A-CDM at airports: what TOBT, TSAT and TTOT mean

The Airbus has been sitting at its stand for minutes with its doors closed, the bridge is gone, the tug is attached – and nothing happens. Then, to the minute, the clearance comes and the jet pushes back. What looks like chance at the fence follows, at many large airports, a fixed schedule of acronyms: A-CDM. This guide translates TOBT, TSAT, TTOT and their relatives, shows you the 16 milestones of a departure and explains what you can read from them as a spotter.

A-CDM in one sentence

Airport Collaborative Decision Making means that everyone involved in a departure works with the same times: airport operator, airline, ground handler, air traffic control and the Network Manager of Eurocontrol. In the past, every partner planned with its own assumptions – air traffic control with the flight plan, ground handling with its status at the gate, network planning with averages. A-CDM pools this information and turns it into target times that everyone aligns to.

In Europe, the concept is jointly backed by Eurocontrol, the airport association ACI Europe, the airline association IATA and the air navigation association CANSO. Eurocontrol names more efficient and robust handling on the ground as its goals, especially for turnaround and departure preparation, as well as more accurate takeoff times for planning in European airspace.

The logic behind the acronyms

Before you dive into individual timestamps, it is worth looking at how they are built. Almost all A-CDM acronyms consist of four letters, and the first one tells you how reliable the time is:

• S – Scheduled: the planned time from the airline’s flight plan, such as the SOBT (Scheduled Off-Block Time), which belongs to the airport slot.
• E – Estimated: an estimate that can change continuously, such as the ELDT (Estimated Landing Time) of an arriving flight.
• T – Target: a target time that the partners agree on and align their processes to.
• A – Actual: the time actually measured, for example the AOBT (Actual Off-Block Time) when the aircraft is really pushed back.

The rest of the acronym describes the event: OBT is off-block (leaving the parking stand), IBT in-block (arriving at the stand), LDT landing, TOT takeoff. Once you have internalized that, you can usually decode unfamiliar acronyms yourself.

Acronym	Spelled out	What is behind it	Who provides it
SOBT	Scheduled Off-Block Time	planned departure time at the stand, matching the airport slot	Airline / flight plan
EOBT	Estimated Off-Block Time	departure time according to the ATC flight plan	Airline
ELDT	Estimated Landing Time	expected landing time of the arriving flight	Network Manager, local radar data
EIBT	Estimated In-Block Time	expected arrival at the parking stand	A-CDM system (landing plus taxi time)
TOBT	Target Off-Block Time	time at which the aircraft is ready for departure	Airline or ground handler
TSAT	Target Start-up Approval Time	expected clearance for engine start / pushback	Departure manager (DMAN)
TTOT	Target Take-Off Time	target time for takeoff	A-CDM system / DMAN
CTOT	Calculated Take-Off Time	ATC slot when airspace is congested	Network Manager
EXOT / EXIT	Estimated Taxi-Out / Taxi-In Time	estimated taxi time to the runway or to the stand	A-CDM system
ASRT / ASAT	Actual Start-up Request / Approval Time	request and issue of the start-up clearance	Air traffic control
AOBT / ATOT	Actual Off-Block / Take-Off Time	actual pushback and takeoff	Air traffic control, sensors

TOBT: the commitment from ground handling

The Target Off-Block Time is the heart of A-CDM. It describes the moment when the aircraft is ready to go at its stand: all doors closed, jet bridge retracted, pushback tug ready. It is maintained by the airline or the contracted ground handler, because only they know how far catering, fueling, baggage and boarding have progressed. How many trades have to mesh here is shown in the guide to the turnaround of an aircraft.

As long as nobody has entered a TOBT manually, the A-CDM system calculates one itself: expected arrival at the stand plus minimum turnaround time (MTTT, Minimum Turn-round Time) gives an automatic TOBT. If it falls before the scheduled departure time in the flight plan, it is set to that EOBT – earlier than planned does not count as a target. As soon as the handler enters a value of their own, it overrides the automatic one.

