MKB

It stands for "Marker Beacon". It's purpose is to turn on the audible tone when you cross over one of these beacons on approach when the switch is on and to turn off that audible signal when the switch is off. Even without the tones, you still see the light signal on the panel light up when you cross one of them. If you look on the panel, you'll see either a vertical or horizontal array of 3 lights, 1 blue, 1 yellow, and one clear. These are the MKB indicators and are almost always marked identifying which beacon you are passing when they are lit.

Anticipating the next question, marker beacons are electronic signals on a final approach course that define specific points on the approach. They appear on the instrument procedure plate for the approach. This helps the pilot(s) know where they are on the approach in terms of distance from the runway.

There are three marker beacon types. The Outer Marker is typically located about five miles from the touchdown zone of the runaway. The Middle Marker is normally about 1/2 mile from the TDZ and the Inner Marker is at the beginning of the TDZ. They are interesting information when you are shooting an ILS approach but they are vital when shooting a VOR or NDB approach and there is no DME associated with that approach. Often, they are used to define a minimum crossing altitude and a point where you can begin a further descent or begin executing a missed approach.

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