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Flow Management Procedures

Normally, on VATSIM, the Reykjavik CTA operates under the principle of “free flow,” meaning there are no restrictions on aircraft movements, departures or arrivals. However, during periods of heavy traffic, flow control measures may be imposed to reduce the strain on local and neighbouring ATC.

Flow control measures may come from:

  • vIFF, the network flow system, which issues slots (CTOTs) automatically when an airport, a sector or an ECFMP measure is over capacity. BIRD capacity scenarios switch on automatically from VATSIM bookings.
  • A local supervisory controller, if one is present (e.g., rostered for an event).
  • ECFMP (European Collaboration and Flow Management Project), where neighbouring vACCs may impose restrictions on flights entering or exiting their airspace via the Reykjavik CTA.

Types of Flow Control

CDM (Collaborative Decision Making)

CDM is run with the CDM plugin for EuroScope, included in the BIRD install pack, at BIKF, BIRK, BIAR, BGGH, BGSF and EKVG. The plugin should always be running, and the lowest position at each airport is CDM master.

Full procedures, phraseology and airport data are in the [CDM – Collaborative Decision Making Tool](https://wiki.vatsim-scandinavia.org/books/software-resources/chapter/cdm-collaborative-decision-making-tool) chapter. Start with the [Controllers Guide – CDM](https://wiki.vatsim-scandinavia.org/books/software-resources/page/controllers-guide-cdm).

The basic principles are:

  • Every departure gets a target start-up approval time (TSAT) from its target off-block time (TOBT), the airport's departure rate and the taxi time.
    • Aircraft must start up within +/- 5 mins of their TSAT.
  • Some departures also get a calculated take-off time (CTOT) from vIFF.
    • Aircraft must take off within -5/+10 mins of their CTOT. A CTOT must always be followed.

This spreads departures out so the airport's capacity is not exceeded, and keeps aircraft on stand until there is room for their departure.

The overall flow of CDM is:

  1. Aircraft calls for clearance.
  2. DEL asks for the aircraft's TOBT (the time they expect to be ready for push and start) if it has not been set on the VDGS, and enters it in EuroScope. BIRD runs CDM with real mode off, so a flight has no TSAT until it has a TOBT.
  3. DEL issues the clearance with the slot (if any) and the start-up time (e.g., "ICE123, ... squawk 2341. Slot 1450, expect start-up at 1435, call ready for start-up.")
    1. During events with slot bookings, DEL applies the booked slot as a manual CTOT from the event slot column. The plugin then calculates the TSAT from it.
  4. When the pilot reports ready for start-up, the controller sets ASRT. Start-up is approved within +/- 5 min of the TSAT.
  5. GND and TWR must ensure that the aircraft takes off within -5/+10 mins of its CTOT. If the slot will be missed, it can be extended once by 10 minutes; otherwise a new CTOT is requested from vIFF.

Radar Release

During periods of high traffic congestion in the airspace surrounding the airport, APP may move an airport into the status of radar releases. When radar releases are in effect, TWR must receive a release from APP for each aircraft before TWR clears it for take-off.

APP and TWR should remain in coordination regarding when the period of radar releases will end.

Minimum Departure Interval (MDI)

A minimum departure interval (MDI) is a minimum amount of time which must elapse between two departing aircraft going in a specific direction. This is generally imposed by either local staff, or by staff in neighbouring FIRs, to manage the flow of departing traffic out of an airport.

MDIs issued via ECFMP are automatically included in vIFF's CTOT calculation. Regardless of whether CDM is in use or not, it is TWR’s responsibility to ensure that aircraft adhere to the MDI requirement.

Note that for an MDI, like wake turbulence separation, the time between departures is counted from when the aircraft become airborne. TWR may, for instance, clear aircraft for take-off slightly early, so that they become airborne just as the MDI is satisfied.

Level Capping

Level capping refers to when flights meeting certain conditions are subject to a level restriction for part (until a certain point) or all of their flight. This is in order to reduce congestion in en-route airspace, particularly in upper sectors.

Rerouting

Flow controllers may occasionally decide to reroute aircraft if necessary to relieve pressure on en-route sectors. If a rerouted aircraft already has a clearance, then the issuing of the re-clearance via the new route is generally handled by DEL.

If the aircraft is already taxiing, then GND should taxi it to an unoccupied area where it will not be blocking traffic, so that it can hold position and receive the re-clearance.

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