Because the entire departure sequence hangs on the TOBT, strict rules apply at A-CDM airports. Zurich Airport describes them in its operational information, for example, like this:

• Changes are only allowed in steps of at least five minutes.
• The last correction must be made no later than five minutes before the current TOBT.
• The crew must report ready to the clearance delivery position (Clearance Delivery) within five minutes before or after the TOBT.
• Anyone who is not ready within the window is not processed further until a new TOBT has been entered.

The exact tolerances can differ from airport to airport. The principle stays the same: an overly optimistic TOBT costs not only your own flight time but also throws the sequence of all the others into disarray.

TSAT: when the clearance comes

The Target Start-up Approval Time is the system’s answer to all the TOBTs combined. A departure management system, DMAN (Departure Manager) for short, sorts the ready flights so that the runway is used as evenly as possible and nobody has to queue up at the holding point. According to Zurich Airport, the calculation includes the following, among other things:

• the flight’s TOBT,
• the available runway capacity and the assigned runway,
• variable taxi times depending on stand and runway,
• aircraft type, wake turbulence category and departure route,
• any necessary de-icing,
• ATC slots (CTOT) and special departure spacing.

According to the Eurocontrol specification, the TSAT is issued between 40 and 30 minutes before the TOBT. From then on, it is the decisive time for the crew. In Zurich, a window of plus/minus five minutes applies here too, and the tug must be coupled and ready five minutes before the TSAT so that the pushback can begin immediately after clearance.

That is exactly why you so often see aircraft at the fence that are ready and still wait: their TOBT has been reached, but their TSAT has not. Instead of standing in a queue in front of the runway with engines running, the aircraft waits at the stand. That saves fuel, noise and exhaust on the apron.

TTOT and CTOT: the view beyond the airport

The Target Take-Off Time is the time at which the aircraft is expected to lift off. The Eurocontrol specification describes it at its core as a simple addition: readiness time plus estimated taxi time, that is TOBT plus EXOT. If there is no TOBT yet, the calculation uses the expected arrival plus minimum turnaround time, and for aircraft without an inbound flight (for example from maintenance), the EOBT. If de-icing is needed, the de-icing time is added.

This TTOT is the airport’s most important gift to the European network. It is sent to the Network Manager in so-called DPI messages (Departure Planning Information):

Message	Roughly when it is sent	What it communicates
E-DPI	from about three hours before EOBT	first departure data, flight is known in the A-CDM system
T-DPI-t	from two hours before EOBT and on relevant changes	target takeoff time based on the TOBT
T-DPI-s	after the TSAT is issued	target takeoff time from the departure sequence
A-DPI	at actual off-block	flight is under way, takeoff time known very precisely
C-DPI	on cancellation, for example a deleted TOBT	previous information is invalid

The Network Manager uses this data to plan airspace. If a sector or a destination airport is overloaded, it issues a CTOT – an ATC slot, that is a narrow time window for takeoff. According to Zurich’s operational information, the standard tolerance for this is five minutes early to ten minutes late. The CTOT in turn feeds into the TSAT. What is behind airport and ATC slots is explained in the article Airport slots.

The 16 milestones of a departure

A-CDM tracks every flight along a fixed chain of events. Each milestone triggers recalculations and can raise warnings if something does not fit together. The list goes back to Eurocontrol’s implementation manual:

Not every airport records all milestones the same way, and some – such as the moment of local radar pickup or the start of boarding – are defined locally. The order can also shift: for a long-haul inbound aircraft that took off before the two-hour mark, milestone 3 comes before milestone 2.

The six building blocks of the concept

The timestamps are only one part of A-CDM. The Eurocontrol manual divides the procedure into six elements that build on each other:

1. Information sharing: a shared data platform for all partners.
2. Milestone approach: the chain of events per flight described above.
3. Variable taxi times: realistic taxi times by stand and runway instead of a flat value.
4. Pre-departure sequencing: the departure sequence from which the TSAT arises.
5. Adverse conditions management: procedures for snow, thunderstorms or reduced capacity.
6. Collaborative management of flight updates: the data exchange with the Network Manager through DPI messages.

For smaller airports, there is a leaner variant called Advanced ATC Tower, which shares only part of the A-CDM data with the network.

