Definition
A combined navigation aid located at the outer marker position of an Instrument Landing System (ILS) approach. It pairs a low-power non-directional beacon (NDB), which transmits a homing signal that an automatic direction finder (ADF) in the cockpit can point to, with an outer marker beacon that transmits a narrow vertical signal triggering a marker beacon receiver indication as the aircraft passes overhead. The LOM is typically located 4 to 7 nautical miles from the runway threshold along the final approach course.
Plain English
A two-in-one ground station near the start of the final approach to a runway. It sends out a steady radio signal that helps the pilot fly toward it, and it also sends a short upward signal that flashes a light and makes a tone in the cockpit when the aircraft passes directly over it.
Context Anchor
Seen on instrument approach charts and in instrument approach discussions, especially where the outer marker also has a locator beacon.
Derivation
Locator refers to the NDB's role in helping the pilot locate and track inbound to the final approach course. Outer Marker identifies the position of the marker beacon — the outer of the three traditional ILS marker beacons (outer, middle, inner).
Why Pilots Care
It establishes the final approach fix, where the pilot begins the final descent segment and verifies position and timing.
Intuition Check
Do not treat a LOM as distance-measuring equipment. It marks and identifies a specific location on the approach path; it does not continuously show your distance from the runway.
Example Sentence 1
Passing the LOM, the pilot saw the blue marker light flash, heard the steady tone, and confirmed the altitude on the approach chart.
Example Sentence 2
The approach plate showed the LOM 5.8 miles from the runway threshold.