Definition
A 75 MHz radio beacon associated with an Instrument Landing System (ILS) that transmits a narrow vertical signal upward from a point on the approach path approximately 3,500 feet from the runway threshold. As an aircraft passes over the middle marker on a standard ILS approach, it receives a coded audio tone (alternating dots and dashes) and a visual amber light indication in the cockpit, signaling that the aircraft is near the decision height for a Category I ILS approach.
Plain English
A radio beacon on the ground near the runway that sends a signal straight up. When you fly over it on an ILS approach, your cockpit gets a beep and a light, telling you that you are close to the runway and near the point where you must decide whether to land or go around.
Context Anchor
Seen during some instrument approaches, especially older ILS procedures and cockpit marker beacon indications.
Derivation
Called 'middle' because it sits between the outer marker (farther from the runway) and the inner marker (closest to the runway) along the ILS approach path.
Why Pilots Care
It provides a clear position and height check that supports the go/no-go decision at decision height, reducing the chance of continuing an unstabilized or unsafe approach in low visibility.
Intuition Check
Do not read “middle” as the middle of the runway or the middle of the whole flight. Here it means the middle marker beacon in a set of approach marker beacons.
Example Sentence 1
As we crossed the middle marker, the amber light flashed and we heard the alternating tone, confirming we were near decision height.
Example Sentence 2
The approach plate showed the MM symbol 0.6 NM from the threshold, so we began our landing flare preparation.