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Minimum Safe Altitude Warning (MSAW)

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Revision as of 14:26, 6 October 2011 by Anonymous

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Category: Safety Nets Safety Nets
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Minimum Safe Altitude Warning (MSAW) is a ground-based safety net intended to warn the air traffic controller (ATCO) about the increased risk of controlled flight into terrain by generating, in a timely manner, an alert of aircraft proximity to terrain or obstacles.

The main purpose of MSAW is to enhance safety and not to monitor adherence to any specified minima. In practice MSAW is a part of the ATC system and from this perspective it can be regarded as a “function”.

The term MSAW can be met in ICAO documents however an ICAO definition has not been established yet.

MSAW logic

The MSAW function monitors the levels reported by aircraft transponders with pressure-altitude reporting capability against defined minimum safe altitudes. When the level/altitude of an aircraft is detected or predicted to be lower than the applicable minimum safe altitude, an acoustic and visual warning is generated to the ATCO within whose area of responsibility the aircraft is operating.

MSAW adds independent alerting logic to the control loop (ATCO-CREW-AICRAFT & AVIONICS-SENSORS & COMMUNICATION-CONTROLLER WORKING POSITION-ATCO) to help prevent controlled flight into terrain by generating alerts of existing or pending situations related to aircraft proximity to terrain or obstacles, which require attention/reaction.

MSAW normally obtains input from the surveillance data processing, the environment data processing and possibly from the flight data processing systems in order to generate alerts. Some examples are presented below:

  • Surveillance data including tracked pressure altitude information can be used to predict hazardous situations;
  • Flight data can be used in the following manner:
    • Type/category of flight: to determine the eligibility for alert generation and possibly also the parameters applied;
    • Concerned sector(s): to address alerts;
    • Cleared Flight levels: to increase the relevance of alert generation.
  • Environment data and parameters include:
    • Terrain and obstacle data;
    • Alerting parameters;
    • Additional items (QNH, temperature, etc.).

Terrain and obstacles

In order to provide the MSAW function with proper data for monitoring, a terrain and obstacles model should be created in the air traffic control system. The following methods could be used:

Use of polygon volumes

These are volumes of airspace set several hundred feet below the lowest applicable minimum safe altitude. As appropriate, that could be the Minimum Vectoring Altitude (MVA), the Minimum Obstacle Clearance Altitude (MOCA) or Minimum Sector Altitude (MSA), or it may be set to follow more closely the terrain.

Use of digital terrain data

Use of Digital Terrain Elevation Data (DTED)[1] usually reduces the number of nuisance alerts since it provides much finer representation of the terrain than a series of hand-made polygons.

If deemed appropriate a mixed model with the use of both polygons and digital terrain data for the description of terrain and obstacles can be used.

MSAW exclusion areas may be defined where no detection of hazardous situations will be done. Such areas can be established by the ATC in order to prevent the MSAW function from unnecessary scan of the airspace.


The MSAW function provides vertical and horizontal prediction for the position of the aircraft using information from the radar and flight data processing systems.

Vertical prediction

Vertical prediction is a straight-line extrapolation made by using the current altitude (with barometric correction) and the vertical track rate.

Vertical prediction with cleared flight level (CFL)

In some ATC systems the MSAW function uses cleared flight level for trajectory prediction to increase the relevance of conflict prediction.

Horizontal prediction

Horizontal prediction is a straight-line extrapolation made by using the current track position and speed in lateral dimension.


Time settings

The future position of the aircraft is extrapolated forward from the current track position for predefined time called “look-ahead time”.

The look-ahead time and MSAW function processing time define the “warning time” – period available for the ATCO to react to a hazardous situation.

The longer look-ahead time may lead to more false alerts generated by MSAW. The shorter look-ahead time could lead to insufficient warning time for the controller.

An “adequate” warning time is the time sufficient for controller’s reaction, communication, pilot’s reaction and aircraft response and is an indicator for the proper setting of the MSAW function.

