Definition
The reciprocal (opposite-direction) signal of a localizer course on an Instrument Landing System (ILS). Because the localizer antenna radiates signals in both directions from the runway, an inverse course extends out the opposite end of the runway, and this reverse signal is called the back course. It can sometimes be used for a non-precision instrument approach to the opposite runway, but it provides no glide slope guidance and the course-deviation indicator senses laterally reversed (a left needle means fly right, unless the avionics have a back-course mode that corrects this).
Plain English
The localizer signal that points out the back of the runway, opposite to the normal landing direction. Some airports use it for an approach to the other end of the runway, but the needle on your instrument moves the opposite way from what you're used to.
Context Anchor
Seen on instrument charts, localizer procedures, and intersection descriptions that use a localizer signal for position fixing.
Derivation
Back' here means 'rear' or 'opposite direction,' and 'course' means the path of the radio signal. So 'back course' literally means the course extending out the back side of the localizer antenna — the opposite direction from the normal front-course approach.
Why Pilots Care
Back-course approaches allow instrument landings to the opposite runway end but lack a glideslope and produce reverse needle sensing, requiring careful procedure briefing.
Grounding Statement
Picture a radio path lined up with a runway: one side is the normal approach path, and the path continuing out the other side is the back course.
Intuition Check
Back course does not mean flying backward or returning along the same route. It means the localizer course on the opposite side from the normal front course.
Example Sentence 1
Tower cleared us for the back course approach to Runway 13, so I briefed the reversed needle sensing before starting the descent.
Example Sentence 2
On the approach plate the back course procedure was noted as available for Runway 18 when winds favored that direction.