Definition
A low-power, low- or medium-frequency non-directional radio beacon installed at the site of the middle marker of an Instrument Landing System (ILS). It transmits a continuous signal that allows a pilot using an Automatic Direction Finder (ADF) to navigate to the middle marker position on the final approach course.
Plain English
A small radio beacon at the middle marker of an ILS approach. Pilots can use it with an ADF to fly toward that point on the final approach.
Context Anchor
Seen on some instrument approach charts and in older instrument landing system procedures, usually near the final part of the approach to a runway.
Derivation
Called a 'compass locator' because in earlier instrument flying the ADF needle was known as the 'compass needle,' and the beacon's job is to 'locate' the marker for the pilot. 'Middle' refers to its position at the middle marker of the ILS — between the outer marker and the runway.
Why Pilots Care
Confirms the aircraft's position and altitude at the middle marker so the pilot can cross-check the glide path and decide whether to continue the approach.
Intuition Check
Do not read “compass” as the magnetic compass in the panel. Here it means a radio-navigation signal that certain aircraft equipment can point toward. “Middle” does not mean the middle of the runway; it means the middle marker point on the approach path.
Example Sentence 1
The pilot tuned the ADF to the middle compass locator to back up the ILS approach into the airport.
Example Sentence 2
Passing the Middle Compass Locator, the crew confirmed they were on glide path before continuing the ILS approach.