Definition
A 75 MHz radio transmitter located approximately 3,500 feet from the runway threshold on an Instrument Landing System (ILS) approach, marking the point where the aircraft is near the missed approach point during a Category I ILS approach. It transmits a vertical fan-shaped beam upward, triggering an amber light and an audio tone of alternating dots and dashes (about 95 Hz) in the cockpit when the aircraft passes overhead.
Plain English
A small ground-based radio transmitter near the runway that sends a signal straight up. When you fly directly over it on an instrument approach, your cockpit gets a flashing amber light and a beeping tone, telling you that you are close to the runway and near the point where you must either see the runway or go around.
Context Anchor
You encounter this term during ILS approach discussions and in the cockpit when a marker beacon receiver shows an amber light and plays its tone.
Derivation
Called the 'middle' marker because it sits between the outer marker (farther from the runway) and the runway itself. 'Marker' simply means it marks a specific point along the approach path; 'beacon' comes from Old English 'beacen', meaning a sign or signal.
Why Pilots Care
It marks the approximate decision height on a standard Category I ILS, confirming the aircraft's position relative to the runway for the landing decision.
Intuition Check
Middle does not mean the middle of the runway. It means the middle marker in the ILS marker-beacon system, positioned near the runway end compared with the outer marker farther away.
Example Sentence 1
As the amber light flashed and the dot-dash tone sounded, the pilot crossed the middle marker beacon and began looking for the runway lights.
Example Sentence 2
The pilot noted the middle marker beacon light while cross-checking altitude and glide slope.