Definition
The reciprocal (opposite) signal of a localizer course transmitted by an Instrument Landing System (ILS). The localizer antenna radiates two signals: the front course, which is aligned with the runway used for the published ILS approach, and the back course, which extends in the opposite direction off the other end of the antenna. A back course can be used for a non-precision instrument approach to the opposite runway when a back course procedure is published, but it provides no glideslope information and the course-deviation indicator displays reverse sensing unless the equipment is set to back course mode.
Plain English
The localizer antenna at an ILS-equipped airport sends its signal in two directions. The main beam guides aircraft to one runway end. The opposite-direction beam, called the back course, can sometimes be used to line up with the other runway end, but it has no glideslope and the needle moves backwards from what a pilot is used to.
Context Anchor
Seen in instrument approach procedures and localizer equipment discussions.
Derivation
Back simply means the opposite direction from the front course. The localizer transmitter physically radiates signals out both ends of its antenna, so the side opposite the published ILS approach became known as the back course.
Why Pilots Care
Allows instrument approaches on the non-standard side of the runway when the front-course ILS is unavailable.
Intuition Check
Back course does not mean flying backward or using a return route. It means using the opposite-direction side of a localizer guidance signal.
Example Sentence 1
With the ILS runway closed for maintenance, the crew briefed and flew the published back course approach to the opposite runway.
Example Sentence 2
On the back course the CDI needle indications reverse from normal localizer behavior.