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 Slot Guidelines (WSG), first issued in 1976.

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.

Airport Coordination
Airport coordination is a means of managing airport capacity through the application of a set of rules contained in the Worldwide Slot Guidelines (WSG). Coordination involves the allocation of constrained or limited airport capacity to airlines and other aircraft operators to ensure a viable airport and air transport operation. Coordination is also a process to maximize the efficient use of airport infrastructure.
Coordination is not a solution to the fundamental problem of a lack of airport capacity. In all instances, coordination should be seen as an interim solution to manage congested infrastructure until the longer term solution of expanding airport capacity is implemented.
The prime objective of airport coordination is to ensure the most efficient use of airport infrastructure in order to maximize benefits to the greatest number of airport users.

For the purposes of airport coordination, airports are categorized by the responsible authorities according to the following levels of congestion:

• Level 1: airports where the capacity of the airport infrastructure is generally adequate to meet the demands of airport users at all times.
• Level 2: airports where there is potential for congestion during some periods of the day, week, or season which can be resolved by voluntary cooperation between airlines. A facilitator is appointed to facilitate the planned operations of airlines using or planning to use the airport
• Level 3: airports where capacity providers have failed to develop sufficient infrastructure, or where governments have imposed conditions that make it impossible to meet demand. A coordinator is appointed to allocate slots to airlines and other aircraft operators using or planning to use the airport as a means of managing available capacity.

source: IATA Worldwide Slot Guidelines para 1.1-4

Airport Coordinator
A coordinator is appointed by the responsible authority, following consultations with the airport managing body, all airlines using the airport and their representative organizations. Coordinators must be functionally and financially independent of any single interested party and act in a neutral, transparent and non-discriminatory way.

source: IATA Worldwide Slot Guidelines para 5.2

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

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