Definition
The signal radiated from the back (opposite) side of a localizer antenna, opposite the published front-course approach to the runway. When usable for navigation, it provides lateral guidance for an instrument approach to the reciprocal runway end, but without glide slope information and with reversed needle sensing on conventional course deviation indicators (a left needle indicates the aircraft is right of course, and vice versa) unless the pilot sets the published front-course inbound heading in the omni bearing selector or uses an HSI configured to correct for it.
Plain English
Most ILS antennas put out a strong signal toward the runway pilots are approaching, but they also leak a usable signal out the back. A back course approach uses that rear signal to line up with the opposite end of the runway. There is no glide slope, and on a basic instrument the left/right needle works backwards unless the pilot sets it up correctly.
Context Anchor
Seen on instrument approach procedures labeled as a localizer back course, often abbreviated in chart titles as LOC BC.
Derivation
"Back course" simply means the course on the back side of the antenna, opposite the main (front) course. The wording reflects the physical geometry of the localizer signal pattern.
Why Pilots Care
Allows a usable instrument approach to the reciprocal runway when no separate procedure exists.
Grounding Statement
Picture using the same runway alignment signal from the opposite side of the antenna: the line is still useful, but the cockpit indication may be reversed.
Intuition Check
Do not assume “back course” means a backup or secondary-quality localizer. It means the opposite-direction side of the localizer signal, and the needle may not behave the way it does on the front course.
Example Sentence 1
Cleared for the LOC BC Runway 13 approach, the pilot briefed the back course minimums and confirmed the HSI was set to the published front-course heading so the needle would sense correctly.
Example Sentence 2
ATC cleared the flight for the localizer back course approach when the front course was not available.