Definition
A 75 MHz marker beacon associated with the back course of an ILS (Instrument Landing System) localizer. When an aircraft flies over a BCM during a back-course approach, the airborne marker beacon receiver picks up the signal and provides a visual and audio indication in the cockpit, alerting the pilot to a specific position along the back-course final approach path.
Plain English
A small radio transmitter on the ground that flashes a light and sounds a tone in the cockpit when you fly over it. It tells you that you've reached a known point during a back-course approach to a runway.
Context Anchor
Seen in ILS and localizer back-course approach discussions, especially when identifying fixes or position points on an instrument approach.
Derivation
Back course' refers to the localizer signal extending behind the runway, in the opposite direction from the normal approach. 'Marker' comes from its job: marking a specific point along the approach path. The name simply tells you what it is and where you'll find it.
Why Pilots Care
Provides essential position awareness on back-course approaches that usually lack glide-slope guidance, allowing accurate timing of descent and missed-approach points.
Intuition Check
Do not read “back-course” as meaning the airplane is flying backward. It means the aircraft is using the opposite-side course of the localizer signal.
Example Sentence 1
As they crossed the BCM, the marker light flashed white and the pilot began the final descent to minimums.
Example Sentence 2
The BCM signal confirmed the aircraft had reached the step-down fix before continuing toward the runway on the localizer back course.