Produce a Definition of Minimum Safe Altitude Warning Systems

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Produce a Definition of Minimum Safe Altitude Warning Systems

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

WP No. 91

Produce a Definition of Minimum Safe Altitude Warning Systems

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 made the observation that whilst IFATCA had a policy for the usage and implementation of a Minimum Safe Altitude Warning System (MSAWS) no definition of an MSAWS system existed.

1.3. The purpose of this paper is to recommend a definition of MSAWS.


2.1. Controlled flight into terrain (CFIT) has, in the past decade, become one of the major factors in aviation accidents. In response to this trend, aircraft manufacturers have sought to develop systems to prevent these events occurring and their efforts are reflected today by the fitment of enhanced ground proximity systems (EGPWS) to all modern aircraft. Other systems designed to enhance pilot situational awareness with relation to terrain are under development, which provide real-time vertical indications of underlying terrain.

2.2. At the same time, air traffic system manufacturers have also sought to develop tools that would provide suitable warnings to a controller if a flight’s trajectory were to take the aircraft below a safe level above terrain. MSAW systems in modern ATC environments are becoming more prevalent and allow some of the terrain monitoring functions traditionally undertaken by controllers to become automated.

2.3. An MSAW system operates by taking the trajectory data of an SSR response from a radar processor and compares the Mode C information, combined with the trajectory model to predict where the aircraft will be at a given time in the future (generally between 45 and 90 seconds). The system compares this projected position (continuously updated) with a digital terrain database and provides alerts to the controller if the aircraft will breach a defined terrain clearance “blanket”.

2.4. An MSAW system is not intended to absolve controllers of general terrain awareness, minimum safe altitudes or radar terrain vectoring requirements. Instead it is designed to provide a safety net or “last line of defence” against a CFIT accident. At times of high workload, both for controllers and pilots, the system is designed to alert when an unusual event, such as a high rate of descent or an erroneous vector or turn creates a potentially dangerous situation. For this reason, the terrain clearance “blanket” generally provides last minute warnings, allowing time for a controller to recognise and react to the situation. By doing this the number of nuisance alerts are significantly reduced and only timely and necessary alerts are generated.

2.5. In certain areas, the MSAWS functionality is also used for alternative applications. For example to provide warnings on the potential penetration of sensitive areas.

2.6. ICAO defines the function of an MSAW system as:

”The objective of an MSAW function is to assist in the prevention of controlled flight into terrain accidents by generating, in a timely manner, a warning of the possible infringement of a minimum safe altitude”

This is consistent with current IFATCA policy (2001) that MSAW should serve as a “last ditch ground based warning system”. In these respects, MSAW is different from other terrain awareness tools such as automatic MSA route checking, radar terrain maps and paper based or procedural methods of terrain avoidance. MSAW systems are also automated, but respond in real-time to provide updated and accurate modelling of aircraft flight paths in relation to terrain, to provide a final safety net to reduce CFIT accidents.

2.7. One major factor that will determine the effectiveness of any MSAW system is the accuracy of the terrain database that is resident in the system. Modern mapping techniques have ensured that much of the world is now digitally mapped with a common standard of map reference. Ideally, this information should be consistent with the database(s) that support EGPWS and other terrain monitoring systems. This would ensure that there was no confusion between controller and pilot when either system generates a terrain warning alert.


3.1. MSAW systems have been developed to act as ground based safety net, providing warning is a timely and accurate manner, to prevent CFIT accidents.

3.2. MSAW systems rely on aircraft trajectory data from automated radar systems to predict aircraft flight paths, which are then compared with a database of terrain. Ideally, these databases should be the same as those employed in aircraft systems for terrain monitoring.

3.3. MSAW systems are not designed to replace traditional methods of terrain monitoring either from the ground or cockpit based.

3.4 SC1 is of the opinion that MSAWS should only be deployed as a safety net. Alternative uses of the MSAW function should be classified as a controller tools and redefined accordingly.

3.5 IFATCA’s investigation identifies that current ICAO definition of MSAW is acceptable.


It is recommended that;

4.1 All IFATCA Member Associations should take account of the potential mis-use of this system.

4.2 The Executive Board of IFATCA brings this issue to the attention of ICAO Montréal.

4.3 This paper is accepted as information material.


ICAO Doc 4444.

WP 88 Toulouse 1999.

WP 88 Buenos Aires 2003.

Professional and Technical Manual of IFATCA (2001).

Last Update: September 29, 2020  

March 24, 2020   273   Jean-Francois Lepage    2004    

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