The outer marker (OM) is usually located 4-7 NM from the runway threshold. The middle marker (MM) is usually located 3,500 feet from the runway threshold. When the aircraft crosses the outer marker a flashing blue light will annunciate on the panel.

What is an outer marker?

Locator outer marker

An LOM is a navigation aid used as part of an instrument landing system (ILS) instrument approach for aircraft. Aircraft can navigate directly to the location using the NDB as well as be alerted when they fly over it by the beacon.

What is the outer marker on an ILS?

The outer marker is located approximately 3.9 nautical miles from the runway threshold and is aligned across the front beam of the localiser. Its purpose is to provide height, distance and equipment functioning checks to aircraft on final approach.