1. The main causes of the air traffic congestion are:
a) accumulation of air traffic during specific periods of the year and also during certain times of the week and hours of the day, due to holiday patterns and travel habits of the public;
b) differences in the capacities of the various ATC systems or parts of these systems affected by traffic accumulations;
c) insufficient advance notice (to ATC units) of likely
traffic demands which may cause overloading of the
system at certain points, in certain areas, and/or during specific time periods; and
d) lack of proven techniques and procedures to restore, in critical situations, a reasonable balance between traffic demand and available ATC capacity by means acceptable to aircraft operators both from an operational and from an economic point of view.

2. The accumulation of air traffic may be due to the fact that operators adapt their services to their customers’ demand and that the choice of routes and flight levels is limited, due to the need to share the airspace with other users, especially the military.
In addition, operators may have to cope with restrictions imposed on them for environmental reasons, i.e. night curfews at aerodromes, noise abatement procedures, etc., which tend to concentrate the traffic in a narrow period of time.

3. The traffic handling capacities of ATC systems may be inadequate as a result of insufficient staffing of existing facilities, in terms of either the number of personnel or their qualifications. Inefficient ATC procedures may also limit ATC capacity,
e.g. inadequate liaison and/or lack of letters of agreement between States, especially those relating to transfer of control of aircraft between adjacent States.
Furthermore, lack of ATC equipment such as primary and secondary surveillance radar and electronic data processing equipment may cause difficulties in coping with the growth of air traffic.

4. Measures to control the flow of air traffic will need to be taken in certain cases to ensure a reasonable balance between the air traffic demand and the ability of the air traffic services to accommodate that demand.
However, it should be emphasized that these measures are restrictive in nature and should be kept to the minimum and, whenever possible, be applied selectively so as to affect only that part of the over-all air traffic which causes the problem.

5. The term “selectively” should not permit the use of any discriminatory practices by the ATC unit concerned.
Any distinction between different parts of the air traffic should be based exclusively on categories such as arriving or departing air traffic or overflights, without any consideration of the type of flights (civil, military, scheduled, non-scheduled, etc.).
When flow control measures are necessary for certain areas, they should be applied only for the period when expected air traffic demand will exceed the capacity in those areas.
Flow control measures should be established and co-ordinated in such a way that they will not cumulatively interact with each other on the same flights.

6. In the context of ATFM, the following types of
flights should be granted exemption from flow control measures:
a) flights in a state of emergency, including flights subjected to unlawful interference;
(b) flights operating for humanitarian reasons;
(c)medical flights specifically declared by medical authorities;
(d) flights on search and rescue missions;
(e) flights with “Head of State” status*;
and other flights as specifically required by State authorities*.

7. The ATFM service should fulfil the following
basic strategic and tactical functions:
a) collection and collation of data on the air navigation infrastructure and on the capacities of the ATC system and selected aerodromes within the “ATFM area”, including runway, taxiway and gates capacities. This embraces those areas in which traffic flow problems are likely to be encountered;
b) collection and analysis of data for all planned controlled flight operations into, out of, within and through the ATFM area;
c) determination of a coherent picture of expected traffic demand, including anticipated ad hoc traffic, comparison with available capacity and identification of areas and time periods of expected critical traffic loadings.

8. d) co-ordination with the appropriate ATS authorities in order to make every possible attempt to increase the available ATC capacity where required. In some particular situations it could be advantageous for national and local scheduling committees to be established, with representatives from national ATS, airport authorities, national and international operators. Such committees can make significant ontributions when developing strategies to reduce the impact of peak demand periods; and
(e) where ATC capacity shortfalls cannot be eliminated, determination and implementation in good time of suitable tactical measures co-ordinated throughout the ATPM area as necessary and with aircraft/aerodrome operators concerned.

9. Whenever measures to control the flow of air traffic have to be applied in the form of delays, they should, if possible, be applied by ATC to aircraft on the ground rather than to aircraft in flight. Whenever application of
such measures in the form of delays to airborne aircraft becomes unavoidable, the flights concerned should be informed as soon as possible.

10. The introduction of flow control measures requires that the air traffic system capacity be accurately determined and an assessment made of the level of traffic demand above which the traffic flow will require regulation.
Having determined this level, ATPM should provide the area control centre (ACC) serving a terminal area with information on an hour-to-hour basis (and to the best accuracy possible) regarding the planned times of
arrival and departure of all aircraft during each period (15 or 30 minutes) when peak traffic conditions may be expected. Preferably, such predictions should be prepared
three hours or more in advance and reviewed and revised periodically thereafter. Since such predictions require information on many aircraft that have not yet taken off or are otherwise not known to the ACC, special arrangements must be made by the ATPM to obtain this information from adjacent ACCs, the operators and/or other sources such as repetitive flight plans.

11. Predicting the capacity of the system starts from a knowledge of capacity available under optimum operating conditions. It is then necessary to make allowance for adverse weather conditions, runway configurations at destination and the effect of forecast wind shift, runway unserviceability, inoperative en-route navigation and/or landing aids or any other factor which could adversely affect flight regularity.
This information is then assessed by the ATPM service, in close co-operation with ATC, so as to arrive at a probable capacity figure for the system at any given time.

12. Measures to regulate demand may take several forms
Measures to regulate demand may take several forms. In simpler systems, ACCs exercising flow control to surrounding area control centres disseminate notices requesting that affected aircraft be spaced at prescribed intervals, e.g. one every ten minutes. Spacing of aircraft for flow control purposes should not be mixed with separation, but rather should be based on an “acceptance rate”, i.e. the number of aircraft accepted in a given period of time. This method of applying flow control is used by a number of ACCs without computer assistance.
An improved and more sophisticated form of regulation of arrivals is possible with automated ATC systems, in which the controller is provided with computer assistance in the sequencing and spacing of aircraft in the terminal area. In this case, calculation of the delays which are caused by the sequencing and spacing operation can be transformed into clearances requiring aircraft to operate at reduced speed while still en route to the terminal area.