Definition
A marker beacon used as part of an Instrument Landing System (ILS) that defines a point along the glide slope, normally located approximately 3,500 feet from the runway threshold. When the aircraft passes over it, the airborne marker beacon receiver activates an amber light and an audio tone of alternating dots and dashes at 1300 Hz.
Plain English
A small radio beacon on the ground near the runway. When you fly over it during an ILS approach, your cockpit gives you a light and a sound to confirm where you are on the approach.
Context Anchor
Seen on some instrument approaches to a runway, especially older instrument landing system approaches that still use marker beacons.
Derivation
Called 'middle' because it sits between the outer marker (farther from the runway) and the inner marker (closest to the runway), forming the middle of three position-confirming beacons along an ILS approach.
Why Pilots Care
It marks the approximate location of decision height, giving the pilot a clear cue to commit to landing or begin a missed approach.
Intuition Check
Do not think of the Middle Marker as a physical sign in the middle of the runway. It is a radio signal marking a point along the approach path near the runway.
Example Sentence 1
As we crossed the middle marker, the amber light flashed and we heard the alternating dot-dash tone, confirming we were close to decision height.
Example Sentence 2
Passing the middle marker confirmed the aircraft was on the glide slope and aligned with the runway centerline.