ILS Backcourse Setting RNAV
A backcourse is when an aircraft makes use of the localiser reverse sidelobes to fly an approach in a reciprocal direction to the main ILSILS —Instrument Landing System approach. These are banned in the UK and many other countries. The "sensing" on the instruments is quite tricky to explain, but essentially the only "correct" sensing is with an HSIHSI —Horizontal Situation Indicator with the front course selected. Because the lobes on the backcoursed have been mirror imaged and are therefore the wrong way around, the selected of a front course (when you want to fly the back course) "reverses the reverse" and means the HSIHSI —Horizontal Situation Indicator senses correctly. This option is not available with an OBIOBI —Omni Bearing Indicator.