its CTOT” (§6.1.1).

3. Taxi time: the minutes between off-blocks and the CTOT

NM works out your take-off time from your off-blocks time plus “an average taxiing time for the runway in use at the airfield concerned” (§8.2.3.1). The SAM gives that value as TAXITIME, in hours and minutes. Where it comes from (§8.1.10.1):

• a default for each runway in NM’s environment database;
• a taxi time filed in the flight plan, which overrides the default;
• at an A-CDM airport, the taxi time the airport sends in its DPI messages, which takes precedence over both.

Here is a made-up SAM:

-TITLE SAM -ARCID GLY21 -ADEP ESSA -ADES LOWW -EOBD 261020 -EOBT 0805 -CTOT 0840 -TAXITIME 0015 -REGCAUSE WA 84

• Estimated take-off: EOBT 08:05 + 15 minutes of taxi = 08:20Z.
• Your ATFM delay: 20 minutes. The ATFCM Operations Manual writes it as “CTOT – (last EOBT received in IFPS + taxi time)” (§5.7.2.7).
• Latest off-blocks: CTOT 08:40 − 15 minutes = 08:25Z. The Users Manual calls this the COBT, “CTOT –Taxi time” (§8.4.3).
• The window: 08:35 to 08:50Z, for ATC’s sequence.

The 15 minutes are NM’s average for the runway in use, not your taxi. A remote stand, a runway change or de-icing can make the real taxi longer, and then you need to be off blocks earlier than 08:25.

4. EOBT, TOBT, TSAT, TTOT, CTOT: who sets each one

Six of these seven times end in “OBT” or “TOT”, and three start with a T. The definitions below are EUROCONTROL’s (the A-CDM manual’s, unless the table says otherwise):

TOBT, TSAT and TTOT exist only at A-CDM airports (section 8). Elsewhere you work with the EOBT and the CTOT. The T stands for target, and the manual means it: a target time “serves as a ‘contract’ between partners who are thus committed to achieving the milestone at this time”.

The EOBT is the time you actually want to be off blocks, not a copy of the slot. Section 7 explains why that matters.

5. The numbers that get mixed up

Most mix-ups are about which number belongs to which time. Here they are side by side:

6. The slot messages, and what each one asks of you

• SAM, Slot Allocation Message: your CTOT. “They shall comply with the CTOT” (§8.2.3.1).
• SRM, Slot Revision Message: a new CTOT, later or earlier: a change of more than 5 minutes, a reply to your DLA or CHG, or an improvement. “They must comply with the NEWCTOT” (§8.2.3.2).
• Improvements arrive by themselves. “All flight plans are by default in RFI status. In case an improvement is possible, flights in status RFI will immediately receive a Slot Revision Message (SRM)” (§7.1). If your passengers can’t leave earlier, ops can send an SWM: the flight then gets a Slot Improvement Proposal (SIP) instead, which they accept (SPA) or reject (SRJ) within 15 minutes (§7.2, App. B). An SWM is rejected for a flight from an A-CDM airport (§2.2).
• SLC, Slot Requirement Cancellation: the regulation no longer applies. “A flight may normally depart without an ATFCM regulation. Departures from A-CDM will have to comply with the relevant TTOT. When the SLC is issued after EOBT + 15 minutes the AO must update its EOBT by sending a DLA to IFPS.” And: “An SLC does not guarantee that the flight will not be subject to further regulations” (§8.2.3.5).
• FLS, Flight Suspension: NM “indicates with an FLS that this flight is considered as not taking off” (App. B), for example after an SMM or when it hasn’t seen you airborne. A DLA or CHG with a new EOBT brings back a DES (de-suspension) or a new slot, and “The TWR of departure should not let the aircraft start-up / depart before such a message (DES or SAM) is received” (§9.3).
• REA, Ready: doors closed and ready before your CTOT? The operator “may ask local ATC to send a Ready (REA) message” (§7.3), and NM looks for an earlier CTOT. “The REA message can only be sent by ATC following a request from AO”, from 15 minutes before the EOBT (§7.3.1).

