Review of Policy on Ground Based Safety Nets

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Review of Policy on Ground Based Safety Nets

42ND ANNUAL CONFERENCE, Buenos Aires, Argentina, 17-21 March 2003

WP No. 88

Review of Policy on Ground Based Safety Nets

Presented by SC1


1.1  As part of the 2002/3 work programme, SC1 undertook to review existing policy on ACAS / TCAS and MSAW, with particular attention to the implication of the definition of “Safety Net”.

1.2  Due to the current focus on ACAS / TCAS policy SC1 has decided to divide the Work Item into two Working Papers. This paper will focus upon policy for Ground Based Safety Nets. A separate Working Paper (W.P. No XX) reviews policy on Airborne Safety Nets.

1.3  The purpose of this paper is to review and, if applicable, amend IFATCA policy on issues concerning the use of Ground Based Safety Nets.


2.1  During the 2002 Annual Conference the following definition of Safety Net was approved:

“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.”


2.2  There is a need to review existing IFATCA policy on Ground Based Safety Nets in light of the Safety Net definition that was determined at the 2002 Annual Conference.

2.3  By analysing the IFATCA definition of Safety Net it is possible to set the scope of what functions and systems fall within the remit of “Safety Nets”. The definition stipulates that a Safety Net consists of a function that has the explicit purpose of alerting the pilot or controller to the imminence of a collision. This collision can be between aircraft, aircraft and terrain or obstacles. The definition also specifies that the function of alerting controllers or pilots of the penetration of dangerous airspace will be included as a ‘Safety Net’. The following are examples of functions that can be considered to be ‘Safety Nets’: Short Term Conflict Alert (STCA), Minimum Safe Altitude Warning (MSAW), and Area Proximity Warning (APW).

2.4  IFATCA already has established policy for STCA. The policy states:

“Ground based safety nets, like STCA, can enhance overall safety in the automated ATC systems. Therefore each automated ATC radar system should be provided with a ground based safety net system such as STCA, as a last resort, that only should be used to advise the controller of potential losses of separation.”


2.5  Whilst there is policy for STCA, there is no definition of what STCA actually consists of. There is however a generic understanding of the purpose of STCA, but the parameters within systems vary from ATS Unit to ATS Unit.

2.6  Existing IFATCA policy on MSAW states:

“MSAW, as a last-ditch ground-based warning system, must be fully implemented without delay, with the necessary operational requirements and appropriate ATC procedures and training on a world-wide basis, in order to significantly reduce the number of CFIT-accidents.”


SC1 had considered modifying policy so as to only require the implementation of MSAW at Air Traffic Services Units that have elevated terrain that may prove hazardous to aircraft. However, the prime purpose of MSAW functionality is considered to be the prevention of Controlled Flight Into Terrain (CFIT). SC1 have included the prevention of collision with obstacles as falling within this primary objective. MSAW functionality should therefore provide alerts to controllers so as to prevent the collision of aircraft with terrain and obstacles. In order to meet this requirement, the inclusion of an Obstacle Clearance database may be a pre-requisite of an MSAW system.

2.7  SC1 discussed whether MSAW functionality should be considered only as a ‘safety net’ and not as a ‘controller tool’ for determining adequate terrain clearance. When first considered, the MSAW function appears to fall within the definition of a Safety Net. MSAW is certainly being proposed to alert the controller of the imminence of collision of aircraft with terrain and obstacles. However, the MSAW function is also being proposed for deployment as a means of assisting controllers to avoid environmentally Noise Sensitive Areas. For example, an alert could be generated by the MSAW function when an aircraft is about to overfly a Noise Sensitive Area below a specified minimum altitude. MSAW was initially designed as a safety net to prevent CFIT. However, the alerting possibilities of MSAW can be used as a ‘controller tool’ for other applications.

2.8  The Safety Net definition includes functionality to alert controllers of the imminence of penetration of dangerous airspace. Some ATS Units are being equipped with a function known as Area Proximity Warning (APW). This functionality alerts controllers when aircraft are about to penetrate specified types of airspace. The types of airspace typically specified include Danger Areas, Prohibited Airspace, Temporary Reserved Airspace. The types of airspace can be considered as ‘dangerous’ due to the type of activity being conducted within them. The provision of alerts to controllers to prevent inadvertent penetration of such types of airspace is therefore considered as a Safety Net.

2.9  There are no definitions established for Minimum Safe Altitude Warning (MSAW) or Area Proximity Warning (APW). There is a need to establish definitions for these functions. The acronyms, namely MSAW and APW, mean different things to different system developers and to different ATS providers. There is a need for standardisation so that there is an understanding as to what core functionality can be expected from MSAW systems and APW systems respectively.

2.10  The provision of safety nets, including STCA, MSAW and APW, should not be taken into account when determining the overall safety of an ATS system. Such systems are safety nets and not intended as tools. This policy is in line with EUROCONTROL’s Safety Review Commission who have drafted the following policy statement:

“Any safety benefit which may be provided by a safety net shall be considered as an additional overlay to that provided by the ATM system. The ATM system must be able to demonstrate whatever ATM safety minima and aviation level of safety are considered to be necessary, without reliance upon the safety benefit expected to be provided by safety nets”.


3.1  The topic of Safety Nets has been sub-divided into Airborne Safety Nets, e.g. ACAS/TCAS, and Ground Based Safety Nets, e.g. STCA, MSAW, APW.

3.2  The introduction of additional safety nets within the ATM system, such as the implementation of STCA, MSAW and APW functionality, is to be encouraged.

3.3  During discussion regarding Ground Based Safety Nets, arguments were put forward whether certain functions should be classed as Safety Nets or whether they are ‘controller tools’. However, whilst there is a general understanding of what type of functionality could be considered as a ‘controller tool’, the lack of definition of ‘controller tool’ sometimes hindered discussions.

3.4  The implementation of safety nets within the ATM system has tempted controllers, managers and regulators to use the application in the calculations to determine if the ATM system is ‘safe’. However, the stance that has historically been taken by IFATCA and ICAO alike, and that needs to be maintained, is that such systems are safety nets and as such are a “last ditch” defence against collision with other aircraft, obstacles, or terrain.

3.5  This review of IFATCA policy on Ground-based Safety Nets needs to be repeated on a regular basis to ensure that policy reflects developments in the applications and the way in which they are used.


It is recommended that:

4.1  The definition of Safety Net, IFATCA Manual page 3 2 1 16, para 1.12., be amended to read:

“A safety net is an airborne and/or ground based function, the primary 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.”

4.2  A definition of Short Term Conflict Alert be prepared during next year’s Work Programme by SC1.

4.3  A definition of Minimum Safe Altitude Warning be prepared during next year’s Work Programme by SC1.

4.4  A definition of Area Proximity Warning be prepared during next year’s Work Programme by SC1.

4.5  Policy be prepared for Area Proximity Warning systems during next year’s Work Programme by SC1.


5.1  Professional and Technical Manual of IFATCA (2001).

5.2  WP 98 – Toulouse 1998.

5.3  WP 88 – Santiago 1999.

Last Update: September 29, 2020  

March 22, 2020   738   Jean-Francois Lepage    2003    

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