Middle Marker

The middle marker (MM) is an ILS marker beacon located approximately 3,500 ft from the runway threshold. It marks the position on an ILS approach where an aircraft on the glide slope is approximately at the standard CAT I decision height of 200 ft AGL. The MM triggers an amber cockpit light and an alternating dot-dash (· − · −) audible tone.

Practical Significance

The middle marker traditionally marked the point at which the pilot makes the CAT I decision to land or go around. With modern avionics and precise GPS altimetry, the MM is less relied upon as a decision cue — the DH is calculated by the altimeter, not necessarily confirmed by marker passage. However, MM passage with the altimeter at the expected DH altitude provides a useful cross-check.

Related Terms

See also: Marker Beacon, Outer Marker, Inner Marker, Decision Height.