Definition
A low-power, low- or medium-frequency non-directional radio beacon (NDB) installed at the site of the middle marker on an instrument landing system (ILS) approach. It transmits a continuous signal that a pilot's automatic direction finder (ADF) can home in on, helping the aircraft intercept and follow the final approach course toward the runway.
Plain English
A small radio beacon placed at the middle marker of an ILS approach. The pilot's ADF needle points to it, giving an extra way to line up with the runway during the approach.
Context Anchor
Seen on some instrument approach charts, especially older ILS-related procedures that use marker beacon or locator information.
Derivation
"Locator" because it helps pilots locate and join the final approach course. "Middle" because it sits at the middle marker position along the ILS, roughly half a mile from the runway threshold. "Compass" because the ADF's needle behaves like a compass pointing toward the beacon.
Why Pilots Care
Gives pilots a reliable position fix during the approach, confirming they are at the correct distance to begin the final descent segment.
Intuition Check
“Middle” does not mean halfway down the runway or halfway through the flight. Here it means the specific middle marker location on an instrument approach.
Example Sentence 1
The approach chart showed an LMM at the middle marker, so the pilot tuned the ADF to use it as a backup reference on final.
Example Sentence 2
Crossing the LMM, the approach plate showed the decision altitude was now only 2.5 nautical miles away.