De-icing: when winter changes the math

In winter, A-CDM becomes especially interesting. If an aircraft has to be de-iced, the system factors in additional times: when de-icing is supposed to begin, how long it takes and when it is finished. With de-icing at the parking stand, the takeoff time shifts accordingly; with de-icing on a separate pad, the system can re-sort the flights and issue new TSATs if more aircraft need de-icing than the facility can handle. Important for understanding: in Zurich, the crew must report ready within the TOBT window even if de-icing is still going on at the stand – the de-icing time is already built into the TSAT. You can read more about fluids, holdover times and procedures in the article De-icing aircraft.

What A-CDM means for you at the fence

For spotters, A-CDM is not an end in itself but a key to understanding what you see and hear.

• Aircraft waiting at the gate: Doors closed, bridge gone, but no pushback – usually the TSAT has not been reached yet. That is not a defect but intentional.
• Shorter queues at the holding point: Because the departure sequence is created at the gate, queues in front of the runway form less often at A-CDM airports. For you, that means taxi shots come at a more even rhythm.
• Radio becomes easier to follow: If you listen in on the delivery or apron frequency, where that is legal, you hear ready reports and clearances in the order of the departure manager. Which frequencies exist is explained in the guide Understanding aviation radio frequencies, and the legal situation is covered under Listening to aviation radio.
• Learning to read delays: A delayed flight at the gate can hinge on a late TOBT (ground handling not finished yet) or on a late TSAT or CTOT (runway or airspace full). The article Why flights are delayed sorts the most common causes.
• Interpreting live data correctly: On flight tracker sites, a flight often only appears with pushback. If you know that the departure sequence is fixed half an hour earlier, you interpret the traffic picture on the map better. Background is provided by the guide Flight tracking with ADS-B for spotters.

At some airports, TOBT and TSAT are also shown on the docking system at the parking stand. In Zurich, for example, the TOBT appears there 30 minutes before the scheduled off-block time, and from five minutes before TOBT the display alternates between TOBT, TSAT and TTOT. That is intended for the crew – from the spot it is usually too far away to read along. But if you wait for the departures from Butzenbüel in Zurich, you benefit anyway: the rhythm of departures there is the result of exactly this planning.

Where A-CDM is in use

According to Eurocontrol, A-CDM is fully implemented at 35 European airports. In Germany, these include Frankfurt Airport, Munich Airport, Düsseldorf, Hamburg, Stuttgart and Berlin Brandenburg. Abroad, large hubs such as Amsterdam Schiphol, Brussels, London Heathrow, Paris Charles de Gaulle as well as Zurich and Geneva are on the list. Airports in Asia, Australia and New Zealand also work according to the concept.

Whether your home airport is on the list is not decisive for spotting itself. But it helps put the behavior on the apron in context. At the Zeppelinheim viewing platform or the Munich Airport visitors’ hill (Besucherhügel), you see the result of A-CDM every day: takeoffs in a dense, even sequence without long queues forming at the runway threshold. How many movements a runway can handle in the first place is explained in the article on airport capacity.

What to keep in mind

• Local rules differ. Tolerance windows, display times and procedures are set per airport. The values from Zurich in this article are an example, not a Europe-wide standard.
• TOBT is not the departure time on your ticket. The time on your boarding pass is the scheduled time. TOBT and TSAT are internal operational times.
• TSAT is not a takeoff time. It marks the expected start-up or pushback clearance. Taxi time and, if applicable, de-icing still come before liftoff.
• CTOT is a hard constraint. An ATC slot applies Europe-wide and cannot simply be moved by the airport. It comes from the Network Manager, not from the local air navigation service such as DFS.
• Not every airport uses A-CDM. Smaller airfields often work without it or with the leaner Advanced Tower variant.

Conclusion

A-CDM turns many individual estimates into a shared timeline. With the TOBT, ground handling says when the aircraft is ready, the departure manager answers with the TSAT when it may go, and the TTOT tells the European network when the aircraft will be airborne. Sixteen milestones hold this chain together, from the activated flight plan to liftoff. Anyone who knows the acronyms sees more at the fence than just waiting and taxiing aircraft – they recognize the planning behind it.