7. When you can’t make the slot

The manual’s answer for an airport without A-CDM is short (§2.2): “As soon as possible send a DLA / CHG stating your new EOBT or send a SMM, if your new EOBT is not known, to ensure that the slot can be reused and to minimise your risk of substantial delay.” A small delay can keep the slot: “If the new EOBT still enables the flight to depart according to its CTOT, the slot will not be recalculated”; if not, “the next available slot will be issued in a SRM”.

Three rules are easy to get wrong, for crews and for ops:

• Don’t move the EOBT to match the CTOT. “An AO should not modify the EOBT to a later time simply as a result of an ATFCM delay (CTOT)” (§6.3.1). The EOBT is the earliest time you can be off blocks; the CTOT already carries the delay.
• A DLA can’t make the EOBT earlier. “The IFPS shall not accept negative delays: should the EOBT of a flight need to be changed to an earlier time that flight must be cancelled and re-filed with the earlier EOBT” (IFPS Users Manual §3). A second flight plan is not a fix either: “Only one Flight Plan shall exist at any given time for the same flight” (ATFCM Users Manual §6.1.1; Regulation 255/2010, Article 7(1)).
• An extension is 10 minutes, once. “Only one CTOT extension per flight is allowed”, and the operator can ask the NM helpdesk for it only “during the 20 minutes prior to the COBT” (§2.2). At an A-CDM airport the helpdesk gives no extension on the operator’s request: once you have start-up approval, “only the pilot can call the TWR (over the frequency) for a slot extension”; before that, the TOBT has to be updated.

For business aviation, NM keeps a telephone line: “The telephone service is reserved for business aviation and those who do not have internet access” (§8.4). The number is in §8.4.2 of the manual and in ENR 1.9.

8. At an A-CDM airport, the TOBT does the work

At an airport with Airport Collaborative Decision Making (A-CDM), the airport, the handlers, the operators and ATC share one set of target times, and the airport sends them to the Network Manager. EUROCONTROL lists 35 airports where A-CDM is fully in place, Geneva, Nice, Vienna, Málaga, Palma, Milan Linate, Paris Orly and London Stansted among them. Paris–Le Bourget, Farnborough and London Luton are not on the list (EUROCONTROL’s A-CDM airports map, read on 17 October 2026).

How the times fit together (EUROCONTROL’s A-CDM specification, 2025, unless noted):

• The TOBT is the operator’s. The operator, or the handler it delegates to, sets it and updates it “with an accuracy of at least +/- 5 minutes” (requirement A-CDM-[DAT]-[290]). The pilot doesn’t: Madrid’s AIP says “The TOBT and/or EOBT can only be updated by the airline or its ground agent, so that pilots shall refrain from making requests of this nature to ATC” (AD 2-LEMD, 20.10.1). If you will be late, tell your handler or ops early.
• The TSAT comes 30 to 40 minutes before the TOBT: “Between 40 minutes and 30 minutes before TOBT, a TSAT is issued by the Pre-departure sequencer” (§5.1.9.3). For a regulated flight the TSAT is worked back from the slot: “TSAT = CTOT – Taxi-Time” (DPI Implementation Guide §4.7.24.1).
• Ask for start-up within TSAT ±5 minutes (§5.1.13.2). Local procedures differ: Heathrow wants the ready call “at TOBT (window of -5 to +5 minutes)”, Madrid a single call from TOBT −5 to TSAT +5. The AIP of the airport is what you fly to.
• Not ready costs the place in the sequence. “If no update of the TOBT is received at TOBT + 10 minutes at the latest, the TOBT is removed from the A-CDM System and a C-DPI is sent to NM” (§5.2.2). The A-CDM manual is plain about what that does: “The C-DPI message suspends the flight in ETFMS” (§3.7.3). The same happens with no start-up request by TSAT +10 (§5.1.12.3). After start-up approval, “If the AOBT is not recorded at ASAT +5 minutes, start-up approval will be revoked” (§5.1.14.2).
• The TOBT moves your CTOT. “If the TTOT in a T-DPI-t or T-DPI-s message is after the CTOT tolerance window, the CTOT will be re-calculated accordingly” (ATFCM Users Manual §6.3.4). Once you are off blocks, “An A-DPI message is sent to NM, which then freezes the CTOT” (§5.1.14.3).
• The DLA is still yours. A TOBT change doesn’t file a DLA by itself. When the TOBT runs more than 15 minutes past the EOBT, the operator files a DLA, unless it has signed up for NM’s EOBT update service, which files it from the TOBT (IFPS Users Manual §118; specification §5.1.8.3). Until then, the A-CDM manual warns, “start up / push-back clearance may not be granted until discrepancy is resolved”.

