Produce a Definition of Area Proximity Warning System

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Produce a Definition of Area Proximity Warning System

43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004

WP No. 92

Produce a Definition of Area Proximity Warning System

Presented by SC1


1.1.  Agenda Item B5.5 from the 2003 Annual Conference discussed the application and usage of “safety nets” within the air traffic environment.

1.2.  The paper discussed the implementation of Area Proximity Warnings (APW) into some ATS units. These warnings are designed to alert a controller to the imminent incursion of a controlled flight into one of a number of specific types of airspace. Presently IFATCA does not have definitions or policy for the usage of such systems.

1.3.  The purpose of this paper is to develop definitions for APW and Area Proximity Warning systems (APWS).


2.1.  IFATA Policy (2003) defines a safety net as:

”A safety net is an airborne and/or ground based function, the sole purpose of which is to alert the pilot or controller of the imminence of collision of aircraft, aircraft and terrain/obstacles, as well as penetration of dangerous airspace”

This definition had been expanded at the 2002 Annual Conference to include systems that provide warnings of airspace incursion. As ATS systems have developed, the ability for automated monitoring of aircraft in relation to special airspace has been enhanced and functional warnings in relation to airspace can now be generated in a similar manner to those of MSAW.

2.2.  In the modern world of aviation there are a variety of different types of airspace that could possibly require protection. There are danger areas, restricted areas, special use areas, prohibited areas, temporary reserved areas, military areas and other locally defined special airspace areas to name a few. All of these have different entry and operational requirements but from an ATS perspective these requirements are immaterial. The airspace will have been established for a specific purpose and there will be a requirement to either keep aircraft operations clear of such areas or at least advise a pilot of the imminent penetration of some types of airspace. For the purpose of this paper, all such airspaces referred to above shall be classified as “special-use” airspace.

2.3.  Traditionally, there have been non-automated systems in place to ensure aircraft do not stray into special-use airspace. In the first instance, the publication of charts and manuals that outline the airspace in terms of position, altitude restrictions and operating requirements have been published. They are often activated or de-activated by the issuance of a NOTAM and pilots and controllers have kept themselves informed of the impact of such areas through briefings.

2.4.  The introduction of more automated systems has meant that at a flight planning stage there exists the ability to automatically check a submitted flight plan for avoidance of special-use airspace. Similarly, as radar displays have developed, the ability to display radar maps reflecting airspace restrictions has developed and allowed controllers to monitor aircraft operations clear of such airspace. Both of these methods however rely on vigilance and perhaps continuous awareness by both pilots and controllers which, from a human factors perspective, is undesirable. In times of high workload or difficult operating conditions it is not always possible to provide the necessary monitoring as priority tasks of separation take precedence.

2.5.  The introduction of an automated APW does not negate the need for these traditional means of monitoring special-use airspace. Both pilots and controllers have professional responsibilities to ensure they are adequately briefed before commencing work and to fully understand the implications and requirements of special-use airspace. An automated system does, however, provide an addition to the safety net of an ATS system that will help prevent inadvertent incursions of airspace in the event that other avoidance methods have failed.

2.6.  An APWS should therefore be considered in the same way as other applications, such as MSAWS. Both of these systems use processed radar data to accurately predict the future positions of controlled flights. This information is then compared with a database of special-use airspaces and, if an aircraft is to penetrate either the airspace or separation minima established from the airspace, then a warning is generated for the controller. These warnings must be accurate and timely, allowing the controller time to recognise the alert, assess the situation and deliver any appropriate instructions to the pilot prior to the incursion occurring. The parameters of the system must be set such that are a minimum number or irrelevant, inaccurate or nuisance alerts to ensure that if an alert is actually generated, the controller will respond in a timely manner.

2.7.  APWS are similar to MSAWS or STCA in purpose but the core functionality is subtly different. They all serve to act as a defence against incidents or accidents and ensure that important tasks are still being undertaken when higher priority tasks demand more attention of a controller. APWS, however, could cover a multitude of possible types of airspaces, each with different requirements. The purpose of an APW is to inform a controller of an airspace incursion, but it may not necessarily result in a controller taking any action. The aircraft may have a need to enter the airspace, may have authority to operate within it or may be able to transit at pilot’s discretion. All these scenarios are slightly different form those that would occur in response to an STCA or MSAW warning.


3.1.  APWS and APW form an integral and important part of a modern air traffic system.

3.2.  There are a large variety of different airspaces that may receive protection from an APWS, however an APW alert may not necessarily require controller intervention.

3.3.  Reliance on an APWS must not replace the traditional responsibilities of pilot and controllers in relation to special-use airspace.

3.4 SC1 is of the opinion that APWS could be deployed as both a safety net and a Controller Tool depending on the activity within the “special-use” airspace.


It is recommended that;

4.1  The definition of an Area Proximity Warning System is:

An Area Proximity Warning System (APWS) is a functionality of an automated ATS surveillance system that uses track projections to predict an aircrafts incursion into “special-use” airspaces. The APWS provides timely warnings of such incursions to allow appropriate actions to be taken by a controller.

4.2  The definition of an Area Proximity Warning is:

An Area Proximity Warning (APW) is an alert provided to a controller of the imminent incursion of a controlled flight into “special-use” airspace. The response to such a warning will be dictated by the nature of the airspace in question and its specific requirements.


Professional and Technical Manual of IFATCA (2001).

WP 88 Buenos Aires 2003.

Last Update: September 29, 2020  

March 22, 2020   585   Jean-Francois Lepage    2004    

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