Introduction
With the onset of congestion during some periods of the day at a few major airports in the early 1960’s, the scope of the schedule discussions was broadened to cover the adjustments needed to reduce anticipated delays to an acceptable level. Where services planned during certain periods at an airport exceeded what the airport could accommodate without unacceptable delays, some services would have to be moved, usually with some economic penalty. This induced IATA (International Air Transport Association) to establish coordination offices at such airports, usually embedded within the organisation of the national airline. Over the years, a consensus developed as to which services should be moved, in fairness to all planning to operate during the period. The recommended procedures and priorities for such schedule adjustments are contained in the IATA Worldwide Scheduling Guidelines (WSG), first issued in 1976.

Within the European Union the airport/schedule coordinations shall be established and operated as independent entities acting according to the EEC95/93 Regulation which came in force 1993. Appendix 7.3 of the WSG recommends also the independence of the coordinator.

Visit also the website of the European Union Airport Coordinators Association.

Definition of "Slot"
The aviation industry knows 2 different types of slot which are not related to each other:

• The Air Traffic Control (ATC) slot is needed by each departing flight on the actual day of operation to avoid congested airways. This slot is only valid for this specific flight and for a specific departure time window (~15 min.). In Europe the ATC slot is centrally coordinated and allocated by EuroControl in Brussels.
• The Airport slot is mandatory at coordinated airports for each movement (arrival AND departure) and is valid for a specific time at a specific weekday and for a specific period applied for! The airport slot is used to plan the runway capacity and/or other capacity constraints (see below) for a whole season to minimize airport congestion and potential delays.

Level of Airport Activity
Depending on the level of activity at airports, certain procedures to ensure acceptance of airline schedules have been developed to cover various situations. For the purposes of schedule clearance, there are three broad categories of airports:

• Level 1 airports: capacities are adequate to meet the demands of users. Such airports are recognised from a schedule clearance viewpoint as non-coordinated.
• Level 2 airports: the demand is approaching capacity and a more formal level of cooperation is required to avoid reaching, if at all possible, an over-capacity situation. These airports are referred to as schedules facilitated.
• Level 3 airports: demand exceeds capacity during the relevant period and it is impossible to resolve the problem through voluntary cooperation between airlines and where, after consultation with all the parties involved, there are no possibilities of resolving the serious problems in the short term. In this scenario, formal procedures need to be implemented at the airport to allocate available capacity and coordinate schedules. Airports with such high levels of congestion are referred to as coordinated.

source: IATA Worldwide Scheduling Guidelines para 2.1-2.4

Capacity Constraints to be coordinated
Following constraints may be coordinated depending on the level of congestion and vary by each airport:

• Runway Capacity
  i.e. performance of runway system, surrounding area (topography), approach and departure routes, Air Traffic Control capabilities, ATC acceptance rate
• Parking Capacity
  i.e. number of available ramp stands
• Terminal/Passenger Flow Capacity
  i.e. “passenger throughput”, check-in, arrival, transfer and baggage sorting/area infrastructure, customs & immigration facilities
• Environmental Restrictions
  i.e. night curfews, noise restrictions, noise quotas
• Other operational restrictions (security, special handling etc.)

see also Coordination Principles