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Here are some NOTAMS for a recent flight I was planning:

LXY 03/002 LXY NAV NDB U/S 2203081935-PERM

CRS 02/008 CRS NAV NDB UNUSABLE 2202141320-2206142111EST

ANB 05/580 SJI NAV VOR NOT MNT 2205201953-2205232000EST

We all see these kinds of NOTAMS all the time. In each case, I would consider each NAVAID to be unfit for navigational planning or flight use purposes, meaning that I now consider all of them out of service and not to be used. But is there any actual semantic difference between the qualifier in these three cases? Or do they all just mean, "it's not working, don't use it!"? If so, why are three separate qualifiers used when UNUSABLE would be sufficient?

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UNSERVICEABLE (abbreviated U/S) is the ICAO version of what the FAA used to call OUT OF SERVICE (abbreviated OTS). To my mind this is less clear and a step backwards, but at least it's in line with the rest of the world now.

UNUSABLE comes out to mean the same thing as UNSERVICEABLE: you cannot use the NAVAID. In my experience UNUSABLE or NOT USABLE is more common when referring to specific radials of a NAVAID where the signal is blocked by obstructions below a certain altitude, but the NAVAID itself is still operational. As you can see from your example, however, the entire NAVAID may be deemed NOT USABLE.

MNT is the abbreviation for MONITORED. To say a NAVAID is NOT MONITORED means that the fault-reporting equipment is itself UNSERVICEABLE, or that the facility responsible for monitoring that equipment is closed. Thus the NAVAID may be perfectly operational, or it may not be; the first person to know if it isn't will be the pilot who tries to use it. Procedures which depend on NAVAIDs that are NOT MONITORED may not be used for alternate-airport planning.

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    $\begingroup$ Can a pilot generally expect that a navigation aid which appears to be functioning will be supplying correct data, i.e. are navigation aids, whether monitored or not, designed in such a way that they will shut down in the presence of any faults that would otherwise cause them to transmit inaccurate data? $\endgroup$
    – supercat
    Commented May 22, 2022 at 17:02
  • $\begingroup$ Not quite sure of that, @supercat; that would be a good question on its own. My vague understanding is that the "monitoring" equipment simply checks for the presence of any signal on the correct frequency, while actual validation of the signal must be performed from the air (i.e. Flight Check). $\endgroup$
    – randomhead
    Commented May 23, 2022 at 1:35
  • $\begingroup$ @supercat, ILS has a failure mode where the instrument will indicate on center, with no fault flag, but the system is actually not working. The monitoring equipment is supposed to check for this case, but without that there are known incidents where that happened. $\endgroup$
    – Jan Hudec
    Commented May 24, 2022 at 21:16
  • $\begingroup$ @JanHudec: Are there fail-safes in ILS to ensure that if an antenna gets knocked out of position, it will transmit either nothing or some kind of error indication, rather than transmitting an incorrect glide slope? $\endgroup$
    – supercat
    Commented May 24, 2022 at 21:26

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