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I know that Minimum safe altitude warning (MSAW) alerts ATC and pilots if an airplane is under the risk of controlled flight into terrain.

But when is this alarm triggered? What is the minimum altitude for it to trigger? If a plane is landing, should it be triggered just before landing?

It will confuse ATC if every plane that is landing triggers that alarm.

There must be some limitations. So, what are they?

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  • $\begingroup$ I think I understand the question(s) here, but paragraph 2 has me, maybe, confused. $\endgroup$
    – CGCampbell
    Commented Feb 5, 2015 at 14:41
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    $\begingroup$ note that Part 91, 135, and 121 all require require onboard terrain awareness device for turbine aircraft of 6 or more pax, so ground radar-based warnings are less important today than they once were EG: law.cornell.edu/cfr/text/14/135.154 $\endgroup$
    – rbp
    Commented Feb 5, 2015 at 15:05
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    $\begingroup$ I think you are trying to refer to a terrain warning system in the ATC facility. If so, you should clarify this in your question. $\endgroup$
    – Maverick283
    Commented Feb 20, 2015 at 8:17
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    $\begingroup$ MSAW will be triggered if the altitude is already low, but also if the predicted altitude, based on current parameters, is low too. See SKYbrary $\endgroup$
    – mins
    Commented Feb 28, 2015 at 17:39
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    $\begingroup$ MSAW will not alert a pilot directly but rather just ATC. As mentioned above, the pilots will have a TAWS or GPWS system onboard. If you are interested in the flight conditions that will cause such a system to create an alarm to the pilots, this FAA document has the test conditions listed. $\endgroup$
    – Porcupine911
    Commented Mar 7, 2015 at 23:14

2 Answers 2

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The system projects the altitude that an aircraft will be at based on recent track and a calibrated "look ahead time" and an expected controller response time. That value is then compared to a calibrated altitude floor. If the aircraft is projected to be below the floor, then the controller will be alerted.

The specific values for all those things are calculated to minimize false alarms, accommodate the type of operation and local operational needs. The values are specific to that facility and that area and so a straight answer isn't exactly possible.

Here are a few interesting resources in case they help.
Minimum Safe Altitude Warning
EUROCONTROL MSAW Guide

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    $\begingroup$ I have rolled this back to revision 2. The name is correctly spelled EUROCONTROL. $\endgroup$
    – SentryRaven
    Commented Apr 30, 2015 at 11:25
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    $\begingroup$ Thank you. I had just made some formatting changes which it looks like you retained. How did I spell it? $\endgroup$
    – SOCPilot
    Commented Apr 30, 2015 at 11:54
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    $\begingroup$ EuroControl, which was fixed in rev2 and undone by you in rev3, looks like by accident if you were doing edits. $\endgroup$
    – SentryRaven
    Commented Apr 30, 2015 at 11:57
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    $\begingroup$ Yep. I just added line breaks to the links. Didn't like the way they ran up against each other. $\endgroup$
    – SOCPilot
    Commented Apr 30, 2015 at 12:01
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MSAW is one of a family of ground-based safety nets designed to provide warnings to air traffic controllers.

There is not a standard or performance specification for MSAW, so the features of each system varies depending on the system manufacturer, influenced sometimes by the needs expressed by the ANSP (Air Navigation Service Provider).

Many modern MSAW systems store the alerting surface as a grid of elevation values which represent the terrain (i.e. a terrain model). A margin above the terrain (typically a few hundred feet) is often included in the terrain model. Almost invariably, MSAW applies a simple linear prediction to determine whether each aircraft under ATC will be lower than the terrain model within a specified warning time or a 'look-ahead time'.

Some older MSAW systems which have no prediction are probably still in operational use - in these cases the MSAW surface may be set slightly higher, although the balance between warning time and nuisance alert rate may be harder to get right.

MSAW inhibition areas are defined to encompass the final approach paths to runways which allows aircraft to land at airports/airfields without triggering an MSAW alert. If APM (Approach Path Monitor) is available, then it often makes sense for the APM approach funnel and the MSAW inhibition area to have the same dimensions so that there is no gap in the protection afforded by the ground-based safety nets.

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    $\begingroup$ Hi Rod. This answer would be much easier to read if it was broken up into paragraphs. Can you edit your answer to add some blank lines here and there, wherever applicable? Doing so will break it up into paragraphs. $\endgroup$
    – user
    Commented Dec 20, 2016 at 15:23

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