Definition
A low-power non-directional beacon (NDB) installed at the site of the middle marker of an instrument landing system (ILS). It transmits a continuous low-frequency radio signal that allows pilots to home or track to the middle marker location using an automatic direction finder (ADF), providing a backup means of identifying the final approach course and a fix near the runway threshold.
Plain English
A small radio beacon located at the middle marker of an ILS approach. It lets a pilot using an ADF receiver point the aircraft toward that spot, giving an extra way to confirm position close to the runway.
Context Anchor
Seen on instrument approach charts and in ILS approach discussions, usually near the final part of the approach to a runway.
Derivation
Locator describes the beacon's job — helping the pilot locate the final approach course. Middle marker refers to the point along the ILS where it is installed, roughly 3,500 feet from the runway threshold. Combined, the term names a beacon that locates the middle marker fix.
Why Pilots Care
Confirms the aircraft is at the correct point on the approach for altitude, timing, or landing decisions.
Intuition Check
Do not picture a painted sign or visual marker on the ground. In this term, the locator middle marker is a radio aid that marks a specific point on an instrument approach.
Example Sentence 1
Crossing the LMM, the pilot confirmed the ADF needle swung through the nose, verifying position near the middle marker.
Example Sentence 2
Passing the LMM, the crew verified they were at the published altitude before continuing the descent.