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Instrument Pilots: confused with Marker Beacons? So was I.

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The FAA materials and tests cover outer markers, marker beacons, locator outer markers (LOM) and compass locators to various degrees. However, I find the material incomplete and a bit confusing. This article will cover what the FAA does not.

First and foremost, Outer Markers are optional and being phased out:
"From the 1930s until the 1950s, markers were used extensively along airways to provide an indication of an aircraft's specific position along the route, but from the 1960s they have become increasingly limited to ILS approach installations. They are now very gradually being phased out of service"

If you don't see an OM published on the approach plate, it does not exist. Don't look for the light at the FaF. It will not happen.

Look at these 2 plates to see what to look for. It is shown in both in the Profile View and Plan View.
The whole system is really confusing. Here's my understanding:

Marker Beacon is the name of group of 3 transmitters along the approach path of a runway: outer marker, middle marker, inner marker.
Outer Marker is the general name of the component 4-7 miles out from runway. It usually is at the FaF but not necessarily.

An OM can be installed as a vertical radar transmitter (a simple outer marker), or as an NDB.
If the OM an NDB, it becomes known as a Locator Outer Marker, or LOM.
But.. the NDB portion of the LOM is called a Compass Locator. This makes some sense since you can navigate to it like any other NDB by tuning its frequency into your ADF, then when you'er directly overhead, your OM in the panel will light up. So 2 different components in your airplane will give you indication.

Remember: The advantage of LOM (the NDB version of an OM) is you can navigate to it with ADF, so it has 2 functions.

The airplane panel receiver is both visual and audio, and it detects both the NDB variation or simple vertical transmitter OM. You can identify a LOM from a distance by tuning into its NDB frequency with an ADF receiver (if you have one) and using IDENT. If you don't have an ADF, you can't really identify the LOM. If it's just an OM (no NDB), you can't identify it at all until you're right over it. Your "test" function in the panel, I think, just tests if the light bulb works.

Substituting OM

What if the OM is inoperative? Simple - it can be substituted with DME, GPS or VOR radial as long as that radial is published on the approach plate. Since the OM is optional and doesn't have to exist at all, you must use one of the substitutes to identify your FaF. This is just common sense.
​
I'm pretty happy they're phasing it out.

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