Wednesday, July 18, 2018
Transport Canada - Aviation Safety Letter A New Layer of Safety—Minimum Safe Altitude Warning (MSAW) - ISSUE 4/2009

A New Layer of Safety—Minimum Safe Altitude Warning (MSAW) - ISSUE 4/2009

by Bill Crawley, Manager, ATS System Integration, NAV CANADA


Pilots flying in mountainous terrain face a number of dangers, including inadvertently flying below the minimum safe altitude and flying in icing situations where the aircraft cannot reach or maintain a safe altitude. Pilots rely on a number of methods to mitigate the dangers, including on-board ground proximity warning systems (GPWS), published minimum vectoring altitudes, as well as pilot and controller knowledge of terrain. NAV CANADA recently developed a new safety net, the minimum safe altitude warning (MSAW), that can be used by air traffic services (ATS) to help prevent flight into obstacles and terrain.

How does MSAW assist controllers?
MSAW produces visual and aural cues to controllers for aircraft whose flight vector puts them in a predicted or immediate conflict with a digital model of surrounding terrain or obstacles. The MSAW functionality also includes tools that can be used by the controller to aid aircraft that are not in an MSAW condition, such as an aircraft that is experiencing icing conditions and cannot maintain its current altitude. In such situations, the controller can initiate the display of terrain contours surrounding any point on the display, or they can be dynamically associated with a manoeuvring aircraft, thereby providing tactical terrain information that can be relayed to the pilot.

If an aircraft is in a predicted or immediate terrain/obstacle conflict situation, the controller is alerted by flashing indications in the subject aircraft’s data tag. The data tag is linked to the aircraft’s on-screen target and contains important information about the flight, such as the aircraft call sign, altitude and speed. The MSAW alert indicates:

  • the height of the offending terrain/obstacle;
  • an indication of the “immediate safe altitude,” which is the height of the highest terrain, plus adapted buffers, within a 2-min look ahead of the aircraft and 45° each side of the aircraft’s track; and
  • in the case of a predicted MSAW event, the time to fly to the object.

These visual indications are accompanied by an audible voice alarm, enunciated at the controller’s display, which further helps to draw attention to the MSAW condition.

Collaboration between NAV CANADA and various operators led to the development of compatible controller and pilot procedures. If a controller receives an MSAW notification, specific phraseology will be used to inform the pilot, depending on the nature of the situation. For example, the controller may verify the pilot’s intentions and/or verify the altimeter setting that is in use:



Or, the controller may ask the pilot about their awareness of terrain:


If appropriate, the controller will provide direction based on the displayed MSAW information:



How does MSAW work?
MSAW performs processing of aircraft trajectories against adapted airspace volumes that define airspace to protect around terrain and known obstacles. An adaptable vertical buffer is added to the ceiling of the digital terrain model and to the height of known obstacles to derive the final height that MSAW will protect against. Different vertical buffers can be applied independently to terrain and obstacles.

MSAW employs adaptable horizontal and vertical “look-ahead” time parameters that are used to predict the trajectory of an aircraft. It is possible to adapt different look-ahead values for different areas. For example, it may be desirable to have greater look-ahead times for en route aircraft than for aircraft operating in the vicinity of an airport.

Test phase
The MSAW functionality was initially turned on in the Vancouver, B.C., area control centre (ACC) in June 2008. During this first on-test phase, the MSAW functionality was limited to a 50-NM radius centered on the Prince George, B.C., airport. The other component of MSAW, the on-demand display of background contours (at 1 000-ft increments) was enabled at all sectors in the Vancouver ACC. On the afternoon of June 19, 2008, in the Airports Specialty, the MSAW background feature was used to help a Caravan that was in an emergency icing situation. The controller was able to relay terrain clearance information to the aircraft through an American B777, and the aircraft was then able to descend below icing levels and land safely.

The Prince George MSAW test phase has been positive, with only a couple of adjustments required to the MSAW adaptation. The test phase is now complete and the next phase for Vancouver will involve the implementation of the MSAW function in the remainder of the Airports Specialty, excluding small areas immediately around all of the airports except Prince George.


This article was published by Transport Canada in TP 185E -. Reprinted with permission

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