Definition
A low-power, low/medium frequency (L/MF) non-directional radio beacon installed at the site of the middle marker of an Instrument Landing System (ILS), used as an aid for navigating to the final approach course. It transmits a continuous identification signal and can be received by a standard ADF (Automatic Direction Finder) in the aircraft.
Plain English
A small radio beacon located at the middle marker of an ILS approach. The pilot can home in on it using the ADF needle to help line up with the runway during an instrument approach.
Context Anchor
Seen on some instrument approach charts and in instrument landing system discussions, especially under compass locator equipment.
Derivation
"Locator" comes from the Latin locus, meaning "place." Its job is to help the pilot locate the final approach course. "Middle" refers to its position at the middle marker of the ILS, roughly half a mile from the runway threshold.
Why Pilots Care
It lets pilots confirm they are at the correct point to check altitude and decide whether to continue the approach or go missed.
Intuition Check
Do not read middle locator as any locator that happens to be in the middle of an airport. In this context, it means a specific radio beacon tied to the middle-marker point of an instrument landing system approach.
Example Sentence 1
The approach chart showed a middle locator at the middle marker, so the pilot tuned the ADF to its frequency as a backup reference.
Example Sentence 2
Hearing the middle locator's identifier confirmed they had reached the point to begin timing for the missed approach.