Inner Marker – Canadian Flight Trainers

The inner marker is an ILS approach aid — a 75 MHz marker beacon that provides a position fix close to the runway threshold, typically 300–500 ft before the touchdown zone. It is associated with CAT II and CAT III ILS operations and indicates that the aircraft is near the decision point for these low-visibility approaches.

ILS Marker Beacons

Outer marker (OM): 4–7 NM from runway threshold; typically at the glide slope intercept altitude; blue light and slow dashes
Middle marker (MM): Approximately 3,500 ft from threshold; at CAT I DH; amber light and alternate dots and dashes
Inner marker (IM): 300–500 ft from threshold; used in CAT II/III; white light and rapid dots

DME Substitution

At many airports, DME distances published on the approach plate can substitute for marker beacon fixes. This reduces reliance on ground-based marker infrastructure, many of which have been decommissioned. Always check the approach plate to determine whether DME substitution is authorized.

Related Terms

See also: Outer Marker, Middle Marker, ILS.