Definition
A 75 MHz radio beacon transmitting a narrow vertical fan-shaped signal upward from a point on the ground approximately 4 to 7 miles from the runway threshold along the final approach course of an ILS. When the aircraft passes overhead, the airborne marker receiver activates a blue light and an audio tone of continuous dashes at 400 Hz, indicating the position at which the aircraft, when on glideslope, should be at the published glideslope intercept altitude.
Plain English
A small ground transmitter that sends a signal straight up. As the aircraft flies over it on an instrument approach, a blue light flashes and a low tone sounds in the cockpit, telling the pilot they have reached a known fixed point along the approach -- usually where the aircraft should begin tracking the glideslope down to the runway.
Context Anchor
Seen during an ILS approach when the aircraft passes over the outer marker and the cockpit marker light and tone indicate that point on the approach path.
Derivation
Called 'outer' because it is the marker farthest from the runway among the ILS markers (outer, middle, and sometimes inner). 'Marker' because it marks a specific point along the approach path. 'Beacon' from Old English beacen meaning 'sign' or 'signal' -- a fixed signal source whose purpose is to be detected from a known position.
Why Pilots Care
It marks the final approach fix where the pilot normally intercepts the glide slope and begins the final descent.
Grounding Statement
As the aircraft flies over the outer marker area, the receiver detects the signal and gives the pilot a cockpit indication that a known approach point has been crossed.
Intuition Check
“Outer” does not mean outside the airport, and “marker” does not mean a sign beside the runway. Here, it means a radio signal marking a specific point on the instrument approach.
Example Sentence 1
Crossing the outer marker, the pilot verified the altimeter showed the published glideslope intercept altitude.
Example Sentence 2
Crossing the outer marker beacon, the crew began timing the approach segment to the missed approach point.