So at a CDM airport the TOBT is how the whole team says when the aircraft will be ready. An honest TOBT, updated early, helps everyone in the sequence, including you.

9. Reading the reason: REGCAUSE

The SAM and SRM tell you why you are waiting, in the REGCAUSE field: a cause letter, a location letter (D departure, E en route, A arrival) and an IATA delay code (§8.2.3.4). The common ones, from Appendix D:

So CE 81 is ATC capacity en route (IATA 81, “ATFM due to ATC ENROUTE DEMAND / CAPACITY”), and WA 84 is weather at your destination (IATA 84, “ATFM due to WEATHER AT DESTINATION”). A slot is not a promise that the destination is open: the manual leaves it to the operator “to check that the airport is fully operational before commencing the flight” (§9.4).

Where you are in the slot window

When a slot is deployed, the departure is already stressful: impatient passengers, a handler waiting for a time, ops on the phone, and a delay none of us chose. That is when I want to see at a glance where I am in the slot window, instead of working it out from a column of times. I built slot alerts in Galley to give that situational awareness.

• The slot comes to you. Add your Galley roster address to ForeFlight’s slot e-mails, or forward them from [email protected] with a mail rule. Galley reads each SAM, SRM and SLC, attaches it to the leg on your roster, and replaces the last one. No ForeFlight? Paste the message from ops or WhatsApp.
• A push only when it changes: Galley notifies you when the CTOT moves or the slot is cancelled (“CTOT 12:25Z (+15)”, “Slot cancelled”). The same message again changes nothing but the “Updated” time.
• Slot mode shows the CTOT and a countdown to take-off. Inside the −5/+10 window the countdown becomes “Window closes in”, and after it “Window closed”. Under the countdown, Galley shows the latest off-blocks time (the CTOT minus the message’s taxi time), a timeline with now and the window, the change since the last message, and when the last update came.
• Your duty with the new CTOT: Galley moves the leg’s off-blocks and on-blocks, checks the FDP against your FTL scheme, and warns when the slot takes it close to or over the maximum. It doesn’t plan with commander’s discretion, which is for unforeseen circumstances after report. When the destination’s forecast at the new ETA is below your minima, slot mode says so.
• Lay the phone down: Galley turns the slot into a black, sideways view with the CTOT in large digits and keeps the screen on. A tap turns it dim red for the night.

Galley shows what the slot messages say. It doesn’t show TOBT or TSAT yet, and it doesn’t replace the conversation with ops, the handler and the tower: their information, the tower’s clearance and your own judgement come first. What it gives the whole crew is the same picture of the slot, at the moment everyone is busy.

Slot alerts are for pilots and part of Galley Pro; new accounts get their first month of Pro free.

Get your slot alerts in Galley

Related

• EASA FTL for business aviation: the rules that catch crews out
• EASA FTL calculator