Flight settings

For optimising the MSAW function different types of flights may be specified for monitoring, such as civil, military, IFR flights, VFR flights, etc.

Secondary Surveillance Settings

The MSAW function can be set not to monitor specified secondary surveillance radar (SSR) code(s) or code blocks.

These can be SSR codes of traffic which is not under ATC or which operates regularly in close proximity to the ground (e.g. airport helicopters, aircraft taking part in special events).

Such settings may reduce the number of false alarms generated by the MSAW function.

Alert Inhibition

It may be necessary to inhibit alerts for predefined volumes of airspace (e.g. exercise areas) or for specific flights (e.g. Calibration Service Aircraft on a defined flight pattern) to suppress unnecessary alerts.

Operating MSAW

Hazardous situations related to aircraft altitude can remain unnoticed by the flight crew and the controller. The controller’s workload and priorities may cause an imminent hazardous situation to remain undetected if not alerted by MSAW. This may happen especially during heavy workload conditions. For the successful implementation of MSAW it is necessary to tune the function taking into account the specifications and local environment and to provide the relevant training to ATCOs and engineers.

ICAO Doc. 4444 PANS-ATM, Chapter 15, Para 15.7.4. specifies MSAW procedures: "Local instructions concerning the use of the MSAW function shall be specified, inter alia:

a) the types of flight which are eligible for generation of MSAW;
b) the sectors or areas of airspace for which MSAW minimum safe altitudes have been defined and within which the MSAW function is implemented;
c) the values of the defined MSAW minimum safe altitudes;
d) the method of displaying the MSAW to the controller;
e) the parameters for generation of MSAW as well as warning time; and
f) conditions under which the MSAW function may be inhibited for individual aircraft tracks as well as procedures applicable in respect of flights for which MSAW has been inhibited. "In the event an MSAW is generated in respect of a controlled flight, the following action shall be taken without delay:

a) if the aircraft is being vectored, the aircraft shall be instructed to climb immediately to the applicable safe level and, if necessary to avoid terrain, be assigned a new heading;
b) in other cases, the flight crew shall immediately be advised that a minimum safe altitude warning has been generated and be instructed to check the level of the aircraft. "Following an MSAW event, controllers should complete an air traffic incident report only in the event that a minimum safe altitude was unintentionally infringed with a potential for controlled flight into terrain by the aircraft concerned."


The performance of the MSAW function can be described as the best balance between warning time and nuisance alert, taking into account local environment. In this way the air traffic controller would be able to rely on the MSAW during the provision of service.


However the operational use of MSAW has not, always led to the best advantage being taken of its potential as a safety net. Investigations of accidents and serious incidents which occurred in an ATS environment where MSAW was available sometimes disclosed problems with the display of MSAW alerts to controllers, its selection and serviceability and with the operational procedures and associated training.


The use of MSAW would depend on the controller’s trust. Trust is a result of many factors such as reliability and transparency. Neither mistrust nor complacency is desirable; training and experience are needed to build trust at the appropriate level. An excessive amount of false alarms can reduce the ATCOs’ confidence in the MSAW.


Good practices of using the MSAW have shown that the increasing complexity of the MSAW and the environment in which it is used is addressed through appropriate training and competency assessment.

The primary goal of the training is to develop and maintain an adequate level of trust in MSAW, i.e. to make controllers aware of situations where MSAW is likely to be effective and, more importantly, situations in which MSAW will not be so effective (e.g. sudden, unexpected manoeuvres).

Statistical analysis

Retaining electronic records of all MSAW alerts generated by the appropriate ATS authority may facilitate the statistical analyses. The data and circumstances pertaining to each alert should be analysed to determine whether an alert was justified or not. Non-justified alerts, e.g. during visual approach, should be ignored. A statistical analysis should be made of justified alerts in order to identify possible shortcomings in airspace design and ATC procedures as well as to monitor overall safety levels.

Further reading


  1. ^ DTED is a standard National Geospatial-Intelligence Agency (NGA) product.