Outer Marker

The outer marker (OM) is the farthest of the three ILS marker beacons from the runway, located approximately 4–7 NM from the threshold. It marks the position at which an aircraft on the ILS glide slope is at approximately 1,400–1,800 ft AGL — typically the point where the aircraft should intercept the glide slope from below during the approach.

Outer Marker Function

The OM identifies the initial approach fix for the ILS approach in many procedure designs. At OM passage, the pilot should be: established on the localizer, at or approaching the glide slope intercept altitude, configured for the approach (or completing configuration), and cleared for the approach. The OM light is blue; the audio is two dashes per second (− −).

OM Replacement

Many outer markers have been replaced by compass locator beacons (LOM — Locator Outer Marker) or by DME fixes. The approach plate specifies the OM equivalent for each ILS. Where the OM has been decommissioned, the approach must be crossed-checked with DME or GPS position.

Related Terms

See also: ILS, Marker Beacon, Middle Marker.