Definition
A low-power non-directional radio beacon (NDB) installed at the site of an Instrument Landing System (ILS) middle marker. The LM transmits a continuous signal that an aircraft's Automatic Direction Finder (ADF) can home in on, providing course guidance to the middle marker location on the final approach path.
Plain English
A small radio beacon placed at the ILS middle marker that an aircraft's direction-finding equipment can point toward, helping the pilot fly to that point on the approach.
Context Anchor
Seen in approach procedure information, navigation facility listings, and notices about older or specially equipped ILS approaches.
Derivation
The letter L stands for 'locator' (a low-power beacon that helps locate the approach course) and M stands for 'middle marker.' Compass locators were called that because pilots originally tracked them using a magnetic-compass-style ADF needle.
Why Pilots Care
It supplies a reliable position fix at a key point on the ILS glidepath, supporting precise vertical and lateral guidance during low-visibility landings.
Intuition Check
LM does not mean a magnetic compass in the cockpit. Here, it means a small radio beacon located at the ILS middle-marker position.
Example Sentence 1
The approach plate showed an LM co-located with the middle marker, so we tuned the ADF to its frequency as a position cross-check.
Example Sentence 2
On the approach plate the LM symbol indicates the location of the compass locator serving the ILS middle marker.