Sources

1. EUROCONTROL Network Manager, ATFCM Users Manual, Edition Wave 2.3 (validity 22 April 2026): §2.2 (FAQ), §6.1.1, §6.3.1, §6.3.3, §6.3.4, §7.1–7.3, §8.1.1, §8.1.5, §8.1.10.1, §8.2, §8.2.1, §8.2.2, §8.2.3.1–8.2.3.5, §8.4, §8.4.3, §9.3, §9.4, §11.4; Appendix B (messages), Appendix D (regulation causes and IATA codes), Appendix F (slot window). https://www.eurocontrol.int/sites/default/files/2026-03/eurocontrol-atfcm-users-manual-wave-2-3.pdf
2. EUROCONTROL Network Manager, ATFCM Operations Manual, Edition Wave 2.3 (validity 22 April 2026): §5.7.2.7 (ATFM delay), §6.1.1 (slot tolerance window). https://www.eurocontrol.int/sites/default/files/2026-03/eurocontrol-atfcm-operations-manual-wave-2-3-external.pdf
3. EUROCONTROL Network Manager, IFPS Users Manual, Edition Wave 2.3 (validity 22 April 2026): §3 (filing time, EOBT changes, negative delays), §117, §118 (DLA, CHG; EOBT update service for A-CDM departures). https://www.eurocontrol.int/sites/default/files/2026-04/eurocontrol-ifps-user-manual-wave-2-3-external.pdf
4. EUROCONTROL, Airport CDM Implementation: The Manual, Version 5.0 (31 March 2017): Abbreviations (EOBT, EXOT, TOBT, TSAT, TTOT), Definitions (target time), §3.5.2.4, Attachment 2 (alert CDM08). https://www.eurocontrol.int/sites/default/files/publication/files/airport-cdm-manual-2017.PDF
5. EUROCONTROL Specification for Airport Collaborative Decision Making (A-CDM), EUROCONTROL-SPEC-198, Edition 1.0 (30 January 2025): §5.1.8.3, §5.1.9.3, §5.1.12.3, §5.1.13.2, §5.1.14.2, §5.1.14.3, §5.2.2; requirement A-CDM-[DAT]-[290]. https://www.eurocontrol.int/sites/default/files/2025-01/eurocontrol-specification-for-acdm.pdf
6. EUROCONTROL Network Manager, DPI Implementation Guide, Edition 2.700 (30 June 2025): §4.7.24.1 (TSAT for a regulated flight). https://www.eurocontrol.int/sites/default/files/2025-06/eurocontrol-dpi-impl-guide-2-700.pdf.pdf
7. EUROCONTROL, Airport collaborative decision-making, and its A-CDM airports map (read 17 October 2026): the 35 airports where A-CDM is fully implemented. https://www.eurocontrol.int/concept/airport-collaborative-decision-making
8. Regulation (EU) No 255/2010 laying down common rules on air traffic flow management, consolidated version of 11 December 2017: Article 2(7), Article 6(6), Article 7. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:02010R0255-20171211
9. UK AIP, ENR 1.9 Air traffic flow management, AIRAC 1 October 2026: paragraphs 7.2, 8.3, 8.5, 11.4. https://www.aurora.nats.co.uk/htmlAIP/Publications/2026-10-01-AIRAC/html/eAIP/EG-ENR-1.9-en-GB.html
10. AIP España, ENR 1.9 Air traffic flow management (AMDT 408/26): ENR 1.9-4 and 1.9-5. https://aip.enaire.es/AIP/contenido_AIP/ENR/LE_ENR_1_9_en.pdf
11. AIP Estonia, ENR 1.9 Air traffic flow management and airspace management, effective 1 October 2026: paragraph 6.2.2. https://eaip.eans.ee/2026-10-01/html/eAIP/EE-ENR-1.9-en-GB.html
12. UK AIP, AD 2 EGLL London Heathrow, AD 2.20, AIRAC 1 October 2026: Ground movement: Reduced Engine Taxi; A-CDM start-up procedures. https://www.aurora.nats.co.uk/htmlAIP/Publications/2026-10-01-AIRAC/html/eAIP/EG-AD-2.EGLL-en-GB.html
13. AIP España, AD 2 LEMD Madrid-Barajas, AD 2.20 (AIRAC AMDT 09/26): 20.10.1 Start-up of engines. https://aip.enaire.es/AIP/contenido_AIP/AD/AD2/LEMD/LE_AD_2_LEMD_en.pdf

Changes

• 17 October 2026: Written and checked against the texts above.